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Biaxial creep and plastic flow of anisotropic aluminium

By David William Alan Rees


Creep tests at room temperature under combined tension-torsion have been performed on thin walled tubes of commercially pure aluminium (ElA) in the annealed, extruded and prestrained conditions. The required creep stresses were achieved by incremental loading along a constant stress ratio path (radial loading). Axial ([epsilon][sub]zz) and shear ([gamma][sub][theta]z) strains were measured throughout deformation. Strains produced by radial loading on annealed and extruded aluminium consisted of an instantaneous component and a short-time creep component. For all stress paths the latter component was the form [epsilon][alpha]t[sup]m where the time exponent m lay in the parabolic range [omicron]<m<1/2. The plastic prestraining of annealed aluminium was either beneficial or detrimental to its combined tension-torsion creep resistance. Forward prestrains (in tension or positive torsion) or large reversed prestrains (in compression or negative torsion) were beneficial to creep resistance by hardening the aluminium in the direction of the stress path. The effect was to eliminate the short-time creep component of radial loading and to reduce the primary creep strains and secondary creep rates of ensuing long-time creep. Small amounts of reversed prestrain were detrimental to creep resistance by softening the aluminium in the direction of the stress path. The effect was to increase the parabolic creep component of radial loading (m lowered) and to increase the primary creep strains and secondary creep rates of ensuing long-time creep. Experimental observations on flow behaviour were not in agreement with predictions from isotropic theory. The anisotrophy in the three conditions of aluminium was examined in the instantaneous plastic strain increment vector of loading (d[gamma][sup]P[sub][theta]z/d[epsilon][sup]P[sub]zz) and in the strain rate vector of short and long-time creep ([gamma][sub][theta]z/[epsilon][sub]zz). For each stress path on annealed aluminium the colinearity in these vectors indicated an initially anisotropic material which hardened uniformly throughout deformation. Stress paths on extruded aluminium produced rotations in the initial instantaneous vectors of loading while stress paths on prestrained aluminium produced rotations in the instantaneous vectors of loading and in the strain rate vectors of long-time creep. With increasing stress of loading or with accumulating creep strain the vectors rotated into colinearity. The behaviour was typical of the nature of anisotropic hardening in material possessing a strain history. Plastic strain increment ratios derived from the yield functions of Edelman and Drucker f=1/2C[sub]ijkl([sigma]ij-m[epsilon][sup]P[sub]ij)([sigma]kl-m[epsilon][sub]kl) and Yoshimura the three conditions of aluminium. Hill’s yield function f=1/2C[sub]ijkl[sup]sigma][sub]ij[sup][sigma][sub]kl was descriptive of the plastic anisotrophy in uniform hardening. This led to an extension of each theory for creep. In general strain rate ratios derived from the extended theories were in good agreement with observations on creep anisotrophy

Publisher: Kingston University
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