Mode of deformation and strength of welded shell with tunnel pipe

Thin-walled welded shells are the main type of structures for pressurized modules and propellant tanks in rocket and space engineering. Shrinking in process of shaping the welded seam leads to a distortion of the shape of the shell and, as a consequence to an essential change in the mode of deformation, which, in its turn, affects the strength and the service life. Stages with load-carrying tanks, as a rule, have a built-in tunnel pipe in the lower tank to provide a passage to the engine for the transfer line of the second propellant component. The paper, taking a propellant tank as an example, discusses the behavior of the shell with a tunnel pipe: shape distortion resulting from the weld shrinking, increased level of stress in the weld due to the shape distortion, reduction in the load-carrying capacity and service life. For the purposes of analysis, a finite-element model of the shell was built. Analytical studies were conducted of the dependence of the subsidence of the tunnel pipe flange on the transverse shrinkage of the weld, of the dependence of gain factors of stress on the subsidence of the flange. A decrease in the load-carrying capacity of the shell made of high-strength material was determined. The analyses, which took into account geometric and physical nonlinearity, demonstrated that the shell made of highly plastic material does not lose strength due to shape distortion, but its service life becomes reduced because of increased level of stress caused by repeated loading cycles. The paper identifies the sources of cyclic loads through all phases of the product life cycle.