Effects of fatigue precracking on stretch zone formation

An experimental study was performed to analyze the effect of the fatigue precracking stress intensity range (ΔK) on the formation of the stretch zone, a potential measure of JIC, in AISI 4340 steel. Compact tension specimens were fatigue precracked at 62%, 100% and 147% of the ΔK limit prescribed by ASTM E813-93, then loaded to levels of J ⩽ JIC. The resulting stretch zone widths (SZW) on the post-fracture surfaces were measured using scanning electron microscopy and plotted against J calculated from load vs load-line displacement. 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 JIC, revealing a constraint factor of m = 1.08. At J > 0.65 JIC, a transition from blunting to ductile crack propagation occurred. Extrapolation of the blunting line proposed in ASTM E813 to the critical stretch zone width yielded values of JIC that were erroneously high compared with extrapolation from experimental data.