Abstract In high cycle fatigue, damage and plasticity occur at the microscale, a scale smaller than the representative volume element (mesoscale). Therefore, the two-scale damage model is used for fatigue life assessment. In this model, a spherical inclusion is assumed to be fully embedded in the matrix. However, experimental observations show that the microplasticity and microcracks nucleate more easily in grains on the free surface rather than in the bulk. Hence, the aim of this work is to extend the two-scale damage model to take into account the nucleation of fatigue cracks on free surfaces. In order to do this, a surface scale transition law is derived to link the micro and mesoscales, considering a hemispherical damaged inclusion at the surface of an elasto-plastic matrix. Furthermore, a numerical scheme is proposed by developing an in-house Python code in conjunction with Abaqus software to compute the damage. The material parameters required in model are identified using monotonic and fatigue test results of smooth specimens. As a validation, for the butt-welded specimens subjected to fatigue loading, the obtained results of the proposed surface law are compared with the results of the pre-existing non-surface laws and the experimental fatigue data. It is confirmed that the use of surface localization law leads to a noticeable reduction in the number of cycles to crack initiation of about 25%.

中文翻译：

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结合表面夹杂物疲劳裂纹形核的两尺度损伤模型的开发

摘要 在高周疲劳中，损伤和塑性发生在微观尺度上，该尺度小于代表性体积元素（中尺度）。因此，采用二尺度损伤模型进行疲劳寿命评估。在该模型中，假定球形夹杂物完全嵌入基体中。然而，实验观察表明，微塑性和微裂纹更容易在自由表面的晶粒中而不是在本体中成核。因此，这项工作的目的是扩展两尺度损伤模型，以考虑自由表面疲劳裂纹的形核。为了做到这一点，考虑到弹塑性基体表面的半球形损坏夹杂物，推导出了表面尺度过渡规律来连接微观和中尺度。此外，通过结合 Abaqus 软件开发内部 Python 代码来计算损伤，提出了一种数值方案。模型中所需的材料参数使用光滑试样的单调和疲劳试验结果确定。作为验证，对于承受疲劳载荷的对接焊接试样，将所提出的表面定律的所得结果与预先存在的非表面定律的结果和实验疲劳数据进行比较。经证实，表面定位定律的使用导致裂纹萌生周期数显着减少约 25%。对于承受疲劳载荷的对接焊试样，将所提出的表面定律的所得结果与预先存在的非表面定律的结果和实验疲劳数据进行比较。经证实，表面定位定律的使用导致裂纹萌生周期数显着减少约 25%。对于承受疲劳载荷的对接焊试样，将所提出的表面定律的所得结果与预先存在的非表面定律的结果和实验疲劳数据进行比较。经证实，表面定位定律的使用导致裂纹萌生周期数显着减少约 25%。