Numerical failure analysis of steel sheets using a localization enhanced element and a stress based fracture criterion

Abstract

Strain localization and fracture initiation of elasto-plastic thin steel sheets is analyzed. A shell element enhanced by embedded discontinuities is developed to improve coarse-mesh accuracy in terms of fracture initiation prediction and to regularize the post-instability response. Discontinuities in the strain field are introduced when instability is signaled by a local bifurcation analysis. The enhancements are implemented for the Belytschko–Lin–Tsay shell element. This is combined with a stress based fracture criterion which relates the magnitude of the stress vector and the first invariant of the stress tensor. A robust experimental procedure based on full-field measurements enable direct calibration in stress space, and provides a flow curve up to large strains. Numerical examples involving tensile samples with different localization behavior are presented to demonstrate significant reduction of spurious mesh dependence. Moreover, the engineering feasibility of the direct use of a stress based fracture criterion in combination with the enhanced element is evaluated by comparison of measured and calculated fracture elongations.