Influence of mean stress on fatigue strength of stainless steel

In order to incorporate the mean stress effect in the design fatigue curve for designing nuclear power plant components, change in fatigue strength due to the mean stress and root causes of the change were investigated for Type 316 stainless steel. First, 20% cold work was applied to specimens of Type 316 stainless steel in order to reduce the cyclic strain hardening and softening during fatigue tests, and then, the specimens were subjected to stress controlled fatigue tests under the mean stress in air at room temperature. Although the mean stress tended to show a beneficial effect on the fatigue life under the same stress amplitude, it had a detrimental effect under the same strain range. It was shown that the reduction in fatigue life was brought about by the change in the effective strain, which was caused not only by the rise in the crack opening point but also by ratcheting deformation during the fatigue tests. The mean stress had little influence on the fatigue limit for the strain range. It was concluded that, although the mean stress reduced the fatigue life in the stress controlled fatigue tests, its effect did not need to be considered in the design fatigue curve because no significant change in the effective strain range occur in actual components. (author)