Analysis of thermal autofrettage for functionally graded thick-walled cylinders

Abstract

In recent times thermal autofrettage has been revealed to be a potential procedure for strengthening thick-walled cylindrical pressure vessels. The process is well-demonstrated for homogeneous solids both theoretically and experimentally. Due to the increasing trend of using functionally graded cylinders in industries, this paper endeavours to explore the applicability of thermal autofrettage to functionally graded cylinders for the first time. A theoretical approach is made to investigate the development of stresses during thermal autofrettage of a thick-walled cylinder made with functionally graded material. Under the application of a through thickness temperature gradient to the functionally graded cylinder, the loading stage of thermal autofrettage is modelled to exhibit elastic-perfectly plastic behaviour incorporating von Mises yield criterion. The cylinder end condition is assumed to be open-ended conforming generalized plane strain condition. The in-loading thermo-elastic-plastic stress formulation is carried out and post-unloading of temperature gradient, the residual stresses induced in the cylinder are evaluated. The current process is exemplified for a typical FG cylinder composed of aluminium alloy (Al A359) and silicon carbide (SiC). The analysis reveals that a significant amount of compressive residual stresses of the order of 0.99 times the yield strength at the inner radius can be induced by thermal autofrettage in FG cylinder which enhances its pressure carrying capacity. For instance, in the present Al A359/SiC FG cylinder of wall thickness ratio 3, the maximum achievable pressure carrying capacity is nearly 40 %.