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IPS e.max CAD and IPS e.max Press : fracture mechanics characterization Alkadi, Lubna T. 2014

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IPS E.MAX CAD AND IPS E.MAX PRESS: FRACTURE MECHANICS CHARACTERIZATION by  Lubna T. Alkadi  DDS, King Saud University, 2010  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (CRANIOFACIAL SCIENCES)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  July 2014  © Lubna T. Alkadi, 2014 ii  Abstract  Objective: To determine fracture toughness (KIC) and fatigue crack propagation (FCP) parameters for IPS e.max CAD and IPS e.max Press.   Materials and methods: For KIC determinations, 20 (6x6x6x12mm) notchless triangular prism (NTP) specimens of IPS e.max CAD and IPS e.max Press were prepared. IPS e.max CAD blocks were cut, ground and then crystallized, while IPS e.max Press ingots were pressed into molds obtained from wax prisms. Each specimen was mounted into a holder and custom grips were used to attach the holder to a computerized universal testing machine (Instron model 4301). The assembly was loaded in tension at a crosshead speed of 0.1mm/min and KIC was calculated based on the recorded maximum load at fracture. Fractured surfaces were characterized using scanning electron microscopy (SEM). The results were statistically analyzed using Weibull statistics and t-test (=0.05). For FCP characterization, a pilot test was done with three Plexiglas NTP samples. A pre-crack was initiated in one of the specimen edges. Several lines were scribed on the side of the specimen to monitor crack propagation. The specimens were mounted in the holder and then attached to custom grips on a servo hydraulic fatigue-testing machine (Instron model 8511). A strain gauge was attached to these grips to monitor crack opening displacement. Each  specimen was cyclically loaded in tension (Mode I) in a load range between 1 and 20 N and crack length was monitored and filmed using a high definition video recorder (SONY HDR-XR550V) attached to a microscope (Edmund Scientific Co, Barrington, NJ). Video recording was terminated once catastrophic fracture of the specimen occurred. Cyberlink Power Director and iii  Image J software were used in data analysis.  Results: KIC values were significantly higher for IPS e.max Press than IPS e.max CAD. The pilot FCP tests on Plexiglas revealed limitations with regards to the applicability of NTP specimen KIC test to FCP studies due to the presence of a trapezoidal crack front in the specimens.  Conclusion: IPS e.max Press is superior to IPS e.max CAD in KIC. Further research should be conducted to evaluate the feasibility of using a trapezoidal crack front in FCP studies.   iv  Preface  This in vitro study was supervised by Dr. N. Dorin Ruse. The research committee members were Drs. N. Dorin Ruse and Caroline Nguyen, from the Faculty of Dentistry, and Dr. Tom Troczynski from the Department of Materials Engineering.  Human or animal subjects and bio-hazardous materials were not used in this study; therefore, ethical approval from the UBC Research Ethics Board was not required. v  Table of Contents  Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iv Table of Contents ...........................................................................................................................v List of Tables ............................................................................................................................... vii List of Figures ............................................................................................................................. viii Acknowledgements ........................................................................................................................x Dedication ..................................................................................................................................... xi Chapter 1: Introduction ............................................................................................................... 1 1.1 Computer-Aided Design/ Computer-Aided Manufacturing (CAD/CAM) ................. 2 1.2 Dental Ceramics .......................................................................................................... 4 1.3 IPS e.max .................................................................................................................. 11 1.4 Fracture Mechanics ................................................................................................... 13 Chapter 2: Research Protocol.................................................................................................... 21 2.1 Purpose ...................................................................................................................... 21 2.2 Hypothesis................................................................................................................. 22 2.3 Expectations .............................................................................................................. 22 2.4 Materials and Methods .............................................................................................. 23 2.5 Results ....................................................................................................................... 37 Chapter 3: Discussion ............................................................................................................... 44 3.1 KIC ............................................................................................................................. 44 3.2 FCP ........................................................................................................................... 47 vi  Chapter 4: Conclusions ............................................................................................................. 52 References ................................................................................................................................. 53  vii  List of Tables  Table 1: Mechanical properties of different ceramics according to their manufacturers' reported values ............................................................................................................................................ 10 Table 2: KIC of IPS e.max CAD and IPS e.max Press .................................................................. 38 Table 3: First Plexiglass specimen (a vs N graph) ........................................................................ 42 Table 4: Second Plexiglass specimen (a vs N graph) ................................................................... 42 Table 5: Third Plexiglass specimen (a vs N graph) ...................................................................... 43  viii  List of Figures  Figure 1: NTP specimen ............................................................................................................... 15 Figure 2: NTP specimen holder .................................................................................................... 16 Figure 3: NTP specimen holder in mounting block with metal spacer ......................................... 16 Figure 4: A typical Paris curve50 ................................................................................................... 