Stress triaxiality and failure in SFRPs: a novel workflow for failure curve determination, considering fibre orientation and specimen geometry

This research investigates the static failure behaviour of a PA6T/6I GF50, focusing on the distribution of the stress triaxiality at critical points where failure is expected. The study explores whether a relationship exists between this stress state indicator and the strain experienced by the material, providing insights into when critical failure conditions are reached. Experimental tests, including uniaxial, biaxial, bending and shear, were conducted on various specimen types and loading orientations relative to their injection moulding direction. Finite element simulations, accounting for material's anisotropic elasto-plastic response, were run to evaluate the critical local stresses at failure locations. Digital image correlation was employed throughout the test campaign to monitor strain evolution. The evolution of the stress triaxiality during loading was computed from simulation results at both the macro (composite level) and micro-scales (matrix level). At the macro scale, it was evaluated using both isotropic (von Mises) and anisotropic (Hill 48) yield models. However, neither approach successfully identified generalized failure curves, as those correlating stress triaxiality with equivalent plastic strain, applicable across all loading scenarios and specimen geometries. To overcome this limitation, a new approach based on the calculation of the stress triaxiality and equivalent plastic strain locally at critical failure points was proposed. This allowed the identification of a failure curve that is independent of loading conditions and specimen geometry, underscoring the importance of triaxiality-based material characterization over traditional uniaxial approaches for accurately predicting failure in anisotropic short fibre-reinforced polymers.