Nondestructive Testing Of Carbon Fibre Reinforced Polymer Composites Using Eddy Current Method

Due to the increasingly extensive application of Carbon Fiber-Reinforced Polymer(CFRP)composite in aircraft,automobile,wind power plant and other industries,the development of Non-Destructive Testing(NDT)techniques for this advanced material have attracted more and more attentions.As motivated by the urgent demand for efficient CFRP inspection technique,this research focused on the scientific and technical issues involved in the NDT of CFRP composites using eddy current method.The CFRP composite is conductive but low in conductivity;and it shows a strong electrical anisotropy.Eddy Current Testing(ECT)is an efficient and non-contact NDT technology,which is based on the electromagnetic induction phenomenon between current-carrying coils and conductive materials.It has high detection sensitivity,particularly suitable for thin-layer structure inspection,without the need to clear surface coating or to provide couplant.ECT method is appropriate for inspecting the raw carbon fiber materials and the finished CFRP products,especially for the quality control of the Resin Transfer Molding(RTM)and the prepreg manufacturing.On the basis of the intensive study associated with the electrical properties and eddy current path,a numerical solver based on the reduced magnetic vector potential and the edge elements was innovatively developed to enable the 3D electromagnetic simulation of CFRP material.The devloped program was used to compute the eddy current field and the coil signals.In addition,the mid-frequency ECT technique for CFRP was proposed creatively,and a real mid-frequency ECT system was implemented.In the system,the electromagnetic coil was used to excite the eddy current in CFRP,and the pick-up signal was extracted by the high sensitivity lock-in amplifier.Finally,several kinds of defects including fiber orientation,misaligned fiber bundles,crack,delamination,impact damage were detected and visualized.Results showed that the mid-frequency could find the defects in CFRP plates and aluminum honeycomb interlayer structure with high precision.The main works and the innovation points performed in this dissertation include:(1)The general solution of the conductivity tensor in unidirectional CFRP and the eddy current path have been analyzed and derived.The impedance characteristics of the unidirectional CFRPs in the longitudinal,transversal and thickness direction were obtained by means of impedance measurement.Combining the measuring results and the coordinate transformation matrix,the general conductivity tensor matrix of the unidirectional CFRP layer was derived,and the current flowing path in CFRP was also obtained.(2)The analytical solutions of the eddy current density and the skin depth in CFRPs were derived by means of Maxwell equation.The impedance characteristics of equivalent circuit of the coupling between CFRP plate and eddy current coil were studied;and then the variations of the resistance and inductive reactance caused by the testing frequency and coupling coefficient were analyzed in detail.The impacts of the factors such as testing frequency,lift-off and defect on the probe signal were discussed from the view point of coupled circuit impedance.(3)Taking into account the laminar structure and the electrical anisotropy of the CFRP plate,the 3D electromagnetic FEM program for CFRP simulation was developed,which was based on the reduced magnetic vector potential and the edge elements.The developed program was then adopted to compute the eddy current induced in CFRPs and the coil impedances.In the program the reduced magnetic vector potential method was used to handle the coil movement,and the edge elements modeling was used to treat continuity problem of the field vector at the interface between adjacent plies.(4)The effects of the stacking sequence and interlaminar interface on the eddy current and coil impedance were studied through the simulation and experiment.The bulk conductivity of the laminar CFRPs and fiber orientations were measured by using eddy current probe.The bulk conductivity could be estimated from the absolute probe signal,and the fiber directions could be measured by drawing the polar diagram of signals extracted from the transmitter-receiver rotating probe.(5)A mid-frequency ECT system for CFRP inspection with the working frequency up to 250 kHz,was developed.The system was mainly involving the weak-signal extraction method using the lock-in amplifier and C-scan imaging technique.Considering the frequency characteristic and the Point Spread Function(PSF)of eddy current probe,a Transmitter-Reciever(T-R)type of probe was chosen to inspect the CFRPs.Signals from various kinds of damages were obtained in the experiment and simulation,and the location and size of the damages were visualized in the scan image.The results indicated that by choosing suitable probe,improving sensitivity and gain of the lock-in amplifier and introducing advanced signal processing approach,the high precision ECT detection and the imaging for CFRPs could be achieved at relatively low frequency.