In many engineering applications, diagnostic techniques are needed to characterize the mechanical properties of internal components that are not readily visible at the surface of an object, as in the use of nondestructive testing to detect sub-surface damage in composite materials. Understanding the role of structural interfaces between two bodies is a key factor in developing these diagnostic techniques because the mechanical and geometric properties of the interface determine the degree to which measurements on the surface can be used to interrogate sub-surface components. In this paper, vibration measurements on a polycarbonate material, henceforth referred to as the buffer, are used to characterize the mechanical properties of a polymer particulate composite, henceforth referred to as the target, which is located beneath the buffer. To this end, a three-dimensional laser Doppler vibrometer and piezoelectric inertial actuator are used to measure the broadband response of the two-body structural dynamic system. Because of the importance of the actuator dynamics to the diagnostic measurements, a descriptive model is developed to better understand these dynamics and interpret the results. The longitudinal dynamics of the two-body system are shown to involve stronger coupling between the target and buffer materials as compared to the transverse dynamics. A Complex Mode Indicator Function is then used to extract the modal deflection shapes, and it is shown that the interface between the bodies introduces complexity in the dynamic response. Changes in the surface velocity of the buffer material are also studied as a function of a key mechanical property — the volume fraction of crystals in the target composite material. It is demonstrated that both the linear and nonlinear vibration characteristics of the buffer material change as a function of the composition of the target material, suggesting that a compositional diagnostic procedure is possible using surface vibration measurements.

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