UBC Theses and Dissertations
Extracting the strain-softening response of composites using full-field displacement measurement Zobeiry, Navid
From both numerical and experimental standpoints, it is very desirable to develop a general methodology that can be used to determine the strain-softening response and characteristic damage properties (e.g. fracture energy, damage height) of quasi-brittle materials. In the absence of a direct methodology, one has to conduct multiple experiments combined with trial-and-error procedures and/or simplifying assumptions regarding the damaging behaviour of the material in order to construct a strain-softening curve. In this study, a new methodology is developed that directly identifies the damaging constitutive response of composite materials using full-field measurements of kinematic variables. Using this methodology, the damage related properties can be extracted and the strain-softening response of composite materials under mode I loading can be obtained directly. Compared to other available indirect approaches, this method invokes fewer assumptions about the behaviour of the material and does not require any prior assumption regarding the shape of the constitutive response, as is required in other approaches. A series of compact compression and over-height compact tension tests are conducted on IM7/8552 quasi-isotropic laminates. Using the digital image correlation technique, full-field displacement vectors of the specimen surface are measured in each test. Based on the acquired data and using the basic principles of mechanics (equilibrium and compatibility), a family of approximate stress-strain curves are obtained. These approximate curves are then used in an iterative process to determine the optimized strain-softening response of the laminate. To validate the capability of the method to capture the local damaging behaviour of the composite laminate, a series of destructive tests such as deplying and sectioning are performed on the damaged specimens. The tested laminates are also simulated using finite element analyses of the specimens that employ the extracted strain-softening curve as input to a damage mechanics based material model. The proposed methodology provides insight into the details of damage propagation in composite materials and is a promising tool for characterizing the strain-softening behaviour of composite laminates in a relatively easy and direct manner.
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