Research On Strength Behavior Of Austenitic Stainless Steel Cryogenic Pressure Vessel Considering Triaxiality Effect

Cryogenic pressure vessels, which are used as normal storage and transportation equipments for cryogenic liquefied gases, are widely used as extreme pressure equipments in the fields of energy, chemical machinery, aerospace engineering and so on. With excellent cryogenic mechanical properties, austenitic stainless steel is now widely used in the manufacturing of cryogenic pressure vessels.To achieve national development concept of "low carbon" and "green", vessels thickness, weight, and energy consumption should be reduced through adopting light weight technologies such as increasing allowable stress, cold stretching technology, and design by analysis. In this context, researches on strength of austenitic stainless steel vessels at room temperature and cryogenic temperature, which reveal the strength margin of cryogenic vessels under different working conditions, have great significance in improving the design method of cryogenic vessels and realizing the balance between security and economy.Ductile rupture due to excessive short term static load is one of the most essential strength failure modes, which is also the focus of this dissertation. Previous research focused on plastic collapse load of simple pressure vessel at room temperature. However, few research has been introduced on triaxiality and its influence on material ductility, which could cause local failure of vessel with complex structure. Vessel strength under cryogenic environment is not fully investigated. Supported by the National High Technology Research and Development Program (863Program Project No.2009AA044801) and the Research Fund for the Doctoral Program from Ministry of Education of China (Project No.20090101120161), this dissertation focused on strength of austenitic stainless steel cryogenic vessels, and investigated the effect of triaxiality on fracture strain, strength prediction of cryogenic vessels and design method by elastic-plastic analysis. The main work is listed below.(1) Tensile tests of smooth specimen and notched specimen from austenitic stainless steel were carried out at room temperature and cryogenic temperature. Based on damage mechanics, the parameterized simulation model of the whole process of tensile test was established applying Gurson-Tvergaard-Needleman (GTN) model to predict the influence of notch sizes on the material ductile rupture failure, and analyze triaxiality effect and fracture strain at room temperature and cryogenic temperature. The results showed that high triaxiality and cryogenic temperature would decrease fracture strain.(2) Based on plastic collapse and local failure modes, strength prediction method of pressure vessels was established and proved by burst test at room temperature. This method was used in strength prediction of vessels under cryogenic environment. The result showed that plastic collapse load increased obviously under cryogenic environment, but high triaxiality of complex structure might cause local failure and the strength increase of complex cryogenic vessel might be limited.(3) Based on the strength prediction method established in this paper and advanced foreign standards, a design method by elastic-plastic analysis was introduced considering plastic collapse and local failure. The true stress-strain curve parameter and local strain limit were discussed for Chinese austenitic stainless steel and cryogenic vessels. Different light weight design methods were compared and strength margin of cryogenic vessels under room temperature and cryogenic temperature were analyzed. The results showed that vessels still had enough strength margin after using light weight design.