| As adhesive bond technology has increasing application in aeronautics and space industries, more and more research interests naturally arise for the nondestructive testing and evaluation of adhesive bond defects and properties. Up to now, there does not appear to be a practical method to evaluate adhesive bond strength. This deters engineers from applying the adhesive bond technique to some important structures, which restricts this technique to further spread its application.It is hard to nondestructively evaluate the adhesive bond properties. Early in the 1970s, significant attention had been paid to this challenging problem, but no satisfying results were obtained. In all techniques for the nondestructive testing and evaluation of adhesive joints, ultrasonic techniques are used by the most researchers and considered as one of the most promising approaches. It is multimode Lamb waves that composed the Acousto-ultrasonic(AU) signals when a thin plate is tested by the AU method. In this research, Lamb modes in AU signals were separated in time-frequency space. On the basis of analyzing the AU signals containing multiple Lamb modes and the Lamb mode conversion, adopting the means of direct contact testing of standard transducers, the adhesive bond strength of weak aluminium lap joints was characterized using Lamb waves, along with the accelerated environmental degradation of adhesive bonds.The AU signals in aluminium were analyzed using time-frequency methods. By comparing the separated components in time-frequency space with theoretical Lamb time-frequency curves, it was confirmed experimentally that the AU signals in a thin plate are composed by multimode Lamb modes. Through reassigning of the time-frequency distribution of AU signals and processing reassigned image, the portions of the group velocity dispersion curves of S0 and A1 modes were extracted, and the experimental results are shown to be in good agreement with the theoreticalcurves. By theoretical and numeric analyzing, it was also found that the "complicated" signals in rear end of AU signals were mainly the superposition of reflected Lamb modes.The AU signals in adhesive joint were analyzed by time-frequency method and the Lamb modes were separated in time-frequency space from AU signal. The components of AU signals attenuated sharply and accompanied by mode conversions when they passed the overlap regions of adhesive joints. The normalized spectrum peak values of the main Lamb modes are correlative linearly with adhesive bond strength. The Lamb wave dispersion in adhesive joints was investigated. The dispersion of Lamb waves changes mainly with the cohesive properties and is not sensitive to adhesive properties. With the change of the bond strength, the relative variations of the group velocities are small, while absolute variations are significant. It is applicable to characterize the adhesive bond strength by dispersion of Lamb waves when the overlap size of the adhesive joints is long enough only. The energy distribution density of S1, the main component in the AU signals measured, tends to move backward on the time axis with the decreasing bond strength. This trend is dominated by unequal attenuation of the Lamb waves in adhesive joints. S1 mode was isolated in time domain by the FFT band pass filtering, and then the centroid of its envelope was extracted. Good correlation of the centroid position with the bond strength was obtained.On the basis of the characterization of the adhesive bond strength, the accelerated environmental degradation of adhesive bonds was also characterized by Lamb modes. The attenuation of Lamb modes is not sensitive to the degradation while the energy transfer, i.e. mode conversion, between Lamb modes can be used to be an indicator of residual strength. In the case of the small lap region size, the relative variation of dispersion is too small to measure accurately. But the dispersion properties depend on the material properties only and are not susceptive to the coupling...
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