Nondestructive Testing System Based On Millimeter-wave Interference Technology

Millimeter waves are very sensitive to boundary conditions and the electromagnetic properties of dielectric materials,and millimeter waves can achieve non-contact measurement,which have good penetration for some low-loss non-metallic materials,therefore,millimeter waves can be used for non-destructive testing(NDT)of dielectric materials.The currently developed millimeter-wave NDT technology mainly focuses on combining millimeter-wave imaging technology and combining AI algorithms in data processing to obtain the structural integrity of materials.The signal testing equipment used in these methods require high precision,complicated operations,and high costs,which are not conducive to large-scale applications.On this basis,we propose an efficient,simple and low-cost millimeter-wave NDT system,which has certain potential to solve the problem that the existing millimeter wave non-destructive testing technology is difficult to apply on a large scale.The NDT system is based on the principle of millimeter wave interference in a specific interval,and uses the combining efficiency to indirectly characterize the electromagnetic properties or structural properties of the dielectric material.The propagation characteristics of millimeter waves in dielectrics were theoretically deduced,and then the relationship between the coherent combining efficiency of millimeter waves and the amplitude/phase consistency of input signals was theoretically deduced.The sensitivity of the combining efficiency to the amplitude difference and phase difference was compared,and phase difference was selected as the test factor of the NDT system designed in this thesis.Based on the theoretical derivation of two parts,a set of NDT system was designed,which greatly simplifies the signal acquisition.That makes the test system simple,high-efficient and low-cost,therefore,it can be applied online on a large-scale.The NDT system was simulated and calculated,and the relationships between the structural characteristics,electromagnetic characteristics of the sample and combining efficiency were obtained.The factors affecting the test sensitivity of the system were studied,which is beneficial to optimize the system to improve the testing sensitivity in the actual test.The simulation results show that the system can be used to detect common defects,thickness and dielectric constant of the samples.In Ka band,the system can detect the minimum width of the surface crack of the material is about 0.04?,the minimum radius of the penetration damage is about 0.07?,and the minimum side length of the internal delamination defect is about 0.2?.The resolution in detecting thickness is about0.01?,and the resolution in detecting dielectric constant is about 0.04.A test system with an operating frequency of 35 GHz was built,the key devices of the system were calibrated and tested,and the error analysis was carried out.It was found that the relative error of the system when testing a same sample was less than 1% and the system stability was good.The verification experiment of material defect detection was carried out using the system,and the experimental results showed that when the system operates at a frequency of 35 GHz it can at least detect surface cracks with a width of about 0.07? and penetration damage with a radius of about 0.1?.The experimental results,simulation results and theoretical derivation have proved the feasibility of the method.