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

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.

Determination of Onset of Yield due to Material Properties in a Heat Generating Two-Layered Compound Cylinder

In this theoretical study, based on Trescas yield criterion and its associated flow rule, the elastic deformation of a centrally heated compound cylinder with fixed ends is investigated analytically by taking into consideration not only the geometrical but also the material parameters such as yield strength, modulus of elasticity, Poissons ratio, thermal conductivity and coefficient of thermal expansion. These material parameters are assumed to be independent of the temperature. The compound cylinder is assumed to be very long such that axisymmetric condition exists. Both of the constituent materials of the two layers are supposed to be elastic-perfectly plastic materials. There is heat generation in the interior solid cylinder but no heat generation in the outer hollow cylinder. Both of the cylinders are assumed to be bounded perfectly at the interface. Elastic stress analysis is performed to prevent yield in the compound cylinder. Keywords: Compound cylinder, elastic stress analysis, thermal stress, yield strength.