Damage mechanics-based failure model for sheet metals under large plastic deformation

Abstract

This research aims to characterize the plastic behavior and ductile failure process of sheet metals commonly used in automotive and industrial applications. Emphasis is placed on the observed hardening behavior of the material at large plastic strain. This requires correction for triaxial stress condition and the damage-based prediction of ductile fracture of the material using Rice-Tracey void-growth model. These essential requirements are quantified and examined in the research for accurate simulation of sheet metal deformation and fracture. For this purpose, low carbon steel (LCS) and dual phase steel (DP600) sheets with a thickness of 2 mm were tension tested using various smooth and notched specimen geometries. The notched geometry introduces a different degree of the stress triaxiality to be quantified with respect to fracture of the specimen. The relationship between local ductility reduction and stress triaxiality is established using hybrid experimental-computational method employing tension test data and finite element (FE) simulations of the test. This relationship represents the damage initiation criterion for Rice-Tracey model. Stress triaxiality values in the range of 0.4 to 0.74 are found to be a strong function of the steel hardening characteristic. In addition, localized failure (necking) in low carbon steel is more pronounced compare to that of the high strength dual phase steel where only diffused necking is observed. The predictive capability of the Rice-Tracey damage model is demonstrated through simulation of the failure process of a spot welded lap joint under shear loading. Complementary test of identical spot welded joint specimen provided data for validation of the FE model. In the large strain region of the uniaxial stress-strain curve, the effect of local stress triaxiality on the fracture strain is eliminated through a novel large strain shear compression test procedure of the sheet metal. A shear compression jig assembly with a machined slot inclined at 35˚ to the transverse plane of the assembly is designed and fabricated. The design fulfills the requirement to establish a uniform distribution of plastic strain throughout the gage section of the Shear Compression Metal Sheet (SCMS) specimen. The global load and displacement data pairs are recorded throughout the test. FE simulation of the test set-up is performed to establish the corresponding internal states of stresses and strains in the gage section of the test specimen. This hybrid experimental-computational procedure, then establishes the true stress-plastic strain curve of the material directly from measured load-displacement readings of the test assembly. A plastic strain level of 49.2 % has successfully been established for 0.0627C steel sheet. The procedure has been validated for metallic materials with relative plastic modulus, Ep/E in the range of 5x10-4< Ep/E < 0.01. In addition, it is able to establish non-linear characteristic hardening of the material through piecewise linear consideration of the measured global load-displacement curve.