Research On Eddy Current Pulsed Thermography Non-destructive Evaluation Technique

Non-destructive testing and evaluation (NDT&E) is important to the structuralhealth monitoring (SHM) and quality control of key structure. Pulsed eddy currentthermography (ECPT), is new emerging NDT&E technique, which combines eddycurrent and thermography. The major advantage of ECPT over other techniques is thepotential in accurate non-contact inspection of a large area with a short time and largestandoff distances.ECPT mainly use a specific manual selected frame to detect the defects. It not onlyrequires experienced operators, but is labor consuming. And worse still, surfaceemissivity variation and heat diffusion causes "blurred effect", which seriously pullsdown the image quality and raises the difficulty of defect detection. To overcome thisissue, this thesis focus on the ECPT features of metallic material, infrared imageenhancement and automatic feature extraction. First, develop an ECPT model to analyzethe transient infrared response behavior in various defects and surface emissivityconditions; following the achievements, reduce the effect of surface emissivity variationto enhance the infrared image; lastly, proposes an automatic image extraction methodfor defect detection.The main contributions are:1) Analyze the influence of different eddy current excitation parameters, such asfrequency, amplitude, time, coil material and so on. Design and develop an ECPTexperiment set-up. Manufacture surface and undersurface defects using materials withdifferent electromagnetic properties. A sample with emissivity variation is also made.2) Through theoretical analyzing, simulation and verified experiment, discuss theinteraction of defect with eddy current and heat diffusion, and point out the mechanismof surface and undersurface defect detection. Not only present eddy current and heatdistribution, but describe the changing behavior of transient response. The effect ofsurface emissivity is also considered in detail, which provides fundamental support forimage enhancement.3) According to electromagnetism, thermology and thermal radiation, the defectfeature under emissivity variation is systematically discussed. Several methods areproposed to extract the pixel level features corresponding to material temperature, such as Fourier transform, two heat balance method, normalization, and similarity oftransient response behavior. Their advantages and disadvantages are also given from thepoint of information integrity, automation and application prospect. All of thesemethods can reduce the effect of emissivity variation, without any further cost andoperation. This benefit pushes ECPT development from a lab technique to in-situapplication.4) Taking the infrared response as an observation, a single channel blind sourceseparation model is developed for ECPT. Since the response of different areas areindependent, both principal component analysis (PCA) and independent componentanalysis (ICA) are employed to enhance image contrast without any training knowledge.Verified experiments are carried out, and the physical meaning of principle componentand independent component are detailly given. Their advantages and disadvantages arealso discussed from both of the mathematical and physical side. Both of these methodscan reduce the "blurred effect", and improve the probability of detection (POD) ofECPT.5) Kurtosis and normalized mixing vector are employed to quantify the spatial andtime features respectively. Both of them are used as indicators for automatic defectdetection. Their efficiency is verified through theoretical analysis and experiment. In theexperiment, defect image is automatically extracted, which shows the potential incutting down labour and raising efficiency.