Signal characteristics of surface-bonded fiber Bragg grating sensor with elastoplastic metallic coating subjected to residual strain

Abstract

This study involved predicting the reflected spectrum of a surface-bonded fiber Bragg grating sensor with an elastoplastic metallic coating wherein residual strains are non-uniformly distributed on the sensor. A profile of residual strains was numerically calculated based on the stepwise numerical technique by adopting strain transfer analysis and was validated by finite element analysis. The reflected spectrum of a metal-coated fiber Bragg grating sensor was consequently simulated to obtain the calculated profile of residual strains through the Transfer-matrix (T-matrix) method and coupled mode theory. The results revealed that a non-uniform profile of permanently induced residual strains along the sensor makes the reflected spectrum distorted. Thus, it is necessary for a metal-coated fiber Bragg grating sensor with a 10-mm Bragg grating to be bonded to a minimum length of 16.0 mm to suppress the signal distortion. The study additionally investigated the effect of parameters of an elastoplastic metallic coating (i.e. thicknesses and mechanical properties) on the spectrum characteristics and proposed minimally required bonding lengths by considering these characteristics. The evaluated spectra of the metal-coated fiber Bragg grating sensors that consider various coating parameters will reduce measurement errors and provide directions to determine geometric parameters when the sensor is actually installed on a structure surface.