Delamination Study of Open‐Hole Thermoset Composites Under Combined Tension–Bending Fatigue Loading Based on Acoustic Emission

ABSTRACT The initiation and propagation mechanisms of delamination in GFRP composites, particularly around laser‐cut holes and interlaminar interfaces, remain inadequately understood under combined tension–bending fatigue loading. This study presents a comprehensive experimental investigation of delamination behavior in open‐hole thermoset GFRP laminates, integrating acoustic emission (AE) monitoring and scanning electron microscopy (SEM) for damage characterization. Fatigue tests were performed at two cycle counts, 40,000 and 60,000, to capture the onset and progression of internal damage. Delamination initiation was identified at approximately 40,000 cycles, coinciding with the first sharp rise in cumulative AE energy. In comparison, extensive propagation occurred by 60,000 cycles, at which point delamination had fully developed around the hole and between the plies. AE features, including cumulative energy and peak frequency, were continuously recorded under mixed‐mode cyclic loading. Notably, this work identifies, for the first time, characteristic AE peak frequency bands correlated with the initiation and growth of delamination under combined loading. Furthermore, the evolution of AE energy and normalized mechanical responses was benchmarked against the Paris Law framework, providing a quantitative basis for describing delamination under complex fatigue conditions.