抄録
A strain controlled two step test was conducted to investigate changes in the surface features of SUS316NG during cyclic variable loading. In the two step test, strain range was switched from Δ_<εH> = 4% to Δ_<εL> = 2% at the usage factor UF= 0.2. The specimen surface was observed by a laser microscope to clarify the surface morphology and surface roughness R=a. Initiation and propagation of the main crack was also observed by using cellulose acetate replica The results in the two step test were compared with those obtained in fatigue tests under constant strain ranges. As a result changes in surface features such as slip bands and surface asperities clearly depended on the value of strain ranges even under the same UF value. Small cracks initiated very early in the fatigue life, however, they hardly propagated until the middle of fatigue life. In contrast, Ra clearly increased from the beginning to the middle of fatigue life. These results suggest that surface roughness can be an effective parameter for fatigue damage assessment. In addition, the increasing rates of Ra during the first and second steps in two step test were similar to those in fatigue tests under constant strain ranges Δ_ε = 4% and Δ_ε = 2%, respectively. Therefore, the history of strain range under cyclic variable loadings can probably be described from the value of R_a and the increasing rate of R_a in the fatigue tests under constant strain ranges.
