Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Liu, Haoyang
Master of Applied Science - MASc
University of British Columbia
For structural components used in leisure industries, such as those in aquaparks, high load-bearing capacity and lightweight materials are of primary interest. For this purpose, rigid fibre reinforced polymer (FRP) composites are currently used widely. However, they often result in large and stiff moulded parts, which lead to logistic challenges as well as added cost associated with transportation and handling. FRP materials are also difficult to recycle, raising concerns over environmental sustainability. One proposed solution may be to replace such legacy materials with flexible, durable, and recyclable alternatives, while redesigning the support structures so that the overall integrity and load-bearing capacity remain unchanged. The high flexibility of the alternative materials can enable them to be rolled or folded (i.e., reduce occupied space) during transportation. Towards the above motivation, this thesis is a preliminary investigation towards replacing glass fibre reinforced polymer (GFRP) composites used in aquatic play structures with flexible, durable, and recyclable alternatives. Two thermoplastic polyurethanes (TPU) and one thermoplastic vulcanizate (TPV) were chosen as candidate materials. The physical and mechanical properties of the legacy (base) and candidate materials were characterized through a series of coupon-level experiments and imaging. Material samples were also subjected to weathering exposures, including water and chlorinated water immersion, hygrothermal aging, and accelerated weathering in xenon lamp chamber. GFRP exhibited superior tensile strength and modulus, while the TPE candidates showed higher flexibility and strain at break. TPU showed better abrasion and puncture resistance than the TPV candidate, but higher water absorption. Yellowing could be observed for TPU after weathering in xenon lamp chamber. No statistically significant changes in the tensile properties could be concluded for all materials upon weathering. Next, building on the above characterization test data, a multicriteria decision making method, namely PROMETHEE II, was implemented to evaluate and rank the legacy and candidate materials based on typical design criteria including cost, mechanical properties, weathering performance, and recyclability. Texin 285A (TPU) was ranked as the top choice. Design ideas to incorporate the flexible material into water slide flumes, including a honeycomb sandwich structure design and a diaphragm design, were also explored.
TOWARDS REDESIGNING LEGACY GFRP-CONSTRUCTED AQUATIC STRUCTURES WITH FLEXIBLE, DURABLE, AND RECYCLABLE ALTERNATIVE MATERIALS: A MULTICRITERIA APPROACH by Haoyang Liu BSME, Purdue University, USA, 2021 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) September 2023 © Haoyang Liu, 2023 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis entitled: Towards Redesigning Legacy GFRP-Constructed Aquatic Structures with Flexible, Durable, and Recyclable Alternative Materials: A Multicriteria Approach submitted by Haoyang Liu in partial fulfillment of the requirements of the degree of Master of Applied Science. Dr. Abbas S. Milani, School of Engineering Supervisor Dr. Joshua Brinkerhoff, School of Engineering Supervisory Committee Member Dr. Ahmad Rteil, School of Engineering Supervisory Committee Member Dr. Solomon Tesfamariam, School of Engineering University Examiner iii Abstract For structural components used in leisure industries, such as those in aquaparks, high load-bearing capacity and lightweight materials are of primary interest. For this purpose, rigid fibre reinforced polymer (FRP) composites are currently used widely. However, they often result in large and stiff moulded parts, which lead to logistic challenges as well as added cost associated with transportation and handling. FRP materials are also difficult to recycle, raising concerns over environmental sustainability. One proposed solution may be to replace such legacy materials with flexible, durable, and recyclable alternatives, while redesigning the support structures so that the overall integrity and load-bearing capacity remain unchanged. The high flexibility of the alternative materials can enable them to be rolled or folded (i.e., reduce occupied space) during transportation. Towards the above motivation, this thesis is a preliminary investigation towards replacing glass fibre reinforced polymer (GFRP) composites used in aquatic play structures with flexible, durable, and recyclable alternatives. Two thermoplastic polyurethanes (TPU) and one thermoplastic vulcanizate (TPV) were chosen as candidate materials. The physical and mechanical properties of the legacy (base) and candidate materials were characterized through a series of coupon-level experiments and imaging. Material samples were also subjected to weathering exposures, including water and chlorinated water immersion, hygrothermal aging, and accelerated weathering in xenon lamp chamber. GFRP exhibited superior tensile strength and modulus, while the TPE candidates showed higher flexibility and strain at break. TPU showed better abrasion and puncture resistance than the TPV candidate, but higher water absorption. Yellowing could be observed for TPU after weathering in xenon lamp chamber. No statistically significant changes in the tensile properties could be concluded for all materials upon weathering. Next, building on the above characterization test data, a multicriteria decision making method, namely PROMETHEE II, was implemented to evaluate and rank the legacy and candidate materials based on typical design criteria including cost, mechanical properties, weathering performance, and recyclability. Texin 285A (TPU) was ranked as the top choice. Design ideas to incorporate the flexible material into water slide flumes, including a honeycomb sandwich structure design and a diaphragm design, were also explored. iv Lay Summary Replacing rigid structural materials with flexible, durable, and recyclable polymers could reduce the current challenges and cost associated with transportation and handling of large components used in leisure industries (e.g., aquaparks), while promoting sustainability. Towards this motivation and industrial application, this research focused on replacing rigid glass fibre reinforced polymer composites in select aquatic play structures with thermoplastic elastomer alternatives. Mechanical properties characterization and weathering performance tests were conducted to establish the baseline performance of the legacy (current) material and investigate the feasibility of the new candidate materials to be used in outdoor aquatic environments. A multicriteria decision making model was then implemented to rank the material alternatives. Finally, preliminary design ideas to incorporate the new top ranked flexible material in place of the rigid materials in water slides, including a honeycomb sandwich design and a diaphragm design, were investigated and prototyped. v Preface This thesis entitled “Towards Redesigning Legacy GFRP-Constructed Aquatic Structures with Flexible, Durable, and Recyclable Alternative Materials: A Multicriteria Approach” presents the original work by Haoyang Liu, under the supervision of Dr. Abbas S. Milani at The University of British Columbia. GFRP (base) material was provided by Waterplay Solutions Inc. TPE materials were supplied by Nexeo Plastics. The compression moulding of TPE materials was done by Haoyang Liu. All sample cutting was carried out by the School of Engineering machine shop technicians at UBC. All coupon-level testing was performed by Haoyang Liu. SEM imaging was done at the Fipke Laboratory for Trace Element Research (FiLTER) at UBC. The manufacturing and mechanical testing of honeycomb sandwich and diaphragm samples were done by Haoyang Liu, Amin Batouei, and Nadia Abdollahi. A summarized version of this thesis is under submission for a journal publication. vi Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ............................................................................................................................... iv Preface ............................................................................................................................................ v Table of Contents ......................................................................................................................... vi List of Tables ................................................................................................................................. x List of Figures .............................................................................................................................. xii List of Symbols ............................................................................................................................ xv List of Abbreviations ................................................................................................................ xvii Acknowledgements .................................................................................................................. xviii Dedication ................................................................................................................................... xix Chapter 1: Introduction ............................................................................................................... 1 1.1 Background ...................................................................................................................... 1 1.2 Motivation and Objectives ............................................................................................... 1 1.3 Thesis Framework ............................................................................................................ 3 Chapter 2. Literature Review ...................................................................................................... 5 2.1 Legacy Material: Glass Fibre Reinforced Polymers (GFRP) .......................................... 5 2.1.1 GFRP Applications ................................................................................................... 5 2.1.2 GFRP Recycling ....................................................................................................... 6 2.2 Candidate Materials Class: Flexible Polymers................................................................. 6 2.2.1 Plasticizers ................................................................................................................ 7 2.2.2 Thermoplastic Elastomers ......................................................................................... 7 2.3 Hysteresis and Mullins Effect ........................................................................................ 14 2.4 Polymer Weathering ....................................................................................................... 15 2.4.1 Photooxidation Degradation ................................................................................... 16 2.4.2 Hydrolytic and Chlorinated Water Degradation ..................................................... 16 2.4.3 Accelerated Weathering Performance .................................................................... 17 2.5 Summary ........................................................................................................................ 18 vii Chapter 3: Materials and Methods ........................................................................................... 19 3.1 Materials ......................................................................................................................... 19 3.1.1 The base GFRP material and aqua structure ........................................................... 19 3.1.2 Santoprene 121-67W175 ........................................................................................ 20 3.1.3 Texin 285A ............................................................................................................. 20 3.1.4 Texin 255D ............................................................................................................. 21 3.2 Sample Preparation ........................................................................................................ 21 3.2.1 GFRP....................................................................................................................... 21 3.2.2 TPE ......................................................................................................................... 22 3.3 Selected Mechanical Characterization Methods ............................................................ 23 3.3.1 Density Measurement ............................................................................................. 24 3.3.2 Tensile Test ............................................................................................................. 24 3.3.3 Three-Point Bending Test ....................................................................................... 25 3.3.4 Cyclic Tensile Test ................................................................................................. 27 3.3.5 Hardness Test .......................................................................................................... 28 3.3.6 Abrasion Test .......................................................................................................... 28 3.3.7 Friction Test ............................................................................................................ 29 3.3.8 Puncture Test .......................................................................................................... 30 3.3.9 Impact Test.............................................................................................................. 31 3.4 Weathering Characterization Methods ........................................................................... 32 3.4.1 Water Immersion .................................................................................................... 32 3.4.2 Chlorinated Water Immersion................................................................................. 33 3.4.3 Accelerated Hygrothermal Aging ........................................................................... 34 3.4.4 Accelerated Weathering in Xenon Lamp Chamber ................................................ 35 3.5 Microscopy Characterization Methods .......................................................................... 36 3.5.1 CLSM ...................................................................................................................... 36 3.5.2 SEM ........................................................................................................................ 37 3.6 Summary ........................................................................................................................ 37 viii Chapter 4: Results and Discussion ............................................................................................ 38 4.1 Control Sample Performance ......................................................................................... 38 4.1.1 Density and Tensile Properties ............................................................................... 38 4.1.2 Hardness, Abrasion Resistance, and Flexural Properties........................................ 42 4.1.3 Cyclic Tensile Response ......................................................................................... 45 4.1.4 Puncture Resistance ................................................................................................ 47 4.1.5 Surface Roughness and Frictional Properties ......................................................... 48 4.1.6 Impact Resistance ................................................................................................... 49 4.2 Weathering Performance ................................................................................................ 51 4.2.1 Appearance, Surface Morphology, and Mass Change from Xenon Lamp Chamber Weathering ............................................................................................................................ 51 4.2.2 Water Absorption .................................................................................................... 55 4.2.3 Tensile Properties.................................................................................................... 57 4.3 Summary ........................................................................................................................ 62 Chapter 5: Material Alternatives Ranking: An MCDM Approach ....................................... 63 5.1 Introduction .................................................................................................................... 63 5.1.1 PROMETHEE II ..................................................................................................... 63 5.2 Application of PROMETHEE II to material ranking for aquatic play structures .......... 67 5.2.1 PROMETHEE II Complete ranking ....................................................................... 67 5.2.2 Sensitivity Analysis ................................................................................................ 70 5.2.3 Consideration of Redesign Cost.............................................................................. 72 5.3 Summary ........................................................................................................................ 74 Chapter 6: Incorporation of the Selected Flexible Material into Actual Aquatic Play Structure: Preliminary Redesign Ideas .................................................................................... 75 6.1 Introduction .................................................................................................................... 75 6.2 Design Concepts for Incorporating the Texin 285A Flexible Material ......................... 75 6.2.1 Concept 1: Extensive Mesh Support Structure ....................................................... 75 6.2.2 Concept 2: Honeycomb Sandwich Structure .......................................................... 75 ix 6.2.3 Concept 3: Flexible Diaphragm .............................................................................. 80 6.3 Summary ........................................................................................................................ 84 Chapter 7: Conclusions, Limitations, and Future Work Recommendations ........................ 85 7.1 Conclusions .................................................................................................................... 85 7.2 Limitations and Future Work Recommendations .......................................................... 88 7.3 Contributions to Knowledge .......................................................................................... 91 Bibliography ................................................................................................................................ 92 x List of Tables Table 1. Performance characteristics of some TPE materials [69]. ............................................. 11 Table 2. Common processing methods for thermoplastics [57]................................................... 13 Table 3. TPE compression moulding parameters......................................................................... 23 Table 4. Exposure cycle parameters for hygrothermal cyclic aging. ........................................... 35 Table 5. Exposure cycle parameters for accelerated weathering in xenon lamp chamber ........... 36 Table 6. Density and tensile properties of legacy and candidate materials; average values are in bold and SD values are given in brackets. .................................................................................... 42 Table 7. Hardness, mass loss from abrasion, and flexural properties of legacy and candidate materials; average values are in bold and SD values are given in brackets. ................................. 44 Table 8. Cyclic tensile test phase 1 results. .................................................................................. 46 Table 9. Cyclic tensile test phase 2 results. .................................................................................. 47 Table 10. Root mean square roughness (Sq) and kinetic coefficient of friction (µk) of GFRP, Santoprene, and Texin 285A; average values are in bold and SD values are given in brackets. . 49 Table 11. Results from drop-weight impact tests of GFRP; average values are in bold and SD values are given in brackets. ......................................................................................................... 51 Table 12. Mass change of specimens after weathering conditioning exposures; average values are in bold and SD values are given in brackets. ................................................................................ 56 Table 13. Two-tailed t-test results for the mass change of materials after immersion in chlorinated water at different free chlorine concentrations (p-values showing statistically significant difference at 95% confidence level are bolded). ............................................................................................ 57 Table 14. Effects of accelerated weathering on sample tensile properties; average values are in bold and SD values are given in brackets. .................................................................................... 58 Table 15. Two-tailed t-test results for the tensile properties of materials after weathering conditioning (p-values showing statistically significant difference at 95% confidence level are bolded). ......................................................................................................................................... 60 Table 16. Sample evaluation table. .............................................................................................. 64 Table 17. Sample weights of relative importance table. .............................................................. 64 Table 18. Evaluation table for material selection for aquatic play structures.  indicates a criterion for which a higher value is preferred, and  indicates a criterion for which a lower value is preferred. ....................................................................................................................................... 67 xi Table 19. Five-point Likert scale employed................................................................................. 