Prediction of Strain Path Changing Effect on Forming Limits of AA 6111-T4 Based on a Shear Ductile Fracture Criterion

Silin Luo,Gang Yang,Yanshan Lou,Yongqian Xu

doi:10.3390/met11040546

Abstract

Strain path changing is a phenomenon in the stamping of complex panels or multiple-step stamping processes. In this study, the influence of the strain path changing effect was investigated and assessed for an aluminum alloy of 6111-T4 with a shear ductile fracture criterion. Plastic deformation of the alloy was modeled by an anisotropic Drucker yield function with the assumption of normal anisotropy. Then the shear ductile fracture criterion was calibrated by the fracture strains at uniaxial tension, plane strain tension and equibiaxial tension under proportional loading conditions. The calibrated fracture criterion was utilized to predict forming limit curves (FLCs) of the alloy stretched under bilinear strain paths. The analyzed bilinear strain paths included biaxial tension after uniaxial tension, plane strain tension and equibiaxial tension. The predicted FLCs of bilinear strain paths were compared with experimental results. The comparison showed that the shear ductile fracture criterion could reasonably describe the effect of strain path changing on FLCs, but its accuracy was poor for some bilinear paths, such as uniaxial tension followed by equibiaxial tension and equibiaxial tension followed by plane strain tension. Kinematic hardening is suggested to substitute the isotropic hardening assumption for better prediction of FLCs with strain path changing effect.

Highlights

The accurate prediction of failure during sheet metal forming is a big challenge in the numerical design of sheet metal forming processes
Reasons are discussed for the improper prediction of fracture-forming limit curve (FFLC) under some strain path pairs, such as uniaxial tension followed by equibiaxial tension and equibiaxial tension followed by plane strain tension
The results show that the fracture predicting error of equibiaxial tension followed by plane strain tension was very large, and approaches should be proposed to better this bilinear strain path

Summary

Introduction

The accurate prediction of failure during sheet metal forming is a big challenge in the numerical design of sheet metal forming processes. Cao et al [8] applied the MarciniakKuczynski model to investigate the effect of strain path changing on FLCs. Stoughton [9] showed that the forming limit for both proportional and nonproportional loading could be explained from a single criterion, which is based on the state of stress rather than the state of strain, thereby introducing the definition of forming a limit stress diagram. Since the beginning of the 20th century, ductile fracture has attracted increasing efforts for failure prediction in sheet metal forming as the wide employment of advanced high strength steels and aluminum alloys. It is because the lightweight metals fail mainly by a ductile fracture with little necking [13,14]. Reasons are discussed for the improper prediction of FFLCs under some strain path pairs, such as uniaxial tension followed by equibiaxial tension and equibiaxial tension followed by plane strain tension

Modeling of Plastic Deformation

Shear Ductile Fracture Criterion

Modeling of Strain Path Changing Effect on FFLC

Conclusions