Fatigue Crack Growth Behavior of Metastable Austenitic Stainless Steel in Cryogenic High Magnetic Field Environments
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AUSTENITIC stainless steels are extensively used to construct cryogenic structural members (e.g., pressure and vacuum vessels, components of the superconducting magnet structures for magnetic fusion energy devices, components of the cryogenic transfer lines, and). In the superconducting application, the alloys sustain high stresses in high magnetic fields at liquid helium temperature (4 K). It is known that plastic deformation at cryogenic temperatures induces martensitic transformation in metastable austenitic steels and that the presence of a strong magnetic field enhances the transformation. If the metastable austenitic steels undergo straininduced martensitic transformation under service conditions, there may be unanticipated effects, such as changes in the mechanical properties that can potentially degrade the performance of the device.[1] Also, the components of the superconducting magnets are subjected to loadings in service that are cyclic in nature,[2] and extension of flaws is possible if such flaws exist in the appropriate size, orientation, and location. While the termination of a component’s life may be based on the critical flaw size for large scale rupture, as calculated from the fracture toughness of materials, it has long been recognized that the total useful life of a cyclically loaded component is dependent upon the growth rate of flaws from a subcritical size to a critical size. Therefore, an understanding of both the fatigue crack growth and fracture characteristics of structural alloys under appropriate service conditions is essential to evaluate the structural integrity and useful life of components or to
establish corresponding nondestructive inspection criteria. Previous works have demonstrated that the presence of a magnetic field can influence the fracture properties of austenitic stainless steels at 4 K. Recently, Shindo et al.[3] investigated the fracture behavior of 304 stainless steel, a metastable alloy, in a 4 K, 6 T (T: Tesla) magnetic field environment using the compact tension (CT) specimens with side-grooves. They found that the precracking temperature affects measured fracture toughness. The fracture toughness was determined in accordance with JIS Z 2284,[4] the first cryogenic standard for elastic-plastic fracture toughness test. On the other hand, no one has examined the fatigue crack growth behavior of austenitic stainless steels at 4 K in high magnetic fields. In general, fatigue crack growth data for cryogenic high magnetic field conditions are difficult to obtain because of the cost of testing as well as the complexity of the experimental setup. In this article, we report examination of the fatigue crack growth behavior of 304 stainless steel in cryogenic high magnetic field environments. Fatigue crack growth tests were performed with the side-grooved CT specimens at 4 K in 0 and 6 T magnetic fields to establish the crack growth rate as a function of the J-integral range. The J-integral range was calculated from an elasticplastic finite element analysis. The martensite measurement a
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