含表面梯度强化层的缺口样品疲劳起源寿命数值分析

以Tanaka和Mura的疲劳模型为基础,引入弹性应变能释放项,构建了新的适用于复杂载荷的疲劳模型.利用这一模型,结合表面梯度强化层的强度、模量和残余应力的梯度分布特征,对含表面梯度强化层的缺口样品的疲劳形核寿命分布及裂纹起源位置进行数值分析.分析结果表明:表面强化会增加样品的疲劳形核寿命,强化层厚度变化会改变裂纹形核位置.存在临界厚度,当强化层厚度小于临界厚度,裂纹形核于强化层与基体的界面;反之,形核于强化亚表层或表面.硬度比增加会导致临界厚度增加,过大的残余压应力会降低疲劳裂纹形核寿命.相同名义应力集中系数值(Kt)的样品在同一强化工艺处理后,其疲劳形核寿命和裂纹起源位置随样品缺口尺寸而改变.

Numerical simulation of fatigue initiation life for notched specimens with gradient surface layer

Based on Tanaka ＆ Mura＇s fatigue model and introduced the elastic strain energy release of the crack initiation, a new fatigue model is proposed for fatigue initiation life under complex loading. Then, with the gradient characteristics of the hardness and residual stress, this new model is used to assess the fatigue life distribution and the crack initiation site for the notched components with a gradient surface layer. The results of numerical simulation indicate that surface hardening treatment increases the fatigue life. And the gradient layer thickness changes the crack initiation site. If the gradient layer thickness is smaller than a critical value, fatigue crack will initiate at the interface of surface and matrix, otherwise at the surface or subsurface. The increase of hardness ratio will lead to an increase of the critical thickness value. Excessive residual compress stress reduces fatigue initiation life. For notched samples with the same stress concentration factor, after the same hardening treatment, the fatigue life and crack initiation site would change with the notch size of the sample.