19 Figure 5: NTP specimen holder .................................................................................................... 25 Figure 6: NTP specimens of IPS e.max CAD in the partially crystallized stage .......................... 25 Figure 7: Crystallized IPS e.max CAD NTP specimen ................................................................ 25 Figure 8: PVS impression of plexiglass NTP ............................................................................... 26 Figure 9: Wax NTP specimen ....................................................................................................... 27 Figure 10: IPS e.max Press specimen ........................................................................................... 27 Figure 11: Custom grips attaching NTP specimen holder to testing machine .............................. 29 Figure 12: A putty mold and metal ruler used to scribe lines on one side of the NTP specimen . 30 Figure 13: Lines scribed on the NTP specimen ............................................................................ 31 Figure 14: The grips used to attach the NTP holder and the strain gauge for testing FCP........... 32 Figure 15: NTP holder attached to grips and illuminated ............................................................. 32 Figure 16: Initial pre-crack in an NTP specimen under cyclic loading ........................................ 33 Figure 17: Crack propagation in an NTP specimen under cyclic loading .................................... 34 Figure 18: The last recorded crack propagation prior to catastrophic fracture ............................. 34 Figure 19: Cyberlink Power Director software used to obtain snapshots from FCP video clip at certain intervals ............................................................................................................................. 35 Figure 20: Calibration of measurements using Image J software ................................................. 35 ix  Figure 21: Measuring the crack using Image J software after calibration .................................... 36 Figure 22: Weibull plot of KIC results........................................................................................... 38 Figure 23: SEM photo of 2 sides of IPS e.max CAD prism x50 magnification ........................... 40 Figure 24: SEM photo of 2 sides of IPS e.max CAD prism x5000 magnification ....................... 40 Figure 25: SEM photo of 2 sides of IPS e.max CAD prism x10000 magnification ..................... 40 Figure 26: SEM photo of 2 sides of IPS e.max Press prism x50 magnification ........................... 41 Figure 27: SEM photo of 2 sides of IPS e.max Press x5000 magnification ................................. 41 Figure 28: SEM photo of 2 sides of IPS e.max Press x10000 magnification ............................... 41 Figure 29: SEM photos comparing IPS e.max CAD (above) and IPS e.max Press (below) x10000 magnification ................................................................................................................................ 46 Figure 30: NTP specimen for FCP................................................................................................ 47 Figure 31: FCP Specimen #3 Y* vs a ........................................................................................... 49 Figure 32: FCP Specimen #3 Y* vs N .......................................................................................... 49 Figure 33: FCP Specimen #3 ΔK vs N ......................................................................................... 50 Figure 34: FCP Specimen #3 da/dN vs ΔK .................................................................................. 51    x  Acknowledgements  I cannot express enough gratitude to my supervisor Dr. N. Dorin Ruse for his continuous support and guidance. No matter how large the obstacle I faced, he always had a solution. His knowledge and dedication were essential for the completion of this project and I thank him deeply for this wonderful learning opportunity.   Besides my supervisor, I offer my deepest appreciation to my committee members: Dr. Tom Troczynski and Dr. Caroline Nguyen for their time and efforts in providing me with their constructive feedback.  In addition, I would like to thank my sponsoring institution, King Saud bin Abdulaziz University for Health Sciences, and the government of Saudi Arabia for giving me this unique opportunity to become a specialist in my field by financially supporting my education over the course of the program.  Finally, I would like to thank Ivoclar Vivadent for supplying the materials used in this project.   xi  Dedication  To my precious family: my mother, Hanan, my father, Tarek and little brother, Mohammad. I thank you for believing in my dream no matter how impossibly far away it seemed in the beginning. Leaving home and being physically and literally on the other side of this planet was not easy, yet your love and support (and skype) made it possible.   To my loving husband, Aziz, thank you for the nights we spent in the coffee shop working together on our projects. Thank you for the days we spent on the beach trying to forget about them. Thank you for everything else in between.       1  Chapter 1: Introduction  Since ancient times, teeth have been considered essential for beauty, function and speech, and therefore, humans have been creative in using materials naturally found to replace their teeth when they were lost by accident or due to disease. The earliest evidence of dental treatment goes back to 4000 BC in Egypt, the medical center of the ancient world. There, archeologists have found what is thought to be the first dental prosthesis consisting of a natural tooth replacing an extracted third molar by tying a gold wire to link it to the adjacent second molar1. Evidence of tooth replacements were also found from the early Greeks, Etruscans and Romans2.   Until the 18th century, only minimal progress was achieved in prosthetic dentistry, until the work of Pierre Fauchard, who is considered the father of modern dentistry, helped make it a profession of high standards. Dentists of that time utilized naturally occurring materials to replace human teeth, including ivory, wood, stone, animal teeth or extracted human teeth1.  In 1774, Alexis Duchateau, who was dissatisfied by discolored ivory dentures, fabricated a ceramic denture, therefore becoming the first person to use ceramics successfully in dentistry1, 3. In 1844, ceramic denture teeth were manufactured by S.S White and in the years that followed many manufacturers also started producing ceramic teeth in an attempt to mimic natural teeth in esthetics1. Ceramics became popular due to their ability to overcome many of the drawbacks of previously used materials, such as corrosion in saliva, discoloration and poor esthetics1.   2  Over the years and until today, new ceramic products have been introduced to the market, each claiming to be superior to the others in esthetic and physical properties. With the introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) techniques, a new category of machinable ceramics became increasingly used to fabricate restorations that could be digitally designed and milled4.  