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.