三层介质超声谐振模式随材料和界面粘接性能变化的演变规律

Evolution of the ultrasonic resonance modes in a three-layer structure with change of material and interface adhesion properties

在用超声波谐振对粘接材料的粘接强度进行无损评估时,不同模式对粘接强度的敏感程度受到众多因素和参数的影响,对检测结果的可靠性至关重要.基于多层介质中声传播和界面弱粘接边界条件的理论模型,将一个上下非对称的金属-粘接剂-金属三层结构的平面波反射系数函数中的谐振模式看作是上下铝金属层各自的Lamb波频散模式通过夹心粘接剂层相互耦合后叠加组成.改变影响结构粘接强度的因素,即粘接剂的性能参数(声阻抗、密度、厚度)和界面切向劲度系数kt来分析三层结构谐振模式耦合方式的变化,得出结论:粘接结构粘接性能的变化基本上不改变与被粘铝层相关的固有部分的Lamb波模式,而它们的耦合模式则在谐振频率上产生平移并会与固有模式进行交换和替代;不同参数的变化引起的模式演变有各自的规律,大多可彼此区分.

The quantitative non-destructive evaluation（NDE） of interface adhesion has long been a challenge for the safe use of bonding structures. It is difficult to predict the adhesion resistance force between adhesive and adhered material without performing destructive testing. Ultrasonic approach seems to be the only potential way for its NDE based on the reason of mechanical nature of the problem.Different ultrasonic techniques, such as bulk wave echography, reflection resonance, and Lamb guided waves, have been used to evaluate the interface adhesion strength. But no direct relation between the interfacial bonding strength and the ultrasonic measurement has been established. The most used compression wave echography and resonance at normal incidence are less sensitive to the interface condition, except for a disbond. It is essential that the interface should be excited with a shear stress component to increase the measurement sensibility. But it is not easy to excite the interface by using shear waves in experiment, while the use of guided waves will encounter the problems of high attenuation and mode selection as all modes are not sensitive to a certain interface in a bonded structure.A previous study has shown that the V（z） inversion technique can be used to perform a multimode measurement on a layered structure, where both compression and shear stress resonance occur. This method has the advantage in using a simple experimental setup working at the normal incidence with a focus transducer of large angular aperture.The inversed angular-frequency reflectance function R（θ, f） gives the resonance modes which are equivalent to the Lamb type guided modes, while it is a local determination of the wave mode, thus the difficulty in guided wave measurement above mentioned can be avoided.The first part of the paper contains the development of the theoretical model for wave propagation in a multilayered structure where three-layer sandwich bonded structures can be considered as a particular case. A weak interfacial adhesion is described by two interface compression and shear stiffness parameters, namely kn and kt. By integrating the transfer matrix formalism under the non-ideal boundary conditions, the plane wave angular（incident angle） and frequency reflection coefficient function R（θ, f） for a liquid immersed asymmetric metal-adhesive-metal three-layer andits dispersion curves of guided mode waves with or without charge are calculated. It is confirmed that the evolutions of the reflection zeros（mode resonances） correspond to the dispersion curves of the guided waves of the same structure without charge. Furthermore, the resonance modes observed in R（θ, f） can be considered as a combination of the respective Lamb modes of the top and bottom single metal layers coupled through the modes conditioned by the middle adhesive layer and the its interface conditions.The second part of the paper shows the behaviors of the resonance modes by changing the parameters related to the bonding strength. The acoustical impedance, the mass density and the thickness of the adhesive layer, which are related to the cohesive property, and the shear interfacial stiffness coefficient kt which conditions the adhesive property,are changed respectively to observe the resonance mode evolutions. The mode evolutions due to each parameter are analyzed and differentiated. It can be concluded that the change in the adhesion strength of the bonding structure does not affect significantly the modes belonging to those inherent to the two adhered aluminum layers, while the coupling modes will be shifted in frequency and exchange with or replace the said inherent modes.It is expected that the obtained results in this study will be of significance for quantitatively characterizing the interfacial properties of an adhesively bonded layered structure by using the V（z） inversion technique.