ON THE CYCLIC FRACTURE TOUGHNESS PARAMETER, KS, FROM THE STANDPOINT OF FRACTURE PHYSICS AND MECHANICS

The paper analyzes the stages and kinetic features of fatigue crack growth. Particular attention is paid to fatigue stage II consisting of two substages, namely, IIa and IIb. The stress intensity factor Ks is proposed to determine the boundary between them (corresponds to the stable crack length ls under plane- strain conditions) and to characterize the cyclic fracture toughness. It is assumed that Ks corresponds to the stress intensity factor KGY estimated by the cyclic yield stress and the length of a focal fatigue crack. Enlargement of the plastic zone at the crack tip and the transition to a plane-stress state at K ≥ Ks change the fatigue fracture pattern, which manifests as knee points in the Kmax dependences of acoustic emission parameters, phase transformation rate in metastable steel, and distance between fatigue striations: after reaching Ks, the fatigue crack grows by the one-striation-per-cycle pattern. In addition, it is shown that the value of K = Ks corresponds to the turning point of the crack growth curve plotted for the steel tested in mixed loading modes.