Effect of autofrettage on the fracture strength of thick-walled high pressure cylinders

Abstract

Armament, hydrostatic extrusion and isostatic compaction equipment are typical examples of very high pressure applications. The cylindrical part of these vessels is usually made out of high strength materials having a diameter ratio between 1.2 and 3.0 with an operating pressure of as high as 400,000 psi. In order to improve the static and fatigue strength of these vessels, an initial compressive stress field must be introduced throughout the wall. Perhaps, the most common method to accomplish this is "Autofrettage", especially when mono-block cylinders are used. The calculation of autofrettage residual stresses is very difficult due to the distinctive Bauschinger effect of the high strength material. As well known, fatigue cracks mostly initiate at the bore of these cylinders and propagate throughout the wall in the longitudinal-radial plane. Elastic and elasto-plastic stress analyses, Linear Elastic Fracture Mechanics, finite element method, and International codes and standards rules have been used to: a) Calculate the design pressure and maximum stresses of a range of thick-walled cylinders b) Evaluate the autofrettage residual stress fields by assuming elastic-perfectly plastic material behavior and Tresca yielding criteria c) Perform correction of the residual stresses for the Bauschinger effect of the material d) Evaluate the Elastic Stress Intensity Factors for a range of crack shapes and sizes, and a range of protected bore cylinders, with and without including the autofrettage residual stress fields e) Repeat step (d) for unprotected bore cylinders The results of the investigation have shown clearly a noticeable improvement in the fracture strength of the autofrettaged cracked cylinders when compared to that of the nonautofrettaged ones. Also, when feasible, preventing the pressure from entering inside the crack, by protecting the bore surface, has an immense beneficial result.