Influence of non-uniform stress states and strongly localized strain distribution on the fatigue behavior of wrought magnesium alloysWednesday (08.11.2017) 16:20 - 16:40 Part of:
The fatigue life time is a crucial design criterion for structural use of wrought magnesium alloy, especially for light-weight design. However, a fatigue model considering the effects of non-uniform stress and strain states for wrought magnesium alloys with consideration of twin formation is still lacking. To cover different stress concentrations, stress gradients, and geometrical size effects, notched specimens with four stress concentration factors, five unnotched specimens of different size, and two bending specimens with different stress gradients are used to investigate the fatigue behavior. Within the high cycle fatigue regime at 2·10^6 load cycles the endurable load is characterized by means of the concept of highly stressed volume. This yields a distinct linear dependency of endurable stresses on the highly stressed volume in logarithmic scale for all used specimens.
Using the concept of highly stressed volume within the low cycle fatigue regime fails because of the unknown elastoplastic stress state in wrought magnesium alloys. Due to the manufacturing process, wrought magnesium alloys exhibit a strong basal texture and, additionally, twinning as a major deformation mechanism within the low cycle fatigue regime is activated only under tension along or compression perpendicular to the c-axis of the lattice system. Furthermore, twinning is not randomly distributed over the tested material, but is rather grouped in bands of twinned grains. Under compressive loading the strain is strongly localized within these bands and is several times higher compared to the strain in adjacent areas. In order to avoid the difficult analysis of the unknown elastoplastic stress state, a newly developed concept of highly strained volume is introduced and applied. To measure the inhomogeneous local strain fields on the tested specimens, an in situ optical strain measurement technique is used.
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