Ductile damage model based on void growth analysis by unit cell

Abstract

The aim of this study is to propose damage model on the basis of the mechanism for ductile fracture related to void growth and applicable to ductile fracture assessment for steels with small initial void volume fraction. In order to determine damage evolution law, void growth behavior in the material was investigated by elasto-plastic finite element analyses using unit cell model with an initial void (f0=0.1%, 0.01% and 0.001%). From the results of unit cell analyses, it was evident that a void in unit cell grew nonlinearly with increasing applied global strain and void growth rate was promoted under higher stress triaxiality condition. In addition, the sudden drop in the stress level coincided with the sudden increase in void volume fraction. Then, in this study, this point was identified as the onset of void coalescence that is ductile crack initiation. It was found that relationships between normalized void volume fraction and normalized strain by each value at the critical point were almost the same and independent of stress-strain relationships of materials and stress triaxiality conditions. Based on this characteristic associated with void growth, damage evolution law showing nonlinear damage accumulation was derived. Then, using the damage evolution law, ductile damage model consisting of critical strain of ductile crack initiation and equivalent plastic strain increment was proposed.