Determining fatigue crack growth parameters under mixed-mode loading is crucial for damage tolerance design, particularly in assessing a component's residual life with defects to prevent disasters. This study introduces a novel methodology for determining fatigue crack growth rate. A digital microscope is utilized to track the fatigue crack tip with respect to local coordinate systems established on the specimen surface. A grid is drawn and subdivided into units with proposed nomenclature. Compact Tension and Shear (CTS) specimens, extracted from AA-7085 plates, undergo fatigue loading in a servo-hydraulic universal testing machine. A fixture proposed by Richard et al. induces planar mixed-mode loading cases in the specimen. During experiments, the digital microscope tracks the crack tip using grid unit corners as local origins. The local coordinates of the crack tip are transformed into the crack tip coordinates with respect to the pre-crack tip. Crack evolution data are used to determine the fatigue crack growth rate by analyzing tangents of a three-parameter fitted curve. The equivalent SIF ranges are computed at different crack lengths in an XFEM script written in Ansys. The modified Paris parameters are found and experimentally observed crack deflection angles are compared with the Maximum Tangential stress, Strain energy density, and Richard’s criterion.

中文翻译：

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平面混合模式疲劳裂纹扩展研究的创新方法

确定混合模式载荷下的疲劳裂纹扩展参数对于损伤容限设计至关重要，特别是在评估有缺陷的部件的剩余寿命以预防灾难时。本研究介绍了一种确定疲劳裂纹扩展速率的新方法。利用数字显微镜相对于样本表面上建立的局部坐标系来跟踪疲劳裂纹尖端。绘制网格并根据建议的命名法将其细分为单元。从 AA-7085 板材中提取的紧凑型拉伸和剪切 (CTS) 样本在伺服液压万能试验机中承受疲劳载荷。理查德等人提出的固定装置。在样本中引起平面混合模式载荷情况。在实验过程中，数字显微镜使用网格单元角作为局部原点来跟踪裂纹尖端。将裂纹尖端的局部坐标变换为相对于预裂纹尖端的裂纹尖端坐标。裂纹演化数据用于通过分析三参数拟合曲线的切线来确定疲劳裂纹扩展速率。在 Ansys 中编写的 XFEM 脚本中计算不同裂纹长度下的等效 SIF 范围。找到修改后的巴黎参数，并将实验观察到的裂纹偏转角与最大切向应力、应变能密度和理查德准则进行比较。