69 Table 20. Importance scores and weights of relative importance for material ranking for aquatic play structures. .............................................................................................................................. 69 Table 21. PROMETHEE II ranking of alternatives for material selection for aquatic play structures. ...................................................................................................................................... 70 Table 22. Original, dilated, and concentrated weights. ................................................................ 72 Table 23. Material alternatives ranking based on original, dilated, and concentrated weights. .. 72 Table 24. Original and new weights with redesign cost considered. ........................................... 73 Table 25. Material alternatives ranking based on original and new weights with redesign cost considered. .................................................................................................................................... 74 xii List of Figures Figure 1. (a) Aquatic play structures made from GFRP; (b) typical GFRP water slide flume parts in a trailer. ....................................................................................................................................... 2 Figure 2. Classification of thermoplastic elastomers, adapted from [57], [65]. ............................ 8 Figure 3. Morphology of styrenic block copolymers [65]. ............................................................ 9 Figure 4. Morphology of multiblock copolymers [69]. ............................................................... 10 Figure 5. (a) Interdispersed co-continuous elastomer and hard polymer phases, and (b) elastomer phases dispersed in hard polymer matrix after dynamic vulcanization; adapted from [65]. ........ 10 Figure 6. Stress–strain responses of a filled rubber submitted to a simple uniaxial tension and to a cyclic uniaxial tension with increasing maximum stretch every 5 cycles [82]. ............................ 15 Figure 7. Water slide flume section made from GFRP. Samples were cut from the curved portion of the flume (circled in the image). ............................................................................................... 20 Figure 8. (a) Santoprene 121-67W175, (b) Texin 285A, and (c) Texin 255D ‘as-received’ pellets........................................................................................................................................................ 21 Figure 9. (a) GFRP sheets and (b) GFRP test specimens after water jet cutting. ........................ 22 Figure 10. (a) Aluminum mould with TPE pellets inside, (b) closed mould, (c) hydraulic press, (d) compression moulded TPE sheets, and (e) test specimens cut. .............................................. 23 Figure 11. (a) GFRP tensile specimen in wedge grips and (b) Texin 285A tensile specimen in scissor grips. .................................................................................................................................. 25 Figure 12. GFRP sample in three-point bending fixture. ............................................................. 26 Figure 13. Texin 285A specimen mounted on the Taber Abrasion Tester, undergoing an abrasion test. ................................................................................................................................................ 28 Figure 14. (a) Friction test sled with polyester fabric clamped to the bottom (setup for Santoprene and Texin 285A), (b) sled with square GFRP taped to the bottom to match the curvature of rectangular GFRP specimen, and (c) polyester fabric clamped over the square GFRP. .............. 29 Figure 15. Coulomb friction test setup......................................................................................... 30 Figure 16. Texin 285A specimen in puncture test fixture............................................................ 31 Figure 17. Impact test setup in Instron CEAST 9340 system. ..................................................... 32 Figure 18. (a) Chlorinating granules in a petri dish, (b) chlorinated granules dissolved into RO water, and (c) specimens immersed in chlorinated water with 3, 5, and 10 ppm free chlorine content (left to right). ................................................................................................................................. 34 xiii Figure 19. (a) Atlas SUNTEST XXL+ xenon lamp weathering chamber and (b) specimens placed inside the chamber prior for weathering as well as repositioning sequence indicated by the arrows........................................................................................................................................................ 36 Figure 20. Example GFRP load-extension curve (a) before toe compensation, (b) with toe region removed, and (c) after horizontal shift to intercept the origin. ..................................................... 39 Figure 21. Load-extension curves of GFRP, Santoprene, Texin 285A, and Texin 255D specimens. The test was repeated three times for each material. .................................................................... 40 Figure 22. Load-extension curves of (a) GFRP, (b) Santoprene, (c) Texin 285A, and (d) Texin 255D specimens. Graphed on separate scales. .............................................................................. 41 Figure 23. GFRP specimen failure under tension. ....................................................................... 41 Figure 24. Candidate material specimens after 1000 abrasion cycles. From left to right: Santoprene, Texin 285A, Texin 255D. ......................................................................................... 42 Figure 25. Typical load-deflection curve (a) prior to toe compensation and (b) after toe compensation. ............................................................................................................................... 43 Figure 26. Flexural stress-strain curves of (a) GFRP, (b) Santoprene, (c) Texin 285A, and (d) Texin 255D specimens. Each test was repeated three times. ........................................................ 44 Figure 27. Stress-strain curves of (a) Santoprene and (b) Texin 285A from cyclic tensile test phase 1. The loading paths are indicated by the arrows. ......................................................................... 45 Figure 28. Stress-strain curves of (a) Santoprene and (b) Texin 285A from cyclic tensile test phase 2..................................................................................................................................................... 46 Figure 29. (a) Puncture force-deflection curves for Santoprene and Texin 285A specimens and (b) puncture load and deflection. ........................................................................................................ 48 Figure 30. (a) Santoprene specimen and (b) Texin 285A specimen after puncture test. ............. 48 Figure 31. GFRP specimens after drop-weight impact: (a) top side and (b) bottom side. .......... 50 Figure 32. (a) Force vs. time from drop-weight impact tests of GFRP; three test repeats are shown........................................................................................................................................................ 51 Figure 33. GFRP (left), Santoprene (centre), and Texin 285A (right) specimens (a) before and (b) after xenon lamp chamber weathering. ......................................................................................... 52 Figure 34. SEM images of surfaces of control unaged samples and samples that were aged in xenon lamp weathering chamber. (a) Unaged GFRP; (b) Aged GFRP; (c) Unaged Santoprene; (d) Aged Santoprene; (e) Unaged Texin 285A; (f) Aged Texin 285A. .............................................. 54 xiv Figure 35. Mass change of specimens after aging in xenon lamp weathering chamber. ............. 55 Figure 36. Mass change of specimens after weathering conditioning exposures. ....................... 56 Figure 37. Tensile strength, (b) Young's modulus, and (c) strain at break of specimens after weathering conditioning, expressed as percentages of control sample performance. .................. 59 Figure 38. SEM images of tensile fracture surfaces of control unaged samples and samples that were aged in xenon lamp weathering chamber: (a) unaged Santoprene, (b) aged Santoprene, (c) unaged Texin 285A, and (d) aged Texin 285A............................................................................. 61 Figure 39. V-shape preference function. ...................................................................................... 66 Figure 40. (a) Texin 285A face sheet, (b) aluminum honeycomb core, and (c) honeycomb sandwich specimen. ...................................................................................................................... 76 Figure 41. (a) Midpoint loading configuration and (b) quarter point loading configuration. ...... 77 Figure 42. Midspan loading specimen (a) at maximum deflection and (b) after load removal. .. 78 Figure 43. Honeycomb sandwich specimens after flexural test: (a) midpoint loading, top side, (b) midpoint loading, bottom side, (c) quarter point loading, top side, and (d) quarter point loading, bottom side. ................................................................................................................................... 78 Figure 44. Honeycomb sandwich structure flexural test load-deflection curves. ........................ 79 Figure 45. Flexural stiffness vs. load for honeycomb sandwich structure. .................................. 80 Figure 46. Conceptual design of water slide flume section: (a) side walls and (b) flexible diaphragm sheet (pink) draped over side walls............................................................................. 81 Figure 47. (a) Aluminum block used in diaphragm specimen, (b) epoxy application and clamping, and (c) finished diaphragm specimen (top view). ......................................................................... 82 Figure 48. Testing of diaphragm prototype on a three-point bending fixture: (a) before test, (b) at maximum displacement, and (c) after load removal. .................................................................... 83 Figure 49. Force-displacement diagram from diaphragm prototype testing. ............................... 84 xv List of Symbols Tg glass transition temperature σ tensile stress P load at a given point on the load-deflection curve A average cross-sectional area Ftu ultimate tensile strength Pmax maximum force before failure R rate of crosshead motion L support span d depth of beam Z rate of straining of the outer fibre σf stress in the outer fibers at midpoint b width of beam εf strain in the outer surface Δ deflection of the centre of the beam EB modulus of elasticity in bending m slope of the tangent to the initial straight-line portion of the load-deflection curve ΔWi* ith cycle relative hysteresis ΔWi ith cycle hysteresis Wi ith cycle work input µk kinetic coefficient of friction Ak average reading obtained during uniform sliding of the surfaces B total sled weight Ep potential energy of impactor prior to drop CE specified ratio of impact energy to specimen thickness h nominal thickness of specimen mcl mass of chlorinating granules Clf free chlorine content Cla available chlorine content Vwater volume of water xvi He radiant exposure Te exposure time Ee irradiance ai set of alternatives gj set of evaluation critera wj weights of relative importance dj difference between alternative values Pj preference of one alternative over another Fj preference function pj preference threshold π aggregated preference index ϕ+ positive outranking flow ϕ- negative outranking flow n total number of alternatives ϕ net flow Sj importance score D flexural stiffness S support span length (for flexural stiffness calculation) E Young’s modulus xvii List of Abbreviations FRP Fibre Reinforced Polymer GFRP Glass Fibre Reinforced Polymer TPE Thermoplastic Elastomer TPU Thermoplastic Polyurethane TPV Thermoplastic Vulcanizate FiLTER Fipke Laboratory for Trace Element Research CRN Composites Research Network MCDM Multicriteria Decision Making UV Ultraviolet CSM Chopped Strand Mat RTM Resin Transfer Moulding PVC Polyvinyl Chloride COPE Copolyester TPO Thermoplastic Polyolefin PP Polypropylene FTIR Fourier-Transform Infrared EPDM Ethylene Propylene Diene Monomer RO Reverse Osmosis CLSM Confocal Laser Scanning Microscope SEM Scanning Electron Microscope SD Standard Deviation PROMETHEE Preference Ranking Organization METHod for Enriched Evaluation LCA Life Cycle Assessment LCCA Life Cycle Cost Analysis xviii Acknowledgements I am grateful to have worked with many incredible people during my MASc studies at UBC who have made this work possible and contributed immensely to my personal and professional development journey. First, I would like to thank my supervisor, Dr. Abbas S. Milani, for his guidance and support during my studies. I have learned so much from his knowledge, experience, and enthusiasm for research and collaborative work. I would like to express my sincere gratitude to members of Waterplay Solutions Inc., particularly Mr. Ray Filipiak, Mr. Kyle Luciw, and Mr. Dennis Scott. This work would not have been possible without their intriguing ideas and continued support. Thank you to all my friends at the Composites Research Network (CRN) Okanagan lab, for enriching my knowledge and experience through fruitful discussions and collaborations. My thanks especially go to Amin Batouei, Nadia Abdollahi, Dr. Bryn Crawford, and Dr. Chungyeon Cho for their significant contributions in all facets towards this work, Olivia Margoto, Amir Nazemi, and Rohith JK for their technical assistance towards sample manufacturing and testing, and Dr. Tina Olfatbakhsh for being an amazing lab manager. Lastly, I would like to thank all the School of Engineering machine shop technicians, especially Jiyee Yoon, Chris Seib, and Raymond Seida, for their help with sample preparation. xix Dedication To my friends and family for their unwavering support and encouragement, with gratitude and love. Chapter 1: Introduction 1.1 Background Non-rigid high-performance polymers are frequently used in a variety of engineering applications where flexibility, rollability, durability, comfort, and ease of storage and handling are desired [1]–[3]. Many polymers can be made more flexible through the introduction of plasticizers [4], [5]. Some polymers, such as elastomers, are intrinsically flexible due to their molecular-level properties [6], [7]. However, due to their high flexibility and comparatively low strength and modulus, non-rigid polymers have rarely been used as structural materials. In comparison, traditional structural materials are usually rigid, but are less versatile and more susceptible to fracture [8]. Replacing rigid structural materials with non-rigid polymers can bring the aforementioned benefits that are unique to flexible materials to a variety of designs and applications. This can also bring the opportunity to promote sustainability by selecting materials that are more recyclable in order to reduce waste and impact on the environment. Replacing legacy (currently used) materials in different structures with an entirely different class of materials, however, requires careful considerations and initial feasibility tests. The new candidate materials need to be thoroughly tested to ensure their properties meet the requirements of the intended application(s). Selecting the most optimal material out of a list of candidates based on their properties can be a challenging decision problem due to the usually conflicting nature of the decision/design criteria [9]. To address such problems, Multicriteria Decision Making (MCDM) methods are known to be efficient mathematical tools to evaluate the criteria subjectively [10]. A variety of MCDM methods have readily been applied to material selection problems for engineering design purposes [9]. 1.2 Motivation and Objectives Aquatic play structures are widely used in aquatic facilities, water parks, hotels and resorts, and community spaces. Examples of such structures include water slides, shown in Figure 1 (a), which have been historically manufactured from glass fibre reinforced polymer (GFRP) composites and stainless steel [11]. GFRP has high strength-to-weight ratio [12], but the high rigidity means the manufactured parts and structures are large and non-collapsible. This makes the handling, transportation, and installation of these structures difficult and costly. Figure 1 (b) shows that just a few GFRP water slide flume parts can take up an entire trailer. Like other composites, 2 GFRP cannot be easily recycled compared to single-phase polymers, and their end-of-life management can be challenging [13], [14]. Figure 1. (a) Aquatic play structures made from GFRP; (b) typical GFRP water slide flume parts in a trailer. One way to tackle these challenges is to replace the legacy rigid GFRP material with a flexible polymer material. During transportation, the parts can be rolled or folded to occupy less space and therefore reduce transportation cost. The non-rigid parts can also be less susceptible to damage during handling, transportation, and installation compared to rigid large-scale parts. Transitioning into an alternative material also allows for consideration for a highly recyclable material that can reduce environmental impact and offer more end-of-life management options compared to GFRPs. To the best of author’s knowledge, no past literature on the evaluation of flexible polymers for structural use in outdoor aquatic environment exists. Given the intended applications in aquatic play structures, the candidate materials should withstand outdoor natural environmental conditions such as varying temperatures, moisture, and ultraviolet (UV) radiation [15], [16]. Additionally, they should be resistant to degradation from immersion in chlorinated water and also be able to support a variety of combined mechanical loads. Sufficient material performance characterization is needed to compare the legacy and candidate materials, and to verify a material’s capability to meet the aforementioned criteria. After the performance data of candidate materials is acquired, a final material selection decision needs to be made to choose the best material for use in aquatic play structures. Additionally, to incorporate the new materials in aquatic play structures, new 3 redesigns are required to accommodate the substantial property differences between the old and new materials. As such, the main objective of this research has been to conduct a preliminary investigation of the performance of the legacy material and selected new material candidates, along with new redesign ideas towards replacing legacy GFRP-constructed aquatic play structures with ones that are constructed from flexible, durable, and recyclable polymer alternatives. Four specific sub-objectives were defined as follows: 1. Characterize the performance of the legacy (base) and selected candidate materials through coupon-level mechanical testing. 2. Characterize the weathering resistance, including resistance to solar radiation, temperature, moisture, and chlorinated water, of the legacy and the candidate materials. 3. Develop a MCDM model for the final material selection based on the performance characterization data from the above steps. 4. Develop conceptual redesign ideas to replace the rigid material with flexible materials in the aquatic play structures. Remark: The highly flexible polymer materials consist of flexible and mobile molecular chains, and therefore expectedly they would not possess strength and modulus that are comparable to GFRP. However, we hypothesize that these differences in mechanical properties can be compensated for by the adoption of new conceptual redesigns to modify supporting structures. The redesigns would ensure that the aquatic play structures with flexible material would still possess the necessary strength and rigidity overall to not sacrifice performance. Therefore, when evaluating candidate flexible materials using MCDM, no lower bound constraints will be set for mechanical property values. This research serves as a preliminary phase of a larger/holistic research program towards redesigning aquatic play structures with flexible (and foldable) materials. 1.3 Thesis Framework There are six chapters following this introduction. The first is the pertinent literature review, followed by two chapters on selected materials information, characterization methods, and test results and discussion. They are then followed by a chapter on MCDM for material ranking 4 and a chapter that presents the design concepts on incorporating flexible materials in aquatic play structures. The final chapter summarizes this research and presents its main conclusions. More detailed descriptions of the chapters are as follows: • Chapter 2 contains a literature review on the legacy and candidate materials, their properties and applications, processing methods, recyclability, weathering performance, and experimental methods used to characterize their weathering performance. • Chapter 3 contains information on legacy GFRP and candidate materials used in this study, test sample manufacturing and preparation, as well as mechanical, weathering, and microscopy characterization methods to quantify the performance properties of the materials in this study. The material samples were subjected to tensile, flexure, cyclic tensile, abrasion, friction, puncture, and impact tests. They were also exposed to accelerated weathering conditions including chlorinated water immersion, hygrothermal environment exposure, and xenon lamp chamber weathering that included simulated solar radiation, elevated temperature, and water spray. • Chapter 4 presents and discusses the results from the characterization tests. The similarities and differences between the materials in this study are highlighted. • Chapter 5 presents the implementation of an MCDM method to rank and select the best material for aquatic play structures. The PROMETHEE II method was used to evaluate the data from materials performance characterization according to various evaluation criteria and to rank the material alternatives. • Chapter 6 introduces the ideas for the new conceptual design of a water slide flume that replaces legacy GFRP material with the top candidate material. Three different ideas that incorporate the candidate material in the flume while providing adequate rigidity for the structure are presented. Some proof-of-concept prototyping and experimental results are also discussed. • Chapter 7 presents a summary of this study as well as its limitations and future work recommendations. 