Concept of the highly strained volume for fatigue modeling of wrought magnesium alloys

Microstructural investigations and mechanical fatigue tests with in situ optical strain field measurement were performed on unnotched, notched, and bending AZ31B magnesium specimens. Twinned and twin-free regions are clearly delimited and compressive strain within twinned regions is much higher compared to the strain in twin-free regions. Furthermore, initial macroscopic cracks are observed to be always located within the twinned regions. Thus, only twinned regions are considered for fatigue modeling. To cover stress concentrations, stress gradients, different types of loading, and strongly inhomogeneous distribution of strain within bands of twinned grains all together, current fatigue lifetime description methods are insufficient. Hence, this work presents a newly developed method, referred to as the method of highly strained volume. The new method bases on the principals of the method of highly stressed volume, which is suitable for modeling the fatigue limit. This method uses the size of the highly strained volume and the effective strain amplitude in the highly strained volume to describe the number of cycles to failure. Based on results of 75 fatigue tests (strain amplitude: 0.30–1.17%, strain ratio: 1.47 to −0.431) the accurate description of the fatigue lifetime by the method of highly strained volume is demonstrated.