Study On Acquisition And Adjusting Method Of Liftoff Point Of Intersection In Pulsed Eddy Current Detection

Thin-walled components with non-conductive coatings are widely used in aerospace and nuclear industries,and its safety and stability are the key to the reliable operation of pressure-bearing equipment.The corrosion of external environment and internal liquid erosion seriously threaten the reliability and safety of thin-walled components.Thus,it is necessary to detect and maintain them regularly.However,non-conductive coatings will limit or affect the application of conventional detection techniques.Pulsed eddy current testing(PECT)technology has the advantages of non-contact,low cost and high safety.So,it is one of the feasible testing methods to solve the problem.Due to the change of the non-conductive coating thickness and the influence of surface dirt,the probe floating,tilting and deviation will cause the change of lift off distance,and affect the accuracy and reliability of PECT.As a signal feature immune to lift effect of probe,lift off point of intersection(LOI)can effectively restrain the influence of non-conductive coating or surface dirt on the PECT signal.However,LOI does not appear in the PECT signal of ferromagnetic materials,which limits the application of LOI in the detection of ferromagnetic materials.Moreover,in the actual detection,there are few methods to obtain LOI and LOI can not meet the requirements of the changing detection range and accuracy,which further limits the application range of LOI.Therefore,the study on acquisition and adjusting of LOI under the ferromagnetic materials is a key subject to expansion LOI application range.Supported by the project of the National Natural Science Foundation of China,this paper adopts the methods of theoretical modeling,numerical simulation analysis,experiment and so on.According to the appearance characteristics and change law of LOI,in-depth study on the acquisition and optimization of LOI is carried out.The main works are as follows:1.According to the detection characteristics of corrosion defects of pressurized thin-walled parts,the thickness detection problem caused by corrosion defects of multi-layer conductive structures is analyzed.Based on the electromagnetic theory,the harmonic eddy current testing(ECT)model of multi-turn coil with multi-layer conductive structure is established.On the basis,combined with the pulse excitation frequency domain signal,the pulse eddy current(PEC)thickness testing model of multi-turn coil with multi-layer conductive structure is established using the inverse Fourier transform method.And the PEC thickness testing model is simulated and analyzed by using MATLAB software.According to the composition of the ECT system,the harmonic ECT and PECT systems are built to verify the correctness of the PECT model and the validity of numerical simulation.2.Combined with the lift off effect influence on PECT signal,the effects of thickness,electrical permeability,magnetic permeability and probe lift off distance on standard deviation of PEC differential signal are analyzed.Through the re-differential processing of the PEC difference signal of the ferromagnetic material,it is found that the LOI phenomenon is raised in the PEC secondary difference signal.Based on the LOI phenomenon,a twice difference method of PEC signal is proposed and the time-domain LOI of ferromagnetic materials is obtained.Furthermore,the feasibility of the obtained time-domain LOI thickness measurement is studied,and the influence factors of the time-domain LOI of ferromagnetic materials are analyzed.Finally,the validity of the twice difference method of PEC signal and the feasibility of the time domain LOI thickness measurement are verified by experiments.3.When the lift off distances of probe are different,the change law of frequency spectra of PEC difference signals of ferromagnetic and non-ferromagnetic materials is analyzed according to the lift off invariance of LOI.It is firstly found that there are also lift off invariance points in the real and imaginary part spectra of PEC difference signals of ferromagnetic and non-ferromagnetic materials.Furthermore,the feasibility of frequency domain LOI thickness measurement of ferromagnetic and non-ferromagnetic materials is analyzed,and the influence of pulse excitation signal on frequency domain LOI is studied.Finally,the existence of frequency domain LOI and the feasibility of LOI thickness measurement are verified by experiments.4.Based on the influence of pulse excitation signal on time and frequency domain LOI,the feasibility of the changing frequency and rise time of pulse excitation signal is analyzed to adjust the thickness measurement curve of time and frequency domain LOI.The effects of the frequency and rise time of pulse excitation signal on the thickness measurement curve of time and frequency domain LOI are determined.Furthermore,the effect of the frequency and rise time of pulse excitation signal on the thickness measurement accuracy and range of time and frequency domain LOI is studied.It is found that the increase of rising time of pulse excitation signal or decrease of frequency of pulse excitation signal can extand the thickness measurement range of time and frequency domain LOI,and enhance the thickness measurement accuracy of time and frequency domain LOI in a proper range.Finally,the adjustment effect of frequency and rise time of pulse excitation signal on the accuracy and range of LOI thickness measurement is verified by experiments.The results obtained in this paper have important significance for extending application of LOI in PECT,and provide an effective and reliable technical for the thickness measurement of pressurized thin-walled parts with non-conductive coatings.It further improves the application value of PECT technology.There are 122 pictures,15 tables and 160 references in this paper.