Enhancing the impact resistance of composite laminates through in-plane mechanical property optimization using the isotropic degree index

This study aims to enhance the impact resistance of composite laminates while maximizing material efficiency. Conventional methods, such as reducing layup angles or adopting helical structures, often overlook material waste and industrial feasibility. Additionally, hybrid laminate designs are challenged by the absence of robust scientific methods for selecting material proportions and layup angles. To address these challenges, this paper introduces the isotropy degree index (IDI), a novel metric quantifying how closely the in-plane tensile properties of a laminate approximate isotropic behavior. Based on prior studies on the tensile properties of multiaxial warp-knitted fabrics, this paper deduces a method for calculating the IDI and integrates it into the design of impact-resistant laminates. A laminate's impact resistance strongly depends on its in-plane mechanical properties, and the reliability of this approach is demonstrated through experimental tests, finite element analysis, and industrial CT scans. The results demonstrate that increasing the IDI significantly improves impact strength, reduces impact-induced displacement, and promotes symmetrical delamination patterns diminishing toward the laminate edges. Moreover, a higher IDI mitigates through-thickness force propagation, thereby reducing matrix damage. This research underscores the critical role of IDI in optimizing laminate performance, providing practical guidelines for the design of next-generation impact-resistant composite laminates.