Analysis of Swage Autofrettage in Metal Tube

Autofrettage (self-hooping) is used to induce advantageous residual stresses into pressure vessels to enhance their fatigue lifetime. The process is achieved by increasing elastic strength of a cylinder with various methods such as hydraulic pressurization, mechanical swaging, or by utilizing the pressure of a powder gas. This research work deals with the swage or mechanical autofrettage of metal tubes. The objective is to attain a bore size of 125mm. Normally such a bore size is achieved with hydraulic autofrettage. However, we have used two-stage mechanical autofrettage to achieve the desired bore size. At first stage the swage diameter achieved is 118mm, and in the second stage, the diameter achieved after machining is 125mm. The temperature variation for swage is 38°C to 50°C. The applied pressure varies from 85 to 180 bars inside the tube. The process was applied to a number of tubes selected randomly. The swage autofrettage process was also analyzed using numerical simulation based on finite element method. The results of numerical simulation are compared with design parameters.