Research On The Air-coupled Ultrasonic Testing Of The Bonded Performance Of Layered Medium

As adhesively bonding has reliably sealing performance and excellent shock absorption capacity,it is increasingly used in automotive and aerospace industries,such as cover plates of airplane engine,airplane wing spoilers,shell heat insulation board in solid rocket motor etc..However,some serious accidents were frequently caused by bonding imperfections,so it is crucial to measure the interfacial stiffness in bonding structures.Air-coupled ultrasound with some merits such as non-contact,non-destructive and security had been wildly used to detect the bonding quality,but it can’t be used to measure accurately the interfacial stiffness for it’s demerits,namely,low signal-to-noise ratio and low resolution.For promoting the resolution of air-coupled ultrasound testing bonding structure,some work has been done as follow in this paper:To begin with based on the spring model and the potential function theory,a theoretical model is established to predict the through-transmission spectrum in double-layer adhesive structures by the boundary condition that the displacements and stresses in the medium boundary are continuous.When the bonded plates are the same materials and equal thickness,the results obtained by numerical calculation of this model shows as follows:Firstly,all the resonant transmission peaks move toward s higher frequency with the increase of the interfacial stiffness;The reason for these movements is related to either the normal stiffness（K_N）or the transverse stiffness（K_T）;Secondly,all these peaks shift towards higher frequency with the increasing interfacial stiffness,but the sensitivity is different due to the variation in the incidence angle and testing frequency;The higher frequency peaks are‘later shift’than lower frequency peaks.Thirdly,when the incidence angle is the second critical angle,the resonant transmission peaks is the most sensitive to the interfacial stiffness;Fourth,the change of the transmission coefficient with the interfacial stiffness is slow first,then faster,then slow again,and after the interfacial stiffness is bigger than 10¹⁸N·m^-3,the transmission peaks is almost stable.Furthermore,When the ultrasound vertical incidence into a double-layer adhesive structure bonded by unequal thickness plates,the regulation of the transmission peaks obtained by numerical calculation of the above model shows as follows:Firstly,the resonant transmission peaks move towards a higher frequency with the increase of the normal interfacial stiffness;Secondly,each of the resonant transmission peaks has different movement distances with the increasing interfacial stiffness.Thirdly,the movement starting points of these peaks are at the specific frequency in which the thickness of either substrate plates equals an integral multiple of half a wavelength.Fourth,when the thicknesses of both plates are an integral multiple of the half wavelength at the same time,the transmission is approximate to 1.In addition,based on the spring model and the potential function theory,mathematical model of the ultrasonic testing stiffness coefficient for multilayer medium has been established by the reflection and transmission of acoustic wave propagation in layered medium and the bonding interface spring model.Ultrasonic reflection spectrums under different bonding interface have been obtained when the substrate material is steel,cast iron,aluminum alloy,and the coating material is Al₂O₃ ceramic obtained by different spraying process.The simulation results show that the resonant frequency is periodic on the condition of the debonded or perfectly bonding interface,but the periodicity is different.When the interface is weakly bonded,many resonant frequencies emerge and move to high frequency with the increment of stiffness coefficient.The lower resonance frequencies move faster than the higher ones.Therefore the relations between resonance frequency and stiffness coefficient have been established.The var iation rules of resonance frequency with the substrate and coating impedance are as follow:the resonance frequencies increase with the increasing of the substrate impedance,and decrease with the increasing of the coating impedance in the same stiffness coefficient.The relation between stiffness coefficient and resonance frequency is fitted using the exponential function.The function including the substrate and coating impedance,stiffness coefficient and resonance frequency is obtained in weakly bonded interfaces.Finally,the air-coupled ultrasonic testing experiments at the normal and oblique incident angle respectively are carried out to verify the theoretical analysis shown in the chapter three.These rules provide relevant the theoretical basis for optimizing inspection parameters.According to the actual situation,it can improve the sensitivity through choosing the ultrasonic testing parameters.