5 Chapter 2. Literature Review 2.1 Legacy Material: Glass Fibre Reinforced Polymers (GFRP) Fibre reinforced polymers (FRP) are a class of composite materials that usually contain two distinct phases: the polymer resin matrix and the fibre reinforcement [17], [18]. The fibres usually possess good strength and act as reinforcing elements, and the polymer resin matrix binds and protects the fibres while acting as a load transferring medium [19]. The overall mechanical properties and durability of the composite material are better than either of the constituent materials alone [18]. FRP composites possess desirable properties such as high-strength, lightweight, and excellent weathering stabilities, and therefore have seen broad applications in the construction, transportation, and sports industries [18], [20], [21]. Fibres used in FRP composites can be sorted into two categories: natural or synthetic. Synthetic fibres are produced through chemical synthesis, in comparison to natural fibres which usually come from living plants [21], [22]. Among all types of synthetic fibres, glass fibres are the most used, with nearly 90% of FRPs made of glass fibres [20]. The resultant composite materials are called glass fibre reinforced polymers (GFRP), also known as fibreglass. There are different types of glass fibres that can be used in GFRP. E-glass, or electrical glass, is the most common and has high strength and electrical resistivity, with a tensile strength of around 3.4 GPa [20], [23]. Other grades include A-glass with high durability and strength, C-glass with high corrosion resistance, and S-glass with high tensile strength [20]. These fibres come in various forms, including continuously long threaded, woven, chopped strand mat (CSM), and chopped fibres [24]. Thermoset resins are typically used as the matrix material for GFRP. Unsaturated polyester resin is the most popular due to its low cost [25], [26]. However, its mechanical properties are inferior to epoxy and vinyl ester [25], [27]. Styrene, commonly used as a reactive diluent in unsaturated polyester resins, is also associated with environmental and health concerns [28], [29]. 2.1.1 GFRP Applications GFRP has seen a variety of applications in aquatic and marine environments such as shipbuilding due to their good specific strength, corrosion resistance, and workability [30]–[33]. GFRP is also lighter compared to other common materials for shipbuilding such as metals. Using GFRP sandwich structures on a vessel instead of aluminum can result in a 10% weight reduction [34]. 6 GFRP has also been used in civil construction in various environmental conditions. In recent years, GFRP rebars, which are longitudinally bundled glass fibres embedded in a resin matrix, have been used to replace steel in concrete reinforcement in coastal environments [35]. GFRP rebars are 75% lighter than steel rebars while having tensile strength 2-3 times higher [35]. Bridge decks have also been built using GFRP, taking advantage of their low density, high strength, corrosion resistance, and long service life [36]. Historically, aquatic play structures such as water slides have also been manufactured from GFRP. Unsaturated polyester resin and chopped glass fibres make up the core of the material and give shape to the structure. It is often manufactured through open moulding processes such as hand layup or spray up, although resin transfer moulding (RTM) and light RTM processes have been adopted recently to produce higher quality parts [37]. To finish, a layer of polyester gel coat is usually sprayed on top of the surface to enhance the aesthetics and frictional properties [11]. 2.1.2 GFRP Recycling The demand and production for FRP composites continue to grow worldwide. In 2020, the global composite market size reached 95.89 billion USD, and this number is projected to grow to 160.54 billion USD by 2027 [38]. The rising production and use of these materials ultimately lead to an increasing amount of waste. Landfill and incineration have been popular methods for disposing FRP waste but have led to concerns for the environment and sustainability [39], [40]. GFRPs, like other composite materials, are difficult to recycle due to several reasons including heterogeneity from mixtures of different material phases, thermoset resins not able to be melted due to chemical cross-links after curing, and conventional recycling methods creating waste and emissions [13], [39], [41]. More technologically advanced methods such as mechanical recycling, thermal recycling, and chemical recycling are being developed, but at the present stage they are all complex, costly, and difficult to implement on a commercial scale [13], [39]. 2.2 Candidate Materials Class: Flexible Polymers Due to their unique and desirable properties, flexible polymers have been used in a variety of engineering applications. Their high flexibility and elasticity give them excellent shape-recovery abilities and make them resistant to permanent plastic deformations [42], [43]. Flexibility also leads to improved comfort and shape conformity [44], enabling flexible polymers to be used in wearable electronics for biomedical applications [2], [45]. Some flexible polymers show 7 outstanding impact resistance and shock absorption [45], [46] and have been used in protective engineering applications such as body armour [47]. Many flexible polymers are durable and can stand against environmental exposures as seen being used in wire sleeving and marine hoses [48], [49]. 2.2.1 Plasticizers One way to increase the flexibility of polymers is to add plasticizers. They are low molecular weight compounds that form secondary bonds to polymer chains and reduce polymer-polymer chain bonding, thus making the macromolecules more mobile and lowering the glass transition temperature (Tg) of the polymers that they are added to [5], [50]. The addition of plasticizers increases the flexibility as well as processibility of the material due to improved flow properties and lower processing temperature [50], [51]. Phthalates, or phthalate esters, dominate the plasticizer market due to their low cost, electrical insulation, high elasticity, and relatively non-volatile properties [5], [52]. Polyvinyl chloride (PVC) is the most plasticized polymer, consuming about 80% of plasticizers in the market [53]. One major concern with the use of plasticizers is related to their leaching and migration out of the polymers, causing the polymers to lose the enhanced properties from the addition of plasticizers and become stiffer and more brittle [5]. More importantly, phthalates have been found to be toxic and can lead to health and environmental issues [53], [54]. Many countries have restrictions on the use of phthalates in products in the food and pharmaceutical industries [55]. 2.2.2 Thermoplastic Elastomers There are polymer materials that are inherently flexible due to molecular bonding and configurations without the help of plasticizers. Rubber-like materials have long and entangled molecular chains with high flexibility and mobility, which lead to high deformations when the materials are under external stress [56], [57]. These molecular chains are connected in a network through chemical or physical cross-linking, resisting irreversible relative movements between chains [56], [57]. When the network of chains is stretched, its entropy is decreased [56], [58]. The tendency of the system to maximize its entropy produces the retractive energy needed to return the material to its original configuration, giving rise to the material’s high elasticity [56], [58]. 8 Elastomers are a class of lightly cross-linked polymers with long polymer chains that are bonded by weak intermolecular forces [59]. They possess high flexibility as well as other desirable properties such as good tear strength, abrasion resistance, and resistance to gas, water, and steam [59]. They have been widely used in sealing and insulation applications [60], [61]. Most conventional elastomers are thermosets with chemical cross-links and covalent bonds between molecular chains, making them unable to be melted or remolded, eliminating many opportunities for recycling [62], [63]. Thermoplastic elastomers (TPE) are a class of materials that combine the properties of semi-crystalline thermoplastics and soft elastomers, and they can be melt-processed like thermoplastics [64]. They are usually multi-phase systems with at least one elastomer phase and one phase that is hard at ambient temperature to act as physical cross-links and restrict the flow of the elastomer phase [57], [65], [66]. Below Tg of the elastomeric phase, the material is stiff and brittle [57]. Above Tg, the elastomeric phase is soft, making the material elastic and behave like vulcanized rubber [57], [65]. When the temperature is high enough to soften or melt the hard phase, the material behaves like a viscous fluid and can be processed as a thermoplastic [57], [65]. Consequently, the lower service temperature of the TPE is dependent on Tg of the elastomer phase, and the upper service temperature is dependent on Tg or the melting temperature of the hard phase [65]. TPEs can be classified into six groups as shown in Figure 2, although only styrenic block copolymers, multiblock copolymers, and hard polymer/elastomer combinations are relevant for current commercial applications [65]. Figure 2. Classification of thermoplastic elastomers, adapted from [57], [65]. 9 Block copolymers represent a majority of thermoplastic elastomers. They consist of blocks that contain two or more polymer chains that are connected through their ends, in linear, branched, or cyclic architectural configurations [67], [68]. The different structures in the polymer chains are usually represented by capital letters A, B, C, etc. [57]. Styrenic block copolymers possess the A-B-A type molecular structure, where A is polystyrene, and B is an elastomeric segment [65]. The polystyrene phase consists of spherical domains that connect the elastomeric chains into a network structure [57], as shown in Figure 3. At room temperature, the polystyrene phase is hard, and when it is heated, it softens and enables the material to flow [57]. Figure 3. Morphology of styrenic block copolymers [65]. Multiblock copolymers have structures represented by A-B-A-B-… configuration [65]. The hard thermoplastic segments (A) and the soft elastomeric segments (B) are joined together, forming elastomer-dominated rubbery amorphous regions and thermoplastic-dominated glassy crystalline regions [65], [67]. The typical phase morphology of block copolymer structure is shown in Figure 4. Common thermoplastic elastomers with multiblock copolymer structure include thermoplastic polyurethane (TPU) elastomers, polyamide elastomers, and thermoplastic polyester elastomers or copolyesters (COPE) [57]. 10 Figure 4. Morphology of multiblock copolymers [69]. Thermoplastic elastomers with hard polymer/elastomer combinations, such as thermoplastic polyolefins (TPO), do not possess the block copolymer structure, and instead have macroscopically co-continuous phases as shown in Figure 5 (a), usually achieved through intensive mechanical mixing [65]. Some materials go through a process called dynamic vulcanization, where the rubber elastomeric phase is vulcanized or cross-linked while being mixed with the molten thermoplastic phase at high temperature [65], [70]. The cross-linking causes immobilization of the rubber particles and they break down due to shear during mixing [70], which leads to discontinuous rubber phases dispersed in the thermoplastic phase as shown in Figure 5 (b). The cross-linked elastomer phase leads to better elastic performance, including higher elastic modulus and lower permanent deformation after unloading [70]. Such materials are termed thermoplastic vulcanizates (TPV) [70]. Figure 5. (a) Interdispersed co-continuous elastomer and hard polymer phases, and (b) elastomer phases dispersed in hard polymer matrix after dynamic vulcanization; adapted from [65]. 11 The properties of elastomers vary widely depending on their chemistry, constituents, and morphology. For most TPEs with two-phase morphology, the strength, stiffness, abrasion resistance, hardness, compression and tension set, and tear resistance at room temperature and above are largely dependent on the hard phase or thermoplastic phase [69]. Elongation, flexure, low-temperature performance, and dynamic properties are dominated by the soft phase or rubber phase [69]. Table 1 illustrates some of the performance characteristics of some types of TPE. Automotive, construction, footwear, and appliances represent the largest markets for TPE [57]. Styrenic block copolymers, TPO, TPV, and TPU are the most used TPE materials by weight [57]. Table 1. Performance characteristics of some TPE materials [69]. Physical/Mechanical Properties Polymer Blends Block Copolymers Olefinic TPO Olefinic TPV Styrenics COPE TPU Polyamide Specific gravity 0.97 0.97 0.94 1.2 1.2 1.02 Hardness range (Shore A-D durometer) 60-90A 35A-50D 20A-40D 40-70D 60-90A 75A-70D Compression set F E – G G G – F G E – P Continuous service temperature (°C) 120 135 100 110 120 80 Lower service temperature (°C) -45 -60 -45 -70 -40 -40 Processing (cycle time) E – G E – G E – G E – G F – G G Recyclable Yes Yes Yes Yes Yes Yes Environmental Resistance Ozone E E – G F E G No data Weather (UV) G E – G G – F E – G G G – F Acids E E – G E P E G – F Alkalis E E E G – F P G – F Lubricating oils P F – P P F F E Gas permeability F F F F G No data Note: E = Excellent, G = Good, F = Fair, P = Poor. 12 2.2.2.1 Processing of Thermoplastic Elastomers Most of the thermoplastic elastomers can be processed through melt-processing methods used for thermoplastic polymers. Some common processing methods as well as their advantages and disadvantages are listed in Table 2. The suitability of the processing methods for each material as well as process parameters are dependent on several material properties. The rheology, or flow properties, of a thermoplastic is usually a function of temperature and shear rate, and the melt viscosity of the material can vary widely depending on the conditions but is usually quite high for thermoplastics [57], [71]. Adequate force is needed to push the material into a mould or through a die to form the shape. Thermal properties are also important, as melt-processing requires the material to be heated to a flow state and subsequently cooled to a solid state [57]. The heating and cooling capacities required should be considered when choosing the processing method. This is especially important for semi-crystalline materials since it takes additional heat to melt the crystalline structure, which amorphous materials do not have [57]. Another important consideration should be given to shrinkage and warping of the part after processing. Again, crystalline materials suffer greater from shrinkage than amorphous materials, and higher crystallinity usually leads to higher shrinkage [57]. There is also usually a difference between shrinkage in the flow direction and shrinkage transverse to the flow direction due to the alignment of molecules during the flow, which can lead to warping of the part [57]. Prior to processing, some steps may need to be taken to prepare the resin and ensure the quality of the finished part. Some polymer resins or pellets tend to absorb moisture from ambient environment. Under high temperature during processing, the moisture can vaporize and form air bubbles near the surface, resulting in defects that affect both visual appearance and mechanical properties [72]. Therefore, if the material is hygroscopic, it is important to dry the resin before processing. Most TPEs are colourless, and sometimes the final product needs to be coloured. There are several ways to introduce colour into the resin. Pre-coloured resin called compounded colour or precolour that already have pigments dispersed in the base resin (pre-process colouring) can be purchased from the compounder [57], [73]. This leads to high quality colouring, but compounded colour is more expensive than base resin and has added heat history due to the compounding process [57]. Colour can also be introduced during the forming process (in-process colouring) through the addition and dispersion of dry or liquid colours into the base resin [57], [73]. The 13 quality of in-process colouring depends on the thoroughness of the mixing of colourant and resin [57]. In-process colouring requires less inventory space than pre-process colouring and shorter lead times for orders [73]. Masterbatch or colour concentrate combines the advantages and disadvantages of pre-process and in-process colouring. It is a thermoplastic resin that is already compounded with high concentration of colourant, and it is usually mixed with the uncoloured base resin at 1 to 5% ratio [73]. Table 2. Common processing methods for thermoplastics [57]. Process Technique Advantages Disadvantages Injection Molding The most precise control of shape and dimensions, highly automatic process, fast cycle times; the widest choice of materials High capital cost; is only good for large numbers of parts; large mold pressures (typically 140 MPa or 20,305 psi) Compression Molding Lower mold pressures (typically 7 MPa, or 1,015 psi); does minimum damage to reinforcing fibers (in composites); large parts are possible Requires more labor; longer cycle than injection molding; less shape flexibility than injection molding; each charge is loaded by hand Transfer Molding Good for encapsulating metal parts and electronic circuits Some scrap with every part and each charge is loaded by hand Blow Molding Can make hollow parts (such as bottles); stretching action improves mechanical properties; fast cycle; requires low labor No direct control over wall thickness; cannot mold small details with high precision; requires polymer with high melt strength Rotational Molding and Rotolining Labware that is shatterproof and will not contaminate samples; small to very large vessels; valve lining, bellows, items with complex geometry, containers A slow process, requires labor, and involves numerous moving parts; a great deal of cleaning and surface conditioning is necessary Extrusion Used for films, sheets, wraps, or long continuous parts (i.e., pipes, wire, and cable insulation) Must be cooled below its glass transition temperature to maintain stability 2.2.2.2 Recycling of Thermoplastic Elastomers There are four main methods to recycle polymers. Primary recycling is the mechanical reprocessing of plastic to produce the same product [74]. Secondary recycling is the mechanical reprocessing of plastic to produce lower quality products [74]. Tertiary recycling is aimed to recover petrochemical components [74]. Energy recovery is the collection of heat through incineration of waste [74]. 14 As mentioned before, thermoset polymers, used in most FRP composites, have chemical cross-links and covalent bonds, and cannot be melted once cured [40]. Therefore, primary and secondary recycling cannot be applied to thermosets. On the other hand, TPEs melt under heat and can be processed like thermoplastics [74], so primary and secondary recycling methods are applicable. However, although primary recycling can be applied to TPEs and general thermoplastics in theory, there are several practical challenges. The plastic should be of a single polymer type, uncontaminated, and overall retain its original properties without much degradation throughout its service life [57], [74]. Additives present further complications as they can potentially degrade the polymer or be released into the environment during reprocessing [75]. These are major reasons why most of the waste plastic ends up in landfill or incineration. Secondary recycling imposes fewer requirements on the properties and conditions of the polymer waste than primary recycling and can be a great way to reduce the need to manufacture new materials, therefore reducing energy consumption significantly [74], [76]. 2.3 Hysteresis and Mullins Effect Hysteresis is a term used to describe the looping behaviour of an input-output response and can be found in many systems [77]. Hysteresis is manifested in viscoelastic polymer materials in the form of loops in the stress-strain diagram during loading and unloading. An example is shown in Figure 6. Force induces movement of polymer chains, but this movement is hindered by friction between the chains, leading to the lagging of strain [78]. The internal friction creates heat, which is then dissipated, causing a temperature increase of the material, and creating the difference between strain energies, or area under the curve of a stress-strain diagram, during loading and unloading [78], [79]. Since the energy dissipation through heat is recoverable unlike through permanent plastic deformation, materials with pronounced hysteresis have been used for damping to dissipate energy from impact or vibration [80]. The effect of hysteresis is dependent on both the load and the rate at which the load is applied [79]. Another phenomenon that can be observed in elastomers under cyclic loading is the Mullins effect, resulting in interesting behaviours shown in Figure 6. Material softening can be seen as repeated loading to the same strain does not lead to the same stress [81]. The softening effect is the most significant after the first loading cycle and diminishes after a few cycles, and the effect increases with increasing maximum strain [82]. When the strain under cyclic test exceeds the maximum strain previously applied, the stress-strain response merges with the simple 15 continuous response [82]. It has been suggested that in lightly cross-linked systems, this phenomenon is caused by macromolecular chain extension and slippage over other chains and fillers, while in highly cross-linked systems covalent bond rupture is the main contributor [83]. There are also other proposed physical interpretations of Mullins effect, including the chain entanglement and alignment with the strain axis [84], but no consensus has been reached [82]. Figure 6. Stress–strain responses of a filled rubber submitted to a simple uniaxial tension and to a cyclic uniaxial tension with increasing maximum stretch every 5 cycles [82]. 