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I.

INTRODUCTION

In recent years, mechanistic and continuum studies on fatigue crack propagation (FCP), particularly in the nearthreshold stress intensity range regime, have highlighted crack closure as having a dominant role in influencing crack propagation behavior.[1–7] The first crack closure mechanism, proposed by Elber in 1971, involves the idea that premature contact between opposing crack faces can occur during the tensile portion of the fatigue cycle, due to the permanent residual displacements arising from prior plastic zones.[5] The consequence of such plasticity-induced crack closure is to reduce the actual stress intensity factor range experienced by the crack tip from a nominal value of DK to an effective value defined as DKeff 5 Kmax 2 Kcl, where Kcl represents the stress intensity value at which two fracture surfaces first come into contact during the unloading portion of the fatigue cycle. Numerous studies on the extrinsic crack closure effect on the FCP rates demonstrate that several mechanisms can cause crack closure, including crack-wake plasticity,[5,8] crack surface roughness or deflections with mode II sliding displacement,[9] crack corrosion debris,[10,11] and crack fluid pressure.[12] Several closure mechanisms are particularly pertinent to environment-enhanced fatigue crack propagation (EEFCP).[13,14,15] An oxide-induced crack closure mechanism has been invoked to explain the observation that, at low stress ratios, near-threshold FCP rates are significantly SANG SHIK KIM, Assistant Professor, is with the Division of Materials Science and Engineering, RECAPT, Gyeongsang National University, Chinju 660-701, Korea. KWANG SEON SHIN, Associate Professor, is with the Center for Advanced Materials, School of Materials Science and Engineering, Seoul National University, Seoul 151-741, Korea. Manuscript submitted June 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A

reduced in corrosive environments compared to those in inert environments. The presence of oxide (or, more generally, corrosion) debris on the fracture surface is believed to enhance crack closure through the earlier contact between two mating surfaces.[4,16–19] The presence of viscous fluid within the pulsating fatigue crack promotes crack closure by the hydrodynamic wedging action of fluid counteracting the closing of the crack.[12] Gangloff and Ritchie speculated that process zone–dissolved hydrogen may enhance plasticity and the extent of crack wake–induced crack closure.[16] Al-Li alloys, particularly those processed by the ingot metallurgy technique, have an excellent resistance to FCP.[20–28] Rao and Ritchie, for example, reported that Al 2090 showed a tenfold decrease in FCP rates compared to those of conventional high-strength aluminum alloy 7075.[28] Mechanisms for the improved resistance to FCP in Al-Li alloys in benign environments are relatively well established.[28] Pao et al. proposed that high crack closure in ingot-metallurgy-processed Al-Li alloys is associated with highly tortuous and deflected crack paths.[29]