Estimation of Low Cycle FAtigue Life under Multiaxial Stresses based on the Analysis of Nucleation and Propagation of Cracks

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Continuum damage mechanics approach is applied in this project to the nucleation and propagation of cracks during multiaxial low cycle fatigue in order to evaluate material life, while the fracture mechanics has played an important role on only the crack propagation. The cracks are assumed to initiate on the surface of metals due to the accumulation of slip bands depending on the stress multiaxiality, and the orientation of the slip bands was theoretically evaluated followed by the experimental verification. The orientation of grain boundary cracking was also analyzed in the framwork of damage mechanics, and the evaluation of fatigue life based on such an analysis was carried out. Results obtained are as follows:1. Schmid tensor in a single crystal subjected to multiaxial stresses was defined, and the orientation of slip bands was analytically given by the tensor.2. By applying the theory to polycrystalline metal, a parameter termed intensity of slip bands was formulated with respect to stress axis, which was verified by the experiments for pure copper.3. Criterion for the nucleation of grain boundary cracking was proposed under the assumption that the crack nucleates due to a parameter defined by the stress and strain range on the grain boundary, which was also confirmed by experiments.4. Life estimation method under steady and varying multiaxial stresses is also proposed by applying the above theory. This method revealed to give better results for failure lives than ordinary method based on plastic strain range.