Fatigue behavior of dissimilar ultrasonic welds between magnesium alloy AZ31 and galvanized steel sheets is investigated experimentally, with stress intensity factor solutions and with a kinked crack growth life estimation model. First, stress intensity factor solutions for welds with various widths in lap-shear specimens with and without kinked cracks are obtained using finite element analyses. The analytical stress intensity factor solutions for lap-shear specimens based on the beam bending theory and the analytical solutions for two dissimilar semi-infinite solids with connection under plane strain conditions are reviewed. Results of the finite element analyses for global stress intensity factor solutions are compared with the analytical stress intensity factor solutions to identify transition weld widths for the analytical solutions.
Finite element analyses were also conducted for the ultrasonic welded lap-shear specimens of magnesium and steel sheets, but with a modified fictitious Poisson’s ratio for the magnesium sheet such that the bimaterial constant is equal to zero. The results indicate that the crack-tip stresses directly ahead of the main crack tip are influenced by the oscillation of the interface crack-tip field. However, the crack-tip stresses directly above the main crack tips are weakly affected, and the oscillatory crack-tip stress distributions for both actual and modified material combinations are quite similar. The results suggest that the stress intensity factor solutions for a kinked crack with a vanishing kink length can be approximated by the available analytical solutions for fatigue life estimation.
Experimental fatigue results for dissimilar ultrasonic welds are presented for lap-shear specimens which have been machined into a dog-bone profile to approximate linear welds and specimens which have not been altered. Optical micrographs of the welds after testing were examined to understand the failure modes. Stress intensity factor solutions were obtained from finite element analyses of a plane strain model and a three-dimensional model. The global stress intensity factor solutions and the local stress intensity factor solutions for vanishing and finite kinked cracks were used for fatigue life estimations using a Paris law kinked crack growth model.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99983/1/terjean_1.pd
