Narrow Results

The crack propagation properties for ultrafine-grained Zn-22wt%Al alloy during low cycle fatigue (LCF) in the superplastic region and the non-superplastic region were investigated and compared with the corresponding results for several other materials. With the Zn- 22wt%Al alloy, it was possible to conduct LCF tests even at high strain amplitudes of more than ±5%, and the alloy appeared to exhibit a longer LCF lifetime than the other materials examined. The fatigue life is higher in the superplastic region than in the non-superplastic region. The rate of fatigue crack propagation in the superplastic region is lower than that in the other materials in the high J-integral range. In addition, the formation of cavities and crack branching were observed around a crack tip in the supereplastic region. We therefore conclude that the formation of cavities and secondary cracks as a result of the relaxation of stress concentration around the crack tip results in a reduction in the rate of fatigue crack propagation and results in a longer fatigue lifetime.

This study aims to build a new bridge between configurational stress/force and material fracture. The migrating control volume and thermodynamics are used to develop the Eshelby relation, and the relationship between the conservative integral in fracture mechanics and configurational stress/force for elastic or elastic-plastic materials is further clarified. Additionally, the configurational stresses, including circumferential configurational stress at the crack tip taking T-stress into consideration, are determined, and the $J$ integral vector is then calculated further. The results indicate that $J_1$ integral is path-independent while $J_2$ is path-dependent when T-stress is considered. We preliminarily present the relationship between the configurational stress and crack initiation and the zero circumferential configurational stress fracture criterion (ZCCS) is proposed based on the local properties of the crack-tip configurational stress tensor and fracture mechanics. To estimate the fracture loads, we also develop two fracture criteria based on the critical area of crack-tip plastic zone determined by the Mises configurational stress (MCSPA) and the principal configurational stress difference (PCSDPA), respectively. It is found that the initiation angle assessed by the ZCCS fracture criterion is in good agreement with that by both the MTS fracture criterion and experimental observations, as well as T stresses could affect the initiation angles for mixed-mode cracks under tension-shear loads. Furthermore, the fracture loads evaluated by the MCSPA and PCSDPA fracture criteria are consistent with that by both the MTS fracture criterion and experimental results. Finally, the initiation angles determined based on the characteristics of crack-tip plastic zone by configurational stress coincide with that by MTS and ZCCS fracture criteria.

The crack propagation properties for ultrafine-grained Zn–22wt%Al alloy during low cycle fatigue (LCF) in the superplastic region and the non-superplastic region were investigated and compared with the corresponding results for several other materials. With the Zn–22wt%Al alloy, it was possible to conduct LCF tests even at high strain amplitudes of more than ±5%, and the alloy appeared to exhibit a longer LCF lifetime than the other materials examined. The fatigue life is higher in the superplastic region than in the non-superplastic region. The rate of fatigue crack propagation in the superplastic region is lower than that in the other materials in the high J-integral range. In addition, the formation of cavities and crack branching were observed around a crack tip in the supereplastic region. We therefore conclude that the formation of cavities and secondary cracks as a result of the relaxation of stress concentration around the crack tip results in a reduction in the rate of fatigue crack propagation and results in a longer fatigue lifetime.