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

本项目采用连续损伤力学方法研究多轴低周疲劳过程中裂纹的形核和扩展，以评估材料的寿命，而断裂力学仅在裂纹扩展过程中起重要作用。根据应力的多轴性，假设裂纹是由于滑移带的积累而在金属表面萌生的，并从理论上对滑移带的取向进行了评估，然后进行了实验验证。在损伤力学的框架下，分析了晶界裂纹的取向，并在此基础上进行了疲劳寿命评估。得到的结果如下：1.定义了单晶在多轴应力作用下的施密德张量，并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，提出了一个关于应力轴的滑移带强度参数，并通过对纯铜的实验进行了验证。提出了晶界裂纹形核的判据，假设裂纹形核是由晶界应力应变范围定义的参数决定的，实验也证实了这一点。应用上述理论，提出了恒定和变多轴应力作用下的寿命估算方法。结果表明，该方法比一般的基于塑性应变范围的失效寿命预测方法具有更好的预测效果。

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.