EFFECTS OF FATIGUE PRECRACKING ON STRETCH ZONE FORMATION

An experimental study was performed to analyze the effect of the fatig ue precracking stress intensity range (Delta K) on the formation of th e stretch zone, a potential measure of J(IC), in AISI 4340 steel. Comp act tension specimens were fatigue precracked at 62%, 100% and 147% of the Delta K limit prescribed by ASTM E813-93, then loaded to levels o f J less than or equal to J(IC). The resulting stretch zone widths (SZ W) on the post-fracture surfaces were measured using scanning electron microscopy and plotted against J calculated from load vs load-line di splacement. Precracking was found to reduce the SZW formed at a given value of J in proportion to the maximum applied J during precracking. Blunting line equations were fitted to experimental data for J < 0.65 J(IC), revealing a constraint factor of m = 1.08. At J > 0.65 J(IC), a transition from blunting to ductile crack propagation occurred. Extra polation of the blunting line proposed in ASTM E813 to the critical st retch zone width yielded values of J(IC) that were erroneously high co mpared with extrapolation from experimental data.