Study on interlayer damage and shear behavior of CFRP twill interlocking and CFRP unidirectional fiber bridging

This study systematically investigates the interlaminar damage characteristics and shear behavior of unidirectional (UD) and twill (TW) carbon fiber reinforced polymer (CFRP) fabrics through experimental and numerical analyses. The research highlights the differences in crack propagation and damage mechanisms between UD and TW CFRP, as well as the influence of hybrid layering on the mechanical properties of composites. Double cantilever beam (DCB) tests were employed to quantify the fiber bridging phenomenon in UD CFRP and elucidate the crack propagation mechanisms in both UD and TW configurations. Four-point bending tests were conducted to explore the interlaminar strength, energy dissipation characteristics, and failure behavior of UD and TW CFRP. DCB test results revealed that UD specimens exhibited high-frequency, small-amplitude load drops associated with fiber bridging and rupture, whereas TW specimens demonstrated low-frequency, large-amplitude load drops primarily driven by matrix cracking and interfacial debonding. Four-point bending tests indicated that, although the interlaminar strength of TW specimens was comparable to that of UD specimens, their energy dissipation capacity was significantly higher. Additionally, the study examined the interlaminar shear behavior of hybrid UD-TW layups. The results demonstrated that hybrid layering led to a notable reduction in bending beam strength due to increased interlaminar shear stress, resulting in delamination and damage within the UD layers. Microscopic observations confirmed that the damage mechanisms in hybrid layups were governed by the combined effects of fiber shear behavior in UD layers and fiber interlocking in TW layers. • Crack Propagation Divergence:UD specimens exhibited high-frequency, small-amplitude load fluctuations due to fiber bridging effects.TW specimens displayed low-frequency, large-amplitude load drops driven by matrix cracking and interfacial debonding. • Energy Dissipation Characteristics:TW specimens demonstrated 585.98% higher energy dissipation capacity than UD specimens, attributed to prolonged crack propagation paths and multi-scale damage mechanisms. • Hybrid Layup Effects:UD-TW hybrid layups suffered significant bending strength reductions (24.98% for UD, 55.97% for TW) due to interlayer shear stress concentration, with UD layers prone to translayer damage. • Microscopic Mechanisms:Optical microscopy revealed fiber fracture dominance in UD versus matrix-dominated failure modes in TW, while hybrid layups showed exacerbated damage from their synergistic interaction. • Innovation and Significance:This work first reveals the competitive mechanisms between TW’s fiber interlocking effects and UD’s shear behavior, providing theoretical foundations for optimizing CFRP layup designs in aerospace and other high-performance fields. It offers critical guidance for enhancing composite reliability under extreme environmental conditions.