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, II_a and II_b. The stress intensity factor K_S is proposed to determine the boundary between them (corresponds to the stable crack length a_S under plane-strain conditions) and to characterize the cyclic fracture toughness. It is assumed that K_S corresponds to the stress intensity factor K_GY estimated by the cyclic yield stress and the length of a focal fatigue crack. Enlargement of the plastic zone at the crack tip and a transition to the plane-stress state at K ≥ K_S change the fatigue fracture pattern, which manifests itself as knee points in the K_max dependences of acoustic emission parameters, phase transformation rate in metastable steel, and fatigue striation spacing: after reaching K_S, the fatigue crack grows by the striation-per-cycle pattern. In addition, it is shown that the value of K = K_S corresponds to the pivot point of the crack growth curve plotted for the steel tested in mixed loading modes.