Mechanism of three-dimension sensing with fiber gratings for structure cracks detection

A method using for the detection and analysis of the three-dimension strain sensing signals of the longitudinal concrete cracks was proposed by utilizing the reflectance spectrum of fiber gratings. The fracture model of three-dimension finite element using bottom-up method based on ANSYS was given. The three-axis stress applied on the fiber grating under uniform and radial force was presented, the three-axis strain function was derived on the basis of above-mentioned stress field, and the relation curve between resonance wavelength of x-polarization, y-polarization and three-axis stress was given. The variation of reflective spectrum of fiber grating of 10 cm length is calculated using transfer matrix method under three-axis strain based on theoretical analysis and numerical calculation, the results show that, the peak wavelength shifts of x-polarization and y-polarization are 10.1 nm and 12 nm respectively under uniform and radial force of 20N. At the same time, the reflection spectra of the fiber Bragg grating split into two main peaks and the spectral difference of the two peaks increases linearly, and the split point shiftes to long-wavelength, the reflectivity of shorter wavelength is higher than the long-wavelength but the bandwidth of shorter wavelength is less than the long-wavelength when the load increases. The (Full Width Half Maximum, FWHM) of the two splitted reflection peaks increases but the spectral peaks space remains unchanged almost when the length of grating increases to 15 cm under same force compared to grating of 10 cm length. The sensitivity of the grating proposed in the paper is up to 0.14 nm/N. The two-dimension sensing is extended to three-dimension sensing based on single grating with the method proposed in this paper.