Traction-separation law for glass/epoxy composite laminates under mixed mode I/II loading condition considering the effects of fiber bridging

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This study analyzed the effects of fiber bridging on fracture behavior in glass/epoxy composite laminates under mixed mode I/II loading conditions and proposed new traction-separation laws (TSLs) for this loading condition. To this end, double cantilever beam (DCB), end notch flexure (ENF), and mixed mode I/II bending (MMB) tests were performed under mode I, mode II, and mixed mode I/II loading conditions. Also, finite element analysis (FEA) was employed to simulate these tests. Under mode I loading condition, the presence of fiber bridging increased fracture toughness significantly, which was modeled using a tri-linear TSL. In contrast, under mode II loading conditions, the effect of fiber bridging was negligible, and a bi-linear TSL was sufficient. On this basis, a novel TSL was proposed for mixed modes that considers the effect of fiber bridging under mode I loading conditions and neglects it under mode II loading conditions. This TSL was defined using a tri-linear TSL for mode I and a bi-linear TSL for mode II conditions, along with the B-K fracture criterion. In the bi-linear TSL, the point τb was defined corresponding to the fiber bridging strength (σb) of the tri-linear TSL, with the assumption that τb cannot exceed σb. To analyze the variation in the FEA results with respect to τb, three cases were defined: τb = 0.0001 σb, 0.5 σb, and σb, respectively. FEA results in the first case were observed to be consistent with load–displacement curves obtained from experimental results.