Verification of void growth-based exponential damage function for ductile crack initiation over the full range of stress triaxialities

The exponential function to simulate the damaging effects of void growth has been widely used in fracture initiation models to describe the effect of stress triaxiality on the fracture strain of ductile metals. Its validity has largely been demonstrated for loading conditions with high stress triaxiality, but there are comparatively fewer tests to validate the exponential model for situations with medium to low stress triaxialities. This paper aims to fill this knowledge gap by investigating the dependence of the fracture strain on the stress triaxiality over the entire stress triaxiality regime (ratios of hydrostatic to von Mises stress from −1/3 to 1). In doing so, care is taken to compare results for test specimens with variable stress triaxiality but near identical Lode angle parameter, which is also known to influence the critical fracture strain. Test data are collected from published literature along with a series of newly designed plane strain specimens developed for this study. By categorizing the data into groups with similar Lode angle parameters, the effect of stress triaxiality is explicitly examined within each group. Overall, these comparisons verify that the void growth-based exponential function is capable of reflecting the effect of stress triaxiality for steels over a broad range of stress triaxialities with reasonably good accuracy.