Modeling and Simulation of Residual Stresses in Mechanical Autofrettage

Abstract

The autofrettage process is used to induce advantageous residual hoop stresses into pressure vessels to enhance their fatigue lifetime. Such pre-stressed pressure vessels are routinely used in power, nuclear, process, armament, and food industries. The autofrettage process can be accomplished by applying hydraulic or mechanical pressure or by the pressure of powder gas to the bore of a thick cylinder to induce residual stresses. The two processes are referred to as hydraulic and mechanical autofrettage respectively. The objective of this research is to analyze mechanical or swage autofrettage, which is achieved by ramming an oversized conical mandrel into the bore, thus driving it into the plastic regime. When the mandrel is removed, the outer elastic portion compresses onto the inner plastic regime, thus causing compressive residual stresses. The percentage of material that undergoes plastic deformation determines the level of autofrettage. A computer code based on finite element method is developed to analyze residual stresses including Bauschinger effect duly incorporating failure criteria such as von Mises and Tresca conditions. The computer code developed is benchmarked using analytical solutions based on Lame’s equations. Using this code, parametric studies are carried out to optimize the depth of penetration of the plastic regime into the material thickness of the bore. The results based on modeling and simulations are validated by using other available computer codes and experimental data.