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

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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.