Near-threshold fatigue crack growth in 8090 Al-Li alloy
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I.
INTRODUCTION
QUANTITATIVE assessment of fatigue crack growth (FCG) properties is a basic requirement of damage tolerant design for aircraft structures. The understanding of FCG in A1-Li alloys may provide guidance for tailoring the microstructure and properties of the future AI-Li alloys. A transgranular fatigue crack growth rate (FCGR) equation was derived previously on the basis of the restricted slip reversibility (RSR) concepts,Eq and this equation was later modified to incorporate some unique fracture features observed in A1-Li alloys,t2~ The reduced FCGRs of 8090 A1-Li alloy have been related to the increased yield strength (YS) and work-hardening rate and, more importantly, to the existence of a strong { 110}(112) texture. It has been shown that the texture effect can be accounted for by introducing a geometrical factor cos2~b to the rate equation for the texture free condition, where ~b is the angle between the loading axis and the slip plane normal. The modified RSR model predicts that FCGR in a textured material is usually slower than in texture-free materials because th > 0 and cos2~b < 1. In this previous study,t2~ only the upper Paris regime (AK > 10 MPa~-m) was studied, though the RSR model was shown to successfully describe the features of crystallographic crack growth and FCGRs in 8090-T8771 A1-Li alloy along various orientations in moist air. In the preceding regime of FCG under AK-increasing conditions, crack closure and environmental effects were shown to be minimal. The present study focuses on near-threshold FCG in 8090-T8771 exposed to moist air. It is now well understood that FCG in A1-Li alloys is controlled by planar slip.[2~] Planar slip primarily proceeds by shearing of coherent ordered 8(A13Li) precipitates, and slip localization is accentuated by strong crystallographic texture. Localized slip planarity promotes slip band cracking (SBC), which leads to macroscopic crack deflection and
X.J. WU, Research Officer, W. WALLACE, Director, and A.K. KOUL and M.D. RAIZENNE, Research Officers, are with the Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council of Canada, Ottawa, ON, Canada K1A 0R6. Manuscript submitted October 28, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
also an extraordinary surface roughness comprised of welldefined slip band facets,t~Sj In the low Ms near-threshold region, where crack opening displacement (COD) is very small, crack deflection and surface roughness may induce significant crack closure. Observations of high crack closure in A1-Li alloys, mostly under AK-decreasing conditions, led to some premature conclusions that roughness-induced crack closure is the main cause for low FCGRs in A1-Li alloys,t6J Studies by Slavik et al.,[5] using a constant Kmax - MC decreasing procedure, where the influence of crack closure was eliminated, have shown that crack closure cannot be the primary explanation for reduced FCGRs in A1-Li alloys, as compared to conventional aluminum alloys. The role of crack closure in
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