Effect of stress history on cumulative fatigue damage of fine-grained isotropic GCR graphite Original Research Article

To investigate the effect of stress history on cumulative fatigue damage of fine-grained isotropic nuclear graphite, two step multi-loading fatigue tests were performed by changing peak load from high to low loading (H-L mode) or low to high loading (L-H mode) using the push-pull loading pattern. A remarkable reduction of fatigue life due to fatigue damage given in the first fatigue step was found in the secondary fatigue step in the H-L and L-H mode. These results suggested that there was fatigue interaction between the stress history of the first and secondary step as observed in the fatigue test of metals. This study shows that these phenomena can be explained by the fatigue damage mechanism of graphite with the concept of the intermediate fatigue process between first and secondary fatigue step. The cumulative fatigue damage rule of the graphite applicable for lifetime evaluation in H-L and L-H fatigue processes are proposed in equations (3)–(7), where a, b, c and d are constants, and a and b are positive and negative, and c and d are negative and positive, respectively, in the H-L mode and vice versa in the L-H mode.