Investigating the Fracture Behavior and Damage Mechanisms of Discontinuous Fiber Composites Under Significant Out-Of-Plane Loading

Thesis (Master's)--University of Washington, 2020This paper qualifies the interlaminar fracture behavior and damage mechanisms of discontinuous fiber composites. Laminates are produced by compression molding randomly distributed, pre-impregnated, unidirectional carbon fiber and epoxy platelets of size 50 mm by 8 mm with a thickness of 0.139 mm. Double cantilevered beam tests on geometrically-scaled specimens are completed, and qualitative and quantitative analysis is described in the following sections. Discontinuous fiber composites show complex damage progression in the form of fiber nesting, fiber bridging, crack bifurcation, and fiber breaking. This is visualized with digital microscopy, photographs during and after the test, and CT-scans that display internal damage. It is shown that the apparent fracture energy is size dependent, with the average interlaminar fracture energy increasing with the size of the coupon from approximately 0.54 N/mm to 0.80 N/mm. These values exceed reported inter-laminar fracture energy of traditional laminated composites by up to 167\%. This is a result of complex damage mechanisms and tortuous morphology of the crac