The hysteresis loop on the near-threshold fatigue crack growth curves generated by stepped load reduction and constant-amplitude loading methods in a Ni-based superalloy

Fatigue crack growth (FCG) tests were conducted on compact tension specimens made of a Ni-based superalloy to investigate the near-threshold FCG behaviors using both the stepped load reduction method (LRM) and the constant-amplitude loading method (CALM) at stress ratios of R = 0.05, 0.5 and 0.7 under ambient condition. After the FCG threshold being approached by the LRM, a remarkable hysteresis plateau occurs on the subsequent FCG curve generated by the CALM for R ≤ 0.5, resulting a hysteresis loop on the near-threshold region, but the situation becomes complicated at R = 0.7. In the appearance of hysteresis plateau, the FCG life to fracture can be over 107 cycles longer than that without hysteresis plateau. In the hysteresis plateau region where FCG rate is lower than 10-9 m/cycle, fractography shows a wide optical dark zone where microstructure-sensitive crystallographic facet feature dominates, in sharp contrast to the normal region with FCG rate higher than 10-8 m/cycle where crack grows in transgranular feature and insensitive to microstructures. It is recommended that the stepped LRM should be used to generate the material basic FCG curves for fatigue life prediction of high durability structures.