Research On Pulsed Eddy Current Thickness Measurement Of Thermal Barrier Coatings

Thermal barrier coatings is a kind of multi-layer thermal insulation component,which is often applied on the surface of turbine blades of aero engines.It is usually used to avoid direct contact between high-temperature gas and blades,improve the temperature resistance of the blades,and thus increase the thrust-to-weight ratio of the engine and extend the service life of the engine.Due to the imperfect manufacturing process and the impact of the environment of high temperature and high pressure during service,the thermal barrier coatings will become thinner and may even fall off,which in turn leads to safety accidents.The thicknesses of thermal barrier coatings directly characterize the quality of the thermal barrier coatings,so it is necessary to measure the thickness of the thermal barrier coatings accurately.Pulsed eddy current testing has the advantages of rich information and low cost,it can test multi-layer structure objects non-destructively,conveniently and efficiently.In this thesis,the relationship between the pulsed eddy current signal and the thickness of the thermal barrier coatings is studied by establishing a pulsed eddy current testing model of the thermal barrier coatings,and a new method for measurement of the thermal barrier coatings thickness is proposed and finally verified by experiments.The main research contents are summarized as follows:First,a finite element model for pulsed eddy current testing based on Fourier transform was established.With the help of the finite element software Comsol,the impandance of the coil under harmonic excitation was calculated,and the calculated results are in agreement with experiments.Based on the linear characteristics of the pulsed eddy current problem,the frequency domain response of the coil is superimposed and then the inverse Fourier transform is performed to obtain the time domain response of the coil under square wave excitation.A pulsed eddy current testing experimental platform was built to verify the developed finite element model.Secondly,an efficient finite element model for pulsed eddy current based on cubic spline interpolation was presented.After analyzing the change of the frequency domain response of the coil with the excitation frequency,a piecewise cubic spline interpolation strategy is proposed to reduce the number of frequency domain responses to be calculated,which improves the efficiency of the pulsed eddy current testing model.The segmentation factors affecting the interpolated cut-off point were studied.The effects of the rise time of the excitation signal and the duty cycle on the number of required frequency domain responses were studied.In addition,a pulsed eddy current testing simulation software based on Matlab was developed.Convenient simulation calculation was realized through the software.Subsequently,a pulsed eddy current testing method for the thickness of the thermal barrier coatings is proposed.Using the pulsed eddy current testing model of thermal barrier coatings thickness,the relationship between the pulsed eddy current signal and the coatings thicknesses was simulated,and the mapping relationship between the voltage peak and the coatings thicknesses was constructed.In this thesis,the coatings thickness detectiting method under known and unknown conductivity of the bonding layer is proposed,and the influence of the rise time of the excitation signal on the coatings thicknesses measurement sensitivity is studied in terms of sensitivity.Finally,the samples of thermal barrier coatings were manufactured,and a pulsed eddy current detection experiment was carried out on the samples.The measured voltage peak was used to determine the thicknesses of the coatings.The experimental results showed that the imposed pulsed eddy current testing method for measurement of thermal barrier coatings thicknesses was feasible.There are 75 figures,15 tables and 82 references in this article.