The Overload Retardation Effect in Fatigue Crack Propagation of Worm-Like Graphite Cast Iron

In this paper, single and multiple overloading tests under cyclic controlled stress were carried out on specimens of Worm-like Graphite Cast Iron (WGCI), a brittle material. The results showed that the overloading had a strong retardation effect on this material when the overload ratio r (ratio between the overload range and the baseline load range) is larger than 1.2. For example, the number of cycles of retardation due to overloading, N’, may be as high as ten times the cycles, N, required for the same crack length under baseline loading. In addition, for the testing conditions examined, there is no significant difference in the retardation effect under single or multiple overloading for this material. An analytical model based on the concept of crack closure and the effective stress intensity range was developed for evaluating the effect of the retardation. The development led to a correlation between the number of applied cycles and the crack size after overloading and permits to calculate the retardation cycles N’. Finally, the mechanisms attributed to the retardation effect for WGCI are discussed.