autofrettage

Autofrettage is a mechanical process used to enhance the fatigue life and burst strength of a component, typically a pressure vessel or a high-pressure pipe. The fundamental principle involves inducing compressive residual stresses on the inner surface of the component. This is achieved by over-pressurizing the component beyond its normal operating pressure, causing the material to yield plastically. When the pressure is subsequently released, the outer layers of the material, which did not yield as much, constrict the inner, yielded layers. This elastic spring-back creates a state of beneficial compressive stress at the bore surface.

The effectiveness of autofrettage is directly related to the amount of plastic deformation that occurs. A higher degree of over-pressurization, within the material's limits, generally leads to greater compressive residual stresses and thus a more significant improvement in fatigue life. The process can be applied using various methods, including hydraulic pressure, thermal methods, or mechanical means. Hydraulic autofrettage, where a fluid is used to generate the internal pressure, is a common technique.

The primary benefit of autofrettage is the significant increase in the fatigue life of components subjected to cyclic loading. By counteracting the tensile stresses that initiate fatigue cracks, the compressive residual stresses effectively retard crack propagation. This is particularly important in applications where components experience repeated pressure fluctuations, such as in hydraulic systems, gun barrels, and high-pressure reactors. Autofrettage also increases the ultimate burst pressure of a component by introducing a more favorable stress distribution.