2.4 Polymer Weathering The durability of a material is of importance when considering its application and service life. Polymers, when exposed to outdoor conditions (sunlight, heat, moisture, etc.) as well as other factors such as microorganisms, ozone, chemicals, and salt, can experience degradation or decline of properties through a process called weathering [85], [86]. There are various types of polymer degradation, including photooxidation, thermal, ozonolytic, hydrolytic, chemical, enzymatic, in-vivo/invitro oxidative, biological, and mechanical degradation [87]. The different weathering exposure effects are usually coupled with each other, leading to complex impact on polymer properties due to degradation [85]. Some degradation mechanisms that would specifically pertain to materials used in outdoor aquatic environments are explained further. 16 2.4.1 Photooxidation Degradation Exposure to UV radiation is a major cause of polymer degradation that can result in yellowing, loss of gloss, and decline in mechanical properties [86]. Photooxidation is primarily a surface reaction that takes place when ultraviolet (UV) radiation between 290 and 400 nm and oxygen are present [88], [89]. UV radiation below 290 nm is filtered out by the ozone layer and therefore is usually not a factor [89]. Light energy is absorbed by carbonyl groups and hydroperoxide, which are found in the polymers, or in impurities or additives introduced during processing [89]. This leads to bond dissociation and free radical production, as well as subsequent chemical changes including chain scission and cross-linking which negatively affect material properties [89]. Chain scission leads to reduction in molecular weight of the polymer [90], and cross-linking can cause embrittlement at the polymer surface which can lead to crazing and fracture [88]. UV stabilizers have been found effective in reducing oxidation rate [88]. They include UV screeners such as absorbers or pigments, excited state deactivators, hydroperoxide decomposers, and radical scavengers [88]. 2.4.2 Hydrolytic and Chlorinated Water Degradation Hydrolytic degradation is a result of hydrolysis which is the cleavage of chemical bonds by water molecules [91]. Hydrolysis is dependent on the susceptibility of the chemical bonds within the polymer, the moisture concentration inside the polymer, as well as the presence of catalysts in the water [91]. Rate of hydrolysis is related to the rate of diffusion of moisture from the surface into the bulk of the polymer [89]. Generally, high cross-linking density can suppress hydrolysis, and amorphous regions are more affected than crystalline regions [89], [91]. Polymers with very low hydrolytic activity include polyolefin and polyether, and polymers with high hydrolytic activity include polyester, polyanhydride, and polyorthoester [92]. Chlorine is commonly found in water as a disinfectant. Chlorinated water contains multiple species including Cl2, HClO, ClO-, 1O2, etc., and the concentrations of species vary depending on pH [93]. Chlorine is a strong oxidizer and can react with polymers to produce free radicals, leading to chain reactions that can result in chain scission, increased variation in molecular weight, and reduction in mechanical properties [93], [94]. The effect of degradation depends on the chlorine concentration, exposure length, temperature, and pH [93]. 17 2.4.3 Accelerated Weathering Performance To establish failure mechanisms and baseline data for polymer weathering, test materials are often subjected to weathering experiments. These experiments can be categorized into natural outdoor exposure and artificial accelerated weathering [95]. Natural outdoor exposure applies all environmental exposure conditions to the material samples at a natural rate, yielding realistic degradation results [95]. However, it has the disadvantages of long duration and variability in exposure conditions [95]. Artificial accelerated weathering simulates outdoor environmental conditions in a light chamber that can be coupled with temperature, humidity, and water spray controls [96]. Xenon arc lamps with filters that filter out the low-wavelength UV radiation closely match the ground level solar spectral power distribution and are the state of the art [95]. Fluorescent UV lamps are also used but lack the visible light spectrum [95]. Artificial accelerated weathering provides consistent and controlled exposure conditions and induces degradation in shorter time, but relating accelerated weathering to natural outdoor exposure is a complex matter, as the artificial environment may not be an accurate representation of the natural environment, and the weathering of different materials will be accelerated differently under the same conditions [95], [96]. However, accelerated weathering is still necessary for product development and service life prediction due to time constraints [95]. One study [97] exposed aromatic and aliphatic TPU films to UV radiation, temperature, humidity, and wind under repeated dry and wet cycles in a weatherometer for up to 500 hours. The tensile properties of the films initially improved due to increased cross-linking but eventually deteriorated over time. The ultimate strength dropped to about 50% of original at the end of exposure. Cracks and increased roughness could be observed on the surface of aromatic films, while only increased roughness was observed on aliphatic films at the end of the exposure period. Both types of films also showed initial decrease and subsequent increase in helium gas permeability, but aromatic films exhibited much higher permeability in the end due to surface cracks. It was concluded that aromatic TPU films are more susceptible to weathering than aliphatic TPU films due to having more reactive groups susceptible to UV [97]. Another study [98] compared the degradation of polypropylene (PP) films from outdoor exposure and from accelerated weathering. Outdoor exposure was conducted in Brno, Czech Republic, and accelerated weathering was carried out in a xenon lamp chamber at black standard temperatures from 40 to 70 °C with no humidity control or spray cycles. The degree of 18 photodegradation was measured through carbonyl index obtained from FTIR spectroscopy. The results showed that at a higher temperature, less UV radiant energy was needed to cause the same amount of polymer deterioration. For PP and its copolymers, a black standard temperature between 32 and 36 °C could lead to the highest correlation in terms of degradation between outdoor exposure and accelerated weathering when keeping the total UV radiation energy constant [98]. In another study [99], ABS films were exposed to accelerated weathering conditions in a xenon lamp chamber under alternating wet and dry cycles for up to 200 hours. Four months of outdoor exposure was also conducted in Lisbon, Portugal. An increase in carbonyl and hydroxyl groups as well as a decrease in butadiene could be observed after both accelerated and natural weathering, but accelerated weathering led to more pronounced chemical modifications, possibly due to higher radiation intensity and temperature. It was also observed that the yellowing of the samples proceeded at a high rate initially under both exposure conditions and gradually stabilized after 150 hours of accelerated weathering and after 105 days of natural weathering. Drastic reduction in elongation at break could also be seen after short periods of exposure due to the photodegradation of the butadiene component [99]. 2.5 Summary The past work on legacy and candidate materials in the context of outdoor aquatic applications was reviewed in this chapter. Namely, the review highlighted the properties and processing of GFRP, as well as the shortcomings of this material category related to its stiffness and recyclability. Thermoplastic elastomers, with their flexibility stemming from their molecular properties and configuration, are melt-processible and recyclable, making them potentially suitable alternatives. Some unique viscoelastic properties of thermoplastic elastomers related to hysteresis and Mullins effect were reviewed. The weathering degradation mechanisms and effect of polymers were also dissected. It was concluded that an evaluation of flexible polymers for structural use in outdoor aquatic environments would be of great importance for industrial applications. 19 Chapter 3: Materials and Methods This chapter describes the materials that were studied in this research, including the legacy (base) GFRP material, and three new candidate TPE materials for aquatic play structures, as well as the methods used for the property characterization and weathering performance assessments. Samples preparation, mechanical characterization methods, weathering characterization methods, and microscopy characterization methods are presented in detail. The characterization results will be presented and discussed in Chapter 4. 3.1 Materials GFRP, the legacy material that has traditionally been used in aquatic play structures, was studied as a benchmark in this research. Three TPEs, including one TPV and two TPU materials, were introduced as candidate materials, due to their advertised good mechanical strength, toughness, and weathering performance from product datasheets [100]–[102], to potentially replace GFRP for benefits such as increased ease of transportation and improved comfort. Belonging to the thermoplastic elastomer class of materials, the candidates are known to be melt-processible and can be recycled through primary and secondary recycling methods [74], promoting sustainability of the product. 3.1.1 The base GFRP material and aqua structure A water slide flume section made from GFRP (Figure 7) was manufactured by FormaShape Composites (Kelowna, BC, CA) and donated to this research by Waterplay (Kelowna, BC, CA). This base composite material contains layers of glass fibre chopped strand mat (CSM) and woven roving in a polyester matrix, laid up in an open moulding process, and finished with a vinyl ester barrier coat and a neopentyl glycol polyester gel coat sprayed on top. Material samples were taken from the curved portion of the flume as shown in Figure 7. The curvature in the specimens may lead to uneven stress distribution and improper seating in fixtures during mechanical tests. GFRP samples were cut to minimize the curvature present in the specimens as discussed in Section 3.2.1. 20 Figure 7. Water slide flume section made from GFRP. Samples were cut from the curved portion of the flume (circled in the image). 3.1.2 Santoprene 121-67W175 Santoprene 121-67W175 was selected as the first new candidate material. The pellets were produced by Celanese (Dallas, TX, US) and donated by Nexeo Plastics (The Woodlands, TX, US). It consists of ethylene propylene diene monomer (EPDM) rubber particles in a PP matrix. According to the product datasheet [102], this is a soft TPV with good flex fatigue resistance and UV resistance. It can be processed through extrusion, thermoforming, and vacuum forming processes. It is commonly used in automotive and outdoor applications. Santoprene 121-67W175 will be referred to as “Santoprene” in the rest of this thesis for simplicity. 3.1.3 Texin 285A Texin 285A pellets produced by Covestro (Pittsburgh, PA, US) and supplied by Nexeo Plastics (The Woodlands, TX, US) was selected as the second candidate material. According to the product datasheet [100], it is a polyester based TPU with good abrasion resistance, impact strength, toughness, and flexibility. It has a reported Shore hardness of 85A and can be processed through injection moulding and extrusion. It is commonly used in tubing, seals, athletic soles, and casters. 21 3.1.4 Texin 255D Texin 255D pellets produced by Covestro (Pittsburgh, PA, US) and supplied by Nexeo Plastics (The Woodlands, TX, US) was selected as the third candidate material. According to the product datasheet [101], it is a polyester based TPU with good oil resistance, abrasion resistance, impact strength, toughness, and flexibility. It has a reported Shore hardness of approximately 55D, harder than Texin 285A. It can be processed through injection moulding. It is commonly used in seals, gaskets, sleeves, casters, and gears. Figure 8. (a) Santoprene 121-67W175, (b) Texin 285A, and (c) Texin 255D ‘as-received’ pellets. 3.2 Sample Preparation The GFRP flume sample was pre-manufactured and obtained as is, while candidate material samples were fabricated from pellets through compression moulding. The samples were then cut into test specimens for characterization. The test specimen dimensions are determined according to the characterization methods described in Section 3.3. 3.2.1 GFRP The curved portion of the GFRP flume was cut into smaller sheets with a jig saw (Figure 9 (a)). The test specimens were then cut from the sheets in a water jet (Figure 9 (b)). Since test specimens should be as flat as possible for all mechanical tests, in order to minimize the curvature in specimens, the orientation of the water jet cutting was chosen so that the lengths of all specimens were perpendicular to the curvature. The specimens were approximately 3 mm thick. 22 Figure 9. (a) GFRP sheets and (b) GFRP test specimens after water jet cutting. 3.2.2 TPE The TPE samples were manufactured through a compression moulding process. The thermoplastic pellets were first dried in an environmental chamber (Thermotron SM-4-8200) at 80 °C for 3 hours to remove moisture. The dried pellets were spread uniformly in a 108 mm × 278 mm rectangular aluminum mould (Figure 10 (a)). Then, the mould was closed (Figure 10 (b)) and placed inside a Carver AutoFour ASTM hydraulic press (Figure 10 (c)). The pellets were heated under a mild pressure to a softened state, and then compressed at a higher pressure for the softened material to conform to the shape of the mould cavity, before being cooled to room temperature. The compression moulding parameters for the candidate materials are summarized in Table 3. The moulded sheets, which were about 3 mm thick, could then be removed from the mould (Figure 10 (d)). Before being put in the water jet, each TPE sheet was securely sandwiched in between two steel sheets. This was to provide rigidity and ensure the polymer sheets would not move during cutting. The test specimens (Figure 10 (e)) were then cut out using the water jet. 23 Figure 10. (a) Aluminum mould with TPE pellets inside, (b) closed mould, (c) hydraulic press, (d) compression moulded TPE sheets, and (e) test specimens cut. Table 3. TPE compression moulding parameters. Material Period Length (min) Temperature (°C) Force (N) Santoprene 1 30 180 500 2 10 180 5000 3 15 Cooling N/A Texin 285A 1 30 180 500 2 10 180 5000 3 15 Cooling N/A Texin 255D 1 30 230 500 2 10 230 10000 3 15 Cooling N/A 3.3 Selected Mechanical Characterization Methods Density measurements were taken as the weight of the product would have important implications on logistics and transportation. Tensile tests were conducted to investigate the tensile strength and stiffness which would be important data for structural design. Three-point bending tests were conducted to measure the flexibility of the materials, ensuring the candidate materials can be rolled or folded. Cyclic tensile tests were conducted since aquatic play structures are often 24 subjected to cyclic loads. Hardness tests and friction tests were conducted to quantify the surface hardness and frictional properties to correlate to user experience on aquatic play structures such as a water slide. Abrasion tests and puncture tests were conducted to study the durability of the materials against abrasive actions or contact with sharp objects, which may occur during regular use or extreme events for aquatic play structures. Impact tests were also conducted since aquatic play structures may experience impact loading from the users. 3.3.1 Density Measurement The densities of the legacy and candidate materials were measured in a Qualitest MDS-300 densimeter. Three specimens were measured for each material. 3.3.2 Tensile Test Using an Instron 5969 system with a 50 kN load cell, tensile tests were performed on legacy and candidate material specimens in accordance with ASTM D638 [103]. Type IV dumbbell shape was chosen for the specimens since it is recommended by the ASTM standard for comparing materials with different rigidities. Instron 2716 Series wedge grips were used for the GFRP specimens. Since the TPE specimens tended to notably thin out when stretched, scissor grips from the Universal Grip Company were used instead of wedge grips. This was to provide continuous positive lateral force on the specimens enough to prevent slippage throughout the tests, without crushing the sample through the thickness. A GFRP tensile specimen installed in wedge grips and a Texin 285A tensile specimen installed in scissor grips are shown in Figure 11. The tests were conducted at a head displacement rate of 5 mm/min for GFRP, and 500 mm/min for the TPE materials, until fracture. The tensile stress at any point during the test was calculated using the equation 𝜎 =𝑃𝐴 (1) where σ = tensile stress (MPa), P = load at a given point on the load-deflection curve (N), and A = average cross-sectional area (mm2). The ultimate tensile strength was calculated using the equation 𝐹𝑡𝑢 =𝑃𝑚𝑎𝑥𝐴 (2) where Ftu = ultimate tensile strength (MPa), Pmax = maximum force before failure (N), and A = average cross-sectional area (mm2). 25 The Young’s moduli for GFRP samples were calculated between 0.1% and 0.6% strain. The TPE’s exhibit nonlinear stress-strain behaviour, with the highest Young’s modulus observed in the low-strain region [57]. The initial moduli of the TPE samples were calculated between 1% and 6% strain. Figure 11. (a) GFRP tensile specimen in wedge grips and (b) Texin 285A tensile specimen in scissor grips. 3.3.3 Three-Point Bending Test To investigate the flexural properties of the materials, three-point bending tests were performed using an Instron 5969 system with a 50 kN load cell and the flexural test fixture from Wyoming Test Fixtures, following ASTM D790 [104]. All specimens were cut into 12.7 mm × 100 mm rectangular pieces. The support span (distance between the bottom supports of the fixture) for each specimen was set to be 16 times the thickness of the specimen. The speed of the test for each specimen was calculated using the equation 𝑅 =𝑍𝐿26𝑑 (3) where R = rate of crosshead motion (mm/min), L = support span (mm), d = depth of beam (mm), and Z = rate of straining of the outer fiber (mm/mm/min) = 0.01. The specimens were centred on the supports with their long axes perpendicular to the supports, and the loading nose was midway 26 between the supports (Figure 12). The deflection of the specimen was measured by the crosshead position. Figure 12. GFRP sample in three-point bending fixture. The flexural stress was calculated using the equation 𝜎𝑓 =3𝑃𝐿2𝑏𝑑2 (4) where σf = stress in the outer fibers at midpoint (MPa), P = load at a given point on the load-deflection curve (N), L = support span (mm), b = width of beam tested (mm), and d = depth of beam tested (mm). The flexural strain was calculated using the equation 𝜀𝑓 =6𝛥𝑑𝐿2 (5) where εf = strain in the outer surface (mm/mm), Δ = deflection of the centre of the beam (mm), L = support span (mm), and d = depth of beam tested (mm). The tangent modulus of elasticity was calculated using the equation 27 𝐸𝐵 =𝐿3𝑚4𝑏𝑑3 (6) where EB = modulus of elasticity in bending (MPa), L = support span (mm), b = width of beam tested (mm), d = depth of beam tested (mm), and m = slope of the tangent to the initial straight-line portion of the load-deflection curve (N/mm). 3.3.4 Cyclic Tensile Test Cyclic tensile tests were performed to investigate the cyclic behaviour of the candidate materials. Understanding the cyclic behaviour and associated Mullins effect would help predict the differences in material behaviour between loading and unloading, as well as between first loading and subsequent loading, to better inform the design of structures using such materials. An Instron 5969 system with a 50 kN load cell and the Universal Grip Company scissor grips were used. Specimens were cut into the same shape as regular tensile specimens. The tests were divided into two phases with different experimental procedures [105]. Phase 1 was designed to highlight the elastic hysteresis that is manifested through the differences in tensile response between the loading path and the unloading path, as well as the differences between first cycle and subsequent cycles. The specimens underwent three load-unload cycles at a crosshead movement rate of 500 mm/min. During each cycle, the specimens were stretched to a maximum strain of 1 mm/mm, then immediately unloaded until the tensile load reduced to 3 N (instead of 0 N to account for the sensitivity of the load cell). The work input for each cycle was calculated by integrating tensile stress over strain from the beginning of the cycle to maximum strain. The hysteresis for each cycle was calculated by integrating tensile stress over strain from the beginning to the end of the cycle. The relative hysteresis was calculated using the equation 𝛥𝑊𝑖∗ =𝛥𝑊𝑖𝑊𝑖 × 100% (7) where ΔWi* = ith cycle relative hysteresis (%), ΔWi = ith cycle hysteresis (MJ/m3), and Wi = ith cycle work input (MJ/m3). The residual strain after each cycle was also recorded. Phase 2 highlights the tensile behaviour after the specimen’s previous max strain was surpassed. The specimens underwent several load-unload cycles at a crosshead movement rate of 500 mm/min. The maximum strain was progressively increased for each cycle, and after reaching the maximum strain, the specimens were immediately unloaded until the tensile load reduced to 3 28 N. The maximum strain during the first cycle was 0.5 mm/mm, and it was increased by 0.5 mm/mm for each subsequent cycle. The Santoprene specimen was able to undergo 3 complete cycles before failure. The Texin 285A specimen was subjected to 5 complete cycles before the test was stopped. The calculations for phase 2 are identical to those for phase 1. 3.3.5 Hardness Test The Shore D hardness measurements of the legacy and candidate materials were taken using a Qualitest HPE II Digital Barcol Durometer following ASTM D2240 [106]. 