Ultrasonic Lamb Wave Inspection Using Fiber Bragg Gratings

A new ultrasonic Lamb wave nondestructive testing (NDT) system is developed in this paper. The basic idea is to use fiber Bragg gratings (FBG) sensors as acoustic receivers of ultrasonic Lamb waves. The spectral response of fiber Bragg gratings (FBGs) subjected to the ultrasonic field has been theoretically and numerically investigated. Taking into account the geometrical and elasto-optic effects, the Bragg effective refractive index modulation under the ultrasonic wave action is deduced. A transfer matrix method is used to analyze the influence of ultrasound on the reflection spectrum of FBGs. Numerical results show that the influence of ultrasonic field on grating spectrum is highly correlated to the ratio between the ultrasonic wavelength and the grating length. Only when the ratio is quite larger, the peak wavelength of reflection spectrum shifts while the shape of the grating remains unchanged. The reason is explained by the strain variation along the grating. The results of this analysis provide useful tools for the determination of gage length of FBG sensor and the choice of Lamb wave frequency when the new NDT system is used for damage detection.