The mechanism behind why internal defects are less competitive than surface roughness in low cycle fatigue (LCF) failure is still an issue for inclusion-containing powder metallurgy (PM) superalloys. Differentiating the differences in applied energy and fatigue resistance at various failure sites is crucial to addressing this issue. This study first captures the dependence of failure site on applied loading from the fractographic observations to quantify the characteristics such as surface roughness, internal defects, and sub-surface facets. Subsequently, an LCF lifetime model is developed based on fracture mechanics principles, considering the difference in applied energy and cracking energy requirements due to underconstraint degree at different sites. A representative volume element (RVE) with similar grain characteristic is then established, and different boundary conditions are applied to describe the energy differences around internal and surface. By comparing the energy at different failure sites, the model predicts the tendency of failure sites under varying loading conditions. The developed LCF lifetime model distinguishes energy input and fatigue resistance differences at surface, sub-surface, and interior of the specimen, which reduces the lifetime prediction error from a scatter band of 9 times to within 3 times.

中文翻译：

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考虑表面欠约束引起的裂纹能量差异的 PM 高温合金的新型 LCF 寿命模型


在低周疲劳 （LCF） 失效中，内部缺陷不如表面粗糙度具有竞争力的机制仍然是含夹杂物粉末冶金 （PM） 高温合金的一个问题。区分不同失效部位施加的能量和抗疲劳性的差异对于解决此问题至关重要。本研究首先从断口观察中捕获了失效部位对施加载荷的依赖性，以量化表面粗糙度、内部缺陷和次表面刻面等特征。随后，基于断裂力学原理，考虑了不同位点因欠约束程度而导致的施加能量和开裂能量需求量的差异，建立了 LCF 寿命模型。然后建立具有相似晶粒特性的代表性体积单元 （RVE），并应用不同的边界条件来描述内部和表面周围的能量差异。通过比较不同失效部位的能量，该模型预测了不同载荷条件下失效部位的趋势。开发的 LCF 寿命模型区分了试样表面、近表面和内部的能量输入和抗疲劳性差异，从而将寿命预测误差从 9 倍的散射带降低到 3 倍以内。