3.3.6 Abrasion Test Abrasion tests were performed to investigate the abrasion resistance of the candidate materials. A Taber Rotary Platform Abrasion Tester, Model 1700, was used to perform the tests, following ASTM D3389 [107]. The test specimens were cut into 100 mm × 100 mm squares with a 13 mm-diameter centre hole for mounting. Taber CS-17 abrading wheels, which provide medium coarse abrasive action according to the manufacturer, were used to abrade the specimen surface. Since the abrading wheels need to lay flat on the specimen surface in order to make full contact, the curved GFRP specimens could not be tested. Wheel loading of 1000 g per wheel was applied, and the test speed was 72 rpm. A total of 1000 abrasion cycles were applied to each specimen, and the mass loss was reported. The abrading wheels were refaced using a Taber S-11 refacing disc for 25 cycles before each specimen was tested to ensure desired wheel surface quality as recommended by the manufacturer. Figure 13 shows a Texin 285A specimen during an abrasion test using the Taber Abrasion Tester. Figure 13. Texin 285A specimen mounted on the Taber Abrasion Tester, undergoing an abrasion test. 29 3.3.7 Friction Test Friction tests were conducted using a Labthink FPT-F1 Friction Tester. For each test, a legacy or candidate material rectangular specimen of approximately 105 × 275 mm was taped onto the base plate. A piece of polyester fabric (commonly used in swimwear) was clamped to the bottom of a 63.5 × 63.5 mm square sled that weighed 200 g (Figure 14 (a)). The sled was connected to a load cell via a string and was placed on top of the sheet specimen. For GFRP, since the samples were curved, a 63.5 × 63.5 mm square GFRP specimen (with matching curvature with the rectangular specimen) was taped to the bottom of the sled (Figure 14 (b)), and the polyester fabric was clamped to the bottom of the square GFRP (Figure 14 (c)), to ensure full contact between the fabric and the curved sheet specimen on the base plate. During the test, the base plate travelled horizontally at a constant speed of 250 mm/min, and the fabric-covered sled and the rectangular specimen slid against each other for a displacement of 130 mm. The load cell measured the friction force. The kinetic coefficient of friction, µk, was evaluated between 30 mm and 130 mm displacement, using the equation µ𝑘 =𝐴𝑘𝐵 (8) where µk = kinetic coefficient of friction, Ak = average reading obtained during uniform sliding of the surfaces (N), and B = total sled weight (N). Three specimens were tested for each material. Figure 14. (a) Friction test sled with polyester fabric clamped to the bottom (setup for Santoprene and Texin 285A), (b) sled with square GFRP taped to the bottom to match the curvature of rectangular GFRP specimen, and (c) polyester fabric clamped over the square GFRP. 30 Figure 15. Coulomb friction test setup. 3.3.8 Puncture Test To investigate the resistance to puncture of candidate materials, puncture tests guided by ASTM F1342 [108] were performed using an Instron 5969 system with a 50 kN load cell and custom puncture test fixture. The test specimens were cut into 110 mm-diameter circles and sandwiched between the fixture base plates (Figure 16). The probe tip was lowered to just above the specimens before the start of each test, and the tests were conducted at a head displacement rate of 508 mm/min. The tests were stopped after a head displacement of 28 mm, after which the tip of the probe had penetrated through the specimens. 31 Figure 16. Texin 285A specimen in puncture test fixture 3.3.9 Impact Test To investigate the resistance to damage due to impact, drop-weight impact tests were performed on the legacy material. The test setup is shown in Figure 17. An Instron CEAST 9340 system was used for these tests following ASTM D7136 [109]. The 150 mm × 100 mm GFRP specimens were clamped onto the support fixture concave down. The impact energy was calculated using the equation 𝐸𝑝 = 𝐶𝐸ℎ (9) where Ep = potential energy of impactor prior to drop (J), CE = specified ratio of impact energy to specimen thickness (J/mm) = 6.7, and h = nominal thickness of specimen (mm) = 3. Therefore, the nominal impact energy was calculated to be 20.1 J. Based on the nominal impact energy as well as the total mass of the impactor of 10.337 kg, the drop height was determined to be 198 mm. This would result in a nominal impact velocity of 1.97 m/s. 32 Figure 17. Impact test setup in Instron CEAST 9340 system. 3.4 Weathering Characterization Methods Water immersion and chlorinated water immersion conditions were prescribed since aquatic play structures are exposed to such conditions during regular operations. Hygrothermal aging and xenon lamp weathering chamber conditions were prescribed because structures installed outdoors would be exposed to solar radiation as well as fluctuating temperatures and humidities. The primary goal of the weathering tests in this research was to directly compare the weathering performances of the materials studied, and not to predict the service life as it is a complex matter and requires separate extensive investigation. 3.4.1 Water Immersion To determine the water absorption characteristics of the legacy and candidate materials, immersion tests were performed following ASTM D570 [110]. Three 76.2 mm × 25.4 mm rectangular specimens of each material were dried in a Thermotron SM-4-8200 environmental chamber for 24 hours at 50 °C. Then, the specimens were weighed (conditioned weight) and immersed in RO water in a beaker, and the beaker was placed in the environmental chamber at 33 23 °C. After 24 hours of immersion, the specimens were taken out, and the surface water was wiped off with paper towel, before the specimens were weighed again (wet weight). The percentage increase in weight during immersion was calculated using the equation 𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑤𝑒𝑖𝑔ℎ𝑡, % =𝑤𝑒𝑡 𝑤𝑒𝑖𝑔ℎ𝑡−𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡× 100. (10) To investigate the amount of water-soluble matter in the materials, the specimens were again dried in the environmental chamber for 24 hours at 50 °C after immersion. Then, they were weighed again (reconditioned weight). The percentage of soluble matter lost was calculated using the equation 𝑆𝑜𝑙𝑢𝑏𝑙𝑒 𝑚𝑎𝑡𝑡𝑒𝑟 𝑙𝑜𝑠𝑡, % =𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡−𝑟𝑒𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡× 100. (11) If the reconditioned weight after immersion is equal to or exceeds the conditioned weight prior to immersion, it would be considered that there is no soluble matter loss. 3.4.2 Chlorinated Water Immersion To determine the weathering resistance to chlorinated water, typical in a water park environment, the materials were subjected to chlorinated water exposure. Chlorinated water at 3, 5, and 10 ppm free chlorine content was prepared by adding REGAL stabilized chlorinating granules with 56% available chlorine content to RO water. The mass of chlorinating granules needed to achieve the desired free chlorine content was calculated using the equation 𝑚𝑐𝑙 =𝐶𝑙𝑓 × 𝑉𝑤𝑎𝑡𝑒𝑟𝐶𝑙𝑎× 100 (12) where mcl = mass of chlorinating granules (mg), Clf = free chlorine content (ppm), Cla = available chlorine content (%), and Vwater = volume of water (L). Therefore, to achieve 3, 5, and 10 ppm free chlorine content in 1 L of water, 5.4, 8.9, and 17.9 mg of 56% available free chlorine content granules would be needed, respectively. The chlorinating granules were added into a petri dish (Figure 18 (a)) and weighed on a precision scale. A small amount of water was then added into the petri dish, and a small spatula was used to mix and dilute the granules (Figure 18 (b)). The solution was poured into a glass jar with 1 L of RO water and mixed. After the chlorinated water was prepared, tensile specimens were weighed and then placed in the glass jar (Figure 18 (c)), fully immersed for 7 days. The chlorinated water was changed every 24 hours to ensure the free chlorine content stayed at the desired level throughout the exposure period. After 7 days, the specimens were taken out, wiped dry, and 34 weighed immediately. They were then conditioned under room temperature and ambient humidity for at least 24 hours before mechanical characterization. Figure 18. (a) Chlorinating granules in a petri dish, (b) chlorinated granules dissolved into RO water, and (c) specimens immersed in chlorinated water with 3, 5, and 10 ppm free chlorine content (left to right). 3.4.3 Accelerated Hygrothermal Aging Legacy and candidate materials tensile specimens were aged under accelerated hygrothermal conditions in a Thermotron SM-4-8200 environmental chamber following modified ASTM D1183 [111]. The specimens were exposed to modified test condition D (exterior, marine) according to the standard, except substitute ocean water was replaced with water with 5 ppm free chlorine content to simulate the water park conditions. The test parameters are summarized in Table 4. After the exposure, the specimens were wiped dry and weighed immediately. They were 35 then conditioned under room temperature and ambient humidity for at least 24 hours before tensile testing. Table 4. Exposure cycle parameters for hygrothermal cyclic aging. Period Length (hr) Temperature (°C) Relative Humidity 1 48 71 5 % 2 48 23 immersed in water (5 ppm free chlorine) 3 8 -35 N/A 4 64 23 immersed in water (5 ppm free chlorine) 3.4.4 Accelerated Weathering in Xenon Lamp Chamber Specimens were subjected to accelerated weathering in an Atlas SUNTEST XXL+ xenon lamp weathering chamber (Figure 19 (a)), following ASTM D2565 [112]. The weathering exposure followed test cycle 1A, specified in the standard and summarized in Table 5. The cycle was 2 hours long and was repeated for a total of 168 hours (7 days). The UV exposure dosage can be calculated using the equation 𝐻𝑒 = 𝑇𝑒 × 𝐸𝑒 (13) where He = radiant exposure (J/m2), Te = exposure time (s), and Ee = irradiance (W/m2). The irradiance set point of 42 W/m2 from 300 to 400 nm corresponds to an hourly UV dosage of approximately 0.15 MJ/m2, and a total UV dosage of 25.4 MJ/m2 for the 7-day test. Daylight filters from Atlas were installed below the xenon lamps to simulate terrestrial sunlight in an outdoor environment. The test specimens were placed on a stainless steel rack inside the chamber (Figure 19 (b)). To ensure the uniformity of exposure, the specimens were repositioned every day following ASTM G151 [113], as illustrated in Figure 19 (b). After the exposure, the specimens were conditioned under room temperature and ambient humidity for 24 hours before mass measurement and tensile testing. 36 Figure 19. (a) Atlas SUNTEST XXL+ xenon lamp weathering chamber and (b) specimens placed inside the chamber prior for weathering as well as repositioning sequence indicated by the arrows. Table 5. Exposure cycle parameters for accelerated weathering in xenon lamp chamber Cycle Description Parameter Set Point Maximum Allowed Operational Fluctuation 102 minutes of light followed by 18 minutes of light and water spray Irradiance 42 W/m2 from 300 to 400 nm ±2 W/m2 Uninsulated Black Panel Temperature 63 °C ±2 °C Air Temperature 47 °C ±2 °C Relative Humidity 50 % ±10 % 3.5 Microscopy Characterization Methods Unaged and aged (from xenon lamp chamber weathering) specimens of GFRP, Santoprene, and Texin 285A were imaged using a confocal laser scanning microscope (CLSM) and a scanning electron microscope (SEM). The images were used to investigate the surface quality, roughness, tensile fracture, and effects of weathering. 3.5.1 CLSM An Olympus LEXT OLS4000 CLSM with a 10x objective lens was used to image the top surface of the specimens. The images were specifically used to measure the surface roughness. 37 The root mean square roughness, Sq, was measured over an area of 1286 µm × 1282 µm at 3 different locations for each specimen. 3.5.2 SEM A Tescan Mira 3 XMU SEM at UBC FiLTER lab was used to image the top surface and tensile fracture surface of the specimens. Unaged and aged (xenon lamp chamber weathering) specimens of GFRP, Santoprene, and Texin 285A were imaged. The images were used to investigate the surface quality and the effects of weathering. The specimens were coated with 10 nm of platinum using a sputter coater before imaging. The acceleration voltage was 5 kV. 3.6 Summary In this chapter, the legacy and selected candidate materials were introduced, and the test samples manufacturing methods were explained. The mechanical characterization methods, weathering exposure and characterization test set-ups, as well as microscopy characterization tools for the evaluation of legacy and candidate material properties were presented. These methods were chosen to characterize the properties and simulate the weathering exposure conditions that pertain to the material application in aquatic play structures (e.g., water slides). The results from these characterizations will be presented and discussed in Chapter 4. 38 Chapter 4: Results and Discussion In this chapter, the results obtained following the characterization methods described in Chapter 3 are presented and discussed. The mechanical performance of unaged samples is discussed first, followed by the performance of the samples that were aged under different weathering conditions. The images of unaged and aged samples obtained through microscopy were also compared to investigate the weathering effects. 4.1 Control Sample Performance The results from mechanical characterization and microscopy characterization on the control unaged samples are presented in the subsequent subsections. 4.1.1 Density and Tensile Properties The density measurements for legacy and candidate materials are summarized in Table 6. All three candidate materials possessed lower density than the legacy GFRP material. The densities of Santoprene, Texin 285A, and Texin 255D were 32.1%, 16.2%, and 14.6% lower than the density of GFRP, respectively. The axial extension and load from the tensile tests were measured by the Instron 5969 system. Some of the GFRP specimens exhibited a toe region (initial flat region) at the beginning of the tensile test that can be observed in their load-extension curve (Figure 20 (a)). This is due to a take-up of slack in the material and seating of the specimen inside the fixtures during the start of the test. Since this behaviour is not a representation of material properties, toe compensation was performed to remove this effect. First, the toe region was removed, and the linear part of the load-extension curve (between 200 N and 500 N load) was extended downwards to intersect the horizontal axis (Figure 20 (b)). Then, the curve was shifted to the left so that it intersects the origin (Figure 20 (c)). The corrected extension values after toe compensation were used to calculate strain. 39 Figure 20. Example GFRP load-extension curve (a) before toe compensation, (b) with toe region removed, and (c) after horizontal shift to intercept the origin. The load-extension curves for all specimens are shown in Figure 21. They were also graphed on separate scales in Figure 22 to be seen better. GFRP initially exhibited a linear elastic region. When the stress reached approximately 50% of the ultimate tensile strength, the polyester barrier coat and gel coat reached their tensile limit and started to crack, causing the uneven stress-strain response thereafter and a decrease in Young’s modulus. At break, all GFRP specimens experienced explosive failure of the CSM/polyester inner layers, which also induced complete fracture of the barrier coat and gel coat surface layers (Figure 23). Santoprene and Texin 285A exhibited typical elastomeric stress-strain response, showing a slow and smooth increase in stress until failure without a definitive yield point. Texin 255D showed a typical tensile response of a ductile polymer, showing a Hookean region at small strains before reaching the yield point at the peak in the load-extension curve, followed by strain softening and strain hardening through large plastic deformation [114], [115]. 40 As shown in Table 6, GFRP has tensile strength (120 MPa) and Young’s modulus (8.7 GPa) that are one to two orders of magnitude higher than the candidate TPE materials, while having strain at break (0.019 mm/mm) that is two orders of magnitude lower. Based on the results from another study [116], the tensile strength of the GFRP investigated in this thesis is close to the tensile strength of E-Glass CSM/polyester GRFP with 40 weight percent fibre and 60 weight percent matrix. It should be noted that for FRP composites, increasing fiber content usually leads to an increase in the strength and stiffness of the composite material, up to a certain threshold [117], [118]. Also, CSM is generally considered as a material with isotropic properties in-plane. Woven roving, on the other hand, is anisotropic, with higher strength and stiffness in the fibre direction and lower properties in the transverse direction (resin-dominant direction). Since the GFRP composite consists of CSM and woven roving layers, the overall material is likely anisotropic. Texin 285A showed higher tensile strength and higher elongation at break compared to Texin 255D, but Texin 255D possessed higher initial Young’s modulus. Santoprene showed a tensile strength one order of magnitude lower compared to the TPU materials, while also having lower stiffness and strain at break. Figure 21. Load-extension curves of GFRP, Santoprene, Texin 285A, and Texin 255D specimens. The test was repeated three times for each material. 41 Figure 22. Load-extension curves of (a) GFRP, (b) Santoprene, (c) Texin 285A, and (d) Texin 255D specimens. Graphed on separate scales. Figure 23. GFRP specimen failure under tension. 42 Table 6. Density and tensile properties of legacy and candidate materials; average values are in bold and SD values are given in brackets. Material Density (kg/m3) Tensile Strength (MPa) Young’s Modulus (MPa) Strain at Break (mm/mm) GFRP 1418.0 (40.1) 120.41 (10.27) 8691.30 (300.03) 0.019 (0.0014) Santoprene 962.7 (2.1) 3.88 (0.35) 12.12 (0.62) 1.67 (0.20) Texin 285A 1188.3 (9.5) 30.11 (0.92) 19.91 (0.70) 5.52 (0.14) Texin 255D 1211.7 (5.9) 24.94 (0.76) 374.19 (4.55) 3.18 (0.12) 4.1.2 Hardness, Abrasion Resistance, and Flexural Properties The Shore D hardness measurements of all materials are shown in Table 7. The hardness of GFRP material can be largely attributed to the polyester surface coating. Santoprene showed the lowest hardness due to the presence of soft vulcanized EPDM particles and the low content of the hard thermoplastic PP matrix [119]. Texin 285A had lower hardness values than Texin 255D, possibly due to Texin 255D having a higher hard-segments-to-soft-segments ratio as well as a higher degree of crosslinking [120]. The abrasion resistance of the candidate TPE materials was investigated using the Taber Abrasion Tester. The mass loss results from the abrasion test are summarized in Table 7. The specimens after 1000 abrasion cycles are shown in Figure 24. The mass loss in Santoprene was unmeasurable due to particles that came off the abrading wheels being stuck on the tacky and rubbery specimen surface, which resulted in increased mass overall. However, it showed the most significant damage visually. Texin 285A showed better abrasion resistance than Texin 255D, having an average mass loss of 0.97 mg after 1000 abrasion cycles, about half of the 1.85 mg average mass loss for Texin 255D. Figure 24. Candidate material specimens after 1000 abrasion cycles. From left to right: Santoprene, Texin 285A, Texin 255D. 43 The flexural properties were investigated through three-point bending tests. The deflection and load were measured by the Instron 5969 system. In a typical load-deflection curve, a small toe region (initial flat region) can be observed (Figure 25 (a)). This was the case for all specimens due to the small distance between the loading nose and the surface of the specimen prior to the test, as well as a take-up of slack and change in alignment of the specimen during the start of the test. Since this behaviour is not a representation of material properties, it was compensated for by removing the toe region from the load-deflection curve (Figure 25 (b)), essentially shifting the curve slightly to the left and correcting the deflection and strain values. Figure 25. Typical load-deflection curve (a) prior to toe compensation and (b) after toe compensation. The flexural load-deflection curves after toe compensation for all specimens are shown in Figure 26. GFRP was the only material that experienced complete fracture within the 5% strain limit. Like in the tensile test, the GFRP surface polyester coats started to crack during the flexural tests, causing the uneven load-deflection response. The ultimate failure occurred when the polymer matrix in the centre failed under flexural bending. The TPE materials, which are much more ductile than GFRP, did not fail within the 5% strain limit when the test was terminated. The flexural properties are summarized in Table 7. Santoprene and Texin 285A have the lowest flexural moduli of elasticity. Although Texin 285A was 142% harder than Santoprene, it was only 32% less flexible. Texin 255D had a flexural modulus of elasticity an order of magnitude higher than Santoprene and Texin 285A. The relatively high flexural rigidity means Texin 255D 44 would not meet the goal of being rolled or folded during transportation. It was excluded from subsequent testing and further characterization. Figure 26. Flexural stress-strain curves of (a) GFRP, (b) Santoprene, (c) Texin 285A, and (d) Texin 255D specimens. Each test was repeated three times. Table 7. Hardness, mass loss from abrasion, and flexural properties of legacy and candidate materials; average values are in bold and SD values are given in brackets. Material Hardness (Shore D) Mass Loss (mg) Modulus of Elasticity in Bending (MPa) Flexural Strength (MPa) GFRP 84.5 (1.2) Untested 8690.3 (504.3) 286.9 (20.6) Santoprene 14.4 (0.3) Unmeasurable 16.7 (0.9) > 0.8 (0.1) Texin 285A 34.9 (0.5) 1.0 (0.2) 22.1 (0.9) > 1.3 (0.1) Texin 255D 60.3 (1.2) 1.9 (0.5) 715.2 (39.8) > 28.0 (0.9) Note: Santoprene, Texin 285A, and Texin 255D specimens did not fracture due to their high flexibility. The flexural strengths for these materials are reported based on the largest stress values during tests. 45 4.1.3 Cyclic Tensile Response The cyclic tensile responses of Santoprene and Texin 285A are presented in this section. During phase 1 of the tests, the specimens were subjected to 3 loading-unloading cycles and stretched to a maximum strain of 1 mm/mm during each cycle. The tensile stress-strain curves from phase 1 are shown in Figure 27, where the loading paths are marked by arrows, and the unloading paths are unmarked. Distinct hysteresis can be observed for both materials from the deviation between the loading and unloading path during each cycle. Also can be observed is the pronounced difference between the first cycle loading path and the subsequent loading paths, which is a result of material softening due to Mullins effect [82]. Figure 27. Stress-strain curves of (a) Santoprene and (b) Texin 285A from cyclic tensile test phase 1. The loading paths are indicated by the arrows. The work input, hysteresis, relative hysteresis, and residual strain during each cycle from the phase 1 tests were calculated according to Section 3.3.4 and Equation 7, and are summarized in Table 8. Texin 285A showed higher work input due to the higher tensile stiffness. Santoprene showed a 69.7% first cycle relative hysteresis, higher than 56.5% for Texin 285A. For the subsequent cycles, the two materials showed similar relative hysteresis, which were noticeably lower compared to the first cycle and serve as evidence that most of the damage and chain slippage that result in Mullins effect happen during the first loading. Texin 285A exhibited higher elasticity and showed lower residual strain compared to Santoprene. For both materials, a slight increase in residual strain can be seen after each cycle. Studies [82], [121] have shown that the residual deformation can potentially be recovered over time after load removal. 