1989 Volume 38 Issue 427 Pages 404-409
Push-pull fatigue tests were conducted on 80kgf/mm2 steel under the combination of regularly applied understresses and intermittently applied overstresses. The mean stress superimposed on understresses changed periodically to form a overstress range pair at the time of the mean stress change. The effects of understress application, mean stress superposition on understresses and the mean stress wave form on the crack growth rate were investigated. Crack initiation and propagation lives in the tests conducted under the combination of over- and under-stresses were much shorter than expected when only the overstress cycles were counted, neglecting the damage of understresses. The stress amplitude even about 50% of the endurance limit of the steel was judged effective to make both of crack initiation and growth lives shorter. Crack initiation and growth lives in the tests with the sporadic down-spike stress change and with the rectangular stress change seemed almost equal to each other, and much shorter than those for the constant amplitude test. In two-step P-N interval tests, the crack growth rate was higher than that for the two-step N-P interval test, and even crack growth rate for the N-P test was higher than that for the completely reversed constant amplitude test at relatively smaller values of ΔK.