1.1 Computer-Aided Design/ Computer-Aided Manufacturing (CAD/CAM)  With the introduction of CAD to the world of industry in the 1950s, the possibilities seemed endless and it was only a matter of time until this technology was incorporated into dentistry. In the beginning, however, the process was far from being smooth and there were many obstacles in the way of digitally designing dental restorations, including the limited computing power. Limitations were also due to CAM devices, which were too large and lacked the dimensional reproduction accuracy needed for dental purposes 5. Despite the difficulties, dental researchers and manufacturers continued to improve this very promising area of dentistry.  In general, a CAD/CAM system consists of three main components: (1) a digitizing tool, or scanner, that transfers the numerical data to a computer; (2) a computer with appropriate software to process the digital data and design the final product to be machined; and (3) a machine that transforms the design into an object 6.  Depending on the location of these three main components, the production of dental restorations by this technology can take place either chair-side, in a dental lab, or in a production center 6.  3  In 1971, Dr. Duret introduced the Sopha system, which was the first CAD/CAM system in the history of dentistry. However, this system was not successful in gaining popularity, probably due to the primitive computer abilities available at that early time 7,8.  Dr. Moermann, a co-developer of the CEREC system (Sirona Dental Systems GmbH, Bensheim, Germany), was one of the pioneers in the development of the currently known dental CAD/CAM systems. The CEREC 1 system introduced in 1983, was the first to succeed in optically and three-dimensionally measuring a dental cavity using a compact intraoral camera with a laser displacement gauge connected to a computer. The design of the restoration was made on the computer and then the numerical data was transferred to a chair-side compact machine to mill inlays from a porcelain block against a grinding wheel 9. The CEREC 2 system was a modification of the previous CEREC 1 design by the addition of a cylindrical diamond bur to enable the milling of partial and full coverage crowns. The CEREC 3 system replaced the grinding wheel with another cylindrical bur, making the machine a 2-bur system for additional milling precision. Along with the hardware development, a parallel development took place in the software used in designing digital restorations. The 2D display was substituted by a 3D display for a better representation of the prepared tooth structure on the screen and an improved restoration designing experience 9.    Another pioneer was Dr. Anderson, the developer of the Nobel Procera CAD/CAM system (Nobel Biocare, Gotenborg, Sweden). In the beginning, the system was used to produce titanium substructures that were later veneered with low fusing ceramic. Nowadays, it is also used in the production of all ceramic crowns from aluminum oxide or zirconium oxide. In this system, the 4  scanner with a contact probe is used to scan a final cast of the prepared teeth and then the data is transferred to a computer where the design of the restoration is finalized. The fabrication of the restoration takes place at a distant production center in Sweden 10.   From that point forward, many other systems were developed to be used chair-side or in the dental lab. In addition, multi-axis milling machines were developed for commercial production of restorations, from metallic and ceramic materials, with increased accuracy and precision. Digitizing tools were also improved and new methods of capturing the data were introduced, making CAD/CAM technology the future of dentistry 7.   The marginal and internal fit of restorations made with CAD/CAM technology was tested in several studies 7,11. The results of these studies show that their accuracy of fit is acceptable compared to the conventional methods of fabrication.  1.2 Dental Ceramics  Ceramics is a very broad term. According to Smith,  “Ceramics are crystalline, inorganic, non-metallic materials which consist of metallic and non-metallic elements bonded together primarily by ionic and/or covalent bonds” 12. Glasses are similar to ceramics in composition and in type of bonds, but differ structurally because their composition is heated to fusion and then cooled to reach a rigid state without crystallization12.   5  1.2.1 Traditional Ceramics and Engineering Ceramics  Ceramics, in general, can be categorized into two classes: traditional ceramics and engineering ceramics 12. Raw materials of traditional ceramics are naturally found in earth. These include clay, feldspar and silica 12. Humans, over 26,000 years ago, learned to form these raw materials into objects that were later processed, by subjecting them to high temperatures, to achieve higher strength and hardness 13. All of these naturally occurring materials are silicates, since silicon and oxygen are the main elements in their structure. The basic building block of these materials is the silicate tetrahedron (SiO44-). One electron per oxygen atom is available for bonding and according to the type of atom bonded to it, many different structures can be produced 12.  Clay is the name given to several earthy materials consisting of very small grains that plasticize upon hydration and solidify upon dehydration. The chemical structure of clay consists of a sheet of interconnected silicate tetrahedra. Three corner oxygen atoms of each silicate tetrahedra are shared among adjacent tetrahedra to produce a negatively charged sheet (Si2O52-), which is neutralized by combining it with a positively charged sheet of aluminum hydroxide (Al2(OH)42-), resulting in the formation of a two-layer clay mineral, such as Kaolinite12.   Silica (SiO2) is formed when all four oxygen atoms of silicate tetrahedra are shared between adjacent tetrahedra to form a 3D network. The three crystalline forms of silica  are quartz, tridymite and cristobalite 12, each stable at different temperatures   6  Feldspar is formed when some AlO33- anions replace SiO44- anions resulting in a negative net charge due to the fact that the former has only three oxygen atoms to share while four are required to keep the structure electrically neutral. Cationic species, such as Na+, K+, Ca2+, Mg2+, Ba2+ , are then incorporated into the network to neutralize it12.  Traditional ceramics lack the required mechanical properties for modern applications, which led to the introduction of engineering ceramics, such as alumina (Al2O3) and zirconia (ZrO2) (as partially stabilized zirconia).  1.2.2 Ceramics in dentistry  One of the earliest uses of ceramics in dentistry was in the fabrication of complete denture prostheses by Alexis Duchateau in 17743. The first successful use of ceramic systems in modern fixed prosthodontics was the use of the traditional feldspathic ceramic in the fabrication of the porcelain jacket crown by Charles Land14. This crown was later reinforced with alumina particles by McLean to improve its properties15.  Due to their natural appearance, biocompatibility and optical properties, ceramic materials have been increasing in popularity ever since and manufacturers have been developing new systems to meet the demand16. The numerous available ceramic materials can be classified in several ways, according to their composition, processing method, firing temperature, microstructure, translucency, fracture resistance or abrasiveness and uses17.  