Study on the Fatigue Life under Double Repeated Stress in Two Stress Levels

Abstract

As the Miner's cumulative-damage theory that the fatigue fracture will occur at Σ(n/N)=1 does not coincide with the test results for the fatigue life under varying stress amplitude in two stress levels, the present writers have presented a correctional equation which is conducted by the test result for fatigue damage.From the result obtained in the previous report that the maximum cycle ratio of no fatigue damage decreased as the excess ratio of overstress increased, we consider that the degree of the effect of oversteress on the fatigue life is inversely proportioal to the maximum cycle ratio of no fatigue damage for that overstress. With this conception of fatigue damage, assuming that the primary stress decreases the fatigue life for the secondary stress at the ratio of n1/N1·id2/id1, we present the following equation for the fatigue life.α·n1/N1+β·n2/N2=1α=id2/id1, β≤1where id1 and id2 are respectively the maximum cycle ratio of no fatigue damage for primary and secondary stress. β is the function of the stress level of primary stress and secondary stress and the cycle ratio of primary stress, etc..With this correctional equation, the results obtained in the previous report on the fatigue life under double repeated stress in two stress levels are graphically elucidated.