Simulations Of Non-destructive Testing Based On Superconducting Quantum Interference Devices

A superconducting quantum interference device, named as SQUID for short, is a device with extremely-high sensitivity and could be used to detect very weak magnetic field. In recent years, relevant research on the SQUID-based nondestructive inspection has been developed rapidly and will be applied in various industry fields.Preliminary analysis of the physical principles of the SQUID-based detection for metal is introduced and consequently numerical simulations of nondestructive inspection for the typical pieces with defects are given.First of all, we briefly overview the research status and summarize various applications of the SQUID-based nondestructive inspections. Due to its high sensitivity for the magnetic field detection, SQUID has been widely used in transportation, civil engineering, industrial inspection, and many other fields. Compared with traditional detection methods, such as magnetic particle detections and ultrasonic detections, the SQUID-based nondestructive inspection is more significant for the depth and slight resolutions of the detected defects.Next, based on the detailed analysis of the physical principles within the SQUID-based non-destructive inspections, we carry out a few typical numerical simulations by using the designed sheet metals as examples. Physically, SQUID is a specific application of the famous Josephson effects in superconducting physics. This device is really the basis of most superconducting electronics applications at present. The basic structure of the SQUID consists of superconducting loop and two superconducting junctions. The model of single superconducting junction is a step-like tunneling junction. Based on such a model, we explain why the SQUID can detect sensitively changing of a magnetic flux quantum, and thus be used to realize the sensitively nondestructive inspections.Practically, the process of using SQUID to achieve metal non-destructive inspection is to determine the presence of defects by detecting magnetic field variation of defect parts in the metal. Our simulations of defect detections are based on the AC magnetic field technology. Based on thr injection current and the three excitation coils models, we use finite element analysis method to carry out the numerical simulations of non-destructive inspections for metal plates with opening defects. For simplicity, the injection current model for steel plates with defects is assumed to be the usual DC bias. After initially determining the accuracy of electromagnetic inversion, more realistic excitation coil models are applied. Then, various data on metal defect length, width and depth are obtained through the electromagnetic inversions.After the simulations on the opening defect, we further develop our numerical method for inspecting the defect in internal conducts of the metal plate. Now, a rectangular coil is assumed to be applied and an excitation coil through electromagnetic inversion is used to extract intensity information of magnetic field along different directions. Our simulations show that, the detections of internal defects are more complicated than those for the opening defects and magnetic induction intensity data related to various directions should be combined for comprehensive analysis.