Fiber Bragg Grating Sensor Based Mode Filtering Using Cosine Distance Metric

Abstract

Fiber Bragg grating (FBG) sensors are considered ideal for structural health monitoring (SHM) due to their small weight and size, ability to be embedded and ability to be multiplexed. So, FBG sensors are commonly used as strain sensors for SHM but their use for ultrasonic guided wave (GW) measurements is not common due to the low sensitivity. In the recent times, a renewed interest is seen in the use of FBG sensors for GW measurements using the edge reflection approach which increases the sensitivity several folds. It has been reported, that the mechanism of the measurement of the incident GW is different based on the relative ratio of the wavelength of the incident GW (GW) and the grating length (L) of the FBG sensor. For ratios GW/L» 1 the propagating wave leads to uniform strain over the FBG resulting in the shift of the Bragg wavelength. For the λGW/L ≈ 1 the FBG experiences non-uniform strain over the FBG which results in the distortion of the spectrum (widening or narrowing) of the peak. By separating these effects on the FBG, mode filtering may be achieved. In the previous work the mode filtering was achieved by scanning the entire reflectivity spectrum of the FBG (over 300 points). This approach is time consuming and hence has limited applicability for in-service SHM. Hence, a novel method which uses only measurements at 2 locations on the spectrum is proposed in this paper. The cosine distance in the waveforms is calculated and then used for identifying the mode. This information is then used for damage localization on a simple plate.