Experimental Study On Fracture Characteristics And Quench Behaviors Of Superconducting Tapes Under Cryogenic Temperature Using DIC

The measurement of multi-field performance and mechanical deformation of materials in extremely complex environments are the main challenges faced by current experimental technology research.The digital image correlation(DIC)method is a new optical test method for obtaining mechanical quantities such as displacements and strains through the correlation of image region.It has attracted a lot of attention due to its remarkable advantages such as non-contact,full-field and anti-electromagnetic interference.Based on the DIC test method,this paper carried out the experimental research on the fracture characteristics of materials under low temperature as well as the phenomenon of overcurrent quench of superconducting tapes.First of all,aiming at the basic problem of fracture mechanics,i.e.,the Brazilian disc with a central crack,the fracture test under radial compression of the disc was carried out.Based on the I-type crack stress intensity theory and combined with the DIC optical measurement method,the disc deformation characteristics and the stress intensity factor at the crack tip were obtained.The experimental results agree well with the theoretical prediction results,which confirm the effectiveness of the DIC method in the fracture test.Secondly,in order to achieve visualized low-temperature test environment and facilitate the application of DIC optical measurement methods,we have independently produced a visualized low-temperature test named Dewar(based on a heat conduction cooling mode,whose temperature interval is 293 K to 77K).Experiments on the mechanical properties of metallic copper strips under different low/varied temperature environments has been done.The DIC method was used to test the external full-field displacement and strain information of the metal strip under external load and low-temperature environment,and the fracture characteristics such as stress intensity factor at the crack tip were obtained.Furtherly,the corresponding numerical simulation was carried out by using the finite element software of Comsol.The quantitative simulation results are in accordance to the experimental test results.Finally,based on the non-contact and full-field strain test of the DIC photometric method,mechanical properties of the high-temperature superconducting tape under the over-current quenching was studied,and the full-field strain map of the superconducting tape during evolution from the superconducting state to the normal state was obtained.The results indicate that the DIC test method can clearly and intuitively present the characteristics of the strain change during the entire quenching process of the high temperature superconducting tape,which is expected to be a novel non-contact quenching method for superconducting structures based on the full field strain.By comparing and analyzing the strain signal with the voltage test signal at the time of the occurrence of quench,the critical criterion for judging the quench of the full-field is proposed.Considering the randomness of the ideal superconducting tape start position at the time of the over-current quench,superconducting tape with a center opening was used.The quenching experiment showed that the strain near the round hole increased firstly when the quench occurred,followed by the deformation of the surrounding area and then gradually expanded to the surroundings,thus characteristics of superconducting quench propagation could be visually demonstrated.A new method and technique for superconducting material's multi-field mechanical properties testing under low-temperature is proposed in this paper,especially for superconductor quenching.