2.2. Fatigue tests
Fatigue tests were conducted on a GPS-100 test machine at room temperature in air ambient based on ISO 1099:2006, MOD standard. The loading frequency was 126 Hz and the stress ratio of -1 (R =\(\sigma_{\min}/\sigma_{\max}\) ). Four groups of fatigue life tests were performed on the standard specimens at four different stress amplitude levels, and at least five specimens were used for each group. To reduce the impact of surface defects on fatigue life, 2000 mesh emery paper was used to polish the middle sections of specimens until there was no obvious scratch exists before fatigue test. Fig. 2 (a) and (b) show the fatigue test specimens and fatigue test process, respectively.