(A) Theoretical and Experimental Study of Thermal Autofrettage Process

Abstract

Thick-walled cylinders/hollow-disks find wide range of industrial applications to withstand very high pressure. Pressure vessels, gun barrels, fuel injection system in diesel engines, nuclear reactor and fastener holes are the typical examples. The cylinder may crack if the working internal pressure exceeds the yield strength of the material. The pressure carrying capacity and fatigue life of the thick-walled cylinders used for high pressure application can be significantly improved by inducing beneficial compressive residual stresses at and around the inner wall of the cylinder. This is achieved by employing a process called autofrettage, prior to their use in service condition. The most commonly employed autofrettage processes are hydraulic and swage autofrettage process. The hydraulic autofrettage is achieved by pressurizing the cylinders to an ultra-high hydraulic internal pressure. The swage autofrettage is achieved by pushing an oversized mandrel through the inside of the cylinder to plastically deform the inner wall and some portion beneath it. The autofrettage can also be achieved by detonating an explosive charge inside the vessel, which is called explosive autofrettage. All these existing processes have certain disadvantages. In order to avoid the difficulties associated with the existing methods of autofrettage, in the present thesis, a thermal autofrettage process is proposed. The proposed thermal autofrettage process is achieved by creating a radial thermal gradient across the wall thickness of a cylinder or hollow disk. The proposed thermal autofrettage process is very simple and easy to handle compared to the existing methods of autofrettage.