46 Table 8. Cyclic tensile test phase 1 results. Material Cycle Work Input (MJ/m3) Hysteresis (MJ/m3) Relative Hysteresis (%) Residual Strain (mm/mm) Santoprene 1 2.04 1.42 69.7 0.20 2 0.94 0.35 37.0 0.22 3 0.88 0.30 34.0 0.23 Texin 285A 1 4.82 2.72 56.5 0.13 2 3.12 1.18 37.7 0.15 3 2.90 1.01 35.0 0.17 During phase 2 of the tests, the specimens were subjected to several loading-unloading cycles with progressively increasing maximum strain. The Santoprene specimen failed during the fourth cycle, while the Texin 285A specimen underwent 5 cycles without breaking. The tensile stress-strain curves from phase 2 are shown in Figure 28. The work input, hysteresis, relative hysteresis, and residual strain during each cycle from the phase 2 tests are summarized in Table 9. Figure 28. Stress-strain curves of (a) Santoprene and (b) Texin 285A from cyclic tensile test phase 2. 47 Table 9. Cyclic tensile test phase 2 results. Material Cycle Maximum Strain (mm/mm) Work Input (MJ/m3) Hysteresis (MJ/m3) Relative Hysteresis (%) Residual Strain (mm/mm) Santoprene 1 0.5 0.73 0.46 62.5 0.10 2 1.0 1.73 1.10 63.7 0.20 3 1.5 2.69 1.70 63.1 0.31 Texin 285A 1 0.5 1.70 0.66 38.7 0.07 2 1.0 4.37 2.29 52.4 0.13 3 1.5 7.11 4.01 56.4 0.24 4 2.0 10.22 5.93 58.0 0.45 5 2.5 13.80 8.03 58.2 0.67 Santoprene exhibited a relative hysteresis of approximately 63% during each cycle. For Texin 285A, the relative hysteresis was the lowest (38.7%) for the first cycle during which the specimen was stretched to a maximum strain of 0.5 mm/mm. There was a significant increase in relative hysteresis between the first and second cycle, where the maximum strain increased to 1 mm/mm. During subsequent cycles, the relative hysteresis slowly increased and gradually stabilized at about 58%. Overall, Santoprene possessed higher relative hysteresis than Texin 285A. The residual strain of both specimens increased after each cycle, due to the greater damage from increasing maximum strain. Consistent with phase 1 results, after undergoing the same maximum strain, Texin 285A showed lower residual strain than Santoprene after load removal. 4.1.4 Puncture Resistance The puncture force and deflection were measured by the Instron 5969 system. The load-deflection curves are shown in Figure 29 (a). Figure 29 (b) shows the deflection and load at puncture of the Santoprene and Texin 285A specimens. The deflections at puncture were similar for the two materials, but the average force at puncture of the Texin 285A specimens (141.1 N) was nearly triple that of the Santoprene specimens (50.3 N). This is in good agreement with the hardness measurements of the two materials. The punctured Santoprene specimen and Texin 285A specimen are shown in Figure 30 (a) and (b), respectively. The puncture hole in the Santoprene specimen nearly closed off after the puncture probe was removed, while the puncture hole in the 48 Texin 285A specimen was visible, and the surrounding area was protruding out, showing lower elasticity. Figure 29. (a) Puncture force-deflection curves for Santoprene and Texin 285A specimens and (b) puncture load and deflection. Figure 30. (a) Santoprene specimen and (b) Texin 285A specimen after puncture test. 4.1.5 Surface Roughness and Frictional Properties The Olympus LEXT OLS4000 CLSM was used to measure the surface roughness of the specimens, expressed through root mean square roughness, Sq. The Labthink FPT-F1 Friction 49 Tester was used to measure the kinetic coefficient of friction between the legacy/candidate materials and a piece of polyester fabric. The results are shown in Table 10. GFRP exhibited the lowest Sq value, which translates to the lowest surface roughness. This can be attributed to the polyester gel coat being sprayed on top, resulting in a smooth and even surface finish. Santoprene showed a higher surface roughness than Texin 285A. Since the specimens of the two materials were manufactured following the same process, the difference in surface roughness is likely due to inherent material properties. GFRP had the lowest kinetic coefficient of friction at 0.23, and Santoprene had the highest at 0.93. This is consistent with the materials’ respective hardness and surface roughness [122]. A softer material has more significant microscopic deformation at the surface when in contact with another object [123]. The attractive van der Waals force across the gap between the contacting bodies also pulls them towards each other, leading to a larger effective contact area, requiring larger force to break the adhesive bonds and leading to higher coefficient of friction [124]. Table 10. Root mean square roughness (Sq) and kinetic coefficient of friction (µk) of GFRP, Santoprene, and Texin 285A; average values are in bold and SD values are given in brackets. Material Sq (µm) µk GFRP 0.81 (0.18) 0.23 (0.02) Santoprene 3.01 (0.09) 0.93 (0.05) Texin 285A 1.75 (0.12) 0.77 (0.05) 4.1.6 Impact Resistance Drop-weight impact tests were performed on GFRP specimens following ASTM D7136 [109]. The photos of the specimens after impact are shown in Figure 31. Small circular dents resulted from contact with the round impactor could be observed at the impact location on the top surfaces of the specimens. On the bottom side, cracks roughly parallel to the long side of the specimens as well as fibre breakage could be seen directly below the impact location. 50 Figure 31. GFRP specimens after drop-weight impact: (a) top side and (b) bottom side. The force over time and force over displacement graphs during the impact event are plotted in Figure 32 (a) and (b), respectively. The specimen thickness, measured impact energy, maximum force, absorbed energy values, as well as top and bottom side damage sizes are shown in Table 11. The measured impact energies were slightly lower than the nominal impact energy of 20.1 J due to friction and air resistance. The average absorbed energy was 14.4 J, which was 75.8% of the measured impact energy. 51 Figure 32. (a) Force vs. time from drop-weight impact tests of GFRP; three test repeats are shown. Table 11. Results from drop-weight impact tests of GFRP; average values are in bold and SD values are given in brackets. GFRP Specimen Thickness (mm) Measured Impact Energy (J) Maximum Force (N) Absorbed Energy (J) Top Side Dent Diameter (mm) Bottom Side Crack Length (mm) Specimen 1 3.08 18.9 2354.2 16.5 13.23 30.07 Specimen 2 3.32 19.1 3087.4 13.2 10.34 35.95 Specimen 3 3.75 19.1 3267.8 13.4 9.18 32.09 Average (SD) 3.38 (0.34) 19.0 (0.1) 2903.1 (483.9) 14.4 (1.8) 10.92 (2.09) 32.70 (2.99) 4.2 Weathering Performance The results from the mechanical and microscopy characterization on the samples that were subjected to various weathering conditions, as described in Section 3.4, are presented in the subsequent subsections. 4.2.1 Appearance, Surface Morphology, and Mass Change from Xenon Lamp Chamber Weathering Specimens of GFRP, Santoprene, and Texin 285A were subjected to accelerated weathering in a xenon lamp weathering chamber for 7 days, during which they were exposed to light radiation, water spray, and elevated temperature. The weathering exposure parameters are described in Section 3.4.4. 52 UV radiation can cause undesirable discolouration in polymers. Photos of the specimens were taken before and after 7 days of xenon lamp chamber weathering, and the surface appearances are compared, as shown in Figure 33. After aging, the originally colourless Texin 285A specimens turned yellow, due to photooxidation in the polymer backbone and production of quinone structures [125], [126]. The GFRP gel coat and Santoprene showed no noticeable colour change. Figure 33. GFRP (left), Santoprene (centre), and Texin 285A (right) specimens (a) before and (b) after xenon lamp chamber weathering. The surface morphology of the unaged samples and samples that were aged in xenon lamp weathering chamber for 7 days was studied through microscopic imaging using SEM. The images of the specimen surfaces are shown in Figure 34. After aging, the specimen surface became rougher, which is less apparent in the GFRP specimens and more apparent in the Santoprene and Texin 285A specimens. The surface degradation is a result of multi-stress aging from a combination of UV radiation, moisture, and thermal aging effects in the xenon lamp weathering chamber. Studies have shown that UV radiation usually causes increased surface roughness and cracking due to backbone dissociation [127], [128]. Moisture absorption leads to swelling [129], 53 and thermal exposure causes chain scission and can lead to embrittlement and cracking [130], [131]. The mass of each specimen was measured before and after weathering, and the mass change is shown in Figure 35. GFRP showed a slight increase in mass and Santoprene showed a slight decrease in mass, both under 0.1 %. Texin 285A showed an average decrease in mass of 0.3%. Since the specimens were conditioned before mass measurements, the moisture absorption effects on mass were mostly eliminated. The decrease in mass for Santoprene and Texin 285A can be attributed to the loss of surface particles after they became water soluble due to oxidation and chain scission [132]. 54 Figure 34. SEM images of surfaces of control unaged samples and samples that were aged in xenon lamp weathering chamber. (a) Unaged GFRP; (b) Aged GFRP; (c) Unaged Santoprene; (d) Aged Santoprene; (e) Unaged Texin 285A; (f) Aged Texin 285A. 55 Figure 35. Mass change of specimens after aging in xenon lamp weathering chamber. 4.2.2 Water Absorption The GFRP, Santoprene, and Texin 285A specimens were wiped dry and weighed immediately after water immersion, chlorinated water immersion, and hygrothermal aging exposures. These measurements were compared to the mass measurements before exposure. The results are shown in Figure 36 and Table 12. All material specimens showed the largest mass gain after the 7-day chlorinated water immersion, followed by the hygrothermal aging (including 4.67 days of chlorinated water immersion) and the 1-day water immersion. The order is consistent with the total immersion time. Texin 285A showed the highest water absorption under all exposure conditions compared to GFRP and Santoprene. After 1 day of water immersion, the average mass increase of Texin 285A specimens was 0.89%, which was 350% higher than the mass increase of GFRP and 760% higher than that of Santoprene. Santoprene showed 48% lower mass increase than GFRP under 1-day water immersion and 29% lower mass increase under hygrothermal aging, but under 7-day chlorinated water immersion, the two materials showed comparable mass increase. Two-tailed t-tests (assuming equal variance) were conducted to investigate if the different free chlorine concentrations resulted in a statistically significant difference in the mass change of specimens. The resultant p-values are shown in Table 13. For Santoprene, the difference between 56 mass change at 3 and 10 ppm free chlorine concentration was significant at a 95% confidence interval. The p-value for the comparison between 5 and 10 ppm was also small for Santoprene, but not small enough to indicate significance at a 95% confidence level. For all other cases, the differences in mass change were also not significant at a 95% confidence interval. Figure 36. Mass change of specimens after weathering conditioning exposures. Table 12. Mass change of specimens after weathering conditioning exposures; average values are in bold and SD values are given in brackets. Material Mass Change (%) Water (1 day) 3 ppm Free Chlorine (7 days) 5 ppm Free Chlorine (7 days) 10 ppm Free Chlorine (7 days) Hygrothermal (7 days) GFRP 0.20 (0.011) 0.28 (0.012) 0.29 (0.009) 0.29 (0.013) 0.27 (0.008) Santoprene 0.10 (0.008) 0.28 (0.004) 0.29 (0.018) 0.34 (0.032) 0.19 (0.006) Texin 285A 0.89 (0.009) 1.14 (0.017) 1.15 (0.012) 1.16 (0.012) 1.01 (0.016) 57 Table 13. Two-tailed t-test results for the mass change of materials after immersion in chlorinated water at different free chlorine concentrations (p-values showing statistically significant difference at 95% confidence level are bolded). Material Samples p-Value (Two-Tailed) GFRP 3 ppm vs. 5 ppm 0.289 3 ppm vs. 10 ppm 0.466 5 ppm vs. 10 ppm 0.775 Santoprene 3 ppm vs. 5 ppm 0.479 3 ppm vs. 10 ppm 0.028 5 ppm vs. 10 ppm 0.060 Texin 285A 3 ppm vs. 5 ppm 0.452 3 ppm vs. 10 ppm 0.345 5 ppm vs. 10 ppm 0.768 It should be noted that due to the highly anisotropic properties of GFRP (stemming from the different phases of constituent materials including the fibres, matrix, and gel coat), the in-plane and through-thickness moisture diffusivities are different [133]. Studies have shown that the fibre diffusivity is generally lower than the resin diffusivity, and that the diffusion coefficient along the fibres (in-plane) in a composite is much higher than the diffusion coefficient transverse to the fibres (through-thickness) [133], [134]. The gel coat acts as a barrier in front of the fibre-resin interface and should further reduce the through-thickness diffusion coefficient. Through-thickness moisture diffusion is usually of higher importance for FRP composite sheets due to the high aspect ratio between length/width and thickness. Since the specimens used in the weathering experiments were tensile specimens with small width, the water absorption of the GFRP specimens shall not be extrapolated for larger sheets due to the significant differences in aspect ratios. Comparing the 1-day water immersion and 7-day chlorinated water immersion results shown in Figure 36, it can be deduced that a significant percentage of the total mass increase happened on the first day, and the water absorption rate decreased as the material approached saturation. This is consistent with experimental data from other studies [135], [136]. 4.2.3 Tensile Properties After aging under different weathering conditions (chlorinated water, hygrothermal, and xenon lamp chamber weathering), uniaxial tensile tests were conducted on the specimens 58 according to the procedures described in Section 3.3.2 to investigate the weathering effects on tensile properties. The tensile strength, Young’s modulus, and strain at break results are summarized in Table 14. To provide a visual way to compare the normalized effects of weathering on these properties, the percentages of the property values relative to the control sample for each material are also graphed in Figure 37. Table 14. Effects of accelerated weathering on sample tensile properties; average values are in bold and SD values are given in brackets. Material Weathering condition Tensile Strength (MPa) Young’s Modulus (MPa) Strain at Break (mm/mm) GFRP Control 120.41 (10.27) 8691.30 (300.03) 0.019 (0.001) 3 ppm free chlorine 121.26 (33.36) 8140.50 (1018.61) 0.021 (0.003) 5 ppm free chlorine 126.60 (12.71) 8114.57 (719.63) 0.022 (0.001) 10 ppm free chlorine 111.26 (10.05) 8194.67 (302.80) 0.020 (0.001) Hygrothermal 124.90 (38.57) 8882.50 (859.35) 0.020 (0.006) Xenon lamp chamber 127.69 (16.57) 8229.53 (503.07) 0.022 (0.001) Santoprene Control 3.88 (0.35) 12.12 (0.62) 1.67 (0.20) 3 ppm free chlorine 3.77 (0.19) 12.30 (0.22) 1.56 (0.13) 5 ppm free chlorine 3.79 (0.02) 12.35 (0.54) 1.48 (0.09) 10 ppm free chlorine 3.71 (0.51) 12.39 (1.34) 1.42 (0.11) Hygrothermal 3.75 (0.15) 11.97 (0.12) 1.52 (0.15) Xenon lamp chamber 3.77 (0.31) 12.08 (0.37) 1.48 (0.17) Texin 285A Control 30.11 (0.92) 19.91 (0.70) 5.52 (0.14) 3 ppm free chlorine 31.45 (1.11) 18.81 (0.38) 5.44 (0.05) 5 ppm free chlorine 33.83 (1.94) 18.92 (0.63) 5.75 (0.17) 10 ppm free chlorine 33.82 (2.47) 18.87 (1.15) 5.65 (0.15) Hygrothermal 31.22 (0.81) 21.35 (0.73) 5.88 (0.06) Xenon lamp chamber 30.12 (0.88) 18.90 (0.64) 5.60 (0.19) 59 Figure 37. Tensile strength, (b) Young's modulus, and (c) strain at break of specimens after weathering conditioning, expressed as percentages of control sample performance. 60 To determine if the prescribed aging under various weathering conditions resulted in a statistically significant change in tensile properties, two-tailed t-tests (assuming equal variance) were conducted to compare the results from each weathering condition to control. The p-values from the t-tests are shown in Table 15. Results show that for GFRP, the difference in strain at break between control and 5 ppm free chlorine immersion was significant at a 95% confidence level. However, the statistically significant difference was not observed when comparing control and 10 ppm free chlorine immersion, indicating that the difference could be due to factors that are unrelated to the weathering conditions. For Texin 285A, at a 95% confidence level, difference in tensile strength was observed when comparing control and 5 ppm free chlorine, and difference in strain at break was observed when comparing control and hygrothermal aging. For all other comparisons, no differences could be observed at 95% confidence. Table 15. Two-tailed t-test results for the tensile properties of materials after weathering conditioning (p-values showing statistically significant difference at 95% confidence level are bolded). Material Samples p-Value (Two-Tailed) Tensile Strength Young’s Modulus Strain at Break GFRP Control vs. 3 ppm free chlorine 0.968 0.420 0.515 Control vs. 5 ppm free chlorine 0.547 0.269 0.025 Control vs. 10 ppm free chlorine 0.332 0.114 0.514 Control vs. hygrothermal 0.855 0.734 0.918 Control vs. xenon lamp chamber 0.553 0.244 0.099 Santoprene Control vs. 3 ppm free chlorine 0.683 0.661 0.463 Control vs. 5 ppm free chlorine 0.688 0.656 0.211 Control vs. 10 ppm free chlorine 0.663 0.767 0.128 Control vs. hygrothermal 0.599 0.712 0.362 Control vs. xenon lamp chamber 0.703 0.943 0.286 Texin 285A Control vs. 3 ppm free chlorine 0.180 0.073 0.435 Control vs. 5 ppm free chlorine 0.040 0.141 0.138 Control vs. 10 ppm free chlorine 0.071 0.250 0.319 Control vs. hygrothermal 0.188 0.069 0.013 Control vs. xenon lamp chamber 0.984 0.138 0.554 61 To further investigate the tensile fracture mechanism of the TPE materials, the fracture surfaces were imaged using SEM. The micrographs are shown in Figure 38. Little differences could be observed before and after aging for the same material, although the differences between the two materials are apparent. Noticeable unevenness and fibrils can be observed on the Santoprene fracture surfaces, indicating ductile failure. The lack of sinusoidal folding shows relatively low plastic deformation before failure due to the high elasticity possessed by the rubber phase [137]. TPU showed long and continuous fracture paths, accompanied by sinusoidal folding that indicates high plastic deformation before failure [137], [138]. The visible wavy crests also indicate a very ductile failure [137], [138]. Figure 38. SEM images of tensile fracture surfaces of control unaged samples and samples that were aged in xenon lamp weathering chamber: (a) unaged Santoprene, (b) aged Santoprene, (c) unaged Texin 285A, and (d) aged Texin 285A. 62 4.3 Summary In this chapter, the results from mechanical characterization, weathering exposure and characterization, and microscopy characterization tests were presented and discussed. GFRP possessed superior mechanical properties, although it had a greater density compared to the replacing candidates. Santoprene and Texin 285A showed high flexibility and would be able to be rolled or folded. Texin 285A showed good abrasion and puncture resistance, but had a higher water absorption compared to Santoprene, and it also showed yellowing (discolour) after weathering exposure. No major statistically significant effect of weathering could be observed on the tensile properties for all tested materials, when compared to the respective control samples. All data from the characterization results will be next used in an MCDM model for material ranking in the next chapter. 