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. The combined Stress Intensity Factors due to internal pressure and autofrettage as a function of crack depth for a large number of internal/external crack configurations were determined by the Finite Element Method . A realistic experimentally based autofrettage model, incorporating the Bauschinger effect, was applied, replicating Hydraulic and Swage autofrettage residual stress fields accurately. Fatigue lives were evaluated by integrating Paris' Law. The following conclusions can be drawn from the results: Hydraulic and Swage autofrettage, have a dramatic beneficial effect in extending the fatigue life of a barrel 4-11 times as compared to an identical non-autofrettaged tube. The fatigue life of overstrained barrels is controlled by internal cracking, for barrels overstrained by up to 100% Hydraulic autofrettage and up to 70% for Swage autofrettage, and by external cracking for 100% Swage autofrettaged. Eliminating or carefully designing stress concentrators on the tube's external face and keeping away corrosive agents thus, enables the increase of the level of Swage autofrettage to up to e=100%. Swage autofrettage is 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.

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