Fracture mechanics behavior of a NiFe superalloy sheath for superconducting fusion magnets. Part 2. magnet life analysis model

From previous results of fatigue crack growth and fracture toughness measurements for a NiFe base superalloy, a primary candidate for the International Thermonuclear Experimental Reactor (ITER) central solenoid (CS) conduit, we derive an improved magnet life analysis model from the framework of Newman and Raju. For the superalloy conduit with an initial semielliptical surface crack in its thickness direction, the model predicts the evolution of crack aspect ratio for a wide range of initial crack geometries under pure tension and bending fatigue. The prediction of final fracture due to fatigue crack growth using the linear elastic fracture mechanics approach is shown to be underconservative. An alternative model based on Newmanʹs elastic-plastic fracture toughness parameter is derived for the base metal with nearly semicircular cracks. The improved life analysis model taking into account the fatigue and fracture behavior is applied to the ITER CS magnet and the results are compared with these from earlier models. Accounting for the crack shape evolution leads to significantly longer life compared to assuming a constant aspect ratio. For the superalloy base metal we find that the expected fatigue life of Engineering Design Activity (EDA) design of the CS magnet is about eight times the design requirement. Even the Conceptual Design Activity (CDA) design with a free-standing CS meets the life requirement when analyzed by the improved model.