Fracture loci of DP980 steel sheet for auto-body at intermediate strain rates

Abstract

This paper introduces the effect of the strain rate on the ductile fracture of DP980 1.2t steel sheet. Tensile tests of DP980 sheet are performed at a range of strain rates from 0.001 s(-1) to 100 s(-1) with three different shapes of specimens: the diagonally notched specimen for the in-plane shear test; the dog bone specimen for the uniaxial tension test; and grooved specimen for the plane strain tension test. To trace the strain on the surface of the specimen, 2-D digital image correlation (DIC) technique is adopted to encompass the remarkable transition of the fracture characteristics including the fracture strain, loading path and strain rate sensitivity. The negative stain rate sensitivity is observed on the equivalent strain to fracture which corresponds to the experimental results at low strain rate ranging from 0.001 s(-1) to 0.1 s(-1). The transition of thermal condition from isothermal to adiabatic comes into play to increase the equivalent strain to fracture at the intermediate strain rate from 0.01 s(-1) to 1 s(-1). The fracture loci incorporating with the Lou-Huh ductile fracture criterion are developed to identify the strain rate effect in the wide regime of triaxiality.