Fatigue and Fatigue Crack Growth Properties of 316LN and Incoloy 908 Below 10 K

Abstract

The cyclic loading characteristics of Tokamak type thermonuclear machines demand for an answer towards the fatigue response of the materials used in critical components. As one of the main outstanding parts of such a device the large superconducting magnets and their superconductors will operate under cyclic mechanical stress conditions. The present paper is biased towards the current superconductor design of the NET (Next European Torus) model coil concept¹. The superconductor of this coil will be a cable-in-conduit Nb₃Sn type with an enveloped stiff external jacket structure. The wall thickness of the jacket structure is within the range of 4–5 mm in accordance with the recent structural mechanics calculations. The manufacturing of the jacket lengths for several hundred meters require an appropriate joining process due to the prefabricated section pieces available only in short lengths of 5–7 meters. At the ongoing technical discussions the recently anticipated solution favors the flash butt welding technique, which seems to be quite reasonable considering the present industrial practice. The performance of the superconductors jacket will strongly depend on the material selection and the proper structural design according to the existing low temperature structural materials data base. The wind and react Nb₃Sn-manufacturing process must also account the materials properties after ageing. To envisage all these aforementioned factors a material test program was set up to elucidate the fatigue-life behavior and fatigue crack growth rate (FCGR) of the recently selected two candidate materials. These materials were the AISI 316LN with a specified low carbon content to avoid the embrittlement after the ageing process² and the material Incoloy 908. Both materials were investigated in aged condition. In addition, the 316LN material in the as received condition was also tested with respect to its fatigue-life for specimens bearing predefined flaws and cracks. For more practical engineering relevance the propagation of surface cracks at 12 K and at 295 K was characterized with non standard specimens. All these tests were performed in a newly developed cryogenic dynamic test facility under helium gas environment between 7 K and 20 K. Using the reference growth laws obtained from all these measurements the total crack propagation starting with the initial crack length of the specimen could be predicted by numerical computation.