63 Chapter 5: Material Alternatives Ranking: An MCDM Approach 5.1 Introduction Engineering decision-making problems, such as design concepts ranking, are often delicate and complex since they usually involve several conflicting criteria, and there is no single ideal option that would best suit all criteria [139]. Material selection is an excellent example of such challenging decision problems in engineering practice, for which conflicting design/decision criteria with different units and/or magnitudes of measurements must be taken into account concurrently [9]. To support such decision process, multicriteria decision making (MCDM) methods have been developed to address and evaluate different objectives and criteria, while accounting for personal preferences of the decision maker/designer by allowing them to provide subjective information [139], [140]. MCDM tools have been applied extensively to the material selection problems in areas such as automotive, agriculture, energy, chemical, maritime, health, construction, and manufacturing [9]. Among different MCDM methods, in this work, PROMETHEE II was chosen as a known sample technique, particularly due to its simplicity in conception and computation, as well as its ability to yield complete ranking of alternatives and successful application in real-life planning problems [141]–[143]. More details of the method are provided next. 5.1.1 PROMETHEE II The Preference Ranking Organization METHod for Enriched Evaluation (PROMETHEE) is an MCDM method that evaluates and ranks alternatives based on multiple conflicting criteria [139], [143], [144]. It has become popular due to its easy implementation and straightforward concepts, allowing decision makers to understand the evaluation and ranking process [144], [145]. It has been successfully applied in many areas such as manufacturing, transportation, and environment management [143]. In PROMETHEE, pairwise comparisons between alternatives are made for each evaluation criterion, and the preferences are calculated and aggregated for each alternative and used to quantify its advantage compared to other alternatives. Input from the decision maker is realized through the selection of preference function and definition of weights for the evaluation criteria. Several versions of PROMETHEE have been developed with varying methods of the final ranking 64 of alternatives. PROMETHEE II is the most popular since it yields a complete ranking of all alternatives [144], [145]. To evaluate and rank the legacy and candidate materials for use in aquatic play structures, PROMETHEE II method, based on [139], [144], [145], was implemented as described by the steps below. Step 1: Create an evaluation table such as Table 16, where A = {ai, i = 1, 2, …, n} is the set of alternatives, and {gj, j = 1, 2, …, k} is the set of evaluation criteria. Table 16. Sample evaluation table. a g1(·) g2(·) … gj(·) … gk(·) a1 g1(a1) g2(a1) … gj(a1) … gk(a1) a2 g1(a2) g2(a2) … gj(a2) … gk(a2) ⋮ ⋮ ⋮ ⋱ ⋮ ⋱ ⋮ ai g1(ai) g2(ai) … gj(ai) … gk(ai) ⋮ ⋮ ⋮ ⋱ ⋮ ⋱ ⋮ an g1(an) g2(an) … gj(an) … gk(an) Step 2: Create a weights of relative importance table such as Table 17, where {wj, j = 1, 2, …, k} is the set of weights of relative importance of the evaluation criteria. A higher weight represents a higher relative importance, and the set of weights should be normalized to satisfy the equation ∑ 𝑤𝑗𝑘𝑗=1= 1 (14) where wj is the weight of criterion gj, and k is the total number of criteria. Table 17. Sample weights of relative importance table. g1(·) g2(·) … gj(·) … gk(·) w1 w2 … wj … wk Step 3: In PROMETHEE methods, comparisons are made between every pair of alternatives for each criterion. For each criterion, the difference between a pair of alternatives, a and b, are calculated using the equation 65 𝑑𝑗(𝑎, 𝑏) = 𝑔𝑗(𝑎) − 𝑔𝑗(𝑏), (15) where gj(a) is the value of alternative a for criterion gj, gj(b) is the value of alternative b for criterion gj, and dj(a,b) is the difference between the two values. To quantify the strength of preference of one alternative over the other from each pairwise comparison, a preference function is defined. If a higher value is preferred (e.g., strength, toughness), the preference function is defined as 𝑃𝑗(𝑎, 𝑏) = 𝐹𝑗[𝑑𝑗(𝑎, 𝑏)] ∀𝑎, 𝑏 ∈ 𝐴, (16) and if a lower value is preferred (e.g., cost, density), the preference function is defined as 𝑃𝑗(𝑎, 𝑏) = 𝐹𝑗[−𝑑𝑗(𝑎, 𝑏)] ∀𝑎, 𝑏 ∈ 𝐴, (17) where Pj(a,b) is the preference of alternative a over alternative b for criterion gj, and Fj is the preference function for criterion gj. The preference satisfies 0 ≤ 𝑃𝑗(𝑎, 𝑏) ≤ 1, (18) where 0 represents no preference of alternative a over alternative b, and 1 represents full preference of a over b. There are several common types of preference functions that have been proposed in [144]. It is up to the decision maker to select or define the appropriate preference function. In this study, the V-shape function, shown in Figure 39, was chosen as the preference function for all criteria. A preference threshold pj, which marks the smallest difference dj that warrants full preference (Pj = 1), is a parameter of the V-shape preference function and needs to be defined by the user. In this study, pj is defined as 𝑝𝑗 = 𝑔𝑗,𝑚𝑎𝑥 − 𝑔𝑗,𝑚𝑖𝑛, (19) where gj,max is the maximum value across all alternatives for criterion j, and gj,min is the minimum, following a similar procedure in [145]. This way, the range of magnitudes of all alternatives for each criterion is considered when calculating the preference. 66 Figure 39. V-shape preference function. Step 4: For each pair of alternatives a and b, the aggregated preference index is calculated using the equation 𝜋(𝑎, 𝑏) = ∑ 𝑃𝑗(𝑎, 𝑏)𝑤𝑗𝑘𝑗=1 ∀𝑎, 𝑏 ∈ 𝐴, (20) where π(a,b) is the aggregated preference index of alternative a over alternative b. This represents the preference of alternative a over alternative b considering all criteria. Step 5: For each alternative a, the positive outranking flow is calculated as 𝜙+(𝑎) =1𝑛 − 1∑ 𝜋(𝑎, 𝑥)𝑥∈𝐴, (21) and the negative outranking flow is calculated as 𝜙−(𝑎) =1𝑛 − 1∑ 𝜋(𝑥, 𝑎)𝑥∈𝐴, (22) where n is the total number of alternatives. The positive outranking flow represents the strengths of a compared to other alternatives, and the negative outranking flow represents the weaknesses of a compared to other alternatives. Step 6: For each alternative a, the net flow is calculated using the equation 𝜙(𝑎) = 𝜙+(𝑎) − 𝜙−(𝑎). (23) After all net flows are calculated, the alternatives are ranked in the order of descending net flows. The alternative with the highest net flow is the best alternative. 67 5.2 Application of PROMETHEE II to material ranking for aquatic play structures This section describes the application of PROMETHEE II to the material ranking task at hand for aquatic play structures, along with the subsequent sensitivity analysis on the perturbation of weights. 5.2.1 PROMETHEE II Complete ranking The PROMETHEE II method described in Section 5.1 was applied to the material ranking task at hand for aquatic play structures, prioritizing flexibility, durability, and recyclability criteria. The alternatives are GFRP, Santoprene 121-67W175, and Texin 285A. The evaluation table that contains 16 evaluation criteria is shown in Table 18. The criteria for which a higher value is preferred and those for which a lower value is preferred were differentiated with arrows. For some criteria, precise values from experimental testing were available. For others, integers from 1 to 5 were used as qualitative indicators. Table 18. Evaluation table for material selection for aquatic play structures.  indicates a criterion for which a higher value is preferred, and  indicates a criterion for which a lower value is preferred. Criteria GFRP Santoprene 121-67W175 Texin 285A  Material price (CAD/kg) 41.20 6.50 11.44  Manufacturing and labour cost (qualitative; 1 – 5 scale) 4 3 3  Density (kg/m3) 1418.0 962.7 1188.3  Tensile strength (MPa) 120.41 3.88 30.11  Young’s modulus (MPa) 8691.3 12.12 19.91  Strain at break (mm/mm) 0.019 1.67 5.52  Flexural stiffness (MPa) 8690.3 16.7 22.1  Abrasion resistance (qualitative; 1 – 5 scale) 5 1 3  Coefficient of friction 0.23 0.93 0.77  Damping (qualitative; 1 – 5 scale) 1 5 4  Puncture resistance (qualitative; 1 – 5 scale) 5 1 3  Water absorption (%) 0.20 0.10 0.89  Weathering – chlorinated water (qualitative; 1 – 5 scale) 4 5 4  Weathering – hygrothermal aging (qualitative; 1 – 5 scale) 5 5 4  Weathering – xenon lamp weathering chamber (qualitative; 1 – 5 scale) 5 4 3  Recyclability (qualitative; 1 – 5 scale) 1 5 5 68 The price of GFRP was obtained from Waterplay (Kelowna, BC, CA). The price of Santoprene 121-67W175 was obtained from Celanese (Dallas, TX, US). The price of Texin 285A was obtained from Covestro (Pittsburgh, PA, US). The manufacturing and labour costs were estimated based on the equipment/tooling cost and labour requirement. The open moulding process of GFRP requires a large mould that shapes the part and is very labour intensive since it involves hand lay-up or spray up of fibreglass material, so a score of 4 was assigned. The majority of the cost of TPE moulding is likely associated to the purchase and operation of the machinery, but the process does not involve much labour, so a score of 3 was assigned. In future phases of the project, accurate full production costs will be considered once detailed designs and manufacturing plans are set by the industry partner. The abrasion resistance, damping, and puncture resistance performances of GFRP were deduced since experiments could not be conducted directly due to shape and property constraints. GFRP should have better abrasion resistance and puncture resistance than the TPE materials due to its protective gel coat and high hardness. It is a brittle material, so it should have lower damping properties. The weathering performances under various exposure conditions were evaluated based on a combination of the tensile test results, change in appearance, and surface morphology through SEM imaging. For recyclability, GFRP was assigned a score of 1 due to the lack of technology for recycling FRP composites, and the TPEs were assigned 5s since they can be melted and reprocessed like thermoplastics. Next, the weights of relative importance were determined for the evaluation criteria. Similar to other studies [146]–[148], an importance score based on a five-point Likert scale was directly assigned to each criterion based on linguistic representation of the importance as shown in Table 19. The scores were then converted into normalized weights using the equation 𝑤𝑗 =𝑆𝑗∑ 𝑆𝑗𝑘𝑗=1, (24) where Sj is the importance score for criterion gj. The importance scores for all criteria were determined based on the focus of the study as well as the author’s expertise. For example, flexibility and recyclability were assigned scores of 5 since they represented the primary focus of this study. Tensile strength and Young’s modulus were also assigned scores of 5 since they are crucial to the load-bearing and shape-retaining capabilities 69 of the structure. Each weathering performance criterion was assigned a score of 2 so they would add up to represent a significant part of the decision but at the same time not overshadow other criteria. The importance scores were normalized into weights with a sum of 1. The importance scores and weights are summarized in Table 20. Table 19. Five-point Likert scale employed. Linguistic Terms Importance score Very high importance 5 High importance 4 Medium importance 3 Low importance 2 Very low importance 1 Table 20. Importance scores and weights of relative importance for material ranking for aquatic play structures. Criteria Importance Score Weight Material price 3 0.055 Manufacturing and labour cost 3 0.055 Density 3 0.055 Tensile strength 5 0.091 Young’s modulus 5 0.091 Strain at break 5 0.091 Flexural stiffness 5 0.091 Abrasion resistance 4 0.073 Coefficient of friction 4 0.073 Damping 1 0.018 Puncture resistance 4 0.073 Water absorption 2 0.036 Weathering – chlorinated water 2 0.036 Weathering – hygrothermal aging 2 0.036 Weathering – xenon lamp weathering chamber 2 0.036 Recyclability 5 0.091 70 After the evaluation table and weights were established, steps 3 to 6 in Section 5.1.1 were followed to calculate the positive outranking flow 𝜙+, negative outranking flow 𝜙−, net flow 𝜙, and complete ranking for the 3 material alternatives. The results are shown in Table 21. Texin 285A was ranked the best alternative thanks to its good mechanical properties and high flexibility. Although GFRP possesses superior mechanical properties and good durability, it was ranked behind the TPEs due to having lower flexibility and lower recyclability, as well as higher material and manufacturing cost. Table 21. PROMETHEE II ranking of alternatives for material selection for aquatic play structures. Alternatives Positive outranking flow 𝝓+ Negative outranking flow 𝝓− Net flow 𝝓 Ranking GFRP 0.406 0.424 -0.017 3 Santoprene 121-67W175 0.299 0.296 0.003 2 Texin 285A 0.294 0.281 0.014 1 5.2.2 Sensitivity Analysis The weights that represent the relative importance of the evaluation criteria come with uncertainties due to the subjective opinions of the decision makers [149]. Small changes in weights can lead to a change in the final output. The stability or robustness of the output against the change in weights can be investigated through a sensitivity analysis [149], [150]. To simulate the perturbation of weights, the dilation and concentration method, adapted from [151], was used as described below. - Dilation: the original weights (w1, w2,…, wk) are replaced by (w1, w2,…, wk)α; α > 1. - Concentration: the original weights (w1, w2,…, wk) are replaced by (w1, w2,…, wk)α; 0 < α < 1. - The new weights are normalized so that they sum up to 1. 71 Dilation increases the distances between large weights and small weights, thereby making the originally significant criteria even more significant, and the originally insignificant criteria even more insignificant. As α approaches infinity, the most significant criterion would dominate the rest. On the other hand, concentration reduces the distances between large and small weights, achieving the opposite effect. As α approaches zero, all criteria would possess equal significance. The α values were chosen based on a similar study [149] and shown in Table 22. The original weights and the new normalized weights after dilation and concentration are also shown in Table 22. The rankings based on the original weights, dilated weights, and concentrated weights are shown in Table 23. After dilation, Texin 285A remained the top alternative, and Santoprene dropped to last. After concentration, Santoprene was ranked as the top alternative. This was not surprising as Santoprene had significantly lower performance in two significant criteria, tensile strength and Young’s modulus, than the other two materials. Dilation worsened the ranking for Santoprene as the originally significant criteria were assigned more significance, and concentration improved its ranking since the originally significant criteria were made less significant. Overall, Texin 285A achieved the highest rankings, ranking 1 with original and dilated weights, and 3 with concentrated weights. 72 Table 22. Original, dilated, and concentrated weights. Criteria Weight – Original (α = 1) Weight – Dilation (α = 2) Weight – Concentration (α = 0.5) Material price 0.055 0.041 0.060 Manufacturing and labour cost 0.055 0.041 0.060 Density 0.055 0.041 0.060 Tensile strength 0.091 0.115 0.077 Young’s modulus 0.091 0.115 0.077 Strain at break 0.091 0.115 0.077 Flexural stiffness 0.091 0.115 0.077 Abrasion resistance 0.073 0.074 0.069 Coefficient of friction 0.073 0.074 0.069 Damping 0.018 0.005 0.034 Puncture resistance 0.073 0.074 0.069 Water absorption 0.036 0.018 0.049 Weathering – chlorinated water 0.036 0.018 0.049 Weathering – hygrothermal aging 0.036 0.018 0.049 Weathering – xenon lamp weathering chamber 0.036 0.018 0.049 Recyclability 0.091 0.115 0.077 Table 23. Material alternatives ranking based on original, dilated, and concentrated weights. Alternatives Ranking – Original (α = 1) Ranking – Dilation (α = 2) Ranking – Concentration (α = 0.5) GFRP 3 2 2 Santoprene 121-67W175 2 3 1 Texin 285A 1 1 3 5.2.3 Consideration of Redesign Cost The initial MCDM above did not consider the cost/efforts associated with the new supply chain and substructure production rework required towards the GFRP material replacement. To account for this overall extra cost, a second MCDM problem is considered in this section. Namely, here the redesign cost is attempted to be added as a stand-alone evaluation criterion. Its value was expressed qualitatively on a 1 to 5 scale, where 1 indicates least expensive and 5 is most expensive. 73 For GFRP option, since no redesign effort would be needed, a score of 1 was assigned. For Santoprene and Texin 285A, a score of 5 was assigned. The weight of the redesign cost criterion was individually controlled (not through an importance score like in Table 20). The weights of the remaining criteria, however, were still determined by their importance score, such that the normalized weights of all criteria including redesign cost, add up to 1 (Table 24). A weight as low as 0.03 for the redesign cost yielded top ranking for GFRP. A weight of 0.02 or below for redesign cost resulted in top ranking for Texin 285A. Table 24. Original and new weights with redesign cost considered. Criteria Weight – Original Weight – Redesign cost weight = 0.02 Weight – Redesign cost weight = 0.03 Material price 0.055 0.053 0.053 Manufacturing and labour cost 0.055 0.053 0.053 Density 0.055 0.053 0.053 Tensile strength 0.091 0.089 0.088 Young’s modulus 0.091 0.089 0.088 Strain at break 0.091 0.089 0.088 Flexural stiffness 0.091 0.089 0.088 Abrasion resistance 0.073 0.071 0.071 Coefficient of friction 0.073 0.071 0.071 Damping 0.018 0.018 0.018 Puncture resistance 0.073 0.071 0.071 Water absorption 0.036 0.036 0.035 Weathering – chlorinated water 0.036 0.036 0.035 Weathering – hygrothermal aging 0.036 0.036 0.035 Weathering – xenon lamp weathering chamber 0.036 0.036 0.035 Recyclability 0.091 0.089 0.088 Redesign cost N/A 0.020 0.030 74 Table 25. Material alternatives ranking based on original and new weights with redesign cost considered. Alternatives Ranking – Original Ranking – Redesign cost weight = 0.02 Ranking – Redesign cost weight = 0.03 GFRP 3 2 1 Santoprene 121-67W175 2 3 3 Texin 285A 1 1 2 The results suggest that with a short-term business perspective, if the redesign cost is considered, GFRP would likely be preferred due to the convenience of not requiring redesign effort. However, an argument can be made by industry for discarding the redesign cost in MCDM, specially when novelty and ‘long-term’ business justification along with environmental benefits of the new foldable product are accounted for. A formal Life Cycle Assessment (LCA) and Life Cycle Cost Analysis (LCCA) modeling would be highly valuable in this regard. 5.3 Summary This chapter presented the application of PROMETHEE II MCDM method to the material selection/alternatives ranking for aquatic play structures. The alternatives were GFRP, Santoprene 121-67W175, and Texin 285A. The evaluation criteria included cost, mechanical properties, weathering properties, and recyclability. Weights were calculated through importance scores determined by the decision maker. Texin 285A was ranked as the top alternative, followed by Santoprene and GFRP. A sensitivity analysis through dilation and concentration of weights was also conducted to investigate the robustness of the model. Dilation (α = 2) still yielded Texin 285A as the top choice, while concentration (α = 0.5) indicated Santoprene as the top choice. The latter was due to the fact that Santoprene had significantly lower performance in two significant criteria, tensile strength and Young’s modulus, than the other two materials; while during concertation the distance between weights decrease and this poor performance is overlooked by the MCDM solution process. A separate MCDM study was conducted and took the redesign cost into consideration. It was determined that a redesign cost criterion weight of 0.03 would yield GFRP as the top ranked material; however long-term business and environmental benefits may justify neglecting the latter criterion. Among the selected polymers, Texin 285A was selected as the top-performing replacement material candidate. 75 Chapter 6: Incorporation of the Selected Flexible Material into Actual Aquatic Play Structure: Preliminary Redesign Ideas 6.1 Introduction As introduced in Chapter 1, replacing legacy GFRP with candidate flexible materials can be challenging due to the substantial differences in strength and stiffness between the two types of materials. In a conventional water slide design, the GFRP flume is divided into sections. They are joined together through their outside flanges using fasteners and are usually supported by metal columns underneath, as shown in Figure 1. GFRP possesses high tensile and flexural modulus, and normal operational loads due to water and riders would not induce noticeable deformation. Flexible materials, on the other hand, have flexural moduli that are a few orders of magnitude lower than the flexural modulus of GFRP, and often would not be able to hold shape against their own weight. For a water slide’s structural performance, it is important that the flume maintains its designed shape and would not significantly deform under operational loads. To replace GFRP with flexible material candidate while not compromising the mechanical performance of the overall structure, a new (re)design concept needs to be pursued. The main goal of this redesign must be to provide adequate rigidity to the flume that is made from flexible materials. Three preliminary ideas were considered in this thesis. 6.2 Design Concepts for Incorporating the Texin 285A Flexible Material 6.2.1 Concept 1: Extensive Mesh Support Structure Concept 1 is to have an extensive mesh support structure underneath the flexible material membrane. The mesh would be made from a rigid material like stainless steel to define the shape of the flume and provide strength to support loads. The support structure would utilize modular design to be shipped in parts and assembled onsite. This idea is comparatively simple and straightforward, and the details were not explored further. 6.2.2 Concept 2: Honeycomb Sandwich Structure Concept 2 is to increase the rigidity of the flume itself. This way, an extensive support structure beneath the entire bottom surface of the flume would not be needed, and the original supporting column design can be kept. One method to increase the flexural rigidity without increasing the weight significantly is to utilize a honeycomb sandwich structure. Honeycomb sandwich structures consist of face sheets, honeycomb core, and adhesive that bonds the face 76 sheets with the core. The core is made up with thin-walled hollow cells, usually hexagonal in shape, and can be made from many different materials such as aluminum, polypropylene, and composite materials [152]. Honeycomb sandwich structures have been widely used in aerospace, automotive, and marine industries for their lightweight, excellent specific strength, and energy absorption properties [152], [153]. Individual flexible components can be assembled in a honeycomb sandwich panel to achieve higher stiffness in a lightweight structure [152]. For this study, the concept tested was to have face sheets that are made from flexible materials. The honeycomb core is semi-flexible, so it can be rolled to reduce size while being transported. The face sheets and honeycomb core will be assembled onsite to achieve a rigid structure. For feasibility and flexural stiffness testing, two 200 mm × 75 mm rectangular honeycomb sandwich specimens were manufactured according to the size specifications in ASTM D7250 [154]. Texin 285A sheets that were 3.3 mm thick were used for the face sheets (Figure 40 (a)). The honeycomb core, purchased from McMaster Carr (Elmhurst, IL, US), was made from 3000 series aluminum, and had 0.5-inch thickness and 0.5-inch cell size (Figure 40 (b)). LOCTITE EA 9309NA AERO epoxy by Henkel (Düsseldorf, NW, DE) was used as the adhesive. First, the face sheets and honeycomb core were cut to size. Then, part A and part B of the adhesive were thoroughly mixed by hand at the manufacturer-specified ratio and applied to the surface of the face sheets. The honeycomb core was placed in between the face sheets. The assembled sandwich specimen was placed in the oven underneath an aluminum block (to keep pressure between the core and face sheets) at 80 °C for an hour for the adhesive to cure. A finished specimen is shown in Figure 40 (c). Figure 40. (a) Texin 285A face sheet, (b) aluminum honeycomb core, and (c) honeycomb sandwich specimen. 