CRITERION OF PLASTIC INSTABILITY AND ITS APPLICATION FOR PREDICTING DUCTILITY IN METAL-FORMING

General criteria for ductility based on plastic instability are presen ted, and unreasonable treatments in the criteria proposed by Swift [1] and by Storen and Rice [2] are pointed out. It is shown that the corr ection of these imperfections makes their criterion consistent with ge neral criteria proposed here. Then, the applications of the criteria t o the prediction of cracking in several metal formings are demonstrate d: the formability under linear strain paths in sheet metal forming, t he free surface ductility and cracking mode in upsetting, and the cent ral bursting (chevron crack) in drawing and extrusion. The analytical prediction in a wide range of processes are found to be in very good a greement with experimental results.