Acoustic emission study of fatigue crack propagation in extruded AZ31 magnesium alloy

The fatigue crack propagation behavior and corresponding AE characteristics of extruded AZ31 magnesium (Mg) alloy were investigated in this study. The effects of specimen orientation and loading frequency were considered. By combining the AE parameter and waveform analysis with the micro-structural and fractograph observations, the findings of the study showed that crack extension and twinning at the crack tip were two major AE source mechanisms during fatigue crack propagation in the Mg alloy. More twinning events were observed in the transverse direction (TD) specimens than in the extruded direction specimens, which contributed to more cyclic deformation irreversibility and cumulative fatigue damage, leading to worse fatigue performance and higher AE counts in the TD specimens. The results also indicated that increasing the loading frequency could slightly increase the fatigue life, and significantly decrease the AE counts in TD specimens due to the frequency or strain rate dependence of twinning activity. These results suggest that twinning plays an important role in the fatigue process in this Mg alloy, and that the AE technique is capable of detecting crack propagation and twinning events during fatigue.