Increased Pressure Vessel Fatigue Life Caused by Decreased Autofrettage

Pressure vessels, while gaining fatigue life at interior surfaces caused by autofrettage residual stresses, suffer a decreased life when fatigue cracking initiates at the exterior. This situation may result in a less than optimum vessel life if maximum autofrettage is applied. A series of controlled experiments have been performed along with analyses to maximize the life of a pressure vessel by optimizing the level of autofrettage.

The experiments used specimens with an outer diameter of 284 mm, a 64-mm wall thickness, and an exterior groove. Autofrettage overstrain levels were varied incrementally from 100 to 50% with fatigue life increases over 100%. Normally, pressure vessels would experience a decrease in fatigue lives with decreasing overstrains of this range. In this experiment, because of the exterior stress concentration of the groove, fatigue lives increased with the reduced overstrains. The work has been subsequently applied to operating pressure vessels with fatigue test data taken on these vessels.