Actual Unloading Behavior and Its Significance on Residual Stress in Machined Autofrettaged Tubes

Abstract

A simple method for obtaining actual unloading behavior of high strength steels is proposed. Based on this method loading-unloading stress-strain curves for NiCrMoV125 steel is obtained. It is shown that the actual unloading behavior tends to be nonlinear as soon as unloading occurs. Using the variable material properties (VMP) method, residual stresses induced by autofrettage process of a tube made of this type of steel are calculated. Significance of employing actual unloading behavior is demonstrated by comparing the results with ideal models such as isotropic hardening and bilinear models. There is at least 30% over estimation of compressive residual stress at the bore if ideal models are used. Also, the process of material removal is simulated by using commercial FEM software to evaluate the recommended scenarios of removal on redistribution of residual stresses at the bore. Resulting stresses after machining are also calculated by the VMP method using actual behavior. It is shown that the final residual stress field differs significantly if actual unloading behavior is used.