Measurement Bonding Interface Characteristic Of Multilayer Structure Using Ultrasound Guided Wave

Because of its advantages of high specific strength,high specific modulus,strong adaptability,simple manufacturing process and high safety,adhesive structure has been widely used in aerospace,armored vehicles and other important military fields,such as aircraft wing skin,solid rocket engine shell / adiabatic layer,aeroengine insulation coating and other key parts.Because the bond structure is affected by process,temperature,curing time and vibration damage,aging,creep and static load in the process of manufacture,it is easy to cause porosity,poor bonding,crack,delamination damage and other defects in the interface.These defects reduce the reliability and safety of bond members,so it is necessary to detect the quality of bond structure.In this paper,the ultrasonic guided wave technology is used to study the bonding structure,and the following work is carried out:Firstly,based on the propagation characteristics of ultrasonic guided wave in double layer medium(plexiglass / aluminum)and the theory of interface spring model,the dispersion equation under different bonding interface conditions is derived.The dispersion curves of rigid bonding,slip bonding and complete debonding are obtained by numerical calculation of the dispersion equation.By changing the thickness of medium,the variation of dispersion curve is analyzed.From the rigid bonding interface to the slip bonding interface,there is no mode conversion between the symmetric mode and the antisymmetric mode of the dispersion curve,but the normal and tangential stiffness coefficients have a great influence on the propagation speed of the guided wave.There is a big gap in the phase velocity of each mode in a certain interval,and the phase velocity of rigid bonding is larger than that of slip bonding.When completely debonding,two kinds of dispersion curves appear,namely,the dispersion curves of aluminum layer and plexiglass layer.In the double layer medium,when the sound velocity of the upper and lower layers is close,the dispersion curve of the medium thickness increases only to the left.When the sound velocity of the two upper and lower layers is different,the thickness of the medium with high sound velocity increases,the dispersion curve moves left and up,and the thickness of the medium with low sound velocity increases,and the dispersion curve moves left and down.The dispersion curve of adhesive structure is transformed into the dispersion curve of monolayer medium with increased thickness.Secondly,finite element simulation software is used to simulate the propagation process of ultrasonic guided wave in double-layer structure.If the center frequency is 2MHz,the propagation law of guided wave in ceramic coating with different thickness is analyzed.The results show that the propagation time of guided wave increases with the increase of thickness.When the coating reaches a certain thickness,the wave of higher order mode appears,but the wave of higher order mode will affect the judgment and analysis of the base mode wave,so there are some limitations in the detection of thicker medium by ultrasonic guided wave.The correctness of the dispersion curve in Chapter 3is also verified.for the double-layer bonding structure,when the center frequency is 0.4MHz,the propagation process of ultrasonic guided wave in three different bonding conditions under the condition of air coupling is simulated.the propagation law of guided wave under different bonding interfaces is analyzed.With the increase of interface defects,the amplitude is also increasing.When the interface is completely debonding,the simulation results are consistent with those of single-layer plate.Finally,an experimental platform for air-coupled ultrasonic guided-wave double-layer bonding structure was established.epoxy resin,water and air were used as bonding media to fabricate three different bonding structures: rigid bonding interface,slip bonding interface and complete debonding interface.The experimental results show that the phase velocity of rigid bond structure is larger than that of slip bond structure when the frequency thickness product is 0.4 MHz·mm?0.6MHz·mm and 0.8 MHz·mm,and the gap is the most obvious at 0.6 MHz·mm.complete debonding is consistent with the results of a single-layer plexiglass plate.The guided wave phase velocity obtained from the experiment is in good agreement with the numerical results.The results of different thickness bonding structures are the same,the amplitude and sound energy of the guided wave are the smallest under the condition of rigid bonding,and the amplitude and sound energy of the guided wave are the largest under the condition of complete debonding.