Transducer characterization for Vibrothermography

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Vaddi, Jyani Somayajulu
Iowa State University; Digital Repository @ Iowa State University
couplant; degradation; Transducer; two port model; Vibrothermography
thesis / dissertation description
Vibrothermography, also known as Sonic IR and Thermosonics, is an NDE technique forfinding 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 developan understanding of the excitation process for vibrothermography so that optimal parametersand 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 canbe characterized using the theory of linear time-invariant (LTI) systems and electro-mechanicalNorton equivalence.This work presents quantitative measurements of the performance of piezoelectric stacktransducers in a vibrothermography excitation system and the effect of transducer perfor-mance and specimen characteristics on the induced vibration in the specimen. We show thatwith compliant coupling, the specimen vibration is directly proportional to the transducer opencircuit velocity and that the system resonances generated because of metal-metal contact ofspecimen and transducer are disconnected by adding a couplant between specimen and trans-ducer. We then give suggestions for transducer and couplant selection for vibrothermographyand suggest methods to flatten the velocity spectrum of the transducer.We extend our analysis to high amplitude transducer behavior and elaborate on the effectof power amplifier saturation on the transducer behavior. The saturation effect negates theeffect of adding an external inductance to flatten the transducer velocity spectrum. Finally,preliminary results are reported on the effect of transducer degradation phenomenon.