Residual Stress Concentration And Its Effect On Notch Fatigue Strength

Author:  Zhang, K. Xu, X. Wang, N. Hu
Source:  Conf Proc: ICSP-3, (p.625-630)
Doc ID:  1987073
Year of Publication:  1987
Abstract:  
It has been concluded that shot peening can greatly improve the fatigue strength particularly for notched parts. Kloos found the notched fatigue strength could be even higher than the highest normal fatigue strength after surface rolling. Yet it is still puzzle how to explain this fact. Finite element analysis showed that residual stress concentrates at the root of a notch. With the decrease of notch radius the maximum value of residual stress shifts away from the surface of the notch root. However, only very few experimental results were presented for the distribution of residual stress at sharp notch, due to the difficulty of measurement. Wohlfahrt found, with the same peening parameter, for a case hardened steel the compressive residual stress value at the surface of the notch was the same as that of the flat specimen, but the maximum value in the layer below the notch surface was much higher than that of flat one. Bergstrom reported no significant difference of residual stress for the smooth specimen from the notched one with 2.5mm radius. The residual stress distribution in the axial direction almost overlapped with the curve in the tangential direction, it was, again, similar to that of smooth specimen after shot peening. In addition, Bergstrom found that shot peening did not contribute much for the notch fatigue limit of high temperature tempered steel, due to the relaxation of compressive residual stress and the relaxing rate was the same for smooth and notched specimen at the same local stress. The difference of experimental results presented in the literature could be, in part, explained by the difference of yield strength for the tested material, if the compressive residual stress is easy to fade away, the fatigue strength will show little improvement. However, there was an appreciable amount of compressive residual stress existed at the load level of fatigue limit for, it is not clear why the notch fatigue limit did not increase much. Usually, loading stress concentration is bigger in the axial direction than in the tangential direction, is there any difference between the concentration of loading stress and residual stress? Since there was no correlation for the compressive residual stress and fatigue behavior for high strength specimen with a notch, it would be interesting to study the concentration factor of residual stress after shot peening and its effect on fatigue strength for a low temperature tempered steel. This paper is focused on the measurement of residual stress distribution for a small radius notch and to find out what is happening during cyclic loading. The notch radius in this experiment is 1mm in a 8mm of diameter specimen. It could be easier to make the stress measurement at a large radius notch, but to keep an appreciable stress concentration factor the specimen size should be bigger, then the fatigue test becomes tough.