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.