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.
Highlights
Ultrasonic guided wave (UGW) detection with optical fiber sensors has received interest in the last decennia due to the ability to investigate larger plate-like surfaces for damage with a lightweight, embeddable, EMI-immune and multiplexed network of sensors [1,2,3,4,5,6,7,8]
They showed by simulation that in the first region, a periodic positive and negative strain is acting along the length of the grating, yielding no net Bragg peak shift (∆λB ) and no change in the shape of the Bragg peaks
In this work we demonstrated for the first time a full spectral reconstruction of the Bragg peaks of different grating lengths during the passage of a UGW
Summary
Ultrasonic guided wave (UGW) detection with optical fiber sensors has received interest in the last decennia due to the ability to investigate larger plate-like surfaces for damage with a lightweight, embeddable, EMI-immune and multiplexed network of sensors [1,2,3,4,5,6,7,8]. Coppola et al [10] and Minardo et al [11] identified three main working regions: when the wavelength of the UGW (λUGW ) is much smaller than the length of the grating (L), when λUGW is of the same order as L, and when the guided wave’s wavelength is much larger than the grating length. They showed by simulation that in the first region (λUGW /L 1), a periodic positive and negative strain is acting along the length of the grating, yielding no net Bragg peak shift (∆λB ) and no change in the shape of the Bragg peaks. The second region (λUGW /L ≈ 1) is a transition zone where partial
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