Surface Coating Thickness Measurement Based On Infrared Thermal Wave Nondestructive Testing Technology

Coating is often used to the protection of industrial products.It is an important part of production and application of modern industry.Reasonable thickness range is a necessary standard to ensure the normal function of the coating.Therefore,the nondestructive testing of coating thickness has become an indispensable part of industrial production.Thermographic non-destructive testing technology has the characteristics of non-contact and fast with large detection area and direct view of the results.Therefore,it is very suitable for coating thickness detection.Thermographic non-destructive testing technology can be divided into many categories according to the type and waveform of excitation source.Lock-in thermography(LIT)is often used to the field of coating thickness measurement.Thermal-wave radar imaging(TWRI)is a new thermographic non-destructive testing technology.LIT and TWRI both have continuous excitation heat sources,but in TWRI frequency modulation and phase modulation are introduced into the excitation waveform.Therefore,it is necessary to compare the detection capabilities of LIT and TWRI in the field of coating thickness measurement.Combined with the theory of heat conduction and radiation temperature measurement,this thesis discusses the one-dimensional heat conduction model of the coating structure and the analytical solution of the coating surface temperature under the sine wave excitation corresponding to LIT and the chirp signal and Barker code signal corresponding to TWRI.And discusses the eigenvalue demodulation algorithm under different excitation waveforms.Using finite element simulation,this thesis set up the structural models of the coating tentatively.According to the principle of TWRI,three waveforms are designed,including frequency modulation,phase modulation and the combination of two method.Including the waveform of LIT,the total energy of the four waveforms is the same when the detection time is also the same.In the simulation,this thesis mainly study the effects of various material factors and modulation factors on the eigenvalue demodulation results,including cross-correlation phase and cross-correlation delay time.The results show that the modulation frequency range has the great influence.Then Gaussian white noise is added to study the influence of frequency range on the detection ability.The coating thickness detection ability under four waveforms is compared.This thesis find that using the two frequency modulation waveforms have large thickness measurement dynamic range but the low signal-to-noise ratio in most coatings.Finally,this thesis discuss the difference of detection ability between sinusoidal carrier wave and square carrier wave,which provide a reference for waveform selection in the experiment.The continuous wave excited Thermographic non-destructive testing system is based on the LIT hardware system.After the software rewriting of the lower computer and the upper computer,it can output the four excitation waveforms with square wave carriers to realize the detection of LIT and TWRI.By detecting the same sample under the same excitation energy and the same detection time,this thesis discuss the basis for selecting the frequency range of each waveform modulation,compare its detection ability according to the detection results of four waveforms,and verify the conclusion in the simulation.These conclusion could provide reference for waveform modulation in industrial production.