The multiaxial stress state caused by geometric discontinuities will result in significant differences in the fatigue performance compared to the uniaxial stress state. It is crucial importance to research the notch fatigue behaviors of nickel-based single crystal (Ni-SX) superalloys at high temperature. This study investigated the impact of stress redistribution and stress triaxiality on the fatigue life of notched Ni-based single crystal superalloy at 760℃. The high temperature fatigue tests were carried out on the specimens under different stress ratios and the stress amplitudes under the same life conditions were obtained. A novel anisotropic damage model, incorporating the influence of stress triaxiality is proposed coupled with crystal plasticity theory. Based on experimental findings, an evaluation of the test parameters was conducted.The results showed that Mises stress, maximum principal stress, normal stress of principal stress along the loading direction, maximum principal strain, and strain along the loading direction were inadequate in reflecting the significant variations of samples observed among different stress ratios. However, the stress triaxiality at the unloading valley exhibited a high sensitivity to changes in fatigue life. To evaluate the fatigue life of notched specimens under different stress ratios, an improved Basquin’s type life prediction model was proposed, utilizing the stress triaxiality at the unloading valley. The proposed model demonstrates a favorable normalization trend for the fatigue life of notched specimens subjected to various stress concentration factor (SCF) conditions.

中文翻译：

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应力再分布和应力三轴度对缺口镍基单晶高温合金760℃疲劳寿命的影响

由几何不连续性引起的多轴应力状态与单轴应力状态相比将导致疲劳性能的显着差异。研究镍基单晶（Ni-SX）高温合金的高温缺口疲劳行为至关重要。研究了应力再分布和应力三轴度对缺口镍基单晶高温合金760℃疲劳寿命的影响。对不同应力比下的试件进行高温疲劳试验，得到相同寿命条件下的应力幅值。结合晶体塑性理论，提出了一种新颖的各向异性损伤模型，结合了应力三轴性的影响。根据试验结果，对试验参数进行了评价。结果表明，米塞斯应力、最大主应力、加载方向主应力正应力、最大主应变、加载方向应变不足以反映试验结果。不同应力比下观察到的样品存在显着差异。然而，卸载谷处的应力三轴度对疲劳寿命的变化表现出高度敏感性。为了评估不同应力比下缺口试样的疲劳寿命，利用卸载谷处的应力三轴性，提出了一种改进的Basquin型寿命预测模型。所提出的模型表明，在各种应力​​集中因子 (SCF) 条件下，缺口试样的疲劳寿命呈现良好的归一化趋势。