DYNAMIC FAILURE CHARACTERIZATION OF LASER WELDS UNDER COMBINED LOADING CONDITIONS

摘要

This paper proposes a dynamic failure criterion for laser welds to describe the strain-rate dependent failure behavior of laser welds in the crash analysis of an auto-body. The failure criterion has been developed from experiments with nine different conditions of combined normal and shear loading including the normal loading and the pure shear loading. The experiments were carried out with a high speed material testing machine at intermediate strain rates ranging from quasi-static to 100 s~(-1). The dynamic failure tests were examined with four cases of the 25 mm stitch-type laser weld of CR340 1.2t, the 20 mm stitch-type laser weld of CR340 1.2t, the 25 mm stitch-type laser weld of SPCC 1.0t and the O-type laser weld with the 8 mm diameter of CR340 1.2t in order to evaluate the influence of the strain rate on the failure loads with respect to the base metal and the bead geometry. Based on the experimental data, a novel failure criterion was proposed for the description of the failure behavior of laser welds on the basis of the quasi-static failure criterion. The failure criteria are divided into two regions: the base metal failure criterion is expressed as a β-norm function; and the interfacial failure criterion is expressed as an exponential function. For the both failure criteria, the strain-rate sensitivity of the normal and the shear failure load are assigned as an exponential function of the logarithm of the strain rate. The change of the shear failure load with the increase of the strain rate is also considered to accurately construct the failure criteria. The failure mode and the onset of failure of the laser welds are determined when the maximum value reaches unity. The dynamic failure criterion proposed provides a fairly accurate description of the failure load obtained from the experiments under dynamic combined loading conditions.