Local approach applied to the fracture toughness of resistance spot welds

Abstract : A recently developed methodology for measuring the nugget fracture toughness in mode I was applied to three high-strength steel resistance spot welds, exhibiting either mixed cleavage/ductile or ductile fracture at room temperature. Fracture toughness tests revealed a difference in ductile-to-brittle transition temperature between the welds. Constitutive equations of weld nuggets were determined and implemented in a finite element model of the fracture toughness test, revealing both in-plane bending and in-plane stretching in front of the crack tip. Brittle cleavage fracture of the nuggets is not conveniently described using a simple Ritchie-Knott-Rice approach but the Rice-Tracey model allowed quantitatively relating the high number density of small particles to the nugget fracture toughness in the ductile regime.