Thick-walled cylinders such as gun barrels, high pressure containers, and rocket shells are designed to withstand high pressure. The cylinder material may crack if the induced pressure exceeds the material yield strength. Therefore, the thick-walled cylinders are autofrettaged in order to withstand very high pressure in service condition. The most commonly practiced autofrettage processes are hydraulic autofrettage and swage autofrettage. Hydraulic autofrettage involves very high internal pressure at the bore of the cylinder, and in swage autofrettage an oversized mandrel is pushed through the cylinder bore to cause the plastic deformation of the inner wall of the cylinder leaving the outer wall at the elastic state. This results in compressive residual stresses at and around the inner wall of the cylinder, which reduces the maximum stress in the cylinder during next stage of loading by pressurization. Both the processes are well established, but still there are certain disadvantages associated with the processes. The present work proposes a novel method of autofrettage for increasing the pressure carrying capacity of thick-walled cylinders. The method involves only radial temperature gradient in the cylinder for achieving autofrettage. The proposed process is analyzed theoretically for thick-walled cylinders with free ends. The numerical simulations of the process for typical cases and preliminary experiments show encouraging results for the feasibility of the proposed autofrettage process.

Autofrettage is a means of generating compressive residual stresses at the inner side of a thick-walled cylinder or hollow disk by causing nonhomogeneous plastic deformation of the material at the inner side. The presence of residual compressive stresses at the inner region of the cylinder/disk enhance the pressure withstanding capacity, fatigue life and the resistance to stress corrosion cracking of the component. Despite the hydraulic and swage autofrettage are the widely practiced processes in industries, there are certain disadvantages associated with these processes. In view of this, in the recent years, researchers have proposed new methods of achieving autofrettage. Rotational autofrettage is such a recently proposed autofrettage method for achieving the beneficial compressive residual stresses in the cylinders. In the present work, the rotational autofrettage is studied for a thick-walled hollow circular disk. A theoretical analysis of the residual stresses produced in the disk after unloading are obtained assuming plane stress condition, Tresca yield criterion and its associated flow rule. The analysis takes into account the effect of strain hardening during plastic deformation. Further, the effect of residual stresses in the typical SS304 and aluminum disk is studied by subjecting them into three different types of loads viz., internal pressure, radial temperature difference, and rotational speed individually. A three-dimensional (3D) finite element method (FEM) validation of the theoretical stresses during rotational autofrettage of a disk is also presented.

Usually, autofrettage is achieved either by applying hydraulic pressure to the bore of the thick-walled cylinder or by pushing an oversized mandrel through the bore to deform the cylinder plastically. This produces the compressive residual stresses on the inner side of the cylinder, enabling the cylinder to withstand higher working pressure. There are certain difficulties in these methods of autofrettage. This work proposes a novel method for achieving autofrettage that involves creating temperature gradient in the wall of the cylinder. The proposed process is analyzed theoretically considering strain hardening. The simulation results and preliminary experiments indicate an encouraging trend for the feasibility of the proposed process.

In this work, an experimental study of the residual stresses present in the thermally autofrettaged thick-walled cylinders is carried out. The idea of thermal autofrettage has been conceived recently, and due to its simplicity, it has the potential to be competitive with the existing hydraulic and swage autofrettage processes. In thermal autofrettage, the beneficial compressive residual stresses at the inner wall are produced by means of thermal gradient across the wall thickness. In this work, the residual stresses setup in the cylinders are determined experimentally using Sachs boring method. The experimental results are compared with the analytically determined residual stresses and found to be in good agreement. In addition to the Sachs boring method, two other methods, namely, microhardness test and the measurement of opening angle as a result of cutting through the wall of the cylinders, are also carried out in order to infer the generation of residual stresses in the autofrettaged cylinders. All three experimental methods advocate the feasibility of thermal autofrettage for producing beneficial compressive residual stresses at the inner wall.

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