Test Proof Of Studying On Space Tank's Crack Enlargement Effects Under Over-loading

In space engineering area, as the limit on the weight of the aero spacecraft, it requires the parts of aero-spacecraft have the least weight. High oxygenic pressure vessels worked as the environmental control and ecological protection has no exception. So material selecting and design are the most important among all the elements such as safety, reliability and weight in the balanceable accept or reject process. According to a general meaning of fracture controlling of pressure vessels, it is the target to raise the ability of resisting fractures of the structures through designing, material selecting and manufacturing. But for a space tank that needs a thorough safety and reliability, an appropriate rule of fracture controlling that can eliminate the unsafe tanks through proof test must be set up. There are several material selecting elements should be considered as follows: (1)In order to guarantee the lightweight, The ratio of material's mass to strength should be as small as possible. (2) Material's performance can be stable in a long period under high oxygenic pressure itself or through defend measures. (3)material have the acceptable performance of molding , jointing and processing. (4)satisfying the flying environmental requests (5)decrease the artifactitious cost as lower as possible.Actually, international aerospace craft loaded with people has had a history of forty years. High oxygenic pressure vessel's manufacture technique and material make a gradual progress in astronauts'vital safeguard and environmental control technologic area. As the resource,financial and the aim of launching are different from country to country and the way of supplying oxygen have a great difference ,so gas tank's structure and material selecting are different. Though many light weight and good security's product are provided because of the development of the space Composite tanks, as the deposited oxygen environment's particularity, Metal tank is still the main space high oxygenic pressure vessel in the world.Ultra-high strength steel is a kind of specific strength structural material which is based on this requirement in order to adapt to the development of Aeronautics and Astronautics technique. As to ultra-high strength steel, with the improving of its strength, the fatalness of working under low-stress circumstances is Ultra-high strength steel's SCC is the most important material character to affect space tank's security as Ultra-high strength steel structure is apparent sensitive to environment. But if it is tested through the usual crack specimens for fracture toughness tests, the result will be obviously affected with the indurations effect and residual stress field caused by the alternating loads in fatigue cracks preparation. This will be a particularity of SCC tests for ultra-high strength steels. Special methods was used in crack preparations in this paper, a KISCC measuring method for ultra-high strength steel of 37SiMnCrNiMoV used on Gas-tanks is studies, by what an influencing rule of overloading levels and overloading times on KISCC can be concluded. with the studies of the plastic zone, residual stresses and work hardening zone caused by overloading around the crack Tips, a reference scale for the optimum overloading parameters is provided by orthogonal test design method, avoid adding overloading times and overloading levels blindfold, quicken the experiment process, saved the experiment cost and reached the aim of deciding the optimized technological parameters combination.Besides, a finite element method calculating software of MSC.MARC2000 is used to make modeling calculation on the surface crack .with the calculation of the variations of the distributions of residual stresses, plastic zone and deformations with the load circulations around the crack tips, fracture conditions of the tanks under different over-loading conditions are analyzed, this can be the bases of establishing the methods of fracture controlling.On the basis of the above discussions through experiments and calculations, the influences of proof test on the crack are analyzed.