7  According to their composition, the spectrum of dental ceramics can be classified into the following categories: (1) feldspathic (glass ceramic, leucite reinforced, mica reinforced, lithium disilicate reinforced); (2) glass infiltrated alumina/ spinell/ zirconia core; (3) alumina core; (4) zirconia core 4.   Feldspathic ceramics are predominantly glass-ceramics, with an amorphous matrix derived from the natural mineral feldspar reinforced with silica. Clay does not contribute to the composition of feldspathic dental ceramics because the formability of clay is not a required feature for dental applications 12.   A subsequent advancement in the traditional feldspathic ceramic composition was the addition of various fillers to improve mechanical properties.  One major improvement was achieved in 1962 when leucite (with a coefficient of thermal expansion (CTE) of 20-25 ppm/oC)18 was incorporated into the composition of feldspathic ceramic (usually with a CTE as low as 7.5 ppm/oC)18 used to veneer metallic substructures (CTE of most alloys suitable for veneering with ceramic is around 14-15 ppm/oC)19. Leucite (K2O.Al2O3.4SiO2) is a potassium alumino-silicate, which crystallizes when feldspathic ceramic is reheated, leading to an increase in the CTE of the feldspathic ceramic, bringing it close to, but slightly lower (about 0.5 ppm/oC lower)20 than that of the metal substructure being veneered, therefore placing the ceramic under slight compression3, 16. When used for all ceramic applications, leucite reinforced glass ceramics are still considered one of the most esthetic ceramics available16.  8  Despite the popularity and acceptance of metal ceramic systems, the increased demand for the superior esthetics of metal-free restorations has encouraged manufacturers to introduce new materials to be used in the fabrication of all ceramic restorations.  Vita Mark I and II (VITA Zahnfabrik, Bad Sackingen, Germany) are machinable feldspathic ceramics marketed to be used in CAD/CAM technology. IPS Empress (Ivoclar Vivadent, Schaan, Liechtenstein) is a leucite reinforced glass ceramic that is processed by melting and pressing, following the traditional lost wax technique. IPS ProCAD (Ivoclar Vivadent, Schaan, Liechtenstein) is a machinable ceramic that has a similar composition as IPS Empress but is intended to be used in CAD/CAM processing. Other fillers have been incorporated into the composition of glass ceramics, such as mica in Dicor (Densply Inc, York, PA, which was discontinued) and lithium disilicate in IPS Empress 2 (Ivoclar Vivadent, Schaan, Liechtenstein) and more recently in IPS e.max CAD and IPS e.max Press (Ivoclar Vivadent, Schaan, Liechtenstein) 4.    The desire for materials with better mechanical properties has led to the introduction of two additional strategies to strengthen ceramics. The first was to reinforce the ceramic with a continuous 3D framework formed by an industrial-type ceramic material, capable of better resisting crack propagation, and the second was to entirely eliminate the glassy matrix21.  In-Ceram Alumina, In-Ceram Spinell and In-Ceram Zirconia (VITA Zahnfabrik, Bad Sackingen, Germany) demonstrate how the first strategy was applied. In-Ceram Alumina has been introduced as a core material for crowns and anterior 3 unit fixed dental prostheses (FDPs). This ceramic core is fabricated through the slip casting technique. In this technique, a porous 9  continuous 3D framework of sintered alumina particles is made, which is later infiltrated with a low viscosity feldspathic glass-ceramic during a second firing, to achieve a high esthetic result4.  To overcome the problem of the high opacity of In-Ceram Alumina, another core material, In-Ceram Spinell (VITA Zahnfabrik, Bad Sackingen, Germany) was introduced. It contains a mixture of alumina and magnesia, which imparts a higher translucency to the material compared with In-Ceram Alumina. On the other hand, the flexural strength of this material is lower than In-Ceram Alumina 4.   More recently, In-Ceram Zirconia (VITA Zahnfabrik, Bad Sackingen, Germany) joined the In-Ceram product spectrum, with the intention of producing a stronger material that could be used to restore the posterior areas of the mouth. This was achieved by the addition of 35% of partially stabilized zirconia to the slip composition of In-Ceram Alumina 4.  Industrial pure monophase ceramics were developed by sintering the crystalline phase together without a glassy matrix in between the crystals to form a dense polycrystalline structure3. This process resulted in ceramic materials with superior mechanical properties but high opacity and therefore, they are best used as cores that are later veneered with feldspathic ceramic for the best esthetic result. The two materials produced by this technique are aluminum oxide (alumina, Al2O3) or zirconium oxide (zirconia, ZrO2) 4.   10  An example of an alumina-based polycrystalline ceramic product is Procera (Nobel Biocare AB, Goteborg, Sweden). Zirconia based polycrystalline products include Lava (3M ESPE, St. Paul, Minn) and Cercon (Densply Ceramco, York, Pa).  The natural esthetic appearance of ceramics is attributed to their translucency. Moreover, compared to gold or amalgam, less plaque adheres to a smooth ceramic surface, resulting in superior hygiene and better tissue response.17, 22 However, there are still considerable concerns regarding their mechanical properties14. The stiff and brittle ceramic materials may exhibit catastrophic fracture, in particular when exposed to tensile stresses23, which remains the main cause of their failure 24.  Table 1: Mechanical properties of different ceramics according to their manufacturers' reported values Material Flexure strength (MPa) Fracture toughness (MPam½) Coefficient of thermal expansion  ppm/ oC Modulus of elasticity (GPa) IPS e.max Ceram25 90 ± 10 NA 9.5 ± 0.25 60-70 IPS e.max Press25 400 ± 40 2.5 – 3.0 10.15 ± 0.4 95 ± 5 IPS e.max CAD25 360 ± 60 2.0 – 2.5 10.15 ± 0.4 95 ± 5 VITA In-Ceram Alumina26 500 3.9 7.4 280 VITA In-Ceram Zirconia26 600 4.4 7.8 258 Lava27 >1100 5-10 10 >205 Cercon28 >1300 9 10.5 210    11  1.3 IPS e.max  IPS e.max is the brand name given by Ivoclar Vivadent to a range of all-ceramic products that include four core materials (IPS e.max Press, IPS e.max CAD, IPS e.max ZirPress, IPS e.max ZirCAD) and a veneering material (IPS e.max Ceram). The products of interest (IPS e.max Press and IPS e.max CAD) are both lithium disilicate glass ceramics that were designed to have two different processing pathways for the fabrication of all ceramic restorations. Both materials can be used to produce cores that can be veneered with feldspathic ceramic (IPS e.max Ceram). They can also be used as monolithic materials in the fabrication of full contour restorations25.   1.3.1 IPS e.max Press   IPS e.max Press is supplied as ingots of lithium disilicate glass ceramic available in four translucencies and two sizes. The microstructure consists of approximately 70% lithium disilicate crystals measuring 3-6 μm in length. This product is processed utilizing the lost wax technique, which involves waxing the restorations to the desired contours, spruing and investing the wax patterns, melting the wax to create a mold within the investment and then pressing the molten ceramic into the mold in a special furnace developed specifically for this product (Programat EP 3000, EP 5000). After that, the restorations are divested, polished, characterized and glazed prior to delivery25.   