The Effect of the Loading Path History on the Fracture Loci

Abstract

Theoretical analysis and experimental studies have shown that the stress triaxiality η is one of the most important factor that characterizes the ductile fracture of metallic materials. However, there is another variable that should be involved in fracture prediction models, which is Lode angle parameter ξ. In this study, two uncoupled phenomenological models are used for predicting the ductile fracture of a cast A356 aluminium alloy. These models define the equivalent strain to fracture in function of variables that characterize the stress state. The first considers only the stress triaxiality, while the other takes into account the mean pressure and the Lode angle parameter. Two groups of experimental tests were considered: (i) tensile tests on smooth and notched round bars and (ii) tensile plane strain tests on flat grooved plates. The numerical results show that both stress triaxiality and Lode angle parameter evolve during the tests. Therefore, it is necessary to understand the impact of this variation on the calibration of the parameters of uncoupled ductile fracture models. Note that the aim is to be able to predict the fracture under non-proportional, complex loading conditions. In order to assess the effect of the evolution of the stress triaxiality on the prediction of ductile fracture, two strategies are proposed for calibrating a 2D fracture locus, for a wide range of positive stress triaxiality, named direct and inverse, since the first is based on the initial value of the stress triaxiality, while the second uses the average. The numerical results show that if ductile fracture occurs before the onset of localized deformation, the stress triaxiality and the Lode angle present quite constant values, although there is a small variation in the transition between the elastic and the plastic regime. However, when ductile fracture occurs after the onset of localized deformation, these variables can present a strong variation, which should be taken into account in the calibration of the 2D fracture loci.