Effect of microstructure on the relationship between fracture toughness and ductility

The relationship between fracture toughness, expressed as JIC, and ductility, given by measurements of the bulge ductility, was studied experimentally for two microstructures of 1045 steel. These microstructures consist of annealed pearlitic structure with hardness Rc = 15 and a tempered matensitic structure with Rc = 23. JIC and bulge ductility measurements were performed in the temperature range of −100-25°C. The tensile properties at these temperatures were also obtained. The results show that the variation in flow stress with temperature is similar for both microstructures. However, the flow stress is higher, and the bulge ductility is lower, at a given temperature, for the martensitic structure. Also shifts in the transition temperature from linear elastic to elastic-plastic behavior are observed. Previously developed models by the authors describing the variation of JIC with temperature and ductility are used to account for the behavior of the different microstructures examined.