Research On The Key Technology Of Dual-frequency Eddy Current Nondestructive Testing

The dual-frequency eddy current testing is a new technology on Electromagnetic Nondestructive Testing(NDT) after the singal-frequency eddy current testing. It not only has the characteristic of the singal-frequency eddy current testing, but also can complete the assignment which the singal-frequency eddy current testing can't do. The most important characteristic of this technology is that it can carry out simultaneous testing of the different parameters and can distinguish useful signals from interfering signals.This technology plays an important role in defect detection on air inlet pipe and skin of aircraft in service.In this thesis, the domestic and foreign research status and development on eddy current testing and dual-frequency eddy current testing is expounded ,and the dual-frequency eddy current testing theory and key technology is analysed. The dual-frequency signal generator of the orthogonal reference signal based on Direct Data Synthesize(DDS) technology is designed and finished. Because of the request of separating different frequency signal in dual-frequency eddy current testing , signal processing and separating electrocircuit based on the technology of dual orthogonal vectors LIA(Lock-in Amplifier)is designed, this design is validated by simulation in SIMULINK and electrocircuit hardware. The impedance data curves of difference transducer is obtained by finite element simulation to the air inlet pipe with support plate,using these data curves parameter separateing arithmetic is realized and validated。The differential transducer in dual-frequency eddy current tesing is designed and producted based on the design theory and the finite element simulation. The formulae of the choosing test frequency is proposed. finally, the Dual-frequency Eddy Current testing system is accomplished .Qualitatively, this thesis has investigated separating method on defect signal with interfering signal, and using this method eliminated support plate signal from test signals. Quantificationally, this thesis has given the relationship between the feature of defect signals and the dimension and location of defects ,such as the amplitude and the phase.In this thesis, on one hand the data supporting are provided to the system design by a large number of finite element simulation, on the other hand, dual-frequency eddy current testing has been performed by experimentation on the sample, the experiment data and the simulation data have the approximately same trend .