Research on Simulating Method for Fatigue Crack Growth Behavior under Random Loading Conditions.

提出了一种有效的简化计算方法，该方法可以模拟恒定和随机载荷下的疲劳裂纹扩展行为。该方法可以分析裂纹张开位移、疲劳裂纹闭合机理、裂纹张开点、有效应力幅比和疲劳裂纹扩展速率图。该方法主要由两部分组成。一个是裂纹附近位移的计算，另一个是基于线性累积损伤规律的每个加载循环的疲劳损伤的计算。该方法还需要材料常数，如杨氏模量、断裂韧性、平均晶粒尺寸、循环应力-应变曲线下的屈服应力、应力集中场的名义应力-局部应变关系以及Manson-Coffin方程的指数等，而目前研究人员还提出了一种绘制裂纹萌生和扩展寿命疲劳强度图的新方法。本研究利用上述两种方法，获得了A5083 P-O铝合金在随机载荷下的疲劳裂纹扩展速率图，并与实验结果进行了比较。此外，裂纹张开点和门槛应力强度因子进行了检查。在载荷控制的轴向脉动拉伸下进行试验。应力水平的频数分布呈指数分布.计算结果与实验结果进行了比较。在等幅加载试验下，裂纹张开点和裂纹扩展速率图具有很好的一致性。而在随机载荷试验下，定性地发现了良好的一致性，但不是定量的。

An efficient and simplified calculating method was proposed, which simulated the fatigue crack growth behavior under both constant and random loading conditions, following each loading cycle. This method can analyze crack opening displacement, fatigue crack closure mechanism, crack opening point, effective stress range ratio and fatigue crack growth rate diagram. The method consists of mainly two parts. One is the calculation of displacement near the crack, and the other is the calculation of fatigue damage of each loading cycle based on the liner cumulative damage law. Also the method requires material constants, such as Young's modulus, fracture ductility, average grain size, yield stress under cycle stress-strain curve, nominal stress - local strain relation at the stress concentration field and the exponent at Manson-Coffin equation.While a new method of developing fatigue strength diagrams for crack initiation and growth life is also proposed by present investigators. This method dombines random loading fatigue tests plus an equation, which has been shown to be useful in estimating fatigue strength under random loading condition.In this research, using the two proposed method mentioned above, fatigue crack growth rate diagram of aluminum alloy A5083P-O under random loading with an exponential distribution were obtained. In addition, a crack opening point and a threshold stress intensity factor were examined. Test were performed under load controlled axial pulsating tension. The frequency distribution of stress levels in exponential in distribution. Results obtained by the calculation method were compared with those obtained by the experiments. Good agreement was found in both crack opening point and crack growth rate diagram under constant amplitude loading tests. While, under random loading tests, good agreement was found qualitatively, but not quantitatively.