An experimental and theoretical investigation of the influence of stress state on cavitation during hot working

Abstract

The effect of stress triaxiality on cavity growth during necking in the hot tension test was investigated and quantified. Experimentally, Ti–6Al–4V specimens deformed over a range of temperatures and strain rates developed necks of different sharpness and thus of varying levels of the ratio of hydrostatic to effective stress. The magnitude of these stresses was calculated using the classical work of Bridgman. Metallographic examination revealed that the average cavity size, cavity population, and cavity volume fraction were higher in the central region of each specimen as compared to that in the edge region. An analytical model was developed to separate the effects of cavity nucleation and coalescence from the apparent cavity growth rate and thus to determine the individual cavity growth rate at the two locations. The ratio of the cavity growth rate at the center of the specimen to that of the edge was correlated to the stress state (i.e., ratio of mean to effective stress) and compared to predictions derived from the work of Rice and Tracey and of Pilling and Ridley. These comparisons demonstrated that model predictions based on the Pilling-and-Ridley work provided good agreement with observations at low levels of hydrostatic stress, while the Rice-and-Tracey approach was better at higher levels. In addition, it was established that the analysis could be applied over a wider range of processing and material parameters than that originally assumed by Pilling and Ridley and Rice and Tracey.