Use of a True Material Constitutive Model for Stress Analysis of a Swage Autofrettaged Tube Including ASME Code Comparison

Abstract

This work reports results from a new finite element analysis (FEA)-based user programmable function (UPF) featuring true material constitutive behavior with proper algorithms for accurate stress analysis of swage autofrettage of high-strength thick-walled cylinders. This material model replicates an existing Bauschinger-effect characterization (BEC). This incorporates elastoplastic material behavior during loading. Reversed loading includes a reduced elastic modulus and nonlinear plasticity resulting from the Bauschinger effect (BE), both depending upon the maximum level of loading plastic strain. This case study identifies the difference in stress distributions based on two different material models, a bilinear kinematic hardening model, and the BEC model. Near-bore residual stresses for the BEC case are noteworthy and reported in detail, e.g., axial residual stress is tensile and hoop residual stress exhibits a distinct slope reversal, unlike hydraulic autofrettage. This indicates the possible need to re-assess the ASME pressure vessel code (correction for BE) regarding swage autofrettage.