Original paper(Vol.49 No.7 pp.754)
Change of Actual Stress Distibution in Shot Peened Steel during Fatigue Process
Shinichi Ohya, Reiko Kojima and Yoshihiko Hagiwara
Abstract:The purpose of this investigation is to clarify the behavior of the compressive residual stress induced by shot peening treatment during fatigue process. Fatigue tests on shot-peened specimen were carried out by using a cantilever type rotary bending fatigue test machine. The distributions of actual stress on specimen circumference at the maximum tensile applied load were dynamically measured by the x-ray stress analysis based on the single exposure technique without stopping fatigue test. The distributions of residual stress were also measured under unloading by the same way.
As the results, the actual stress at every position on specimen circumference was roughly the same value as the algebraic sum of the compressive residual stress and the maximum tensile applied stress until crack initiation. However, after a macro crack was appeared in the surface of specimen, the circumferential distribution of actual stress was showed V-shape at crack position. The actual stress at the bottom of V-shaped distribution was the same value as the compressive residual stress when crack length reached greater in size than a width of x-ray irradiation area. It was confirmed that change of actual stress distribution on specimen circumference could indicate the positions of crack initiation. Meanwhile, the average compressive residual stress on specimen circumference was relaxed drastically in initial stage of fatigue life, thereafter it declined barely with increasing a number of stress cycles until the specimen was ruptured. However, circumferential distribution of residual stress remained constant even then there were macro cracks in the peened surface. It was confirmed that observation of the average residual stress and the circumferential distribution of residual stress could not detect the fatigue damage in the peened surface.
Key Words:X-ray stress analysis, Shot peening, Fatigue damage, Rotary bending fatiguetest, Actual stress, Residual stress.