Determination of the Values of Critical Ductile Fracture Criteria to Predict Fracture Initiation in Punching Processes

Punching processes are widely used for producing automobile parts, mechanical components, and other parts. To produce highly accurate parts, it is important to estimate the ratio of the sheared surface to the cut surface. Many researchers have applied the finite-element method (FEM) to analyze the ratio of the sheared surface to the fracture surface on cut surfaces by using ductile fracture criteria. However, it is difficult to determine the fracture criteria on the cut surface by tensile tests or bending tests because the punching process involves many complicated steps. In this study, FEM was applied to the punching process to determine the values of critical fracture criteria (C) by using the ductile fracture criteria proposed by Cockcroft and Latham, Oyane, and Ayada. The ductile fracture criteria were compared with the boundary between the shear surface and the fracture surfaces using experiments performed with a simple punching system. The values of the ductile fracture criteria for the fracture initiation of the formed cut surface were predicted under various clearances between the punch and the die with various punch diameters. The influence of stress triaxiality and the effect of punch diameter on the sheared surface length are also discussed.