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In this paper, a nondestructive, in-service structural integrity monitoring methodology that can detect and characterize local
structural damages of contact-type, i.e. damages and failures which come along with generation, growth and/or changes of
contacting surfaces, such as cracks, debonding, preload-loss in bolted joints, etc., is presented. The presented monitoring
system consists of piezoelectric elements bonded on the structural surface, a high-frequency harmonic voltage source,
and a current detector. When the structure is subjected to a vibrational load such as operational load at low-frequencies,
the scattering conditions for the high-frequency elastic waves in the vicinity of the contact-type damages will change in
synchronization with the structural vibration because of the fluctuation of the contact conditions. This nonlinear effects
of vibro-acoustic interaction between the low-frequency vibration and the high-frequency wave field causes the change
in the driving-point impedance of the structure at the high frequency range, which leads to the significant modulation of
the coupled electro-mechanical impedance (or admittance) of the piezoelectric elements. Therefore, if the piezoelectric
elements are driven by a fixed amplitude high-frequency harmonic voltage source, the nonlinear fluctuation of the coupled
admittance can be observed as the amplitude and phase modulation of the current flowing through the piezoelectric element.
A modeling and analytical study of the nonlinear piezoelectric impedance modulation is presented for a beam structure
including a crack, utilizing a linear time-varying system theory. A damage evaluation measure is presented based on the dimensionless modal stiffness fluctuation estimated from the instantaneous admittance reconstructed from the demodulated current responses. Furthermore, fundamental strategies and future directions for damage localization based on the nonlinear piezoelectric impedance modulation are briefly discussed.
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