FATIGUE THRESHOLD REGIME OF A LOW-ALLOY FERRITIC STEEL UNDER CLOSURE-FREE TESTING CONDITIONS .2. HYSTERESIS IN NEAR-THRESHOLD FATIGUE CRACK-PROPAGATION - AN EXPERIMENTAL ASSESSMENT

Near-threshold fatigue crack propagation behavior of a high strength steel was investigated in laboratory air under closure-free testing conditions at R = 0.7 (= R(eff)) and at two different K-gradients. Depending on the criterion assumed, the threshold value differed; the criterion of non-propagation gave a lower threshold value than that assumed by the propagation criterion. Nevertheless, the subsequent propagation following a load increase was discontinuous in both cases, and da/dN versus DELTAK curves obtained on the same specimen during the K-decreasing and the K-increasing test were not necessarily identical in the threshold regime. This behavior, hysteresis, is analyzed mainly from the experimental viewpoint, and it is shown that hysteresis is not an artifact.