Understanding High-Frequency Modes in Electromechanical Impedance Measurement Using Noncontact Vibration Testing

Electromechanical impedance (EMI) measurements have been used for several decades in noninvasive health monitoring across various domains. Typical EMI measurements are recorded via a bonded piezoelectric transducer, at a high-frequency range, typically 30 kHz and above. Because EMI measurements are single input single output, the peaks in these measurements can be related to either mechanical, electrical, or coupled electromechanical modes, especially at higher frequencies. An attempt to move towards understanding these higher frequency modes is made in this study. To undertake this, noncontact vibration testing is carried out using a laser Doppler vibrometer (LDV). The specimen under investigation is bonded with a piezoelectric transducer with a prescribed voltage applied across the transducer. A comparison between the peaks in a recorded EMI measurement and the noncontact measurement is presented. A better understanding of these modes can be utilized for a better association between high-frequency measurement and physical attributes. Furthermore, tracking variations can lead to physical insights and mode transitions that more accurately inform material changes (damage). This work discusses the first step in this building association by exploring the relation between directional information from the noncontact measurement and coupled electromechanical information from the EMI measurement. This chapter also highlights some challenges and future work needed to make this technique robust and versatile for different specimens.