A FBG sensor is used to monitor the propagation of crack in the repaired patch under cyclic loading condition.As the number of cyclic loading increases,the crack length will extend and the reflection spectra of FBG sensor are found to change.The reflection spectra of the FBG for different crack lengths have been calculated by the finite-element model of the strain field that results from a crack.The relationship between the spectral change and the crack length is verified by the theoretic analysis and the e...