Weld Residual Stress Modeling of and Fracture Assessment of Layered Pressure Vessels

NASA has hundreds of non-code layered pressure vessel (LPV) tanks that hold various gases at pressure. Many of the NASA tanks were fabricated in the 1950s and 1960s and are still in use. An agency wide effort is in progress to assess the fitness for continued service of these vessels. Layered tanks typically consist of an inner liner/shell (often about 12.5 mm thick) with different layers of thinner shells surrounding the inner liner each with thickness of about 6.25-mm. The layers serve as crack arrestors for crack growth through the thickness. Most tanks have between 4 and 20 layers. Cylindrical layers are welded longitudinally with staggering so that the weld heat affected zones do not overlap. The built-up shells are then circumferentially welded together or welded to a header to complete the tank construction. This paper presents results which consider weld residual stress and fracture assessment of some layered pressure vessels. This is part of the much larger probabilistic fitness for service evaluation of these tanks. All fabrication steps are modeled, and the high-level proof testing of the vessels has an important effect on the final WRS state. Because the tanks have low toughness the weld residual stress state has an important effect on the fitness for service of these tanks and are tabulated for use in the probabilistic code.