Current models for predicting the fatigue endurance of notched solids use the stresses along a straight line, beginning at the notch root, as a simplification of the real crack propagation path. In this work, the experimental crack paths for hollow notched samples were analyzed through different microscopy techniques, with the objective of establishing high cycle fatigue crack growth directions in a mild steel. Fully reversed tension-compression fatigue tests $(R=-1)$ of thin-walled tube specimens with a passing-through hole were carried out. The crack paths observed in the outer cylindrical surface were studied in each case, with special attention to the crack initiation point and the crack direction along the first grains. Moreover, the analysis of the fracture surfaces allowed the same analysis to be performed to determine the internal crack paths. It was observed that the crack initiation point was close to the maximum principal stress point at the hole contour as obtained from linear elastic finite element analysis, and the crack direction in its initiation was generally close to Mode I direction, contrary to the conventionally accepted 45${}^{\circ }$ crack growth direction.

当前用于预测缺口实体疲劳耐久性的模型使用从缺口根部开始的直线应力作为实际裂纹扩展路径的简化。在这项工作中，通过不同的显微镜技术分析了空心缺口样品的实验裂纹路径，目的是确定低碳钢中的高周疲劳裂纹生长方向。完全反向拉压疲劳试验$(R=-1)$对带有通孔的薄壁管试样进行了研究。在每种情况下都研究了在外圆柱表面观察到的裂纹路径，特别注意裂纹起始点和沿第一晶粒的裂纹方向。此外，断裂表面的分析允许执行相同的分析以确定内部裂纹路径。观察到裂纹萌生点接近线弹性有限元分析得到的孔轮廓处的最大主应力点，裂纹萌生方向总体上接近模式 I 方向，与传统公认的 45${}^{\circ }$裂纹生长方向。