Study the Effect of Anisotropy of Elastic-Plastic Properties on Residual Stress Development in Autofrettage of Thick Cylinder

Abstract

Thick-walled cylinders subjected to high internal pressure and/or elevated temperatures are widely used in the aircraft, defence, nuclear and chemical industries especially for military applications e.g. the gun barrel in a ballistic event. In the absence of residual stresses, cracks usually form at the bore where the hoop stress developed by the working pressure is highest. To prevent such failure and to increase the pressure-carrying capacity of the pressure vessel, autofrettage process is used. During autofrettage a plastic region within the thick wall of the pressure vessel is produced by loading the pressure vessel in the plastic domain. Upon unloading, a residual compressive hoop stress is established within the plastic zone. This residual compressive stress counters the tensile hoop stress introduced due to service loading, thereby reducing the overall tensile hoop stress at inner surface. Therefore, autofrettage is used to introduce advantageous favourable compressive residual hoop stress inside wall of a cylinder and result in an increase in the fatigue lifetime of the component. Several researchers have studied the autofrettaged cylinders, both analytically and FE simulation. In this paper, effect of orthotropic anisotropy in the material properties will be studied on the stress field generated in the autofrettaged cylinder. Anisotropy in the material properties may result due to manufacturing processes like extrusion or pilgering. Plastic deformation leads to the development of preferred orientation of grains in the components due to slip occurring only in preferred slip planes in a crystal especially in a HCP material like Zr 2.5%Nb. In this paper, a thick cylinder made from Zr 2.5% Nb is loaded till elasto plastic interface is achieved and then unloaded to study effect of anisotropy on the development of residual stresses. Two material models are considered in the analysis i.e. isotropic and anisotropic plastic behaviour. The effect of anisotropy on the residual stress is investigated.