Fracture Behavior of 3D Printed Carbon Fiber-Reinforced Polymer Composites

Abstract This study examines the inter-layer and cross-layer fracture behavior of 3D printed short CFRP composites using both experimental and numerical methods. First, the orthotropic mechanical response of the 3D printed CFRP samples was characterized by tensile tests. The out-of-plane Young’s modulus and yield strength were measured to be about 25% and 32% of the in-plane counterparts, respectively. Second, compact tension fracture tests were performed to characterize the fracture behavior of the CFRP composite samples. The crack resistance curves of inter-layer and cross-layer fracture were independently derived using the elastic-plastic fracture theory. Both cross-layer and inter-layer fracture exhibited stable crack growth with the rising crack resistance curves. Finite element simulations incorporating a cohesive zone method was used to numerically determine the critical cohesive strength and fracture toughness from the experimental results. The experimentally measured and numerically calibrated cross-layer fracture toughness was found to be three-fold of the inter-layer counterparts. In addition, fracture surface morphology and the crack-tip strain field evaluated by the digital image correlation technique revealed three important observations: (1) the cross-layer fracture resulted in a larger fracture process zone ahead of the crack tip compared to the inter-layer fracture, (2) the cross-layer fracture exhibited a fibril-bridging toughening mechanism along with fiber pull-out failure at the micro-scale, and (3) the inter-layer fracture showed a uncracked-ligament toughening mechanism arising from the locally enhanced inter-layer adhesion due to short fiber-bridging. The fracture mechanisms reported in this study provide strategies for designing and 3D printing composites with superior fracture-resistance.