12  1.3.2 IPS e.max CAD  IPS e.max CAD is a lithium disilicate glass ceramic designed to be used in CAD/CAM technology. A process called pressure casting leads to the production of partially crystallized IPS e.max CAD blue blocks. These blocks are composed of 40% lithium metasilicate crystals, ranging in size between 0.2 to 1.0 μm, embedded in a glassy matrix. Their partially crystallized state facilitates the milling process with minimal wear to the milling burs25.  After milling the restorations to the desired shape and contour, they are tempered to 850 °C in a furnace developed by the manufacturer for this material (Programat 300, 500). In this process, lithium metasilicate crystals are transformed into lithium disilicate crystals (70 % volume fraction), which are responsible for the material’s high strength. The coloring ions responsible for the blue color in the partially crystallized stage also show a different oxidation state when tempered, leading to the desired tooth color and opacity25.  1.3.3 Clinical performance of IPS e.max    Clinical evidence shows that IPS e.max Press has a survival rate of approximately 96.6% in 3 years for single crown restorations29. A study that evaluated fixed dental prostheses (FDPs) fabricated from monolithic IPS e.max Press and observed for a mean period of 121 months found the survival rate to be 100% after 5 years and 87.9% after 10 years. The success rate was 91.1% after five years and 69.8% after 10 years30. A clinical evaluation of single IPS e.max CAD 13  crowns showed 100% success after 2 years31. There is a lack of long or short term controlled clinical trials comparing IPS e.max CAD and IPS e.max Press.  1.4 Fracture Mechanics  Although fractures have been a problem for as long as man-made structures have existed, fracture mechanics is relatively young as a science. Following World War II, when Liberty ships (EC-2) fractured in half 32, people became more aware and interested in fracture mechanics. The ships, which were made from carbon steel, became brittle when exposed to lower temperatures in the Atlantic Ocean, which led the steel to undergo a ductile to brittle transition and fracture32. Because the consequences of such drastic fractures were detrimental to humans’ lives, the discipline of fracture mechanics began to flourish. It was soon realized that understanding fracture mechanics could lead to better materials selection and could help design structures and new materials able to resist fractures where fabrication defects and cracks are difficult/impossible to avoid 32.   1.4.1 Fracture Toughness (KIC)  Fracture toughness is an intrinsic material property used to characterize dental materials in vitro. It describes the material’s ability to withstand unstable crack propagation 33.   When a crack exists in a structure, stresses are concentrated at the crack tip upon loading and the stress intensity created by this situation is designated by “K”.  When the load is tensile and 14  creates a purely straight opening of the crack, it is termed “mode I”. When the stress reaches a critical point designated by “C”, the crack becomes unstable and quickly propagates to lead to the catastrophic fracture that separates the structure into two pieces. Thus, fracture toughness (KIC) represents the critical stress intensities for mode one opening and is used to characterize materials regardless of the size of the crack22.  To ensure reproducibility, international standardization of the testing procedure has been accomplished through detailed description of specimen configuration and dimensions as well as tests protocols34. There are several commonly used configurations in dental materials testing. One of the most common ones is the chevron-notched short rod (CNSR) specimen34. However, preparing a sample to be tested according to the specifications of this test’s specimen could be cumbersome and, therefore, a new testing method has been developed to overcome these difficulties, which is called the notchless triangular prism (NTP) specimen KIC test. 35  1.4.1.1 The Notchless Triangular Prism specimen KIC Test  As mentioned earlier, one of the common methods to test fracture toughness is to utilize the CNSR specimen configuration developed by Barker36. To prepare a specimen of this configuration, a chevron notch is cut into a cylindrical specimen. This process can be difficult to achieve, particularly with brittle materials such as ceramics35.  In order to overcome the difficulties associated with specimen preparation in the CNSR method, the NTP specimen was developed35. The NTP specimen (Figure 1) measures 6x6x6x12 mm and, 15  when fitted into the specifically designed holder, achieves a final configuration similar to that of the CNSR specimen, therefore, eliminating the difficult notching procedure 35. The specimen holder (Figure 2) consists of 4 parts: two symmetrical half cylinders, with a loading collar at one end and a triangular prismatic groove at the other end from the base; two symmetrical half disks fastened with screws across the triangular prismatic grooves of the corresponding half cylinders restrain the specimen. To mount a specimen, a special mounting block consisting of two halves is used. The two halves slide horizontally, which allows a 20 – 500 μm spacer to fit in between the two halves of the holder. A ~0.1 mm deep crack is initiated into the triangular prism specimen at a point midway along one of its edges. This crack is aligned with the split line created by the spacer between the two halves of the holder. The screws are then tightened on the holder to hold the specimen in place 35. The specimen can then be used for fracture mechanics testing, specifically KIC.   Figure 1: NTP specimen    6 mm 6 mm  6 mm 16   Figure 2: NTP specimen holder   Figure 3: NTP specimen holder in mounting block with metal spacer  After its introduction, this test was validated through finite element analysis. In addition, a calibration study was done in which materials of known KIC were tested using the NTP and the CNSR methodologies and comparable results were obtained 35.  It was determined that NTP was 17  a suitable testing methodology for materials as well as adhesive interfaces 35. Several investigators have utilized this testing modality in mechanical characterization of dental tissues and materials 37-40.  1.4.2 Fatigue Crack Propagation  When a crack, of length a and crack tip curvature  exists in a structure, the stress at the crack tip (max) is significantly increased relative to the stress applied (a) (see equation below), which leads to a decreased strength22.           √   However, it is rare for a critically large crack to exist initially; rather, a smaller crack propagates under cyclic loading, eventually leading to catastrophic failure of the structure under stresses considerably lower than its strength34, a process called fatigue. For this reason, the determination of the fatigue crack propagation (FCP) parameters in addition to KIC is an essential part of the fracture mechanics design approach41.   