Ultrasonic evaluation of interfacial properties in imperfect anisotropic layered substrates

Interface conditions in layered substrates are critical to the performance of these materials. Most studies of interface conditions in layered substrates are confined to the extreme cases and give no details about the intermediate cases. In this study the lowest order mode of the generalized Lamb wave was used to examine the interfacial properties of imperfect layered substrates. Anisotropic specimens of various configurations with controlled interface conditions were made with a meshed intervening layer technique. It was noted that at very low and very high frequencies the effect of anisotropy on the dispersion curve in a silicon layered substrate was significant. The results also showed that the position of the turning point in the dispersion curve was sensitive to bond quality variation. As the quality of bonding decreased, this turning point shifted toward low velocities and frequencies and finally resembled that of the completely misbonded case. Furthermore, empirical formulae which relate the stiffness constant and the position of the turning point for a partially bonded material were also obtained. It is noteworthy that the position of the turning point does not vary significantly with orientation. By incorporating an effective interface layer into a multiple layer formulation and comparing the calculated results with the experimental data, the interfacial properties of the layered substrate were evaluated. Moreover, the shape of the dispersion curve of this lowest order mode was found to be dependent on the bond quality and anisotropy, but the dominating factor was the bond quality. Therefore, the turning point in the dispersion curve of the lowest order mode may be used to assess bond quality in an anisotropic layered substrate.