Influence of Primary Stress on Fatigue Life on Rolled Bolt-Nut Joints under Two-Step Loading

Fatigue tests of rolled bolt-nut joints were carried out under constant-amplitude loading and two-step loading with three primary stresses. The bolted joints failed in the threaded region at the root of the first thread where the nut and bolt were engaged. Experimental results indicated that cycles to failure of the secondary nominal stress amplitude, where the nominal stress was the load divided by the area of net section in the threaded region, depended on the primary stress and its cycle ratio. In order to know the local stress under elastic-plastic condition at the thread root where fatigue failure occurred, a simplified method based on Neuber' rule and Glinka's rule was investigated. Equivalent stress amplitude, which was a fully reversed stress amplitude, was investigated from the estimated local stresses under the loading cycles and the relation of stress amplitude with mean stress. As a result of investigation of experimental data using the equivalent stress, cycles to failure of the secondary equivalent stress amplitude depended on the cycle ratio of the primary stress and did not depend so much on the primary stress. The fatigue life prediction method based on the above result was proposed. The estimated fatigue life was generally in good agreement with the experimental results.