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

Abstract

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.

Application of Finite Element Method to Analyze the Influences of Process Parameters on the Cut Surface in Fine Blanking Processes by Using Clearance-Dependent Critical Fracture Criteria

“…In the above criteria, σ max is the maximum principal stress, is the equivalent strain at fracture initiation, σ H is the hydrostatic pressure, σ eq is the equivalent von Mises stress, and α is the constant, the value of which depends on the material. In this study, an α value of 0.55 [15] was used.…”

“…Some authors assume that the critical fracture criterion (C) is a material constant for one ductile fracture model, and that the value can be determined by experiments, such as tensile or compression tests. However, it was found in [15] that the value of the critical fracture criterion (C) varies with the clearance between the punch and the die. In that work, the values of the critical fracture criteria were determined by the combination of experiments and numerical analyses.…”

“…The results indicated that the critical value depends on the clearance. The relationships between the critical ductile fracture value (C) and clearance for SPCC at a maximum punch diameter of 25 mm are described in Figure 1a [15].…”

“…The parameters that give a fully sheared surface of SPCC in the fine blanking process were selected by using a punch diameter of 10 mm and a blank diameter of 20 mm. The predictions of the behavior of the cut surfaces were performed using the same clearance of 0.15%t by using a larger blank diameter (100 mm) and punch diameters (10,15,20,30, 40 and 50 mm) to verify whether the optimized parameters and clearance-dependent ductile fracture criteria can be used for other conditions, such as larger blank diameters and larger punch diameters. The initial element length of 0.08 mm and the element length after remeshing, 0.01 mm, are the same as the former case of a blank diameter of 20 mm; the total number of elements is 31,250.…”

“…Hence, experiments with long durations are required to obtain the critical damage value, and experimental costs are high. In order to reduce these conditions, the clearance-dependent critical ductile fracture criterion (C) determined by Cockcroft-Latham and the Oyane fracture criterion obtained from a previous publication [15] were used in this study. Hence, the costs and time required for the experiments to obtain the value of the critical ductile fracture criterion can be reduced or eliminated.…”