What is autofrettage and how does it increase pressure vessel capacity?

Autofrettage involves pressurizing a thick cylinder beyond yield to induce controlled plastic deformation at the inner surface. Upon unloading, the outer elastic region compresses the inner plastic region, creating beneficial compressive residual stresses. This effectively pre-stresses the cylinder, allowing higher operating pressures without yielding. Used in gun barrels, hydraulic cylinders, and high-pressure vessels. Optimal autofrettage pressure yields about 50% of wall thickness.