Analysis of plastic instability during sandwich sheet rolling

Abstract

In this study, plastic instability occurring in the hard inner layer during symmetrical sandwich sheet rolling was investigated. Two criteria for diffusion necking and local necking under a tri-axial stress state were developed to predict the occurrence of plastic instability in the hard inner layer during sandwich sheet rolling. Stress states in the inner hard layer at the exit of the roll-gap, where plastic instability is most likely to occur, were employed to formulate the plastic instability criteria. The effects of various rolling conditions such as the thickness ratio of sheets, yield shear stress ratio of sheets, etc. upon the critical thickness reduction, under which plastic instability can be avoided, were discussed systematically. Furthermore, experiments on sandwich sheet rolling were also conducted by employing aluminum and mild steel as layers of sandwich sheets. It was found that the theoretical predictions of the critical reduction were in good agreement with the experimental measurements. Throughout the study, it is clear that the proposed plastic instability criteria are able to offer useful knowledge in designing the pass-schedule of sandwich sheet rolling processes.