Research On Near-field Confinement Based Microwave Surface/Subsurface Nondestructive Testing

With the rapid development of material science and technology,new materials are used in all fields,including industrial production and daily life,and as a result,stringent requirements have been imposed on their safety and stability.Nondestructive testing(NDT)of materials has attracted much attention.With the advantages of no coupling agent,contactless,and dielectric penetrability,microwave NDT plays an important role in the field of NDT.Generally,microwave NDT can be categorised into near-field and far-field detection.The former,which could be further divided into resonant and non-resonant methods,is widely studied,because it could break the diffraction limit and get a high sensitivity and resolution.The traditional sensing structures of microwave near-field NDT mostly adopt the non-resonant open-ended waveguides,open-ended trans-mission lines,and traveling-wave antennas.The near-field energy of the non-resonant sensing structures is weak and not focused,therefore,their detection resolution and sensitivity are limited.For their attractive features of flexible design,strong near fields and localized en-ergy,the resonant sensing structures have a great potential in near-field inspection.Re-garding researches in this field,in terms of theoretical mechanism,from both the field and the circuit concepts,there is still much room for further study.In terms of applica-tion design,there is a lack of complete design rules and evaluation criteria in resonant near-field sensing structure designs,meanwhile,their sensitivity and resolution are still low.To overcome the aforesaid problems,based on a complementary spiral resonator(CSR)structure,which has strong near-field vertical confinement,this thesis conducts the research of resonant near-field NDT.The main research contents include the fol-lowing four parts:1.In this thesis,the key factors restricting the sensitivity and resolution are anal-ysed from the perspective of field.Firstly,according to the analysis of the near-field de-tection process,the sensitivity is divided into structure sensing sensitivity and network sensing sensitivity.The structure sensing sensitivity is directly related to the near-field characteristics of the sensing structure.Based on the studies of the near-field character-istics for several resonant structures,it is illustrated that the high transverse localization and high vertical confinement of the near fields play important roles in improving the sensitivity and resolution of the surface and near-surface defect detection.The network sensing sensitivity is directly related to the characteristics of sensing networks.Based on the studies of single-port and two-port sensing networks consisting of simple exci-tation circuits,the methods to improve the network sensing sensitivity are offered.2.This thesis investigates the interaction mechanism of near-field detection from the perspective of electromagnetic field.First,based on the perturbation theory of elec-tromagnetic field,the mechanism of defects disturbing the resonant frequency of the sensing structure is analysed.The defects in the ideal lossless medium disturb the electric field energy,which is shown as the disturbance of equivalent capacitance in the equivalent circuit.The surface defects of the ideal lossless conductors disturb the electric and magnetic field energy,which is shown as the disturbance of equivalent ca-pacitance and inductance in the equivalent circuit.Then based on Lorentz reciprocity theorem and microwave network theory,the mechanism of defects disturbing scatter-ing parameter[S]for single-port and two-port networks is analysed,and the correlation between the scattering parameter[S]and the electromagnetic fields in the location of the defects is clarified.3.Firstly,in order to solve the problems of low sensitivity and the difficulty in the omnidirectional inspection for metal surface crack detection,through the theoreti-cal analysis for the near-field detection of the equivalent inductance perturbation type,it is revealed that the sensitivity of metal surface crack detection is relevant to the di-rection of the induced currents on the metal surface and the direction of the crack.This thesis proposes a compact CSR sensing structure based on six spiral slots to generate a ring current on the metal surface under test,and a high-sensitivity omnidirectional crack detection is accomplished.According to the simulation and experiment,it was verified that the sensing structure detecting a crack with a width of 0.2 mm(?/257.3)and a depth of 2 mm(?/25.7)could achieve a resonant frequency shift of around 400 MHz,which confirmed its high-sensitivity performance.Exhaustive simulations were carried out to further demonstrate the ability of the sensing structure for detecting the omnidirec-tional crack and a high resolution(3mm,?/17.2)was also provided.Moreover,as the microwave near field can penetrate through the surface of non-metallic materials,this thesis studies the influence of the structure parameters on the detection depth,resolu-tion and sensitivity of the CSR sensing structure.A high-sensitivity sensing structure is proposed for the surface and near-surface defect detection of non-metallic materials.For internal defect detection,the detection depth of the sensing structure is evaluated,and the methods to improve the detection depth of the sensing structure are presented.4.This thesis studies near-field scanning imaging of material defects.According to the requirements of the near-field characteristics for the high-resolution surface and near-surface defects detection,parametric studies of the CSR with a single spiral slot are carried out,and a resonant microwave near-field scanning imaging probe with a high resolution(3mm,?/308)working in ultra high frequency(UHF)band is designed.The experiments proved that the probe structure was capable of detecting and imaging?/1538 wide cracks,?/513 diameter vias on the surface of a Polyformaldehyde(POM)board,and ?/3846 wide cracks,?/513 diameter vias on the surface of an aluminium board.Besides,it had the ability to detect and image the defects in the foam core under the skin layer of the multi-layer composite and the defects under the metal corrosion.To sum up,based on the electromagnetic field theory and microwave network the-ory,this thesis conducts a deep study on the detection mechanism,sensing structure design and implementation form of the resonant near-field detection for surface and near-surface defects,and according to the requirement of the high detection perfor-mance,provides a set of effective design methods for the actual NDT of materials.