Fatigue Testing Machine For Cryogenic Environment And Fatigure Properties Of Cold Stretched Austenitic Stainless Steel

With the development of cryogenic technology and the growing energy demand, the storage and transport of cryogenic liquid gases have become an important and effective way for transporting energy. The storage of LNG and other liquefied gases have been an important step in the development strategy of global energy. To transport LNG, for example, cryogenic pressure vessels are commonly used in the field of medical treatment, chemical engineering, and aerospace, playing an important role in the development of energy plan about our country’s "Twelve Five-Year Planning Program".Cold stretched cryogenic pressure vessels from austenitic stainless steels have the advantages of thinner thickness, lighter weight (the inner vessel is about30-50%lower in comparison with the conventional ones), bigger volume, lower cost, and lower energy consumption, which are consistant with the development concept of "low carbon","green", and "light". It is great and meaningful to impell the development of cold stretched cryogenic pressure vessels, and to fulfill the concept of safe and economy by researching on behaviors of cold stretched cryogenic pressure vessels and responses of cold stretching on strength, ductility and fatigue properties of materials or structures.Fatigue response is important for the design of cold stretched cryogenic pressure vessels and risk accessments in vessels’entire lives. Nowadays, these products have been included in the standard of Europe, Australia, America, ISO, and China, though there are no design details about the fatigue for cold stretched pressure vessels, and no relative design about the effect of cryogenic temperature on material properties. Furthermore, it is found that there are few methods to test the material’s fatigue properties at cryogenic temperature, and there are no equipments for such functions in China.Based on the research of fatigue design of cold stretched cryogenic pressure vessels, this paper aims to design and manufacture the fatigue testing machine for cryogenic environment, so the fatigue properties at room and cryogenic temperature environment could be obtained. The main contents and work of this paper are described here: (1) Based on the mechanical response of austenitic stainless steel’s tensile and fatigue tests, the parameters of the fatigue testing machine for cryogenic environment are determined such as the load, the temperature, and data detecting methods. Considering the loading structures, temperature control system, machine control system, data collecting system, and safe security, some integrated methods to mix the temperature control system and the testing function are proposed. The fatigue testing machine for cryogenic environment is manufactured, and is justified to be workable in tensile and fatigue tests from room temperature to-196℃. Strain detecting method is found by comparing crosshead control method and strain gauge control method for cryogenic environment.(2) Fatigue tests of cold stretched austenitic stainless steel and vessels are conducted, and the mechanical behavior is gained such as material fatigue response, cyclic stress-strain curve and strain-life curve. The mechanism of improving the fatigue resistance by cold stretching is analyzed from material’s mechanical properties, strain-induced martensite transformation, and dislocation’s movement. The S-N curves about cold stretching are gained to analysis the effect of cold stretching, and the curves are compared with the ones in standards. Further, the use of ASME VIII-2design curves for cold stretched vessels are justified.(3) Based on the tests’data at room and cryogenic temperature, the response of tensile and fatigue under the influence of cryogenic temperature and cold stretching are obtained. Under the test parameters of this paper, the strengthening effect in tensile and fatigue behavior by cryogenic temperature and cold stretching have been discussed from the points of view with micro and macro behavior in phase structure and mechanical properties respectively. The improving effect of cryogenic temperature and cold stretching on material’s fatigue life has also been discussed.We would like to acknowledge the financial support from the key projects in the National High Technology Research and Development Program (863Program)(Project No.:2009AA044801).