Influence of R-ratio and fracture process zone development on mode II fatigue delamination

Certification of composite structures remains a significant challenge in the aerospace sector. These materials exhibit various failure mechanisms under load, complicating the prediction of crack growth. Delamination is the most common and critical failure, typically triggered by combined tensile and in-plane shear loadings corresponding to Mode I and Mode II, respectively. Characterisation of Mode II remains particularly difficult due to the unstable crack propagation exhibited in many test configurations. This manuscript presents an experimental study of Mode II fatigue delamination at various R-ratios using the End-Loaded Split specimens, which enable stable in-plane shear-driven delamination. A multi-method approach utilising Digital Image Correlation (DIC), Acoustic Emissions, and post-mortem fractography analysis was adopted to provide a comprehensive description of how delamination behaves across varying R-ratios. The study was centred on the fracture process zone, measured via DIC, due to its significant impact on energy dissipation. Variations in the length of this zone throughout the fatigue life revealed an imbalance between the damage mechanisms affecting the growth of the true crack length and the effective crack length. This evolution of the fracture process zone was correlated with trends in acoustic energy dissipation and the morphology of the fracture surface. These findings provide new insights into Mode II fatigue delamination and enhance our understanding for the design of damage-tolerant structures.