An approach to structural reliability evaluation under fatigue degradation and shocks

Fatigue is one of the most typical failure mechanisms, which can be regarded as a degradation process. The non-linearity in fatigue damage due to shocks cannot be disregarded. The mechanical shock is a transient physical excitation, in which the structure is nonequilibrium caused by a sudden applied load with high stress level or an abrupt change in the direction or magnitude of velocity. After the shock, the rate of fatigue damage accumulated decreases for a long time. When shock exists, the degradation could be generally retarded. The retardation phenomenon has been proven by the experimental observation. Therefore, a new approach is proposed to evaluate the structural reliability, in which the fatigue strength degradation process considering the retardation effect is explained by a crack-closure based fatigue model. The lifetime will be extended due to the applied shock loads. This fatigue crack growth model is validated by the experimental data of aluminum alloy D16. In addition, considering the material parameter and the shock loads are indeterminate, the reliability assessment under the fatigue loading with shocks is performed. A case study is given and two shock scenarios are discussed. In the first scenario, shocks occur in a fixed time period; while in the second scenario, shock occurs with varying time periods. The results indicate that the proposed approach can account for the retardation and evaluated the fatigue damage more appropriately.