Prediction of Ductile Fracture Based on Mechanics of Plasticity. 1st Report, Intrinsic Criterion.

Abstract

In the framework of mechanics of plasticity plastic instability has been taken up as the onset of ductile fracture. Prediction of ductile fracture begins from the analysis of necking phenomena. Swift's diffuse necking and Hill's localized necking are very famous for assessing sheet formability. However the criteria proposed for necking and shear banding so far have not been useful so much for evaluating ductility. The concept of plastic instability is based on the possibility of multiplicity in deformation under an identical boundary condition. However, unloading cannot be accompanied at plastic instability. While one of us proposed the concept of load instability, which corresponds to the loss of load control where the load is imposed over the value what material can endures. The criterion of load instability was applied for assessing surface ductility in upsetting. It was proved that the criterion gave very good agreement with the experimental results. But, the criterion did not provide enough terms to determine whether ductile fracture occurs or not in the deformation process in question. This paper presents the annexed condition making up for insufficient aspect in the criterion. The condition examines the possibility of both plastic loading and unloading. The criterion supplemented has applicability for assessing ductility limit in almost deformation processes. The correlation between load instability and plastic instability is discussed.