A high frequency impedance-based qualitative non-destructive evaluation (NDE)
technique has been successfully applied for structural health monitoring at the Center for
Intelligent Material Systems and Structures (CIMSS) [1-3]. This new technique uses
piezoceramic (PZT) patches as actuator-sensors to provide a low-power driven constant
voltage dynamic excitation, and to record the modulated current flow through the
structure. Therefore, it relies on tracking the electrical point impedance to identify
incipient level damage. The high frequency excitation provided by the PZT, ensures the
detection of minor changes in the monitored structure. It also limits the sensing area to a
region close to the PZT source, therefore only changes in the near field of the PZT are
detected, enhancing the ability of this technique to localize incipient damage.
The phenomena of the PZT's sensing region localization has been the driving
motivation for this research. More fundamental analytical research should be performed
before full application of this technique is possible. Thereby, a wave propagation
continuum mechanics based approach has been applied to model the high frequency
vibrations of one dimensional structures. Energy dissipation mechanisms, such as bolted
connections and internal friction, are considered to have a major role in the attenuation of
the PZT's induced wave, therefore these mechanisms has been extensively studied.
To analyzed bolted connections, linear and nonlinear joint models have been used
to describe the wave interaction with such nonconservative discontinuities. Also, with the
use of an impedance based model, the electromechanical coupling of the PZT and the host
structure is added into the formulation. The wave interaction and energy dissipated at the
bolted discontinuity has been assessed with energy flux computations of the incident,
transmitted, and reflected waves. The effect of loosening the bolted joint has been also
analyzed by reducing the spring stiffness and increasing the damping in the dash pots for
the linear joint model, and reducing the Coulomb stiffness and shearing force at the
interface for the nonlinear case.
A scheme based on the correspondence principle has been applied to calculate the
specific damping capacity of a system, at any given frequency, as a quantification of the
energy dissipated through the system. The material damping was added into the
formulation assuming the modulus to have a complex representation, and therefore the
corresponding loss factors were found with active measurement of the material properties
of the specimen via a wave propagation method, that monitories the wave's speed at two
locations.
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