Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes are promising for high-energy and high-field applications. In epoxy-impregnated REBCO superconducting windings, during the cooling process, the delamination induced by thermal mismatch stress significantly threatens stable operation due to the weak c-axial strength of REBCO CC tapes. In this study, a two-dimensional axisymmetric multilayer delamination finite element model with main layers of REBCO CC tapes and insulation materials was developed based on the bilinear cohesive zone model. Based on the proposed model, the stress distribution and delamination properties of an epoxy-impregnated REBCO winding induced during the cooling process were investigated. Furthermore, the effects on the structural configuration on the delamination and optimisation schemes of fabrication are discussed. The model results indicate that during the cooling process of the REBCO winding, when the radial tensile stress induced by the thermal mismatch stress is greater than the delamination strength, interface cracking will occur. Interface failure occurs in regions containing several turns of coils and not just a single-turn coil. Our analyses indicate that structural failure caused by delamination depends on the radius ratio of the outer to the inner winding, where a small radius ratio is preferred to reduce the risk of delamination. An excessive radius ratio can result in multiple delamination failures during the cooling process. The optimisation scheme, which divides the winding into several sub-windings with a consistently small radius ratio, is an effective method for mitigating the risk of delamination failure, which is consistent with available experimental results. Additionally, by reducing the thermal expansion coefficient of the epoxy resin, the risk of delamination failure during cooling can be significantly reduced. For the winding structure considered in this study, if the coefficient of thermal expansion of the epoxy resin is reduced to 5 ( ×106 K−1), then delamination failure caused by thermal mismatch will be eliminated. Our model results are consistent with those of several valuable experimental phenomena and numerical calculated in the literature.

Delamination model of an epoxy-impregnated REBCO superconducting pancake winding

A new test method for the measurement of shear interfacial delamination strength of Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes was implemented in this study. A novel shear specimen was designed and constructed. The shear specimen of epoxy resin was cured by three-dimensional printing technology and embedded with a short REBCO CC tape. The tape plane is parallel to the longitudinal axis of the specimen to realize pure shear load to REBCO CC tape through the universal uniaxial tensile load. The torque effect, non-interface elastic-plastic mechanical deformation and overflowed solder appearing in the traditional testing method are omitted. The experimental results show that the valid shear delamination strength is from 10.30 MPa to 11.98 MPa with the average value of 11.27 MPa, indicating the serious scattering of the tested data in traditional testing methods is eliminated. Through the analysis of delaminated interfaces morphology and the energy dispersive spectroscopy, it is observed that the interface delamination mainly occurs within the REBCO superconducting film itself, and a small portion of buffer layers are also peeled due to the brittle nature. In addition, based on the analysis of invalid data and the corresponding morphology after delamination, it is shown that the initial defects and failures significantly affect the experimental data, resulting in a lower testing value than the real shear interfacial delamination strength.

Measurement of Shear Delamination Strength of REBCO Coated Conductors

In this paper, mechanical responses and electro-thermal characteristics of a rare earth barium copper oxide (REBCO) high-temperature superconducting (HTS) insulated pancake coil during the quenching process are investigated through finite element modeling (FEM). Firstly, a two-dimensional axisymmetric electro–magneto–thermal–mechanical FEM model with real dimensions is developed. Based on the FEM model, a systematic study on the effects of the time taken to trigger the system dump, background magnetic field, material properties of constituent layers, and coil size on quench behaviors of an HTS-insulated pancake coil is implemented. The variations in the temperature, current, and stress–strain in the REBCO pancake coil are studied. The results indicate that an increase in the time taken to trigger the system dump can increase the peak temperature of the hot spot but has no influence on the dissipation velocity. An apparent slope change of the radial strain rate is observed when the quench occurs regardless of the background field. During quench protection, the radial stress and strain reach their maximum values and then decrease as the temperature decreases. The axial background magnetic field has a significant influence on the radial stress. Measures to reduce peak stress and strain are also discussed, which indicates that increasing the thermal conductivity of the insulation layer, copper thickness, and inner coil radius can effectively reduce the radial stress and strain.

Numerical Study on Mechanical Responses during Quench Protection in High-Temperature Superconducting Coils

In high-temperature superconducting coils, the high transport current density and magnetic field generate a large electromagnetic force on REBCO-coated conductor (CC) tapes. A large Lorentz force can cause interlayer delamination damage to the CC tape under radial or transverse tensile loads. The structural delamination characteristics of multilayered REBCO CC tape are essential for the fabrication of mechanically reliable CC coils. In this study, the electromechanical delamination behaviors of practical REBCO CC tapes were characterized using an anvil test method. Our recently established continuous critical current, I c measurement system was used to minimize the test time and can directly evaluate the effect of the transverse tensile load on I c behavior without time-consuming intermittent I c measurements during testing. Characterization of the delamination mechanism, including damage initiation, was performed through scanning electron microscopy observation of the fractured sample. Two-parameter Weibull distribution analysis was used to specify the data scattering of the electromechanical delamination strength.

Characterization of Electromechanical Delamination Behaviors in Practical REBCO Coated Conductor Tapes Under Transverse Tension at 77 K

The invention provides a device and a method for testing fracture toughness of a solid-ice interface of a coating layer material surface under a large-area icing condition. The testing method adopts asingle cantilever beam loading principle, utilizes bending stress of a metal substrate to induce initiation and expansion of microcracks at a solid-ice interface, and aims to observe an interface fracture behavior between the surface of a coating layer material taking a metal as a substrate and an ice layer so as to obtain the interface fracture toughness between the ice layer and the surface ofthe substrate. The method has the advantages that the anti-icing performance of a large-area component coating layer material can be accurately evaluated, operation is easy and convenient, the purposeof objectively evaluating the fracture toughness of a solid-ice interface under a large-area icing condition is achieved, and important significance is achieved for industrial application and development of the anti-icing surface; and the testing device is simple in structure, ingenious in design and easy to operate, and has a good using effect.

Device and method for testing fracture toughness of solid-ice interface of coating layer material surface under large-area icing condition

Measurement of Transverse Tensile Interfacial Strength of REBCO-Coated Conductors

The invention provides a method for testing the interfacial fracture toughness of a second-generation high-temperature superconducting strip. The method comprises the steps of: (1), solidifying an enhancement layer, the thickness of which is much greater than the superconducting strip, at two sides of the superconducting strip, so that a composite beam structure is formed and used as a sample to be tested; (2), coating speckles at the narrow side of the sample in the length direction; (3), cutting the initial crack at one end of the superconducting strip of the sample, then, loading the sampleon a stretcher, and aligning a CCD camera to one side of the sample, on which the speckles are coated; (4), respectively developing type I and II interlayer fracture experiments; and (5), obtaining type I and II interlayer fracture toughness of the sample by calculation by using a digital image correlation method, wherein the result is the type I and II interlayer fracture toughness of the superconducting strip. According to the method provided by the invention, the enhancement layer, the thickness of which is much greater than the superconducting strip, is increased at two sides of the superconducting strip; therefore, the fracture toughness of the superconducting strip can be measured by a conventional experimental method; and furthermore, the experimental method is simple and high in accuracy.

Geng Xin

Papers

Measurement of Transverse Tensile Interfacial Strength of REBCO-Coated Conductors

Method for testing interfacial fracture toughness of second-generation high-temperature superconducting strip