Abstract
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 J IC, 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 ⩽ J IC. 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 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. Extrapolation of the blunting line proposed in ASTM E813 to the critical stretch zone width yielded values of J IC that were erroneously high compared with extrapolation from experimental data.
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