77 The specimens were subjected to flexural tests following ASTM D7250 [154] in order to determine the flexural stiffness. The tests were carried out in an Instron 5969 system with a 50 kN load cell and the flexural test fixture from Wyoming Test Fixtures. According to the test standard, two loading configurations should be selected for the testing and calculation of flexural stiffness. Midspan loading (configuration #1, shown in Figure 41 (a)) was used for one specimen, and quarter point loading (configuration #2, shown in Figure 41 (b)) was used for the other. For both configurations, a support span of 150 mm was used, and the tests were conducted at a speed of 6 mm/min crosshead displacement. Figure 41. (a) Midpoint loading configuration and (b) quarter point loading configuration. Both specimens exhibited great elasticity, nearly returning to the original flatness after load removal, as shown in Figure 42. For the midspan loading specimen, damages were concentrated underneath the loading nose. The cracking and debonding of the adhesive can be observed on both sides of the specimen as shown in Figure 43 (a) and (b). The honeycomb core was also deformed and crushed in the centre. For the quarter point loading specimen, the damages were distributed between the two loading noses, consistent with the distribution of the maximum bending moment in the same region. Some local debonding can be observed on the top side, and larger areas of debonding can be seen on the bottom side (Figure 43 (c) and (d)). Honeycomb core deformation can be observed but not as significant as in the midpoint loading configuration. 78 Figure 42. Midspan loading specimen (a) at maximum deflection and (b) after load removal. Figure 43. Honeycomb sandwich specimens after flexural test: (a) midpoint loading, top side, (b) midpoint loading, bottom side, (c) quarter point loading, top side, and (d) quarter point loading, bottom side. From the 3-point bending test data measurements, the flexural stiffness of the honeycomb sandwich could be calculated using the equation 79 𝐷 =𝑃1𝑆13(1 −11𝑆228𝑆12 )48𝛥1(1 −2𝑃1𝑆1𝛥2𝑃2𝑆2𝛥1), (25) where D = flexural stiffness (N-mm2), P = total applied force (N), S = support span length (mm), Δ = beam mid-span deflection (mm), and the subscripts 1 and 2 represent configuration #1 and configuration #2, respectively. According to the test standard [154], flexural stiffness should be calculated at multiple load levels over the linear load-deflection range. The load-deflection curves from the tests are shown in Figure 44. Both specimens showed an initial linear region. The flexural stiffness values were calculated at 10 N intervals from 50 N load to 140 N load (within linear region for both specimens), and the results are shown in Figure 45. The flexural stiffness increased with load and the response was initially linear, followed by an increase in slope, possibly associated with the deformation of the core. Figure 44. Honeycomb sandwich structure flexural test load-deflection curves. 80 Figure 45. Flexural stiffness vs. load for honeycomb sandwich structure. For comparison, the flexural stiffness of a GFRP beam with the same width of 75 mm was calculated using the equations 𝐷 = 𝐸𝐼, (26) 𝐼 =𝑏ℎ312, (27) where D = flexural stiffness (N-mm2), E = Young’s modulus (MPa), I = second moment of area (mm4), b = width of beam (mm), and h = height of beam (mm). The flexural stiffness value of GFRP, using the previously determined Young’s modulus and specimen dimensions, was calculated to be 2.14 MN-mm2. The honeycomb sandwich structure made with flexible Texin 285A face sheets was able to achieve higher flexural stiffness than GFRP, thanks to the honeycomb core and increased thickness. Therefore, using honeycomb sandwich structures can be a viable method that incorporates flexible materials and increases rigidity for water slide flume design. 6.2.3 Concept 3: Flexible Diaphragm Concept 3 focused on incorporating flexible materials into the structure in a diaphragm-type configuration so that they are loaded in tension horizontally under operational loads. This would result in a lower magnitude of deflection since both sides of the diaphragm would be fixed 81 as explained in the subsequent paragraphs. One preliminary conceptual design was proposed to accomplish this in a water slide flume. The side walls of the flume are made from a rigid material such as stainless steel (Figure 46 (a)). They can be attached to supports underneath if needed. A diaphragm membrane made from the flexible material is draped over and attached to the side walls to provide the bottom of the flume (Figure 46 (b)). The sheet should be sized so that it remains flat with near-zero internal stress when it is not subjected to loads from water and riders. The thickness of the sheet should be sufficient to prevent excess deformation from the weight of the rider. Figure 46. Conceptual design of water slide flume section: (a) side walls and (b) flexible diaphragm sheet (pink) draped over side walls. For feasibility testing, one prototype was manufactured according to this design. A Texin 285A sheet with a thickness of 1.1 mm was glued to two aluminum blocks (Figure 47 (a)) using LOCTITE EA 9309NA AERO epoxy by Henkel (Düsseldorf, NW, DE) following the same mixing and curing procedures as the honeycomb sandwich structure specimen. Clamps were used to apply pressure between the flexible sheet and aluminum blocks during the curing of the epoxy (Figure 82 47 (b)). In the end, two aluminum plates were bolted to the aluminum blocks to prevent their relative movement, as in reality, the side supports that the aluminum blocks represent would be fixed. The finished specimen is shown in Figure 47 (c). The length, width, and height of the aluminum blocks were 118.3 mm, 11.9 mm, and 30.1 mm, respectively. The distance between the centres of the blocks was 130 mm. These dimensions roughly translate to a 1:9.1 scale of a section from an actual water slide design. Figure 47. (a) Aluminum block used in diaphragm specimen, (b) epoxy application and clamping, and (c) finished diaphragm specimen (top view). A test was performed to apply vertical load to the diaphragm in order to simulate a rider’s weight on the water slide. The specimen was placed on a three-point bending fixture from Wyoming Test Fixtures in an Instron 5969 system with a 50 kN load cell. The aluminum blocks rested on the two bottom supports, and the top loading nose traveled downwards at a speed of 25 mm/min. The test was terminated at a loading nose displacement of 65 mm. Figure 48 shows the specimen before, during, and after the test. The high elasticity of the material is apparent from comparing Figure 48 (b) and (c), where the specimen experienced high deformation but instantly returned to almost the original shape upon load removal. 83 Figure 48. Testing of diaphragm prototype on a three-point bending fixture: (a) before test, (b) at maximum displacement, and (c) after load removal. The force-displacement diagram is shown in Figure 49. The slope was initially low and increasing until around 20 mm displacement, and stayed relatively constant thereafter. The force reached 874 N at maximum displacement of 65 mm. These results are promising for a diaphragm with 1.1 mm thickness. An actual scale design will have a thicker diaphragm and will be able to sustain higher loads. In this case, the load was also applied in a straight line due to the shape of the loading nose. In reality, the load from the weight of the rider would be distributed among their contact area with the diaphragm which should lead to lower load per area. The main challenge of this design is associated with securely joining the flexible sheet and side walls. Adhesive was used in the prototype, but applying the adhesive to a large structure onsite is a challenging task. It is difficult to ensure the adhesive is applied uniformly and cured properly in an uncontrolled environment. This problem can be avoided by using mechanical joining methods such as fixturing or bolting. However, joining a rigid part and a flexible part together can be tricky, and the possible resulting complications, such as stress concentrations, should be analyzed further. 84 Figure 49. Force-displacement diagram from diaphragm prototype testing. 6.3 Summary To incorporate flexible materials in aquatic play structures such as a water slide flume, three (re)design ideas/concepts were proposed. Concept 1, which is to have an extensive mesh support structure underneath to provide the shape and rigidity of the flume, was deemed as the simplest and should require the least amount of design effort. However, additional materials and parts for the support structure are required. Concept 2, using flexible material face sheets in a honeycomb sandwich structure, can create a rigid flume without the need for a new support structure design. The main challenges associated with Concept 2 are related to the transportation as well as onsite manufacturing and installation of the honeycomb sandwich structure. Concept 3 utilizes flexible diaphragm and side walls to create a semi-rigid flume that would be in tension under operational loads. It is important to ensure that the bonding between the diaphragm and the side walls is strong and has an adequate service life. 85 Chapter 7: Conclusions, Limitations, and Future Work Recommendations 7.1 Conclusions Rigid structural FRP composite materials often lead to large and stiff manufactured parts that are difficult and costly to transport and handle. In addition, these composites are hard to recycle due to the lack of affordable technologies by small-to-medium size industries. To resolve these issues, flexible polymers can be used as a replacement in FRP structures, providing the target can accommodate the change through redesign of sub-structure of the system. Their flexibility allows the manufactured parts to be rolled or folded to reduce the cost and challenges related to transportation. Replacing legacy materials also brings the opportunity to transition into using more sustainable materials to reduce impact on the environment. Since flexible materials do not possess strength and stiffness that are comparable to GFRP, redesign of supporting structures would be necessary to ensure the satisfactory performance of the overall aquatic play structure system. In this research, a study was conducted on aquatic play structures that have been traditionally made from GFRP, a rigid composite material that is hard to recycle. The goal was to replace GFRP with flexible, durable, and highly recyclable materials. To start, a comprehensive literature review was presented in Chapter 2 which identified thermoplastic elastomers as a promising class of materials in terms of flexibility, durability, and recyclability. Santoprene 121-67W175 (TPV), Texin 285A (TPU), and Texin 255D (TPU) were selected as candidate materials for this study. Legacy material and candidate material sample preparation, as well as the mechanical, weathering, and microscopy characterization methods used to evaluate the material performances for use in aquatic play structures, were described in Chapter 3. Specifically, the mechanical tests included density, tensile, hardness, abrasion, flexure, cyclic tensile, puncture, friction, and impact. The weathering exposure conditions included water immersion, chlorinated water immersion, hygrothermal, and xenon lamp chamber. The microscopy characterization methods included CLSM and SEM. The results from the experiments were presented and discussed in Chapter 4. In Chapter 5, the MCDM method PROMETHEE II was used to analyze the previously obtained material properties to rank the material candidates based on a list of evaluation criteria. Lastly, several concepts to incorporate flexible materials in place of rigid GFRP in aquatic play structures were described in Chapter 6. The main conclusions are summarized below. 86 • Legacy and candidate materials characterization (Chapter 3 and 4) Control samples o The candidate TPE materials showed densities 14.6% to 32.1% lower than the density of GFRP. o GFRP possessed significantly higher tensile strength and Young’s modulus than the candidate TPE materials, while having significantly lower strain at break. GFRP showed typical brittle and linear-elastic behaviour in tension. The candidate TPE materials showed elastomer and ductile type behaviour with an initial high modulus followed by stress softening. o The flexural modulus of elasticity for all materials were on the same order of magnitude as their Young’s modulus. The two TPU’s showed better abrasion resistance than Santoprene. Texin 255D was eliminated from the list of candidate materials due to its flexural modulus being too high to be rollable or foldable. o Both Santoprene and Texin 285A showed pronounced hysteresis and Mullins effect from cyclic tests. When the specimens were stretched to the same strain for multiple cycles, Santoprene showed higher relative hysteresis during the first cycle compared to Texin 285A, but the two materials showed similar relative hysteresis for subsequent cycles. In a water slide application, this would translate to higher percentage of energy dissipation for the first rider load for Santoprene, and more consistent energy dissipation for first and subsequent riders for Texin 285A. o Puncture tests showed that Santoprene and Texin 285A specimens were punctured at similar deflections, but it required higher force to puncture Texin 285A. o GFRP showed the lowest surface roughness, followed by Texin 285A and Santoprene. The coefficients of friction for these materials were ranked in the same order. This can also be attributed to the differences in hardness between the materials. 87 Weathered samples o After 7 days of xenon lamp radiation and weathering, Texin 285A exhibited yellowing due to photooxidation. GFRP and Santoprene did not show observable changes in colour. Noticeable increase in roughness on the surface could be seen for Santoprene and Texin 285A. GFRP showed comparatively less surface degradation, likely due to the protective gel coat. o GFRP showed a slight increase in mass after 7 days of weathering in xenon lamp chamber, while Santoprene and Texin 285A showed slight decreases at 0.1% and 0.3%, respectively. This is likely associated with oxidation and chain scission happening on the surface. o Texin 285A exhibited the highest water absorption after 1 day of immersion, showing a mass increase of 0.89%. The mass increases for GFRP and Santoprene were lower than 0.2%. A weak relationship between increasing free chlorine level and increasing water absorption was obtained for Santoprene. The other materials showed no relationship between free chlorine level and water absorption. o The accelerated weathering exposures resulted in no statistically significant conclusions on their effects on tensile properties of the materials. Longer exposure time is expected to lead to deterioration in tensile properties due to further material degradation. • Material alternatives ranking using PROMETHEE II (Chapter 5) o Based on the objectives of this study, tensile strength, Young’s modulus, strain at break, flexural stiffness, and recyclability were assigned the highest weight in the PROMETHEE II analysis. o Texin 285A was ranked first because of its low flexural stiffness, high recyclability, and comparatively higher tensile properties than Santoprene, despite not having great weathering performance. Although GFRP possessed superior tensile strength and Young’s modulus, its poor flexibility and low recyclability resulted in its last-place ranking. 88 o A sensitivity analysis using the dilation and concentration of weights method was done. Dilation (α = 2) strengthened Texin 285A’s top ranking, while concentration (α = 0.5) yielded top ranking for Santoprene and bottom ranking for Texin 285A. The instability was likely due to the large differences in property values between GFRP and TPEs. o A separate MCDM study considering the redesign cost was conducted. The added criterion weight was controlled directly, and it was determined that a weight as low as 0.03 would reverse the rank of GFRP as the top material. This means strong business justifications are needed by industry towards neglecting the redesign cost, e.g. by considering the novelty and ‘long-term’ cost and environmental values of the new foldable product. A formal Life Cycle Assessment (LCA) and Life Cycle Cost Analysis (LCCA) modeling would be highly valuable in this regard. • Ideas/redesign concepts to incorporate the selected flexible material into aquatic play structures (Chapter 6) o Concept 1 was to design an extensive mesh support structure to provide shape and structural support to the flexible flume from underneath. The structure should be modular so that it can be transported in small parts and assembled onsite. o Concept 2 incorporated flexible face sheets and aluminum honeycomb core into a honeycomb sandwich structure to provide rigidity. Little changes to the legacy support structure would be needed in this design. However, transporting the materials and assembling the honeycomb sandwich structure onsite could be challenging. o Concept 3 used the flexible material as a diaphragm, attached to rigid walls on both sides, to be mainly loaded in tension during operation. The main challenge associated with this idea is to join the diaphragm and side walls securely. 7.2 Limitations and Future Work Recommendations As discussed in the first chapter, this research is a beginning phase of a holistic approach towards redesigning aquatic play structures with flexible materials. The limitations to this study as well as work recommendations for future phases are summarized below. 89 • The GFRP specimens were cut from an actual water slide flume part. The specimens were not completely flat due to the curvature of the part. Steps were taken to minimize the effect of curvature during testing. However, abrasion test could not be conducted on GFRP due to the curvature. Flat specimens would eliminate the curvature effect on test results and also enable experiments that would not be possible with curved specimens. • All TPE specimens were manufactured through compression moulding due to simplicity and availability of equipment. It would be interesting to explore other manufacturing methods such as extrusion, which may work better for manufacturing a part with constant cross-sectional profile. • Only accelerated weathering exposures were conducted in this study. In the future, it would be beneficial to increase the exposure period until a statistically significant degradation could be observed in GFRP for a better comparison between legacy and candidate materials. Outdoor natural weathering exposure should also be considered, and the results can be compared to the accelerated weathering results for service life predictions. • Additives and coatings for candidate materials were not considered in this study. Additives such as UV stabilizers as well as protective coatings can improve the weathering performance of TPEs, such as against the yellowing observed for Texin 285A, but may jeopardize the recyclability of the materials. • To improve sustainability, biocomposites that are formed by bio-based resin and natural fibres can be considered as a replacement for GFRP. • Computational methods and simulations, such as finite element analysis, can be utilized to analyze and optimize aquatic play structure designs. Experimental data from material testing can be used as material property input for the simulation models. • Machine learning methods, such as reinforcement learning, can be used to optimize the redesign of aquatic play structures through iterations. Data that is needed can be generated through computer simulations of sample redesign concepts. Machine learning may also be used for manufacturing plan development, risk assessment, cost optimization, etc. 90 • At a later stage/future research phases, to evaluate all aspects of the project holistically, a larger MCDM study can be conducted, which should include additional relevant criteria such as full production cost, environmental impact, cost of redesign, maintenance, etc. • A detailed substructure design was not available within the timeframe of this project. After a detailed design is finalized, complete small-scale and actual-scale prototypes should be constructed for testing. • If adhesives are used in the structure, the strength and chemical and water resistance of the adhesive should be evaluated. The diffusion of water with chemicals through the flexible material should also be evaluated to estimate the time before they reach the adhesive. Methods to ensure quality control of onsite assembly should also be studied. • If mechanical joining methods are used in the structure, fixturing and bolting methods to join the parts should be explored. Analyses for stress concentrations, damage tolerance, fracture toughness, and tear resistance will also be important, especially if joining requires holes in the parts. • Long term testing under realistic service conditions with combined effects of mechanical loading and environmental exposure should be conducted, as well as fatigue testing to understand the material performance under repeated cyclic loading. • Manufacturing and assembly strategies can be researched to develop a production plan, and cost model and supply chain can be considered subsequently. • Investigation of recyclability and development of recycling strategies of the candidate materials can validate the sustainability claim and aid in the development of circularity and recycling roadmap of the product. • LCA and LCCA can be used to quantify the environmental impact in terms of resources used and emissions, as well as the cost of ownership, over the entire life cycle of a product. The results can be used to compare the overall environmental impact, sustainability, and cost of the use of different materials and different designs. 91 7.3 Contributions to Knowledge This research served as a preliminary investigation towards redesigning legacy GFRP-constructed aquatic structures with flexible, durable, and recyclable alternative materials. Important mechanical properties that pertain to the application in aquatic play structures of GFRP and several candidate TPE materials were characterized. An MCDM framework using PROMETHEE II method was followed to rank the legacy and candidate materials. The feasibility of some preliminary aquatic play structure redesign ideas to replace GFRP with flexible materials were shown. The study paved the way for the future phases of the perceived holistic approach towards redesign of aquatic structures with flexible and foldable elastomer materials. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Milani, Abbas S.
Towards redesigning legacy GFRP-constructed aquatic structures with flexible, durable, and recyclable alternative materials : a multicriteria approach
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