Volume 20, Issue 3 (May 1992)
Fatigue Threshold Regime of a Low Alloy Ferritic Steel under Closure-Free Testing Conditions: Part II—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 closurefrec testing conditions at R = 0.7 (= Rcff) 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 ΔK curves obtained on the some 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.