Stress Corrosion Susceptibility of Aluminum Cartridge Cases

Stress corrosion testing of production 5.56 mm aluminum alloy cartridge cases was investigated. In this study, empty cases of 7475 aluminum alloy, tempered to T6 or T73 condition, were assembled with projectiles to represent the stressed condition of finished cartridges; standard methods for application of stresses (such as bent beam, U-bend, C-ring, tensile) were not directly applicable to finished forms of cartridge cases. Stresses applied to the mouth, rim, and neck of the cases were calculated from the interference, i.e., projectile diameter versus internal diameter of the case mouth, and from the case neck inside and outside diameters. Times to failure of stressed cases immersed in boiling 1N NaCl solution were determined by visual evidence of cartridge case cracking. To evaluate the effect of different levels of applied stress, with respect to failure times, cases in the T6 temper were examined over the stress range of approximately 20 to 80 ksi (138 to 552 MPa). For cases in the T73 temper, the stress range employed was approximately 40 to 80 ksi (276 to 552 MPa) in view of the low incidence of failure and long time to failure at stress levels below 40 ksi (276 MPa). Trends of stress corrosion cracking (SCC) susceptibility were determined. Plotting of results on log-probability scale (log time to failure versus cumulative % failed) indicates a linear relationship. Comparison of the results reveal the T73 cases to be appreciably less susceptible to stress corrosion failure than the T6 cases, as would be expected. This method of testing finished cartridge cases is considered feasible as an accelerated means for assessing SCC susceptibility of aluminum cartridge cases.