Abstract
As a key technique, reinforcement of type-II superconducting bulks with metal rings can efficiently improve their mechanical properties to enhance the maximum trapped field. In this paper, we study the magnetostrictive and fracture behaviors of a finite superconducting ring bulk reinforced by three typical reinforcing structures composed of metal rings during the magnetizing process by means of the minimization of magnetic energy and the finite element method. After a field-dependent critical current density is adopted, the magnetostriction, pinning-induced stress, and crack tip stress intensity factor are calculated considering the demagnetization effects. The results show that the mechanical properties of the ring bulk are strongly dependent on the reinforcing structure and the material and geometrical parameters of the metal rings. Introducing the metal ring can significantly reduce the hoop stress, and the reduction effect by internal reinforcement is much improved relative to external reinforcement. By comparison, bilateral reinforcement seems to be the best candidate structure. Only when the metal rings have particular Young’s modulus and radial thickness will they contribute to improve the mechanical properties the most. In addition, if an edge crack is pre-existing in the ring bulk, the presence of metal rings can effectively avoid crack propagation since it reduces the crack tip stress intensity factor by nearly one order of magnitude.
Highlights
By means of the minimization of magnetic energy and the finite element method, this paper presents an investigation of the magnetostrictive and fracture behaviors for a type-II superconducting ring bulk reinforced by external, internal, and bilateral metal rings in an axial magnetic field produced by an infinite solenoid coil
The results show that after introducing the metal rings, a vertical flipping behavior of the magnetostriction curves for the radial wall thickness is found, and the largest hoop stress occurring at the inner surface of the ring bulk is significantly reduced
Bilateral reinforcement seems to be the best candidate structure, where the radial and hoop stresses should be simultaneously considered to evaluate the mechanical stability of the ring bulk due to their equal importance
Summary
Type-II superconducting bulks can trap magnetic fields, and have considerable potential in the provision of a strong magnetic field for engineering applications and in replacing conventional permanent magnets.[1,2] In particular, the ring-shaped bulk cylinder with a concentric hole has been widely used as key components in devices such as magnetic bearings or inductive fault current limiters.[3,4,5] More recently, with the improvement of the trapped field from a view of superconducting properties and the enlargement of the bulk size, the ring bulk has extended its application to a nuclear magnetic resonance spectrometer and a magnetic resonance imaging apparatus.[6,7,8] To promote the resolution of these devices, it is necessary to achieve a stronger trapped field in the bore of the ring bulk. As a key technique to improve the mechanical properties, reinforcement of the bulk with metal rings can effectively reduce the mechanical stress and inhibit the crack generation or propagation.[1,19,27] Johansen et al analyzed the pinning-induced stress of a long bulk cylinder, and found that an ideal clamping ring can reduce the maximum tension by 22% and 40% for field-cooled and pulsed-field activation, respectively.[28] Yang et al investigated the stress and magnetostriction of an infinite hollow cylinder with a filling in its central hole, and indicated that the hoop stress concentration induced by the pinning force is effectively suppressed.[29] Mochizuki et al simulated the hoop and radial stresses during the pulsed field magnetization of an infinite GdBaCuO ring bulk reinforced by an external aluminum alloy ring, and confirmed that a huge hoop stress is concentrated at the innermost peripheral edge of the ring bulk.[30] These works focused on the bulk sample by assuming that it has a sufficiently large axial length within the frame of the critical state model. The fracture behavior induced by the pinning-induced force is investigated
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