Zhang Wei, Liu Huili, Wang Qiang, He Jingjing
School of Reliability and Systems Engineering, Beihang University, Beijing 100191, China.
Materials (Basel). 2017 Jun 22;10(7):689. doi: 10.3390/ma10070689.
In this paper, a strain-intensity-factor-based method is proposed to calculate the fatigue crack growth under the fully reversed loading condition. A theoretical analysis is conducted in detail to demonstrate that the strain intensity factor is likely to be a better driving parameter correlated with the fatigue crack growth rate than the stress intensity factor (SIF), especially for some metallic materials (such as 316 austenitic stainless steel) in the low cycle fatigue region with negative stress ratios R (typically R = -1). For fully reversed cyclic loading, the constitutive relation between stress and strain should follow the cyclic stress-strain curve rather than the monotonic one (it is a nonlinear function even within the elastic region). Based on that, a transformation algorithm between the SIF and the strain intensity factor is developed, and the fatigue crack growth rate testing data of 316 austenitic stainless steel and AZ31 magnesium alloy are employed to validate the proposed model. It is clearly observed that the scatter band width of crack growth rate vs. strain intensity factor is narrower than that vs. the SIF for different load ranges (which indicates that the strain intensity factor is a better parameter than the stress intensity factor under the fully reversed load condition). It is also shown that the crack growth rate is not uniquely determined by the SIF range even under the same R, but is also influenced by the maximum loading. Additionally, the fatigue life data (strain-life curve) of smooth cylindrical specimens are also used for further comparison, where a modified Paris equation and the equivalent initial flaw size (EIFS) are involved. The results of the proposed method have a better agreement with the experimental data compared to the stress intensity factor based method. Overall, the strain intensity factor method shows a fairly good ability in calculating the fatigue crack propagation, especially for the fully reversed cyclic loading condition.
本文提出了一种基于应变强度因子的方法来计算完全反向加载条件下的疲劳裂纹扩展。进行了详细的理论分析，以证明应变强度因子可能是比应力强度因子（SIF）更好的与疲劳裂纹扩展速率相关的驱动参数，特别是对于一些在低周疲劳区域具有负应力比R（通常R = -1）的金属材料（如316奥氏体不锈钢）。对于完全反向循环加载，应力与应变之间的本构关系应遵循循环应力 - 应变曲线而非单调曲线（即使在弹性区域内它也是非线性函数）。基于此，开发了SIF与应变强度因子之间的转换算法，并采用316奥氏体不锈钢和AZ31镁合金的疲劳裂纹扩展速率测试数据来验证所提出的模型。可以清楚地观察到，在不同载荷范围内，裂纹扩展速率对应变强度因子的散点带宽比对应力强度因子的散点带宽更窄（这表明在完全反向加载条件下，应变强度因子是比应力强度因子更好的参数）。还表明，即使在相同的R下，裂纹扩展速率也不是唯一由SIF范围决定的，还受到最大载荷的影响。此外，光滑圆柱试样的疲劳寿命数据（应变 - 寿命曲线）也用于进一步比较，其中涉及修正的Paris方程和等效初始缺陷尺寸（EIFS）。与基于应力强度因子的方法相比，所提出方法的结果与实验数据具有更好的一致性。总体而言，应变强度因子方法在计算疲劳裂纹扩展方面表现出相当好的能力，特别是对于完全反向循环加载条件。
