Amplification of Lamb-wave detection via fiber Bragg gratings using ultrasonic horns

Fiber Bragg Grating (FBG) sensors are often applied as Lamb wave detectors for structural health monitoring systems. Analyzing the measured signal for the identification of structural damage requires a high signal-to-noise ratio because of the low amplitude Lamb waves. This paper applies a two-dimensional ultrasonic horn between the structure and a remotely bonded FBG sensor to increase the amplitudes of the measured signal. Experimentally we test a variety of ultrasonic geometries and demonstrate a 100% increase in the measured ultrasonic signal amplitude using a metallic ultrasonic horn with step-down geometry. A bonding procedure for the combined ultrasonic horn and optical fiber is also developed that produces repeatable signal measurements. For some horn geometries, an additional vibration signal at the Lamb wave excitation frequency is observed in the measurements. Laser Doppler vibrometry measurements and finite element analysis demonstrate that the signal is due to the natural vibration of the horn. The experimental results demonstrate that using an aluminum ultrasonic horn to focus wave is an excellent method to increase the sensitivity of the FBG to the small amplitude Lamb wave, provided the horn vibration characteristics are taken account in the design of the measurement system.