Until recently, the vast majority of published literature about fatigue related to metal fatigue42,43. As this type of testing was adopted in dentistry, testing FCP of dentin was done by several investigators44-46. Lately, there has been an increasing interest in the fatigue testing of high strength, brittle materials, such as ceramics47, 48.  18  In a typical fatigue test, the specimen is pre-cracked and crack propagation is achieved by cyclic loading of the specimen. Crack length is measured, as a function of elapsed fatigue cycles, either visually, using a low-power microscope, or by an equivalent method, such as monitoring changes in the stiffness of the specimen, changes in the voltage field, or monitoring ultrasonic waves that are reflected off the crack33.  Numerical analysis of the data is done to calculate fatigue crack growth rate, da/dN. Subsequently, da/dN is expressed as a function of stress intensity factor range, ΔK, obtaining a relationship that is called Paris Law33, 49.   Paris law is used to describe the crack growth behavior of a certain material. Test data are plotted into a curve represented in Figure 4.33 19   Figure 4: A typical Paris curve50   A typical curve consists of three main segments. The middle segment is often a straight line with a relationship represented by the following equation.      = C (ΔK)m This equation was found by P.C Paris in the 1960s and has influenced the application of fracture mechanics in FCP determination ever since33.  20  Below ΔKth, called the fatigue crack growth threshold, crack growth does not normally happen. When the crack growth rate is high, a steep portion of the curve appears, which represents unstable crack growth prior to catastrophic failure of the specimen33.    Temperature and hostile environmental factors can affect the results of fatigue testing49 and this is to be taken into consideration when applying test results to clinical reality.   21  Chapter 2: Research Protocol  2.1 Purpose  IPS e.max Press and IPS e.max CAD are marketed by the manufacturer as clinically identical materials indicated for the same clinical uses. However, as mentioned earlier, different crystal sizes of lithium disilicate are formed during the manufacturing/processing of each of these materials.  In addition, different processing methods are involved in producing a dental restoration from each material. Both of these factors could lead to significantly different mechanical properties.  It would therefore be of interest from both a materials science point of view and a clinical usage point of view to characterize the materials in the lab as a first step towards arriving to some clinical implications regarding whether one should be used over the other in any given clinical scenario.  Because KIC is an intrinsic material property that correlates to clinical performance (fracture and wear), its determination could be useful in comparing IPS e.max CAD and IPS e.max Press. Moreover, since IPS e.max CAD and IPS e.max Press fail clinically under cyclic loads rather than a static load, it would be valuable to also compare their FCP parameters. Therefore, the purpose of the current study was to determine KIC and FCP parameters of IPS e.max Press and IPS e.max CAD.  22  2.2 Hypothesis  There is no significant difference between IPS e.max CAD and IPS e.max Press with regards to their fracture toughness (KIC) and fatigue crack propagation (FCP) parameters. Ho = There is no difference between IPS e.max CAD and Press in KIC and FCP parameters. Ha = There is a difference between IPS e.max CAD and Press in KIC and FCP parameters.  2.3 Expectations   While the two forms of IPS e.max are essentially lithium disilicate glass ceramic produced by the same manufacturer, the different methods of preparation and processing may lead to statistically significant differences in their mechanical properties. The larger crystals in IPS e.max Press and the Pressing procedure that requires complete melting of the ceramic ingot may give IPS e.max Press an advantage over IPS e.max CAD with regards to the measured properties (KIC and FCP parameters).  Due to the brittle nature of ceramics, it is expected that the FCP test would be challenging. The difficulty would be to capture the crack propagation phase before it progresses into a catastrophic fracture.     23  2.4 Materials and Methods  2.4.1  Sample Size Determination  A power analysis was used to calculate the sample size (n) needed for KIC test with α = 0.05, power of 80%  and a standard difference (Δ) of 0.4  We used Lehr’s basic formula to obtain n:        where        Where δ is the target difference and σ is the standard deviation. Thus, in order to be able to detect a difference of 20 % between the groups, using the mean and standard deviation of a pilot sample (mean = 2 MPa·m1/2 and =0.3), with α = 0.05 and a power of 80 %, n was calculated to be 9.    For materials susceptible to brittle fractures such as ceramics, Weibull statistics are recommended. In Weibull statistical analysis, the probability of failure Pf is related to the fracture stress () by the following exponential relationship51:           [ (      ) ] 24  However, in order to enable analysis of the results by Weibull statistics, at least 20 samples are necessary52. Therefore, 20 samples of IPS e.max CAD and 20 samples of IPS e.max Press were prepared.   2.4.2 Fabrication of IPS e.max CAD Triangular Prisms  IPS e.max CAD blocks were supplied by the manufacturer (Ivoclar Vivadent, Amherst, NY). Twenty 6x6x6x12 mm NTP test specimens were prepared by cutting and grinding commercially obtained IPS e.max CAD blocks. Each IPS e.max block was mounted, using sticky wax (Kerr, Italia, Srl), to be cut into equal rectangular blocks using diamond impregnated slicing wheels (UKAM, Valencia, CA) mounted on an Isomet low speed saw (Buehler, Lake Bluff, IL), under continuous water irrigation.  Each of these smaller blocks was then ground, using a custom built holder (Figure 5) on SiC paper discs (Buehler, Lake Bluff, IL) mounted on a grinding machine (Buehler, Lake Bluff, IL) until a perfect (6x6x6x12) mm NTP was produced (Figure 6). Consecutively finer grit SiC paper, up to 600 grit, was used to polish the prisms. The prisms were fully crystalized in the Programat furnace (Ivoclar Vivadent, Amherst, NY) at 840°C (1544°F) for 20 minutes, following manufacturer’s recommended protocol (Figure 7).  25   Figure 5: NTP specimen holder   Figure 6: NTP specimens of IPS e.max CAD in the partially crystallized stage   Figure 7: Crystallized IPS e.max CAD NTP specimen  26  2.4.3 Fabrication of IPS e.max Press Triangular Prisms  IPS e.max Press ingots were supplied by the manufacturer (Ivoclar Vivadent, Amherst, NY). Twenty 6x6x6x12 mm NTP test specimens were prepared by waxing, investing, and pressing them in IPS e.max Press. To create these wax prisms, a 6x6x6x12 mm plexiglass prism was impressed using Aquasil (Dentsply, York, PA) medium body polyvinylsiloxane impression material (Figure 8). The prism was then retrieved, leaving a mold into which melted dipping wax (Whip Mix, Louisville, KY) was poured to produce 6x6x6x12 mm wax prisms (Figure 9).    Figure 8: PVS impression of plexiglass NTP  27   Figure 9: Wax NTP specimen  Utilizing the lost wax technique, these wax prisms were invested and IPS e.max Press was pressed into the molds (Figure 10). This pressing process was done at the Ivoclar Vivadent headquarters in Amherst, NY. The prisms were then polished to final dimensions using 600 SiC paper.   