3.2 Result analysis
FGH96 test results indicate that the residual fatigue life can be
increased by low-amplitude loading history. This phenomenon, often
referred to as the ” strengthening effect ”27, can be
considered as a specific situation in variable amplitude loading. It
should be noted that the low-amplitude load in this experiment is higher
than the fatigue limit. At the same time, the strengthening effect is
usually considered to be induced by loads below the fatigue
limit38,39. In fact, some existing tests also
demonstrated similar phenomena. Two-level test
results17,44–53 for various materials in low-high
sequence were collected and plotted in Fig 2. The low-amplitude loads in
these tests are all higher than the fatigue limit. The results of the
FGH96 test in this study are also plotted in Fig 2 for comparison.
As shown in Fig 2, test results with residual life fraction exceeding
unity can be regarded as exhibiting the strengthening effect. For
residual fatigue life fractions lower than unity, only the damaging
effect is exhibited39. It is evident that the
strengthening effect exists only when the number of preload cycles is in
a specific range. All the preload cycle life fractions that exhibit
strengthening effects are less than 0.8. For FGH96 test results in this
study, 0.75Nf is the maximum number of preloading cycles
that shows the strengthening effect, and the residual fatigue life is
nearly equal to the life of virgin material.