Size effect on the damage evolution of a modified GTN model under high/low stress triaxiality in meso-scaled plastic deformation

Abstract

Micro-forming is a micro-manufacturing method with broad application prospects, in which the size effect on damage development under different stress states is an allimportant problem, but has not been systematically explained. This study established a coupled model by the combination of mechanism-based strain gradient plasticity (MSG) and a shear modified GTN damage model. The proposed model is implemented in UMAT subroutine of ABAQUS and validated by experimental observation and numerical simulation of notched specimens with different shapes. Fracture photographs suggested that the failure mechanism of materials under high/low stress triaxiality is nucleation, growth and coalescence of voids and shear-induced slip respectively. The fracture displacement and loading force obtained by the numerical simulation considering the coupling model are basically consistent with the tensile test results. Furthermore, the size effect on two damage parameters is qualitatively described, that the MSG theory can promote the evolution of shear damage and inhibit the development of microvoids. In addition, the influence mechanism of the strain gradient is discussed in detail, and the relationship among the equivalent stress, stress triaxiality and damage parameters is revealed. The present work thus facilitates the in-depth understanding of the size effect on damage evolution and fracture formation in meso-scale of metallic materials.