Cyclic-Tension Fatigue Behavior in a Rolled AZ31B Magnesium Alloy Studied Using Ultrasonic Shear Waves

Nondestructive evaluation of cyclic-tension fatigue in a rolled magnesium alloy, Mg-3Al-1Zn, was performed using vertically polarized shear wave (SV) reflection and shear horizontal wave (SH) transmission methods. Internal friction measured by SV reflection increased rapidly in the early stages of the fatigue and finally saturated, showing dominating interactions of movable dislocations and twinning boundaries with the waves as acoustic nonlinearities. The propagation time and logarithmic damping ratio in the SH transmission method followed a repeated increase and subsequent sudden decrease pattern, and finally converged toward fatigue failure due to acoustoelasticity, which represents the interaction with residual stresses. The wave and phase data were determined using an optical microscope, a scanning electron microscope, a surface roughness tester, and X-ray diffraction. The results demonstrated that during the fatigue process, residual stress accumulated on the compressive side of the specimen, despite the applied cyclic-tension loading. Brittle cracks that originated in inclusions provided sudden relief from the residual stress.