Possibility of mechanical fracture of superconducting ring bulks due to thermal stress induced by local heat generation during pulsed-field magnetization

Abstract

During quasi-static magnetization of bulk superconductors using field-cooled magnetization (FCM) from high fields at low temperatures, such bulks are sometimes broken, which is believed to be mainly due to an electromagnetic force – and subsequent stress – larger than the fracture strength. However, a ring bulk can break, even during pulsed field magnetization (PFM), from relatively lower pulsed fields and at relatively higher temperatures. Previous simulation results suggest that the ring bulk should not break due to the electromagnetic force during PFM. In this paper, taking experimental and numerical results into consideration, we propose the possibility of mechanical fracture of a ring bulk during PFM due to thermal stress induced by local heat generation, which has not been considered and investigated to date. Two numerical models with different sizes of heat-generating region were constructed for the ring bulk with a relatively large inner diameter (60 mm outer diameter, 36 mm inner diameter, 17 mm height). For Model-1, with a large heat region, the bulk fracture due to the thermal stress results from the tensile stress along the radial direction in the neighboring heat region. The risk of bulk fracture is enhanced at the inner or outer edges of the bulk surface, compared with that inside the bulk. For Model-2, with a small heat region inside the bulk, the bulk fracture due to the thermal stress results from the compressive stress along the radial direction in the neighboring heat region. These results strongly suggest the possibility of mechanical fracture of an actual ring bulk due to thermal stress induced by local heat generation. This idea is also applicable more generally to the fracture mechanism during FCM of superconducting bulks.

Flux jump and mechanical response in inhomogeneous high-temperature superconductor under the pulsed-field magnetization

“…For the second PFM, the maximum thermal stress is smaller than the electromagnetic stress at the beginning of the magnetization, as shown in figure 12(a). Before and after the flux jump, the maximum thermal stress is larger than the electromagnetic stress, confirming that thermal stress is more significant than electromagnetic stress for the breaking of ReBCO bulk with a flux jump [68]. Figure 12(b) shows the stress distributions at different environmental temperatures.…”

“…The coupled magnetic-thermal model and phase-field fracture model were proposed to simulate the flux jump and mechanical failure behaviors of bulk superconductors during PFM [67]. Shinden et al investigated the effect of thermal stress caused by temperature rise on the HTS bulk, and they assumed thermal stress to be the main reason for the fracture of the bulk during flux jumps [68]. Multi-physical fractures in bulk HTS during magnetization have been studied using the phase-field cohesive zone model [69].…”

Analysis of critical current inhomogeneity in r–z plane of GdBCO superconducting bulk and simulation of flux jumps during pulsed field magnetization

“…They showed that flux jumps take place through the notch area and result in the breaking of shielding current. Recently, Shinden et al [33] constructed local heat regions, and applied only 5 J external heat to make the local temperature rise exceed T c , which successfully simulated the loss of shielding ability of ring bulks during flux jumps. For the disk bulk, numerically enhancing the J c characteristics [17] is an effective way to reproduce the breaking of shielding current in bulks and the high trapped field during PFM, but its multi-physical responses may differ from the experiments [26,34].…”

Modelling and mitigating flux jumps in bulk high-temperature superconductors during quasi-static, high-field magnetisation

“…Second, the motion of flux through the sample during magnetisationeven under slow, quasi-static conditions-can induce significant heat, which may generate thermomagnetic instabilities such as flux jumps, leading to quenching (and often catastrophic mechanical failure) of the sample [11][12][13]. It has been shown in the literature that not only can thermal stress generated during a flux jump lead to the mechanical failure of a bulk [14], but also the induced electromagnetic stresses can cause mechanical failure in unreinforced bulks at 7 to 9 T [15][16][17].…”