Figure 10: IPS e.max Press specimen 28  2.4.4 Testing Procedure  2.4.4.1 Testing KIC  For determining KIC, 20 (6x6x6x12) mm NTP test specimens of each material were tested. In each specimen, a pre-crack was initiated using a sharp surgical blade mounted on a wooden block. Each specimen was then mounted in the specimen holder in the fashion described in the previous NTP section of this thesis. Custom designed grips (figure 11) were used to attach the holder to a computerized universal testing machine (Instron model 4301, Instron Canada, Inc) with a 1kN Instron load cell of and an accuracy of 0.25 % of the indicated load, the assembly was loaded in tension at a crosshead speed of 0.1mm/min, and the load and displacement were monitored and recorded 35.   29   Figure 11: Custom grips attaching NTP specimen holder to testing machine  Fracture toughness was then obtained by utilizing the following equation: KIC =          Y*min Where Pmax = maximum load recorded during testing, D= specimen diameter (12 mm), W= specimen length (10.5 mm) and Y*min = the dimensionless stress intensity factor coefficient minimum 35. The value for Y*min is 28, as proposed by Ruse et al 27. Fractured surfaces were characterized using scanning electron microscopy (SEM).     30  2.4.4.1.1 Scanning Electron Microscopy  Selected KIC fractured specimens were gold coated and characterized using a Hitachi, S-3000N (Hitachi, Japan) scanning electron microscope (SEM).  2.4.4.2 Testing FCP  Because FCP testing using the NTP specimen KIC test methodology had not been previously used, a pilot test was done with plexiglass NTP samples. Plexiglass NTP specimens, 6x6x6x12 mm, were obtained by grinding precut blocks, as described previously for IPS e.max CAD samples. A pre-crack was initiated in one of the specimen edges using a sharp surgical blade. Several lines, 1 mm apart were scribed on the side of the specimen to facilitate monitoring crack propagation. To scribe these lines, a putty mold that contained an impression of a triangular prism was used. The prism was placed in its impression in the putty mold and a metal ruler with grooves 1 mm apart was placed on top of the prism (Figure 12). A fresh surgical blade was used to scribe the lines through the grooves (Figure 13).         Figure 12: A putty mold and metal ruler used to scribe lines on one side of the NTP specimen 31   Figure 13: Lines scribed on the NTP specimen  The specimen was then secured in the NTP holder, in the same manner described previously for KIC testing. The holder was then mounted on specifically designed grips (according to ASTM E647 specifications) attached to a servo hydraulic fatigue-testing machine (Instron model 8511, Instron Canada Inc).  The grips are designed with ledges that accommodate the knife edges of a strain gauge (Figures 14 and 15).  32   Figure 14: The grips used to attach the NTP holder and the strain gauge for testing FCP   Figure 15: NTP holder attached to grips and illuminated 33  The specimen was cyclically loaded in tension (Mode I) in a load range between 1 N (Pmin) and 20 N (Pmax) (R = Pmin/Pmax = 0.05) and crack length on the scribed side of the prism was monitored and filmed using a high definition video recorder (SONY HDR-XR550V) attached to a microscope (Edmund Scientific Co, Barrington, NJ). Crack illumination was achieved by two white beam lights focused on the specimen.  The crack propagation was associated with an increase in compliance (deformation at crack tip as a function of load) and video recording was terminated once catastrophic fracture of the specimen occurred. Figures 16, 17 and 18 below are snapshots acquired from one of the video clips to illustrate the propagation of the crack.   Figure 16: Initial pre-crack in an NTP specimen under cyclic loading 34   Figure 17: Crack propagation in an NTP specimen under cyclic loading   Figure 18: The last recorded crack propagation prior to catastrophic fracture  Over specific intervals of fatigue loading and until complete specimen fracture, snapshots of the video clip were obtained using Cyberlink Power Director, a commercially available video editing software (Figure 19). Image J software was used to calibrate these snapshots to the known distance between each 2 scribed lines (1 mm) and then used to measure the crack length (a) (Figures 20 and 21). These measurements were used to calculate the change in crack length (Δa) between the investigated frames.  35   Figure 19: Cyberlink Power Director software used to obtain snapshots from FCP video clip at certain intervals  Figure 20: Calibration of measurements using Image J software  36   Figure 21: Measuring the crack using Image J software after calibration  The number of cycles between the assessed snapshots (ΔN) was determined based on the time at which the snapshot was taken (directly related to the number of cycles). The change in crack length vs number of cycles was plotted and the results were used to determine the fatigue crack growth rate (da/dN). A final curve of       versus ΔK was plotted on log/log scales and used to calculate FCP parameters, based on Paris’ Law      = C (ΔK)m or log(da/dN) = mlog(K) + logC Where ΔK is the stress intensity range, C (the intercept) is the fatigue crack growth coefficient and m (the slope) is the fatigue crack growth exponent.   37  2.4.5 Statistical Analysis  For KIC, Weibull statistical analysis was used to evaluate the characteristic strength and the reliability of each material51. An independent student t-test was also performed to compare the results.  2.5 Results   2.5.1 KIC results  Weibull modulus, represented by the slope of the curves (Figure 22) gives an indication of the reliability of each material. The steeper the curve, the more reliable the material is.  Characteristic Weibull KIC  is obtained from the plot at 63.2 percentile point.  As seen in table 2 and in the Weibull plot in Figure 22, the results of the KIC test revealed that IPS e.max Press had higher values of both Weibull modulus m and characteristic Weibull KIC than IPS e.max CAD. A t-test showed that IPS e.max Press had a significantly higher KIC than IPS e.max CAD (p < 0.05).     38  Material KIC (in MPa·m1/2) Weibull modulus ɱ Characteristic Weibull KIC (in MPa·m1/2) IPS e.max CAD 1.79 ± 0.26a 8.41 1.90 IPS e.max Press 2.50 ± 0.31b 9.74 2.64  Table 2: KIC of IPS e.max CAD and IPS e.max Press   Figure 22: Weibull plot of KIC results     y = 9.7448x - 9.4073 R² = 0.9581 y = 8.4088x - 5.3585 R² = 0.9579 -5.000-4.000-3.000-2.000-1.0000.0001.0002.0003.0000.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400ln(ln(1/(1-Pf))) ln(KIC) IPS e.max Press and IPS e.max CAD e.max Presse.max CAD39  2.5.1.1 SEM images of IPS e.max  Figures 23 to 28 present characteristic SEM micrographs of fractured KIC specimens. Figure 23 (A,B) shows two halves of a fractured IPS e.max CAD prism with an arrow pointing at the initiation area. In examining the fractured surfaces at higher magnification, Figures 24 and 25 (A,B), they look smooth, possibly indicating a crack propagation through the glassy matrix.  