Weld Residual Stress (WRS) Modeling of Layered Pressure Vessels (LPV) and Fracture Assessment Available to Purchase

NASA has numerous non-code layered pressure vessel (LPV) tanks that hold various gases at pressure. Since replacement costs of the tanks would be high an assessment of the pressure vessels’ capabilities for continued use is desired. Layered tanks typically consists 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. The number of thinner layers required depends on the thickness required for the complete vessel. All cylindrical layers are welded longitudinally with staggered welds so that the weld heat affected zones do not overlap. The built-up shells are then circumferentially welded together or welded to a header (or nozzle) to complete the tank construction. This paper presents computational weld residual stress (WRS) modeling results of two representative layered tanks; (i) a small 4-layer tank and (ii) a large 14-layer tank. Contact between the layers must be considered which led to some convergence difficulties that were overcome. These predictions are compared with the corresponding monolithic tanks (non-layered) of the same size and thickness along with comparing to some compiled API-579 [1] WRS solutions. In general, the WRS fields in layered tanks are quite different from those in corresponding monolithic tanks and the effect of layering is necessary to include in the modeling. In addition, since the toughness of some aged tanks can be low the effect WRS on cracks may important. This is examined by introducing cracks into the tanks at locations where cracking may occur using the finite element alternating method (FEAM). Comments about the effect of WRS fields on fracture are also made.