Effect of Strain-Hardening Exponent and Strain Concentrations on the Bursting Behavior of Pressure Vessels

Studies by the Subcommittee for Effective Utilization of Yield Strength of the Pressure Vessel Research Committee of the Welding Research Council have provided a better understanding of the behavior of pressure vessels in the bursting mode of failure. Specifically, these studies have shown that high-strength steels can be more effectively utilized in pressure vessel applications, and with appropriate safety. However, before specific Code changes are recommended, the possible influence of undetected sharp flaws on the burst pressure, as predicted by the modified Svensson equation, should be established. Accordingly, a study of six notched pressure vessels was conducted to establish the limitations of the Svensson equation with respect to severe strain concentrations, namely, sharp longitudinal notches. Three steels (A517, A516, and 304SS) having a wide range of strain-hardening exponents (0.09, 0.19, and 0.59) were used to fabricate thin-walled pressure vessels (16-in. (406 mm) O.D., 1/2 in. (13 mm) wall thickness, 48-in. (1.22 m) length). Each vessel had a 15-in. (381 mm) long sharp machined notch with flaw depths ranging from 15 to 35 percent of the wall thickness. These vessels were tested hydrostatically to burst at room temperature. All failures were ductile. The results indicate that for pressure vessel steels having nominal yield strength up to 115 ksi (793 MN/m²) and normal ductility and toughness, the modified Svensson equation can be used to predict burst pressure very reliably as long as the flaw depths are less than 25 percent of the wall thickness. On the basis of these test results, as well as burst tests of vessels with moderate strain concentrations such as nozzles and flat end closures, it is recommended that the terms F_cyl and F_sph (factors that describe the effect of strain-hardening exponent on the bursting behavior of cylinders and spheres) be incorporated into the appropriate Code provisions. It is further recommended that the appropriate Code committee consider a possible reduction in the factor of safety against bursting on the basis of the results of this investigation.