Study On Dielectric Measurement For Composite Material Defects

Composite materials is made up of two or more different substances with different ways, which can make full use of kinds of material advantages. With the advantages of light weight, high strength, easy processing, good elasticity and resistance to chemical corrosion, composite materials have been widely used. However, in the process of manufacturing and application, composite materials prone to defects, which cause the performance changes. For dielectric composite materials, the defects will cause dielectric property changes. Dielectric measurement can be used for dielectric material defects detection. In this thesis, the dielectric measurement method based on planar capacitance sensor is studied, then which is used for dielectric composite materials defects detection.The measurement effect is determined by the performance of planar capacitance sensor. The relationship among signal strength, sensitivity, penetration depth and the structure parameters, such as electrode width w to electrode space g ration and electrode numbers N is studied, which is based on Ansoft Maxwell 3D model. The optimization method is also proposed.And then, PCB type interdigital sensors with different structure parameters have been fabricated. After being calibrated, the sensors are used for FR4 plane damage experiments. RO4350/RO3003 plane experiments and XLPO power cable experiments. The results show that as w/g and N increase, the capacitances are larger, defect detection results are better. On the other side, sensor with smaller N has larger penetration depth.The single-wavelength sensor array imaging is studied for square hole defects and circular hole defects detection of single sample. With array movement interval and with lmm movement interval respectively, combined with different image interpolation algorithm, defects location and defects contour detection have been conducted. Then, the defect whose size is smaller than sensor unit is studied, which is based on LBP algorithm. The two-wavelength sensor array is also studied for defects detection of multilayer samples, validating that the two-wavelength sensor array can detect defects of different layers.