Influence of irregular R-curve behaviour on failure prediction of interface-toughened composite joints

Interlaminar toughening is increasingly used to enhance the fracture toughness and strength of composite bonded joints. However, certain toughening strategies can introduce irregular fracture resistance behaviour during debonding, such as falling–flat or rising–falling–flat R-curve characteristics, whose influence on joint performance prediction remains unexplored. This study investigates the effect of such irregular R-curve behaviour on the modelling and strength prediction of interface-toughened composite joints by simulating debonding in single-step joints toughened with core–shell rubber (CSR) nanoparticles, polyphenylene sulfide (PPS) micro-fibre veils, and hybrid CSR–PPS systems using a bilinear cohesive zone model, validated against existing experimental results. Fracture energies are assigned to cohesive elements along the bondline using either experimentally measured crack-length-dependent fracture energies or crack-length-independent constant values. The results demonstrate that irregular R-curve behaviour introduced by PPS-veil toughening significantly influences the predicted debonding process and joint strength, and neglecting this behaviour leads to unreliable modelling. Furthermore, a selective local PPS-veil toughening strategy is explored, exploiting the elevated fracture resistance at early stages of crack growth. Appropriate localised toughening is shown to alter the debonding process and achieve improved joint strength compared with full-length toughening.