Failure of Laser Welds and Formability of Bimaterial Thin Steel Sheets.

Abstract

In the first part of the thesis, failure modes and fatigue behavior of laser welds in lap-shear specimens of HSLA steel sheets are investigated based on experimental observations and finite element analyses. The optical micrographs of failed specimens show that laser welds failed in the base metal due to the necking/shear of the load carrying sheets towards weld face under quasi-static loading conditions and in the HAZ due to the kinked fatigue crack propagating through the load carrying sheets towards the weld root under cyclic loading conditions. Elastic-plastic finite element analyses are conducted to explain the ductile failure and effects of the sheet thickness on the failure modes under quasi-static loading conditions. A kinked fatigue crack growth model based on the computational global and local stress intensity factor solutions for finite kinked cracks and a structural stress model based on the closed-form structural stress solutions of the beam bending theory are adopted to estimate the fatigue lives of the laser welds. Approximate stress intensity factor solutions as functions of the normalized weld width are 2 proposed for the right pre-existing crack tip of the load carrying thinner sheet. The effect of weld gap is also investigated based on the computational stress intensity factor solutions and an analytical structural stress model. In the second part of the thesis, Niobium-clad and polymer-graphite coated stainless steel sheets were examined for their ductility and formability. The effects of annealing temperature and time on the mechanical behavior and failure mechanisms of Nb-clad 304L stainless steel sheets were investigated under uniaxial tensile, bending and flattening tests. Nano-indentation tests indicated that the intermetallic layer formed due to the annealing process exhibits much higher values of hardness and elastic modulus as compared to those of the stainless steel and Nb sheets. Uniaxial tensile, bend and flattening tests and ball punch deformation tests were carried out to determine the mechanical behavior and failure mechanisms of EB-815 coated 316L stainless steel sheets under different straining conditions. The experimental results obtained from these tests will be helpful in future application of these bimaterial sheets as bipolar plates in PEM fuel cells.