Internal Versus External Cracking—Their Impact on the Fatigue Life of Modern Smoothbore Autofrettaged Tank Gun Barrels

Abstract

An extensive analysis of the fatigue life of a typical modern autofrettaged smoothbore tank barrel, cracked either internally or externally, in terms of the initial crack depth and shape, type and level of autofrettage, was conducted. Five overstraining cases were considered: no-autofrettage, 70% and 100% hydraulic autofrettage, and 70% and 100% swage autofrettage. KINmax, the maximum combined stress intensity factor (SIF) KINmax = (KIP + KIA) max, due to both internal pressure and autofrettage, as a function of crack depth for a large number of internal and external crack configurations was determined by the finite element method (FEM). A novel realistic experimentally based autofrettage model, incorporating the Bauschinger effect, was integrated into the finite element model, replicating both the hydraulic and swage autofrettage residual stress fields (RSFs) accurately. Fatigue lives were evaluated by integrating Paris' Law using the above values of KINmax. The following conclusions can be drawn from the results: hydraulic and swage autofrettage have a dramatic beneficial effect in extending the fatigue life of an overstrained barrel 4–11 times as compared to an identical nonautofrettaged tube. The fatigue life of overstrained barrels is controlled by internal cracking, for barrels overstrained by up to ε = 100% hydraulic autofrettage, by up to ε = 70% in the case of swage autofrettage, and by external cracking for ε = 100% swage autofrettaged. Eliminating or carefully designing stress concentrators on the tube's external face and keeping away from corrosive agents thus, extending the fatigue-crack initiation life of an external crack, enables the increase of the level of swage autofrettage to up to ε = 100%. Swage autofrettage is much more superior to hydraulic autofrettage. The fatigue life of a 70% swaged autofrettaged barrel is 1.5 times higher than that of a 100% hydraulically autofrettaged tube. If full swage autofrettage is permissible, the fatigue life of such a barrel is twofold that of a fully hydraulically autofrettaged tube. Unlike the commonly accepted concept, the level of hydraulic autofrettage should not be limited to 70%, and full hydraulic autofrettage should be used. Similarly, in the case of swage autofrettage, if the detrimental effect of external cracking is removed by proper design and maintenance of the tube's outer surface, the level of autofrettage can be increased to up to ε = 100%, thus, gaining an increase of 33% in the fatigue life as compared to overstraining the barrel to only ε = 70%. Initial crack depth and shape are major factors affecting the fatigue life of the barrel. The deeper the initial crack depth, a0, and the slenderer its shape, a/c→ 0, the shorter the fatigue life of the barrel.