New concept for initiation of nonlinear fracture and their experimental background

Two new fracture initiation parameters are proposed to establish relations between the uniaxial tension properties of materials and the fracture toughness of ductile steels, titanium, and aluminum alloys. To this end, the ratio between the dimensionless local strain energy density (SED) per unit volume in elastic-plastic deformation around the crack tip and the normalized total SED at standard uniaxial tension, denoted as damage factor FW, as well as the ratio of the separation at material decohesion to crack advance, denoted as δ0/Δac, are introduced as new nonlinear fracture initiation parameters. The value of FW for all the alloys considered was found to be proportional to the total SED at uniaxial tension, and an empirical relationship describing FW as a function of W¯C was established. Furthermore, it is suggested that with the knowledge of the tensile properties and the estimated range of FW ratios for different materials, the application of this method could be appropriate for predicting fracture toughness. A new automatic algorithm for fracture toughness assessment is presented based on the concept of the plastic stress intensity factor (SIF). The effectiveness of the proposed algorithm accounting for constraint effects and employing the crack-tip distance dependence of governing elastic-plastic parameters is confirmed by fracture toughness testing of different structural materials.