Studies on the Fatigue Strength Under Two or Three-Dimensional Stress Conditions

Abstract

Fatigue strength criterion in intermediate cycle range was presented under two or three-dimensional stress conditions. Parameters affect to fatigue crack initiation life under these conditions are mainly stress biaxiality, stress gradient, mean stress and mean strain, etc. In this paper, effects of stress biaxiality and stress gradient on the life were investigated experimentally, and equivalent strain function ε_D, e_q was derived. Using this strain function, the fatigue life under various stress conditions can be evaluated with good accuracy, and the function ε_D, e_q is expressed as follows. ε_D,e_q=ε_1+3/4ε_2 where ε_1 and _2ε are maximum and medium principal strain amplitude at crack initiation point respectively. It was found that the effect of stress gradient is to increase the fatigue life, and amount of the increase is approximately constant irrespective of maginitude of the stress gradient.