Microstructural effects on fracture toughness in AA7010 plate
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I. INTRODUCTION
THE fracture toughness of high strength aluminum alloys is known to depend on many parameters, including flow strength, work hardening rate, slip character, dispersoid content, intermetallic content, grain structure, and grain boundary structure.[1] Furthermore, microstructural anisotropy associated with wrought materials may influence failure mode depending on load and crack orientation. Various models have been put forward in the literature to predict the influence of microstructural or mechanical parameters on the fracture toughness. Following from Hahn and Rosenfield’s[2] and Rice and Johnson’s[3] work, where unstable crack extension is assumed to proceed when crack tip opening (characterizing the extent of the highly strained region ahead of the crack) reaches the length of the unbroken ligaments separating cracked inclusions, the following expression may be derived:
F
KIC ' 2syE
p 6
1/3
12
D
G
1/2
fv21/6
[1]
where D is the diameter of the large inclusions; fv is their volume fraction; sy and E are the yield stress and Young’s modulus, respectively; and KIC is the plane strain fracture toughness. This model has been shown to give a reasonable ¨ M, Research Scientists, are with B. MORERE and J.-C. EHRSTRO Pechiney CRV, Voreppe, France 38340. P.J. GREGSON, Professor of Aerospace Materials, and I. SINCLAIR, Lecturer, are with the Department of Engineering Materials, University of Southampton, Southampton, SO17 1BJ, United Kingdom. Manuscript submitted August 17, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
prediction of the effect of the volume fraction of inclusions for constant yield strength and constant particle size in a number of systems, but does not agree with experimental results concerning the influence of yield stress. In an earlier model, Hahn and Rosenfield[4] considered the effects of strain hardening coefficient and yield stress on toughness, particularly in terms of the increased flow localization, and hence crack tip “damage,” that occurs with decreasing workhardening rate. Garrett and Knott[5] reviewed the derivation of this model leading to the relationship KIC '
2CE«*c syn2 1 2 n2
!
[2]
where C is a constant, «c* is the critical crack tip strain at which unstable propagation occurs, n is the work hardening exponent, and n is the Poisson ratio. The term «c* is taken to be a function of the volume fraction of void nucleating particles.[6] The predicted n!sy dependency of the fracture toughness for a constant distribution of particles has been shown to provide a reasonable description of toughness behavior as a function of aging between under- and overaged conditions (i.e., varying yield strength) for several Albased alloys. Chen and Knott[7] have studied the effect of grain refining dispersoid particles on the toughness of 7xxx-series alloys, indicating that strain localization within shear bands in the plastic zone ahead of the crack tip could lead to decohesion of the interface between the matrix and dispersoids. Fast shear coalescence of the primary voids t
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