Outer Surface Prior to Inner Surface Double Autofrettage Technique for a Compound Cylinder

Abstract

The autofrettage and shrink-fit processes are used to enhance the load carrying capacity and fatigue life of the pressure vessels. Previous works were mainly concerned with increasing the compressive residual stress at the near bore area of the compound cylinders, ignoring the harmful high tensile residual stress developed at the outer part of the cylinder. Also, the idea of multiple or re-autofrettage had been only used at the inner surface of the cylinder to increase the magnitude of compressive residual stress at the near bore area. In this study, a new design approach is proposed by applying an autofrettage cycle on the external surface of the cylinder prior to an autofrettage cycle on the internal surface of the cylinder. It is shown that this double autofrettage cycle not only increases the magnitude of compressive residual stress at the near bore area but also decreases the tensile residual stress at the near outer surface area. Moreover, the proposed double autofrettage process has been combined with shrink-fit and standard inner surface autofrettage processes to produce new combinations of a two-layer compound cylinder. The fatigue life for these new combinations has been evaluated to verify the improvement while using the double autofrettage process.