This report describes the results of a Task Group effort of E-24 Subcommittee II on Fractography. It presents the results of a study to determine quantitative correlation between fracture toughness, K_Ic, and the fractographically derived width of the stretched zone preceding fracture in fracture toughness specimens. The correlation was obtained for two highstrength aluminum alloys, a 6Al-4V titanium alloy and a 18Ni maraging steel heat treated to a strength level range of 170,000 to 260,000 psi.

It is observed that the width of the stretched zone increases with increasing fracture toughness and material ductility, and exhibits good correlation with the calculated crack opening displacement, that is, the nominal displacement of the crack surfaces normal to the plane of the crack. The width of the stretched zone shows no apparent dependence on, or correlation with, material grain size. Features of the 7075-T6 alloy, for which fraetographic features were defined most clearly and documented most extensively, suggests that the stretched zone represents a region of local strain at the crack tip which forms on loading and prior to incipient cracking. The features of the transition from the stretched zone to the fracture region in this alloy suggests that fracture is induced primarily by achieving critical conditions beyond the crack tip, and is not simply associated with tearing at a critical crack tip strain level.