A Research On Thickness Measurement Method Of Multilayer Heterogeneous Films Based On Ultrasonic

With the rapid evolution of science and technology,the coating material and structure of aircraft skin surface are increasingly complex.However,the thickness and covering evenness of coating will directly affect the temperature distribution,anti-corrosion ability as well as stealth property of aircraft body.Therefore,nondestructive evaluation of multilayer heterogenous film has become an important research topic of high-end equipment manufacturing industry.Featuring in low cost,high sensitivity,rapid detection,defect location and defect ration,ultrasonic testing technology has been widely applied in the field of aerospace and nuclear industry.In this thesis,the coating of aircraft invisibility is very thin,which causes the common ultrasound probe can’t achieve the requirements of experiment.Thus,the straight probe of high frequency,short pulse,and high broadband is used to transmit ultrasonic pulse signal and receive the reflection echo signal.According to the fundamental principle of ultrasonic pulse echo method,if the transmitting speed of ultrasonic wave in coating material is known,we should just evaluate the round-trip time that ultrasonic wave has spent in coating material so that the thickness of coating can be reached.Thus,how to estimate the transmitting time that ultrasonic wave spends in coating efficiently and accurately is key point of this thesis.The research topic of this thesis is listed as following aspects:(1)Research the acoustic parameters of coating material as well as the transmitting rule of ultrasonic wave in it.According to definition of acoustic parameters,such as,acoustic impedance,sound velocity,and sound attenuation,the author analyzes the characteristics of related acoustic parameters of the coating material of aircraft invisibility and studies the situation that ultrasonic wave transmits in coating interface due to the rule of sound transmission,which provides theoretical foundation for the author to study the transmission rule when ultrasonic pulse signal is in multilayer coating interface.(2)The author aims to build Gaussian model of ultrasonic pulse signal and Convolution model of multiple overlapping echo signal and study the processing method of the coating interface echo signal.The Gaussian model,flexible and convenient,is used to be the reference signal of ultrasonic pulse echo signal,which provides evidence for studying simulation of ultrasonic signal.Because of the thinness of coating,there will be aliasing of echo signal,which is from the up and down echo signal of coating.In order to improve recognizability of echo signal and build convolution model of multiple overlapping echo signal,the author applies the method of minimum entropy blind deconvolution to realize separation of the multiple overlapping echo signal,which provides convenience for obtaining other information of ultrasonic pulse echo signal.(3)The way to study the thickness of coating.Estimating the thickness of coating is estimating the round-trip time that the time of flight(TOF).From the perspective of accuracy and calculation amount,this thesis uses parameter estimation which is based on signal envelope extraction and curve fitting.Also,the thesis verifies the estimating results of the parameter TOF from simulation and experiment.(4)The testing experiment of the aircraft invisibility coating is on the basis of monolayer and multilayer of base so as to prove the feasibility of analyzing the method of measuring the thickness of coating by using ultrasonic wave.