Fracture behavior of quaternary Al-Li-Cu-Mg alloy under mixed mode I/III loading
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I.
INTRODUCTION
gluminium-lithium alloys have become prime candidate materials for aerostructural applications because of their superior specific strength and specific modulus properties, t~J The use of these alloys has been impeded because of their low ductility and low fracture toughness.t2~ The influence of various microstructural features on the fracture resistance of these alloys has been studied extensively. I2-~~ These studies are limited to the mode I or tensile loading conditions. However, more often than not, practical structures are subjected to a combined mode of loading that necessitates the study of fracture resistance under such conditions. Mixed-mode I/III fracture has become the focus of many recent studiesI'-~9~ because numerous observed failures include antiplane shear (i.e., mode III) components. The studies on the mixed-mode I/III fracture resistance have shown conflicting results. Pook t111had shown in case of several high-strength alloys that the imposed mode III components hardly influence the fracture resistance of the alloys, and the failure occurs when the resolved mode I component equals K~c, the mode I fracture toughness. The studies on aluminium-matrix composites t12,131, too, have shown that the total fracture toughness under mixed-mode conditions is higher than that under pure mode I loading. On the other hand, the studies pertaining to steels t~4-~8~revealed that the mixedmode I/III fracture behavior depends upon their strengthductility (or toughness) combinations. In steels with high strength and low ductility (brittle steels), the effect of imposed mode III components on mode I fracture resistance was found to be marginal, t16,171 whereas, in steels with low strength and high ductility (tougher steels), the effect was found to be significant, t~4'~5,~81 In the case of A1-Li alloys, the fracture toughness data under mixed-mode I/III loading have not so far been evaluated. In a recent study, t2~ the authors have found N, ESWARA PRASAD, Scientist, is with the Defense Metallurgical Research Laboratory, Hyderbad-500258, India. S.V. KAMAT, on leave from the Defense Metallurgical Research Laboratory, Hyderbad, is Visiting Scientist, Department of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920. Manuscript submitted November 5, 1993. METALLURGICAL AND MATERIALSTRANSACTIONS A
that the mode III fracture of a 8090 AI-Li alloy is lower than the mode I fracture toughness when the alloy is in underaged temper. On the other hand, when subsequently aged to peak strength, the alloy showed higher mode III fracture toughness as compared to the mode I fracture toughness. The objective of the present study was to characterize the mixed-mode I/Ill fracture toughness of the quaternary AI-Li-Cu-Mg (AA 8090) alloy in underaged as well as peak-aged conditions.
II.
EXPERIMENTAL
The material used in this investigation was a 8090 A1-Li alloy with nominal composition (in wt pct), A1-2.2Li1.07Cu-0.72Mg. The alloy plates were in the form of 20-mm-thick plate in und
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