Non-destructive testing technique plays a very important role in ensuring structural integrity and is of great significance for the safety in production.Eddy currents are used as the excitation source in alternating current field measurement and eddy current thermography,which sets a good foundation for the combination of the two techniques and developing a combined testing technique.In this dissertation,research on alternating current field measurement and eddy current thermography is carried out.A combined testing technique,which combines the advantages of the two techniques,is developed.An in-depth research is conducted from the aspects of detection mechanism,defect recognition,defect location and defect reconstruction.The main contents of the dissertation are as follows:1.Based on the mathematical models of alternating current field measurement and eddy current thermography,the distribution laws of magnetic field and temperature field around the defect induced by eddy currents are obtained by using numerical simulation.According to the characteristics of the two testing techniques,an experiment platform is developed for the combined testing technique and an inducer suitable for the two techniques is designed.2.The x component of the magnetic flux density(B_x)around the defect is obtained by using the experiment platform.The wavelet de-noising method and the double moving average method are respectively used to de-noise and smooth the signal of B_x.Methods for defect recognition and location based on extreme points on the curve of B_x are proposed according to the morphological characteristics of B_x.3.The distribution laws of temperature field around defect are obtained in the stationary and moving modes of eddy current thermography.A thermal image processing method based on line scanning method is proposed.Study on defect feature extraction and defect location based on line scanning method is also carried out.4.Two methods are introduced to suppress the effect of uneven surface emissivity for the moving mode of eddy current thermography,which are the temperature gradient based method and the wavelet transform method.The two methods are applied to two different materials and good results are obtained by using both methods.5.Two-level GRNN(Generalized Regression Neural Network)model is constructed to get the matching relationship between B_x and defect depth.The inversion of defect’s depth is realized by using this model.The temperature data are processed by edge detectors and the edge of defect mouth on the surface is obtained,thereby determining the area of the defect mouth. |