Spectral Verification of the Mechanisms behind FBG-Based Ultrasonic Guided Wave Detection

Abstract

Ultrasonic guided wave (UGW) detection with fiber Bragg grating (FBG)-based sensors has received increasing attention in the last decades due to the ability to perform non-destructive inspection (NDI) of large plate-like surfaces with a network of lightweight and multiplexed sensors. For accurate UGW measurements, several studies concluded that the ratio between the wavelength of the UGW and the length of the FBG should be above 7. However, shorter FBGs suffer from a lower FBG reflectivity and less steep slopes in the reflection spectrum. In this work we experimentally verified the effect of a passing UGW on the Bragg peak of FBG sensors of different lengths. By performing edge-filtering interrogation throughout the FBG’s reflection spectrum, we were able to reconstruct the FBG’s spectral response to a UGW in function of time. Our experimental findings are partially in line with those in the literature considering the UGW wavelength to FBG length ratio and the corresponding Bragg peak changes. We experimentally show for the first time that for shorter FBG sensors, the strain modulation is translated mostly into Bragg peak shifting, while for longer FBG sensors, Bragg peak deformation takes over as main mechanism. Despite the different mechanism for the latter, the UGW can still be detected by edge-filtering on the steepest slope, and with a much higher sensitivity.

Optical Fiber Sensors for Ultrasonic Structural Health Monitoring: A Review

“…In the transition regime where λ GW /FBG L 1, the Bragg wave length undergoes a shift, but also the bandwidth of reflection spectrum increases with decreasing L. In this region, the sensitivity increases approximately linearly with λ US /L. Some recent work by Goosens et al [110] has given insight into the response of the FBG spectrum to the GW in the regime II. The result investigates waves at 50 kHz and 250 kHz with sensors from five different gauge lengths, namely 2, 3, 5, 8, and 10 mm.…”

“…Figure 22 shows the results for the excitation of 250 kHz wave in a simple aluminum plate with a PZT actuator at 0.125 m from the FBG sensor. The sweep of the spectrum of the FBG was obtained similarly to Goossens et al [110] and the mechanisms for the coupling of the A and S mode were determined. For the 1 mm aluminum plate, the wavelength of the A 0 wave is 5.6 mm while that of the S 0 wave is 20 mm.…”

Enhancing FBG Sensing in the Industrial Application by Optimizing the Grating Parameters Based on NSGA-II

“…Fulfilling the aim of this research, which is controlling the bandwidth and the power of the reflected wave of FBG by fetching the optimum grating parameters, it is important from the beginning to show the correlation between the control variables and the prospected optimized variables. Therefore, Figure 1 represents data obtained from one study in the literature showing the resulting bandwidth and reflected power of five FBG sensors with different grating parameters [ 22 ]. It depicts the effects of changing the grating parameters on the reflectivity and bandwidth.…”

“…For deriving a relation between bandwidth, reflectivity, sidelobe and grating length, and the change in refractive index. The bandwidth can be determined by Equation (2), which shows the bandwidth depends on the grating parameters, such as the variation in the refractive index [ 22 ]. where…”

“…In addition, the reflected and transmitted power can be determined by Equations (3) and (4) that also show the reflectivity depends on the grating parameters as follows [ 22 ]: where…”