Fig 2 Two-level test results for different materials17,44–53 under the low-high loading sequence
The mechanism of material fatigue resistance improvement is still under investigation54. In Ishihara’s investigation45, the strengthening effect is considered to be due to the improvement of the crack growth threshold. Observations show that a small number of loading cycles can make the microstructure within the material more compact and uniform, hence improving the crack growth resistance55. There are different explanations for other materials, including comprehensive residual stress56, microstructure changes29, and grain refinement57.
It can be seen that the residual life fraction of FGH96 alloy is much higher than other materials. The highest residual life fraction of FGH96 is near 5, while all other materials are less than 2. In Li et al.’s research44, the K417 alloy also exhibits significant strengthening effects. Fig 2 shows that the residual fatigue life of K417 was improved when the preload cycle life fraction was extremely low. Li et al. found that low amplitude loading within a specific number of cycles can reduce the strain response under subsequent loading. The specimens with decreased strain response have a longer fatigue life than the virgin specimens. In this study, FGH96 has similar test conditions to K417, including elevated temperatures and high mean stresses. Therefore, the strengthening effect of FGH96 may be attributed to the cyclic hardening induced by previously cyclic loading.
In the microscope, the increase in fatigue strength may be attributed to the strengthening of grain boundaries. Zheng et al.54,58 pointed out that low amplitude cycles increase the dislocation density near grain boundaries and form dislocation walls. Hence the material’s resistance against deformation increases, and the fatigue strength is improved. When the number of preloading cycles is small, only few hardened structures are formed in the material. It can be seen that the residual fatigue life is only slightly higher than the life of virgin material when the preload cycle life fraction is 0.25. As the number of preloading cycles increases, many dislocation tangles are generated, creating barriers to dislocation movement. Thus the rate of plastic strain accumulation during subsequent loading is reduced, which may result in longer fatigue life than virgin materials59. When the preload cycle lifetime fraction nears 1, the damaging effect from dislocation accumulation offsets the strengthening effect caused by cyclic hardening. Therefore, the residual fatigue life will decrease to 0 with further increases in preload cycles.
To investigate the effect of the number of preload cycles on the failure mode, the fracture surfaces were observed. The fracture surfaces of specimens 7-9 are shown in Fig 3. There are three distinct areas on the macro fractography for each specimen. The area boundary is indicated by dashed lines. Stage Ⅰ represents the crack initiation and crack stable propagation area. This area is brighter and smoother than the others. Stage Ⅱ represents the fast crack propagation area, and stage Ⅲ represents the final fast fracture area60. The stage I region of specimen 9 is smaller than specimens 7 and 8. This indicates that a high number of preload cycles may result in a shorter crack initiation and stable propagation time.
In addition, the stage Ⅰ area of each specimen shows a semi-circle shape, which suggests that the crack initiated from the surface. Fig 4(A), Fig 4(B), and Fig 4(C) illustrate the crack initiation areas of specimens 7-9, respectively. Crystallographic facets can be observed in the crack initiation area, which indicates that all cracks all initiated at the twin grain boundary61,62. Therefore, the variation in residual cycle life is independent of the cracking mode. Fig 4(D) shows the crack initiation area of specimen 3, which was tested under constant high amplitude load. It can be seen that there are multiple crystallographic facets on the specimen surface. Thus the cracking mode of the specimen under constant load is similar to that of the two-level load.