Fiber reinforced polymer (FRP) composite materials are widely used in automotive and aerospace industries. The traditional FRP composites have a laminated construction in which layers are reinforced with fibers in unidirectional (UD) or fabric forms. Such materials possess good mechanical properties in-plane but relatively weak in the through-thickness direction. Due to their low interlaminar strength, composites are prone to delamination. The quasi-three-dimensional (Q3D) composite are designed to improve the interlaminar strength. In this work, the Q3D composite is made by a special braiding process in which the bias tows are braided into adjacent layers. Contrary to the conventional composites in which plies are bonded by the polymer matrix only, the plies of Q3D composites are bridged by fiber tows. As a higher load is required to break the bridging tows for crack growth, the delamination resistance will increase. This has been proven by interlaminar fracture experiments under Mode I and Mode II conditions. This thesis is focused on numerical simulations of UD and Q3D composites under Mode I and Mode II loadings. This investigation is needed for the development of simulation methods for crash safety simulations of Q3D composite structures in automotive applications. In this work, the interlaminar delamination was modeled with cohesive elements. Both the Bilinear and Trilinear Cohesive Zone Models (CZM) were investigated. The CZM parameters were determined from the Mode I and Mode II fracture toughness values measured in experiments. The simulations were performed using explicit finite element (FE) code LS-DYNA. It was observed that in Mode I simulations, the Bilinear CZM predicted a stable crack growth, which agreed with the experimental observation. On the other hand, the Trilinear CZM predicted a relatively unstable crack growth. In Mode II experiment with an end-notched flexural (ENF) configuration, the delamination tends to be unstable. This behavior was better predicted with the Trilinear CZM. In FE models for Q3D composite, the cohesive elements were assigned with two sets of CZM parameters. The first set of parameters was the same as that for the UD model. The second set with higher facture toughness values was assigned to the cohesive elements corresponding to the bridging tows. This method captured the stick-slip behavior of the Q3D composites observed in Mode I experiments. In general, the prediction was better for the UD composite than for the Q3D composite. The predicted load at the delamination initiation was within +/-10% of the experimental values for the UD and within +/-15% for the Q3D.
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