Modeling fatigue crack growth behavior in rolled AZ31 magnesium alloy using CTOD based strip yield modeling

Fatigue crack growth behavior of a rolled AZ31 magnesium plate is investigated and modelled in this study. Fatigue crack growth tests were performed on compact tension specimens at load ratios of R = 0.1 and R = 0.7 to provide data for a strip-yield based fatigue crack growth model. Minimal differences in crack closure were observed between the two load ratios. Threshold values for the stress intensity factor range were found to be often less than those reported in previous literature. The reason for lower threshold values could be related to the nontraditional compression pre-cracking method used in this study, which was employed in an attempt to produce a more accurate measurement of the fatigue crack threshold. In addition to the long crack fatigue crack growth testing using compact tension specimens, load controlled fatigue tests were conducted on flat, reduced gage specimens at load ratios of R = 0.1 and R = −1.0 to study the microstructurally small crack growth behavior and to extend the model capability to predict the microstructurally small crack growth behavior. The reduced gage specimens were found to spend the majority of their life in the crack growth stage. Cracks primarily grew in a planar fashion other than where multiple cracks coalesced. The microstructurally small crack growth data from these experiments were also compared with predictions from the fatigue crack growth model. Crack growth modeling revealed that traditional calculations using plasticity-induced crack closure concepts and an effective stress intensity factor range were not able to predict the microstructurally small crack growth behavior of these specimens. In contrast, computing crack growth rate from crack tip opening displacement was shown to give satisfactory results. Crack opening stresses for the fully reversed tests revealed that the compressive loading largely nullified the effect of the plastic wake on fatigue crack growth.