基于小裂纹理论的多轴疲劳荷载下金属裂纹扩展行为分析

This project is aimed to study the multiaxial fatigue loading crack growth problem by employing the short crack theories, accumulate the experimental data, investigate the short crack growth law, quantify the influences of material properties, loading types and other factors on the short crack growth with the help of short crack threshold, and analyze the non-propagating short crack forming conditions as well. It consists of two main parts. The first one is the experimental measurements of the short crack growth behaviors under multiaxial loading, which include the measurement of the growth direction and rate of the short crack on the thin-walled tubes made of different materials under constant multiaxial proportional and non-proportional loading and the measurement of the length of the non-propagating short crack on the thin-walled tubes made of different materials under constant multiaxial proportional and non-proportional loading. The second one is the study of the fundamental theory problems about the short crack growth behaviors under multiaxial loading. In this part, we will reveal the short crack growth law under constant multiaxial proportional and non-proportional loading based on the experimental data, predict the short crack growth direction with the help of numerical method, build the analytic or semi-analytic expressions of the short crack growth thanks to the short crack threshold, quantify the influences of sizes, materials, and other parameters on the short crack growth, analyze the forming conditions and length of the non-propagating short cracks and correct the theoretical results according to the experimental data.

本项目旨在借助小裂纹理论研究多轴疲劳荷载下裂纹扩展问题，积累试验数据，并在此基础上结合小裂纹门槛值,探讨多轴荷载下小裂纹扩展规律，量化材料性质、荷载方式等因素对小裂纹扩展的影响，分析不扩展小裂纹形成条件。主要包括两部分内容：（1）多轴荷载下小裂纹扩展行为的试验测量。测量不同材料薄壁圆管表面小裂纹在等幅多轴比例、非比例荷载下的扩展方向及速率；测量不同材料薄壁圆管表面在等幅多轴比例、非比例荷载下不扩展小裂纹长度。（2）多轴荷载下小裂纹扩展行为基本理论问题研究。根据试验结果揭示小裂纹在等幅多轴比例、非比例荷载下的扩展规律；借助数值方法预测小裂纹扩展方向；结合小裂纹门槛值研究多轴疲劳荷载下小裂纹扩展的解析或半解析表达式；量化尺寸、材料等参数对小裂纹扩展的影响；分析不扩展小裂纹形成条件及长度大小；根据试验数据对理论结果进行修正。

本项目借助小裂纹理论研究了多轴疲劳荷载下裂纹萌生扩展规律，积累了试验数据，并在此基础上结合小裂纹门槛值,探讨了多轴荷载下材料的本构关系，小裂纹扩展规律，量化了材料性质、荷载方式等因素对材料本构关系的影响，分析了不扩展小裂纹形成条件,推演出了一种针对多轴疲劳数据的数据前处理方法。主要包括四部分内容：（1）针对非比例加载中最重要的非比例度问题，结合转动惯量方法与塑性增量法，得到了反映加载路径形状的非比例度计算方法，并数值化了转动惯量法计算非比例度过程,结合极转动惯量法把此方法推广到一般情况。分析了等效应力强度因子，应力集中系数等不同参数对小裂纹扩展速率的影响。（2）根据推导出的小裂纹扩展解析方法，量化了门槛值、疲劳极限等材料参数对小裂纹扩展速率及不扩展小裂纹长度的影响，并进一步把不扩展小裂纹研究扩展到3维情况。（3）推演出了适用于非恒定荷载下的多轴疲劳本构关系计算流程。引入非比例度对背应力演化的影响，提高了对于多轴荷载下材料本构关系的预测精度. 较好的描述了不同形状、不同幅值的非比例应变路径下的应力响应，同时此方法对非对称加载下的棘轮效应或应力松弛现象均有较好的预测效果。该模型较好地反映出在多轴非比例加载下材料响应对于路径的依赖性。（4）提高了随机荷载数据前处理能力，可提前过滤采集过程中的噪音及对材料不产生损伤的小幅值荷载数据，大大提高了数据处理能力。与偏应力空间结合进一步提高了运算能力。与力学参数结合得到了可对路径追踪并自定义过滤半径的多轴过滤方法，并实现了其数值化。通过实验方法验证了上述相关计算结果的有效性，部分研究结果已经发表。