The actual unloading behavior effect on thermo-mechanical stress intensity factor and life of autofrettage tubes

Life of autofrettage tubes are estimated more accurately if the actual material properties are used. It has now become evident that tubes made of high strength steels exhibit a profound Bauschinger effect. Due to this fact, none of the idealized hardening models are capable of modeling such material’s unloading behavior properly. Therefore, actual material behavior should preferably be employed for an accurate account of residual stresses induced by autofrettage and thermo-mechanical stresses induced by operational conditions. The resulting stresses calculated based on the actual material behavior can then be used to calculate stress intensity factors by weight function method for fatigue life determination. Using standard fatigue crack growth relationships, life of the vessel is then calculated based on recommended initial and final crack length. Thermal loading induced by firing of gun barrels is considered in this analysis. It is shown that thermal stresses have a significant effect in the proximity of the bore (at a distance of about 0.5 mm from the inner surface) and combined effect of the thermal and autofrettage stresses cause initiation of a crack of length 0.5 mm in the first few cycles of loading, significantly reducing the vessel’s life. Calculated life span results based on the above procedure showed very good agreement with available experimental results.