The accurate determination of the cyclic R-curve of the threshold of stress intensity factor range is a demanding experimental task. The main challenge concerns the introduction of a pre-crack that is open under tension loading and free of residual stresses. When pre-cracks are generated by cyclic compression-compression loading, a zone of tensile residual stresses up to the material yield strength is generated ahead of the pre-crack tip. These stresses may affect crack propagation at the beginning of fatigue crack growth and should consequently be removed especially for high-strength materials. However, subsequent stress-relief annealing is impractical for numerous metallic material classes due to undesired microstructural alterations at the temperatures required for stress relief. In the present contribution, the above-described issue is addressed for a high speed steel using single-edge notched bending specimens pre-cracked at various stages of a typical high speed steel heat treatment process. It is shown that stress-relieving via a heat treatment route is a significant improvement compared to the conventional compression pre-cracking procedures to measure material-specific cyclic R-curves.

中文翻译：

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通过退火降低残余裂纹尖端应力来确定循环 R 曲线


准确确定应力强度因子范围阈值的循环R曲线是一项艰巨的实验任务。主要挑战涉及引入在拉伸载荷下打开且无残余应力的预裂纹。当循环压缩-压缩载荷产生预裂纹时，在预裂纹尖端之前会产生高达材料屈服强度的残余拉应力区域。这些应力可能会在疲劳裂纹扩展开始时影响裂纹扩展，因此应将其消除，特别是对于高强度材料。然而，由于在应力消除所需的温度下会发生不希望的微观结构变化，因此随后的应力消除退火对于许多金属材料类别来说是不切实际的。在本贡献中，使用在典型高速钢热处理工艺的各个阶段预裂的单边缺口弯曲样本来解决高速钢的上述问题。结果表明，与测量材料特定循环 R 曲线的传统压缩预裂纹程序相比，通过热处理途径消除应力是一项重大改进。