Fatigue strength improvement of notched structural steels with work hardening

In this study, the effect of plastic work on the fatigue strength of a structural steel was investigated in detail. Fatigue tests were performed using specimens made of a conventional structural steel, JIS S25C. One set of tests employed monotonically pre-strained specimens with pre-strain ranging from 2% until 8% in tension. In this case, the fatigue limit compared to that for non-pre-strained samples, decreases under small pre-strain and then gradually increases with pre-strain. The increase in the fatigue limit is only 7% relative to that for non-pre-strained specimens for a pre-strain of 8%. The other set of tests employed roller worked specimens with the roller deformation value varying from 0.25 to 1.50 mm in the diameter direction. In this case, the fatigue limit remarkably increases to about 230% of the original value with increased roller working with the effect gradually saturating and then deteriorated a little, so that there exists an optimum roller working value for improving the fatigue limit. This remarkable increase in fatigue limit is caused by the existence of compressive residual stress, elongated microstructure and work hardening.