FRACTURE-MECHANICS BEHAVIOR OF A NI-FE SUPERALLOY SHEATH FOR SUPERCONDUCTING FUSION MAGNETS .2. MAGNET LIFE ANALYSIS MODEL

From previous results of fatigue crack growth and fracture toughness m easurements for a Ni-Fe base superalloy, a primary candidate for the I nternational Thermonuclear Experimental Reactor (ITER) central solenoi d (CS) conduit, we derive an improved magnet life analysis model from the framework of Newman and Raju. For the superalloy conduit with an i nitial semielliptical surface crack in its thickness direction, the mo del predicts the evolution of crack aspect ratio for a wide range of i nitial crack geometries under pure tension and bending fatigue. The pr ediction of final fracture due to fatigue crack growth using the linea r elastic fracture mechanics approach is shown to be underconservative . An alternative model based on Newman's elastic-plastic fracture toug hness parameter is derived for the base metal with nearly semicircular cracks. The improved life analysis model taking into account the fati gue and fracture behavior is applied to the ITER CS magnet and the res ults are compared with those from earlier models. Accounting for the c rack shape evolution leads to significantly longer life compared to as suming a constant aspect ratio. For the superalloy base metal we find that the expected fatigue life of Engineering Design Activity (EDA) de sign of the CS magnet is about eight times the design requirement. Eve n the Conceptual Design Activity (CDA) design with a free-standing CS meets the life requirement when analyzed by the improved model.