The orientation dependence of fatigue-crack growth in 8090 Al-Li plate
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I.
INTRODUCTION
THE development of AI-Li alloys has been motivated primarily by their potential for use in the next generation of aerospace structures. Interest lies in their 7 to 10 pct lower density and high stiffness, as compared with traditional high-strength AI alloys. These alloys often exhibit outstanding fatigue-crack propagation resistance but show extremely marked anisotropy in their fracture behavior, t2-61 Such performance has been traced to the intense inhomogeneous slip along certain preferred planes and in certain directions resulting from the presence of coherent shearable 8' precipitates and strong textures in these alloys.J6] Localized slip planarity promotes macroscopic crack deflection and leads to an unusual fracture mode with an extraordinary surface roughness comprised of well-defined slip-band f a c e t s . 17'8'91 It has been observed that the volume fraction of 8' in Licontaining A1 alloys does not affect this mode of fatiguecrack propagation in a vacuum. 19) Experimental studies have addressed fatigue-crack growth in many AI-Li-X alloys. 12-5.7-9) Based on the high level of crack closure measured Venkateswara Rao, et al. ~7] suggested that the superior fatigue-crack growth resistance of AI-Li alloys is due to an extrinsic shielding mechanism such as roughness-induced crack closure. In a recent study on the intrinsic fatigue-crack growth rate (FCGR) for 2024 AI alloy and AI-Li-Cu-Mg alloys in vacuum, Slavik et al. 191 arrived at the conclusion that crack closure is not the primary explanation for reduced FCGRs in A1-Li alloys. Various normalizing parameters ch as elastic modulus, E, yield strength, O-y, and (Eo-y) were evaluated to determine whether the intrinsic fatigue-crack growth rates could be identified and described by a single crack growth law. Their analysis shows that the normalization by E or ~/(Eo-y) does not X.J. WU, Visiting Fellow, W. WALLACE, Director, and M.D. RAIZENNE and A.K. KOUL, Senior Research Officers, are with the Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council of Canada, Ottawa, ON, Canada KIA 0R6. Manuscript submitted May 24, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
appreciably reduce the differences, suggesting that FCGR models based on an energy argumentt~~ or cyclic crack-tip opening displacement (CCTOD) t~t] are not suitable for description of fatigue-crack growth in these alloys. The reduction in FCGR for A1-Li alloys cannot be attributed to fi'-favored slip reversibility, because reducing the amount of 8' in Li-containing alloys by increasing the Cu/Li ratio (X2095-T8) and by aging heat treatment (AA8090-DA) does not increase the fatiguecrack growth rate; in fact, the opposite trend was observed. Having excluded the effects of 8' and of elastic modulus as not being highly significant, there remain several other variables, such as the degree of texture, and anisotropic material properties, such as yield strength (YS) and work-hardening rate, that may well be more prominent. The effect of
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