Inhomogeneous Cyclic Deformation Behavior of Polycrystalline Aluminum

Synopsis

Fully reversed strain-controlled push-pull tests were performed on polycrystalline specimens of commercially pure aluminum to investigate the influence of specimen geometry and strain amplitude in promoting inhomogeneous cyclic deformation. The specimens were cylindrical with an effective gauge section 10 mm in diameter and 20 mm long. The radius of the specimen shoulder was systematically varied between 20 mm and 60 mm. The fatigue tests were run under constant strain amplitude in air and room temperature using a symmetrical sawtooth waveform. The longitudinal strain amplitudes used for the testing were 0.10, 0.18, and 0.40% with a constant strain rate of 0.004 sec⁻¹. The results show that instability, in terms of off-center necking or off-center barelling, is a strong function of the geometry and, to a lesser degree, of the strain amplitude. The tested specimens which did not exhibit shape instability showed intense localization of the deformation, i.e. shear bands uniformly distributed in the gauge section. It is shown that the general condition that appears necessary for shape instabilities and/or localized deformation in shear bands, is a positional variation in the cyclic stress-strain field caused by geometry and material microscopic variations. The observed behaviors are also shown consistant with the hypothesis that localization of deformation into shear bands results form an instability in the constitutive description of an homogeneous deformation field.