Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantage but their use has been limited for the GW sensing due to its limited sensitivity. FBG sensors in the edge-filtering configuration have overcome this issue with sensitivity and there is a renewed interest in their use.The key challenge to the use of the GW based techniques is the complexity of the signal processing due to the presence of multiple modes (Symmetric and Antisymmetric modes and their higher harmonics. Although through selection of proper frequency range, the number of modes can be reduced, the A0 and the S0 modes still are present and with the mode conversions at the defects and structural artefacts such as stiffeners, the signal processing still is challenging. Thus a method for filtering of these modes is desirable.It has been seen that depending on the ratio of the wavelength of the propagating wave to the gauge length of the FBG, the mechanism of the transduction of the wave measurement is different. A large ratio leads to a more uniform strain over the fibre leading to a shift in the frequency, while a non-uniform strain due to a short wavelength wave, leads to the peak widening. In a sub MhZ frequency range the S0 and A0 modes have considerably different wavelengths. By using a proper setup this difference in the mechanism may be leveraged for the separation of the modes. The present paper studies this phenomena and develops and signal processing technique for the separation of the modes.

Guided waves mode filtering using fiber Bragg grating sensors

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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