In this research, we conducted impact fatigue tests in which the magnitude of the impact stress and the duration time of the stress were varied on the smooth specimens of JIS SS400 by using a repeated impact tensile loading machine. To investigate the difference between the behavior of impact fatigue and that of standard fatigue up to about 50 000 times at room temperature, two types fatigue testing machine were used. To continuously obtain the true strain of the specimen during the impact fatigue test, the minimum diameter of the specimen was measured using a two dimensional displacement measurement system. The relationship between the ratio of number of cycles and true strain was investigated. It became apparent that the fracture mode transition from the ductile type to the crack type when the number of cycles to failure was around 10 000 times. It was also found that impact fatigue was lower in fatigue life than standard fatigue when the number of cycles to failure exceeded 10 000 times. As a result of observation of the fractography by SEM, dimples patterns were observed in the cross-sectional of ductile type fracture specimen, and striations patterns were observed in the cross-sectional of crack type fracture specimen.
