Impact of Intensity of Residual Stress Field Upon Reyielding and Re-Autofrettage of an Autofrettaged Thick Cylinder

Abstract

Re-autofrettage has been identified as a significant, cost-effective method to achieve higher reyield pressure (RYP) and/or weight reduction in large caliber gun tubes. For a given overstrain, residual stress profiles for hydraulic and swage autofrettage may differ significantly in their intensity. The simplest representation of this “intensity” effect is the magnitude of the bending moment “locked in” via the residual hoop stress. Hill’s analytical, plane strain, Von Mises analysis predicts a larger “locked-in” moment than does the equivalent open-end condition. By assuming a range of stress-field intensities (f) scaling from 1.0 to 1.4 times that were produced by open-end hydraulic autofrettage, it was possible to assess reyield behavior following initial autofrettage via a generic numerical study. In cases where Bauschinger effect is absent, reyield initiates at the original elastic–plastic interface. This includes the ideal Hill distribution. When Bauschinger effect is present, reyield for f≤1.1 initiates at the bore and after further pressurization at the original elastic–plastic interface within two zones. For f≥1.2, the reverse is the case, with initial yield at the original elastic–plastic interface and subsequently at the bore. RYP increases with increasing f up to f = 1.175 and then decreases significantly. This loss of RYP may be mitigated by hydraulic re-autofrettage. At f = 1.0 re-autofrettage increases RYP by 4%. At f = 1.4, RYP is increased by 19%. There are modest increases in safe maximum pressure (SMP) as a result of re-autofrettage. RYP closely approaching re-autofrettage pressure is achievable for f≥1.3. Within this range, re-autofrettage offers a significant benefit. Re-autofrettage also produces beneficial effects via increased bore hoop compressive stress, this increase varying from 20% for f = 1% to 0% for f = 1.4. Such increased compression will benefit fatigue lifetime for fatigue cracks initiating at the bore. Conversely, tensile outside diameter (OD) hoop stress increases, with increasing f, by a maximum of 6%.