Effects of Strain Path Changes on Damage Evolution and Sheet Metal Formability

Abstract

The concept of the Forming Limit Diagram (FLD) has proved to be useful for representing conditions for the onset of sheet necking, and is now a standard tool for characterizing materials in terms of their overall forming behavior. In this study, the M-K approach, in conjunction with Gurson model, is used to calculate FLDs. The influences of mechanical properties, including strain hardening, strain rate sensitivity, as well as the void nucleation, growth and coalescence, on the FLDs are examined. Most sheet metals undergo multiple deformation modes (strain paths) when being formed into complex manufacturing parts. When the strain path is changed in the deformation processing of metal, it's work-hardening and flow strength differs from the monotonic deformation characteristics. As a consequence, sheet metal formability is very sensitive to strain path changes. In this study, the hardening behavior and damage evolution under non-proportional loading paths are investigated. The effect of strain path change on FLDs is studied in detail. FLDs are conventionally constructed in strain space and are very sensitive to strain path changes. Alternatively, many researchers represented formability based on the state of stress rather than the state of strain. They constructed a Forming Limit Stress Diagram (FLSD) by plotting the calculated principal stresses at necking. It was concluded that FLSDs were almost path-independent. In this work, the FLSD has been constructed under non-proportional loading conditions to assess its path dependency when damage effect is included.