Transducer characterization for Vibrothermography

Vaddi, Jyani Somayajulu
Major Professor
Stephen D. Holland
Committee Member
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Aerospace Engineering
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Aerospace Engineering

Vibrothermography, also known as Sonic IR and Thermosonics, is an NDE technique for

finding cracks and flaws based on vibration-induced frictional rubbing of unbonded surfaces.

The vibration is usually generated by a piezoelectric stack transducer which transduces electri-

cal energy into large amplitude mechanical vibrations. The purpose of this study is to develop

an understanding of the excitation process for vibrothermography so that optimal parameters

and transducers for the testing can be selected. The amplitude and impedance transfer charac-

teristics of the transducer system control the vibration of the sample. Within a linear contact

(no tip chatter) model, the interaction between the transducer system and the specimen can

be characterized using the theory of linear time-invariant (LTI) systems and electro-mechanical

Norton equivalence.

This work presents quantitative measurements of the performance of piezoelectric stack

transducers in a vibrothermography excitation system and the effect of transducer perfor-

mance and specimen characteristics on the induced vibration in the specimen. We show that

with compliant coupling, the specimen vibration is directly proportional to the transducer open

circuit velocity and that the system resonances generated because of metal-metal contact of

specimen and transducer are disconnected by adding a couplant between specimen and trans-

ducer. We then give suggestions for transducer and couplant selection for vibrothermography

and suggest methods to flatten the velocity spectrum of the transducer.

We extend our analysis to high amplitude transducer behavior and elaborate on the effect

of power amplifier saturation on the transducer behavior. The saturation effect negates the

effect of adding an external inductance to flatten the transducer velocity spectrum. Finally,

preliminary results are reported on the effect of transducer degradation phenomenon.