Introducing S-parameters for ultrasound-based structural health monitoring.

Scattering parameters (S-parameters) are the most common tools for characterizing microwave devices and high-frequency systems but have not been used for ultrasoundbased structural health monitoring (SHM) so far. In this paper, we present the theory, signal processing algorithms, and experimental results to demonstrate the applications of S-parameters for three common ultrasound-based SHM techniques, i.e., ultrasound pitch-catch, pulse-echo, and impedance/admittance measurements. The concept of the S-parameters is first introduced, followed by the discussions of extracting the time-domain ultrasound signals from the frequency-domain Sparameter measurements. The time-domain signals calculated from the measured S-parameters were compared with the oscilloscope measurements. Excellent agreements between these two sets of measurements were achieved. In addition, time-frequency analysis of the ultrasound inspection system based on the S-parameter measurements has also been performed. This work demonstrated that the S-parameters can serve as a versatile tool for ultrasound-based SHM.