New Failure Prediction Model for a Ni-Fe Superalloy Sheath for Superconducting Fusion Magnets

posted Apr 15, 2013, 10:15 PM by Seunghyun Kim   [ updated Apr 15, 2013, 10:18 PM by Seung Chang Yoo ]
J.H. Kim, I.S. Hwang and R.L. Tobler
Advances in Cryogenic Engineering (Materials) 44 (1998) 33-40
From fatigue crack growth rate and fracture toughness measurements for a Ni-Fe base superalloy, a new failure prediction model for superconducting magnet sheath is developed. For the superalloy sheath with a semielliptical surface crack growing in its thickness direction, the fatigue crack growth behavior including the evolution of crack aspect ratio is predicted with a good accuracy for a wide range of initial crack geometries under pure tension and/or bending. The existing prediction model for final fracture based on the linear elastic fracture mechanics is shown to be underconservative due to large scale yielding at the crack tip. An alternative model based on the elastic-plastic fracture mechanics is derived for the conduit geometry. A three dimensional J-integral based on the behavior of the conduit material at 4 K was calculated by using the finite element method. Crack initiation stresses determined from 3-D J-integral agree well with measured residual strength at 4 K whereas the KIC-based existing model overestimates the strength by several times. While the failure of the linear elastic fracture mechanics and the success of J-integral approach have been demonstrated for the Ni-Fe superalloy, the finding leads to a generalization that much higher toughness would be required for all superconducting magnet structural materials.