Damage initiation and post-damage response of composite laminates by multiaxial testing and nonlinear optimization
Schmitt, James Tyler.
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Fiber reinforced plastics are increasingly being used in the construction of primary structures in the aerospace and energy industries. While their elastic behavior and fatigue response have been the subject of considerable research, less is known about the performance of continuous fiber composites following initial damage. Several competing models for the post-damage response of orthotropic composite materials are explored in this thesis. Each of these models includes only the in-plane loads experienced by the material and characterizes damage based on the local state of strain. Starting with previous work performed at the Naval Research Laboratory and at MSU, the energy dissipated in multiaxially loaded coupons was used to optimize an empirical function that relates the three in-plane strains to the local dissipated energy density. This function was used to approximate a three dimensional damage initiation envelope as well as to quantify the severity of damage following first ply failure in a fiberglass laminate. Carbon fiber reinforced epoxy was characterized using an assumed bilinear constitutive response. The elastic properties of the material were first optimized to minimize deviation from experimental data and then the necessary coefficients for a per-axis strain softening response were found using a similar optimization. This model provides detailed insight into the residual strength of significantly damaged material, as well as dissipated energy as a direct consequence. To facilitate the need of these models for diverse local in-plane loading configurations, the MSU In-Plane Loader (IPL) was utilized. The tests performed in the IPL for this thesis were instrumental in validating a new image-correlation-based displacement monitoring system.