Research On The Propagation Characteristics Of Ultrasonic In Layered Solid Medium

Solid layered structure generally refers to composite material which is formed two layer,three layer or multilayer structure by bonding,welding or other methods to get different sheet materials—metallic or non-metallic—join together.Since it has the advantages of good fatigue resistance,superior damping performance and high specific strength and specific modulus,so it is widely used in the key parts of aerospace,automotive,military industry and other important areas,such as an airplane wing spoilers and engine cover plate,aircraft and automobile skin,solid rocket motor shell heat insulation board and etc.The ultrasonic nondestructive testing technology in Solid layer is a new multidisciplinary cross technology.It have higher application value for the production and deep processing of sheet material and other aspects.Ultrasonic propagation characteristic in the solid layer have been studied in this paper,and details are as follows:(1)Taken potential function method and transfer matrix method and combined with,mathematical model reflected ultrasonic propagation in layered medium have been established.According to boundary conditions of stress and displacement continuity on the layered medium gas-solid interface and solid-solid interface,mathematical model reflected ultrasonic propagation in single,double,three layers medium have been established,and then dispersion curve of ultrasonic propagation in layered medium have been obtained.At the same time,we have studied the influence of physical parameters on dispersion curves of ultrasonic propagation.The results show that every frequency-thickness must exist two or more kinds of modes in the single layer medium,In addition to A0 and S0 mode,the others all exist cut-off frequency,In the double-layer dielectric,when the thickness of thin layer increased from 1mm to 3mm,dispersion curve move to the left as a whole,the dispersive curves whose substrate is steel move to the right as a whole compared with the dispersive curves whose substrate is aluminum.Lamb wave is non-sensitivity to the density of substrate,when the thickness of substrate increased from 1 mm to 3mm,dispersion curve move to the left as a whole,A0 and S0 mode is sensitivity.In the three layers dielectric,the dispersive curves whose substrate is aluminum move to the right as a whole compared with the dispersive curves whose substrate is steel,S0 mode is sensitivity in all area.When the thickness of substrate increased from 1 mm to 3mm,the dispersive curves whose thickness of substrate is 3mm move to the right as a whole,A0 and S0 mode is sensitivity.In addition,the horizontal stress of ultrasonic is the form of symmetry in the thickness direction of the medium,however the vertical stress of ultrasonic is the form of anti-symmetric in the thickness direction of the medium,Theory basis of ultrasonic nondestructive testing and performance testing in layered media have been offered.(2)Mathematical model of non-contact type ultrasonic wave in layered media have been established,and the air coupling ultrasonic propagation law have been got by the method of numerical analysis.The results show that the mode of dispersion curve under the condition of air coupling move to the left as a whole compared with the mode of dispersion curve under the condition of coupling agent.When the frequency is less than1 MHz,there are four modal curve,group velocity of some modal at the same frequency-thickness is very close.When the air coupling ultrasonic wave vertical incidence of layered medium,Thickness is integer times of half wavelength,transmission coefficient is 1.Thickness is not integer times of half wavelength,the transmission coefficient is very small in wide frequency range.Air coupling ultrasonic can only be maximum through the measured medium only at a particular frequency-thickness,and the value of frequency-thickness has certain periodicity.The cycle is integer times of the half of the product of velocity and thickness measured medium,When the ultrasonic wave oblique incidence silicon nitride ceramic materials,the first critical angle is 3.7°,and when the incident angle exceeds 3.7°,Silicon nitride ceramic material internal only exist shear wave;When the incident Angle reaches the second critical Angle is 6.4°,total reflection of ultrasonic will occur.In addition the transmission coefficient of sound waves appear more modal form,there is cutoff frequency for every kind of mode.First and second critical Angle,which is obtained from theory called Law of Snell and the simulation calculation of ultrasound in the air coupling in the silicon nitride,are identical,and the errors are within 0.1°.Elastic modulus which are obtained by the simulation results is 43.52 GPa,Compared with the theoretical results of error is 1.96%.Therefore,the simulation calculation precision is high.(3)Finally,the contact type and non-contact type ultrasonic testing experiment system have been set up.When the center frequency is 1MHz and 2.5MHz respectively and the distance of ultrasonic transducer is 80 mm and 100 mm respectively,the time domain waveform have been given in the contact type ultrasonic testing,in which there are two signals,one is defect wave,the other is direct wave from board.The results show that amplitude value of both direct wave from board and defect wave has great difference.Diameter of defects hardly have effects on the amplitude of the direct wave from board,the amplitude of defect wave decreases gradually with the increase of defect diameter.When the center frequency is 400 kHz and the distance of ultrasonic transducer is 48 mm and 68 mm respectively,the time domain waveform have been given in the air couplingtype ultrasonic testing.The results show that diameter of defects hardly have effects on the amplitude of the direct wave from board,the amplitude of defect wave decreases gradually with the increase of defect diameter.In addition,in the contact type and non-contact type ultrasonic testing,ultrasonic can arouse single mode at a certain incident angle,the received echo signal will produce the dispersion phenomenon,which have been found in the experiment.