Application of strain energy based approach for evaluation of fatigue crack growth retardation effect under random overload

Fatigue crack growth in structural components subjected to random amplitude loading is a significantly complex problem. Many models have been proposed to estimate the retardation effect due to a single overload, though for the random overload sequence, there is still a lack of effective quantitative description methods. In this research, a new time to crack initiation (TTCI) type for the calculation of equivalent initial flaw size (EIFS) is proposed based on the change in net-section strain energy density. For a certain number of fatigue cycles and current crack lengths, the value of EIFS can be obtained by solving the corresponding integral equation. The proposed EIFS calculation method was applied for the prediction of fatigue crack growth under single overload and random overload sequences. The prediction results agree well with the experimental data for different conditions. For a certain single overload or overload spectrum, the retardation effect is reflected in the decrease in EIFS, which appears to propagate from a smaller initial crack length. The proposed method shows that the new TTCI type of EIFS can be used as the quantitative characterization of the retardation effect due to the random overload sequence.