Figure 26 (A,B) shows two halves of a fractured IPS e.max Press prism. At higher magnification, shown in Figures 27 and 28 (A,B), rough and irregular fractured surfaces are seen with minimal glassy matrix visible in the micrographs and almost entirely crystallized surfaces.         40   Figure 23: SEM photo of 2 sides of IPS e.max CAD prism x50 magnification   Figure 24: SEM photo of 2 sides of IPS e.max CAD prism x5000 magnification   Figure 25: SEM photo of 2 sides of IPS e.max CAD prism x10000 magnificationA A A B B B 41   Figure 26: SEM photo of 2 sides of IPS e.max Press prism x50 magnification   Figure 27: SEM photo of 2 sides of IPS e.max Press x5000 magnification   Figure 28: SEM photo of 2 sides of IPS e.max Press x10000 magnification A A A B B B 42  2.5.2 FCP results  Three plexiglass specimens were tested in the manner previously described. The following tables and graphs summarize the data obtained from these three experiments. # of cycles (N) a (mm) a versus N graph 00 0.00562  19 0.00565 29 0.00567 39 0.00573 49 0.00578 59 0.00603 69 0.00680 71 0.00693 74 0.00796  Table 3: First Plexiglass specimen (a vs N graph) # of cycles (N) a (mm) a versus N graph 2 0.00575  49 0.00575 79 0.00577 129 0.00579 149 0.00580 719 0.00580 899 0.00581 1019 0.00582 1079 0.00596 1199 0.00597 1259 0.00598 1319 0.00602 1499 0.00614 1559 0.00626 1619 0.00640 1679 0.00650 1719 0.00656 1769 0.00668 1799 0.00674 Table 4: Second Plexiglass specimen (a vs N graph)  0.000000.002000.004000.006000.008000.010000.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01a vs N 0.005500.006000.006500.007000 500 1000 1500 2000a vs N 43                     Table 5: Third Plexiglass specimen (a vs N graph)  In the attempt to calculate Y* and subsequently KI, we found that their values did not follow the expected gradual increase with increasing number of cycles. Rather, the values fluctuated between increasing and decreasing, and the resulting graph did not follow Paris law graph.  # of cycles (N) a (mm) a versus N graph 0 0.00574  41 0.00575 80 0.00577 130 0.00579 150 0.00580 320 0.00580 360 0.00580 420 0.00580 480 0.00580 540 0.00580 600 0.00580 720 0.00580 900 0.00581 1020 0.00582 1080 0.00586 1140 0.00586 1200 0.00588 1260 0.00596 1320 0.00597 1410 0.00598 1500 0.00602 1560 0.00604 1620 0.00614 1680 0.00626 1720 0.00640 1770 0.00650 1800 0.00656 1830 0.00668 1878 0.00676 1894 0.00685 1910 0.00707 1920 0.00710 1936 0.00716 1940 0.00733 1942 0.00738 1945 0.00748 1948 0.00756 1949 0.00767 0.005700.005800.005900.006000 500 1000 1500a vs N 44  Chapter 3: Discussion   3.1 KIC  The results of the KIC test showed that IPS e.max Press had higher values of both Weibull modulus m and characteristic Weibull KIC than IPS e.max CAD. The values for Weibull KIC for IPS e.max Press and CAD were 2.64 and 1.90, respectively.  These results are in agreement with the manufacturer’s in-house testing reported values which are 2.5-3 for IPS e.max Press and 2.0-2.5 for IPS e.max CAD. There were no studies found comparing KIC of both materials in vitro. In addition, there were no studies reporting on the individual KIC values of either material.  Several studies were found reporting on KIC of IPS Empress 2, which is IPS e.max Press’s predecessor. IPS Empress 2 is also a lithium disilicate pressable glass ceramic manufactured by Ivoclar Vivadent. The results of these studies for KIC of IPS Empress 2 were 3.453 and 3.1454.   The difference we found between KIC values for IPS e.max CAD and Press can be attributed to the difference in crystals size reported by the manufacturer. When examining the SEM images (Figure 29), IPS e.max Press appears to have a rougher surface, almost entirely crystalized with minimal glassy matrix. On the other hand, IPS e.max CAD had a smooth surface and the crack seemed to propagate within the glassy matrix. This could mean that more energy was needed to 45  penetrate the rougher, more irregular IPS e.max Press surface than what was needed for IPS e.max CAD.  In addition, it may be necessary to revise the crystallization cycle recommended by the manufacturer for IPS e.max CAD, as the fractured surfaces did not appear to be fully crystallized when examined under SEM. 46   Figure 29: SEM photos comparing IPS e.max CAD (above) and IPS e.max Press (below) x10000 magnification 47  3.2 FCP  The results we got from this test varied from what was expected, as Y* and KI values were not gradually increasing as one would expect with increasing number of cycles.  To explain these results, we compared the NTP specimen to the compact tension specimen commonly used in FCP tests. Unlike a compact tension specimen, where the crack front (b) progresses in a constant width, in the short rod chevron notch (SRCN) specimen, and consequently in the notchless triangular prism (NTP) specimen, b is trapezoidal, which implies a constant crack front increase with crack growth. The crack front is calculated using the following equation:                 Figure 30: NTP specimen for FCP  As has been shown by Bubsey55 and others56-62, Y* reaches a minimum value, which is dependent on specimen geometry only. During fracture toughness (KIC) determination, the load a1 ao a D b W= a1 48  required to drive a crack reaches a maximum at the minimum value of Y*. Since the stress intensity factor, KI, is given by,           √   Where     (                    )1/2  KI’s magnitude, under constant load, will follow the same trend as Y*, i.e. it will first decrease to a minimum before starting to increase. This became obvious during the attempt to use the NTP specimen KIC test for fatigue crack propagation (FCP) studies. Three Plexiglas specimens were tested and the results have been analyzed according to the guidelines of ASTM E64749.  Specimen 1 and 2 had too few points to be “useful”. Specimen 3 was adequate. The analysis of the results showed the expected behavior of Y* as a function of , the normalized crack length (=a/W).  The behavior of Y* as a function of number of cycles, and therefore crack length, was paralleled by K.  49   Figure 31: FCP Specimen #3 Y* vs a   Figure 32: FCP Specimen #3 Y* vs N  20.00022.00024.00026.00028.00030.00032.00034.00036.0000.500000 0.550000 0.600000 0.650000 0.700000 0.750000Y*  #3 Y* vs  20.00022.00024.00026.00028.00030.00032.00034.00036.0000 500 1000 1500 2000 2500Y* Nr cycles #3 Y* vs N 50   Figure 33: FCP Specimen #3 ΔK vs N  If only the data points that occurred after Y* (or K) reached its minimum were considered, a characteristic FCP da/dN vs K log/log curve could be obtained (see below). The slope of the curve, which is the exponent m in Paris’ Law, is 13.5, a value which is close to values reported in the literature for poly(methyl methacrylate).63  At this stage, however, further research should be conducted to evaluate the feasibility of the NTP specimen KIC test to be used for FCP studies. 0.3000.3500.4000.4500.5000.5500 500 1000 1500 2000 2500ΔK  Nr cycles #2 ΔK  vs N 51   Figure 34: FCP Specimen #3 da/dN vs ΔK  1.000E-081.000E-071.000E-061.000E-051.000E-041.000E-030.100 1.000da/dN ΔK #3 da/dN vs ΔK (log/log) , N>1500 52  Chapter 4: Conclusions   For fracture toughness, IPS e.max Press was found to be superior to IPS e.max CAD, therefore, the null hypothesis was rejected.  For the FCP test, experimental pilot specimens were tested to verify the methodology for testing FCP using the NTP specimen fracture toughness test. 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