Thick walled pressure vessels are often autofrettaged in order to impart favorable near bore compressive residual stresses which can significantly increase the life of the vessels. These stresses can be imparted via a thermal shrink process in which there is no loss of residual stresses due to the Bauschinger Effect, or more economically with a mechanical swage or hydraulic overload process in which the Bauschinger Effect is present.
In some cases these vessels have holes bored through the wall in order to take advantage of the escaping gasses for actuation of external peripherals associated with the vessel. These through holes, which can be angled or perpendicular to the centerline of the pressure vessel can significantly reduce the fatigue life of the vessel depending on the wall ratio of the vessel as well as the, angle of inclination of the hole to the centerline of the vessel.
This study utilizes the classic stress based Paris Law fatigue life approach which takes into account the residual stresses as a function of radial location to assess the life of the vessel in the region of both configurations of through holes. It quantifies the concentration of stresses associated with the perpendicular and angled through wall holes, and the amount of pressure that actually enters the hole. The analysis is employed to ensure that the life within these through holes meets or exceeds the safe life of the vessel without holes which has been determined by assuming a log normal distribution from of a sample size of six with the 90% lower confidence bound on the 0.1th percentile of the population.