Research On Novel Approaches For Defect Characterisation And Evaluation In Electromagnetic Nondestructive Testing

With the growing interest to use metal alloy and composite structures in aerospace,marine, traffic and other industrial fields, much attention is devoted to the developmentof non-destructive testing (NDT) techniques. Transient electromagnetic (EM) NDTtechniques have a lot of advantages, such as high speed, great depth, high sensitivity,width spectrum, low cost, and easy to quantification, which are widely investigated inthese fields. Two typical transient EM NDT techniques pulsed eddy current (PEC)testing and eddy current thermography testing (ECPT) are investigated in the thesis. Thebrief of these researches and the novel approaches are listed as follows.(1) The PEC features representing material properties are proposed andutilized to solve out the detection problem of the typical defects in aluminium alloy,honeycomb sandwich, steel and carbon fiber reinforcement plastic. PEC response isa complex mix of many factors including conductivity, permeability, lift-off andmaterial thickness variation, which should be all taken into account in PEC testing. Theinfluences of material perperties on PEC responses in time domain and frequencydomain are investigated and normalization technique is used to reduce the lift-off effect.After this, two time-domain features, representing conductivity and permeability(magnetic field intensity) are extracted. These features are utilized to measure stress inaluminum alloy, to detect defect in honeycomb sandwich structure, to evaluatelow-energy impact defect in CFRP material, and to characterize atmospheric corrosionon steel samples in Chapter2. At alst, the methods for evaluating these defects are built.(2) The PEC features representing longitudinal locations are proposed andutilized to solve out the classification problem of the typical defects in multi-layerstructures wildly used in aircrafts. In Chapter3, the directional PEC probe providinguniform eddy current is designed. Then, the PEC feature extraction techniques arestudied in both time and frequency domains. PEC frequency response optimization isinvestigated and used in combination with principal component analysis (PCA) toeliminate the lift-off and interlayer air gap and to classify the defects in multi-layerstructures. Current PEC defect classification methods require highly trained personneland the results are usually influenced by human subjectivity. Therefore, automateddefect classification is desirable in a PEC instrument. The optimized support vectormachine (SVM) is used to build the classifier model and predict the types of defects.PCA and independent component analysis (ICA) are investigated for feature extractionand compared for classification results using SVM. Two-layer Al-Mn alloy specimenswith four kinds of defects are used for classification. The experimental results show thatthe proposed methods have great potential for in-situ defect inspection of multi-layeraircraft structures. (3) Eddy current step heating thermography (ECSHT) testing and relatedquantification methods are proposed in order to solve out the characterizationproblem of subsurface defects and parallel surface defects. Because the conventionalmethods based on eddy current field interruption is invalid to evaluate the subsurfacedefects whichs are beyond the skin depth, Chapter4proposed eddy current step heatingthermography testing based on heat diffusion after long time eddy current excitation. Ifneglecting the skin depth, the ideal temperature-time1/2line for defect-free are is a linearline, while the temperature-time1/2line for defect will separate from that of defect-free.Thus, the temperature-time1/2line can be used to detect the defect. The characteristicfeature separation time is extracted and two features representing separation time aredefined to realize the defect quantification. The conventional methods based on eddycurrent field distribution are difficult to detect the defects which are parallel to theinductive coil. This Chapter also proposed eddy current pulsed thermography (ECPT)based on the lateral heat conduction for detection of these defects. The character bylateral heat conduction is addressed to detect the parallel defects. Due to significanttemperature gradient in the direction of lateral heat conduction, the spatial derivative ofthe thermogram is proposed to improve the defect detectability.(4) Defect quantification methods of eddy current pulsed thermography(ECPT) in time domain and logarithm domain are proposed to solve out thequantification problem of subsurface defects. Chapter5proposed the defectcharacterization methods of eddy current pulsed thermography (ECPT) based on heatdiffusion after eddy current pulsed excitation. The proposed methods are investigatedunder transmission mode and reflection mode through1D analytical analysis,3Dnumerical studies, and experimental studies. Time-based feature from ECPT transientresponse is an effective way to predict the defect depth in steel. Transmission mode issuitable for wall thinning defect quantification, while reflection mode is more suitablefor inner defect quantification. The relationship between peak time and residualthickness is linear when thickness ratio y<0.5. Under reflection mode, the logarithmicanalysis of ECPT is proposed. The ideal temperature curve in logarithm domain is linearand can be used to detect defects. Two features representing separation time inlogarithm domain are defined. At last, the mild steel specimen providing subsurfacedefects are tested and the quantification method is built.(5) The advanced signal processing methods for transient eddy currentthermography are proposed to improve the detection effectiveness anddetectability for deep defects and micro defects. Firstly, Chapter6proposed an eddycurrent pulsed phase thermography (ECPPT) technique combing eddy current excitation,infrared imaging and phase analysis. The experimental results show that this proposedmethod can eliminate non-uniform heating and improve defect detectability. Severalfeatures like blind frequency, min phase, and frequency to min phase are extracted from differential phase spectra and the preliminary linear relationships are built to measurethese subsurface defects’ depth. Chapter6also presented the PCA/ICA based imagereconstruction approach for eddy current pulsed thermography to avoid the severelateral heat diffusion (Blur effect). In the proposed image reconstruction approach,several hundred frames of raw data representing the time history are processed andsome principal components and independent components are selected to improve thedetectability for micro defect and deep defects. By analysis of induction heating andheat diffusion, the early stage thermal images are proposed to improve the proposedmethod.(6) Transient eddy current thermography is investigated for CFRP testingand defect evaluation through the analytical analysis and experimental studies. Thedetection mechanism for carbon fiber structure, delamination, impact and thickness areanalyzed and compared under reflection mode and transmission mode. At last,delaminations with different depth from0.5to3mm, thickness variation from1to3.5mm, and impacts with different energy from4J to12J are characterized and tested usingeddy current thermography. The results show that carbon fiber structure and impactleading to lower conductivity can be detected directly in the heating phase. Thedelamination can be detected using the later phase in transient temperature response.Impact shows the different hot spot shapes at the thermograms. The impact behaviourfor real damages are drawn. The hot area by impacts with10J and12J is like circleshape; the hot area by impact with6J and8J is concentrated. Two detection modes arecompared. Reflection mode is more suitable for in-situ inspection, because there is nodirect access to both sides for many practical components. However, the transmissionmode is more suitable for manufacturing and testing, because the coil doesn’t affect thecamera view to object under this mode.At last, the conclusion and further work are outlined in last chapter.