Fracture Mechanics Model for Compression-After-Impact and Bending-After-Impact Strength of Composites

In this study, we employ the use of a linear elastic fracture mechanics (LEFM) model to understand and predict the post-impact performance of composites. Compressive-after-impact (CAI) and bending-after-impact (BAI) strength of carbon fiber reinforced polymer (CFRP) composites are important design considerations in the industry. Although CFRP composites are light-weight and high-strength, their susceptibility to impact results in barely visible impact damage (BVID), consisting of complex damage modes (matric cracking, delamination, fiber breakage, etc.) often hidden beneath the surface of the composite material. BVID subsequently leads to drastic reduction in CAI and BAI strength.

            We will first present our experimental results of CAI and BAI tests on CFRP composite foam-core sandwich structures. Our results show the effect of impact energy on post-impact performance of composite structures. Additionally, we study the influence of low temperature Arctic conditions. The motivation of this study is fueled by the recent global trend to explore the Arctic region due to greater accessibility to the area, as new sea routes are opened as a result of Arctic ice melting. Our study demonstrates a significant influence of low temperature on impact behavior and after-impact performance.

            We will next utilize a LEFM model to understand how impact damage affects post-impact performance. In particular, we study how impact damage depth (d), damage width (l) and impact-induced delamination (a) affects brittle fracture of composites during compressive and flexural loading. Strain energy release rate (SERR) is calculated to predict crack growth based on fracture mechanics. Results show that increasing damage depth results in higher SERR. Moreover, decreasing undamaged depth, h, also causes higher SERR, thereby easing CAI brittle fracture. Interestingly, when the total depth is kept constant, an exponential increase in SERR is evident when damage depth reaches around 60% of total depth. The effect of low temperature can be considered in terms of the changes in material properties. Specifically, when Young’s Modulus decreases, the SERR also decreases. This implies that stiffer materials are less susceptible to crack propagation based on the CAI model. For the BAI model, the effects of damage depth and Young’s Modulus on SERR are similar to the CAI model. Moreover, the BAI model elucidates the effect of damage width and impact-induced delamination. Results show that when damage width and delamination length increase, SERR increases. It is interesting to note that there is no change in SERR when the total size of damage width and delamination length is kept constant.