Analysis of Fatigue Crack Initiation and Propagation in Cold Forging Tools by Local Approach of Fracture

Abstract

The local approach of fracture based on continuum damage mechanics (CDM) combined with the finite element method (FEM) was applied to establish a more precise method of estimation of service life of cold forging tools by simulating the initiation and growth of a fatigue crack. After a brief explanation on the local approach of fracture based on CDM combined with FEM, modeling of elastic-plastic-damage behavior of tool steel was conducted in the framework of thermodynamics. In the modeling, the salient unilateral property of the tool material is considered by introducing a modified stress tensor. The proposed constitutive equations can be found to describe the experimental behaviors under uniaxial tension and compression properly. Then, the initiation and propagation of a fatigue crack in the cold forward extrusion die was analyzed by the local approach of fracture. The initiation of the fatigue crack in the vicinity of the die radius was found to occur when the number of extrusion increased. Then, the crack propagated along the direction perpendicular to the surface of the die radius. Furthermore, the proposed approach was found to estimate the actual behavior of the fatigue crack growth in good agreement with the calculated change of the rate of crack propagation to the number of extrusion. The calculated crack propagation rate also tended to decrease due to a decrease in tensile principal stress at the crack tip as the crack propagates.