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

本课题采用连续损伤力学方法研究多轴低周疲劳裂纹的成核和扩展，以评估材料的寿命，而断裂力学仅对裂纹扩展起重要作用。假设裂纹起源于金属表面，由于滑移带的积累取决于应力的多轴性，和滑移带的方向进行了理论评估，然后通过实验验证。在损伤力学的框架下分析了晶界裂纹的方向性，并在此基础上进行了疲劳寿命评估。主要研究结果如下：1.定义了多轴应力作用下单晶体中的Schmid张量，并利用该张量解析地给出了滑移带的取向.将该理论应用于多晶金属，得到了关于应力轴的滑移带强度参数，并通过纯铜的实验进行了验证.提出了晶界裂纹形核的判据，即裂纹形核是由晶界上的应力应变范围所定义的一个参数决定的，并得到了实验的证实.应用该理论还提出了稳态和变多轴应力下的寿命估算方法。结果表明，该方法比基于塑性应变范围的普通方法能更好地计算出失效寿命。

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.