A New General Fatigue Limit Diagram and Its Application of Predicting Die Fatigue Life during Cold Forging

Abstract

Traditional fatigue fracture theory and practice focus principally on structural design. It is thus too conservative and inappropriate when used to predict the high-cycle fatigue life of dies used for metal forming, especially cold forging. We propose a novel mean stress correction model and diagram to predict the high-cycle fatigue lives of cold forging dies, which focuses on the upper part of the equivalent fatigue strength curve. Considering the features of die materials characterized by high yield strength and low ductility, a straight line is assumed for the tensile yield line. To the contrary, a general curve is used to represent the fatigue strength. They are interpolated, based on the distance ratio, when finding an appropriate equivalent fatigue strength curve at the mean stress and stress amplitude between the line and curve. The approach is applied to a well-defined literature example to verify its validity and shed light on the characteristics of die fatigue life. The approach is also applied to practical forging and useful qualitative results are obtained.

Effect of shearing on production stability and die life in automatic multi-stage cold forging of an automobile wheel nut

“…In another study [35] that investigated die mechanics and HCF fracture formation in the die insert, a similar test process was employed, excluding the rod shearing stage. Notably, an ideal axisymmetric material in the form of a cylindrical billet was assumed in the model [35].…”

“…The die inserts were assumed to be rigid when simulating the test forging process. The ow curve of the coil material [35] shown in Fig. 4 was utilized for Stages 3 and 4 of the forging process.…”

Ejector pin-related high-cycle fatigue fracture of the critical die corner during automatic multistage cold forging of an automotive wheel nut

“…This study focused on Stage 4 of the AMSCF process because it has the hottest issue of the die's HCF fracture at its CDC region [ 38 ]. Notably, the ejector pin at Stage 4 not only vacates the forged material but also experiences the most severe stress state while forming the bottom side of the material.…”

“…The punches and die inserts were assumed to be rigid when simulating the entire forging process. The flow curve of the coil material [ 38 ] shown in Fig. 4 was utilized for all the stages of the AMSCF process.…”