Time–frequency analysis of electro-mechanical impedance (EMI) signature for physics-based damage detections using piezoelectric wafer active sensor (PWAS)

Abstract

This paper investigates the physical connections between the electromechanical impedance (EMI) signature and the time-domain pulse-echo signal. The EMI signature, acquired from a piezoelectric wafer transducer bonded on a structure, is frequently used for damage detections. Although the EMI signature is very sensitive to damage, it cannot provide any physical insight about the details of the damage, e.g. its type, location, etc. Moreover, legitimate condition changes such as thermal fluctuations may be mistaken as damage. In this paper, we demonstrate that the EMI signature is essentially a pulse-echo signal represented in the frequency domain. Therefore, analyzing the EMI signature in both the time and frequency domains enables us to perform physical-based damage detection and characterization. A digital signal processing algorithm was introduced to convert the frequency-domain EMI signature to a time-domain pulse-echo signal at any given excitation frequency. Analyzing the ‘resonant phase’ and the ‘echo phase’ of the time domain signal separately revealed that the resonance signal is more sensitive to the bonding conditions while the time-of-flight of the echo signal can be correlated to the damage location. In addition, the damage detection sensitivity is significantly enhanced when the damage indices are calculated from the spectra of the resonance and echo signals instead of the as-acquired raw EMI signature.