Non-linear mechanical response of various metals. II. Permanent length changes in twisted tubes

Abstract

For pt.I see ibid., vol.10, no.4, p.519 (1977). The Swift (1947) effect, observed here as the permanent lengthening of a tubular specimen along the principal axis about which it is being examined. Permanent length changes in twisted tubes are considered in relation to the Poynting effect and incremental theories of plasticity. It is shown that the increasing body of experimental evidence, including the present measurements on aluminium, copper, iron and an aluminium alloy, cannot be accounted for by an incremental theory of plasticity which assumes incompressibility and interprets the displacement gradients in terms of small strain components. This is considered to arise from the fact that incremental theories based on small strain components do not consider a component of axial strain which is entirely geometric in origin, an observation which is independent of whether the twisted tube is to be regarded as isotropic or anisotropic. Satisfactory correlation of the observed length changes of twisted tubes with the shear strain is obtained with an incremental-type constitutive equation for an isotropic incompressible strain-hardening elastic-plastic solid with large deformation.