Investigation On The Stress Fatigue Failure Characteristics Of Fiber Bragg Grating

Fiber Bragg grating (FBG) has many excellent characteristics, and it has beenwidely used in civil engineering, aerospace, petrochemical and some other strainmonitoring areas. However, in practical engineering applications, the FBG suffers fromthe long-term cyclic strain field; so the stress fatigue effect can lead to gradualdegradation of sensing properties of the FBG, even complete failure. The error signalfrom the fatigued FBG may cause wrong actions of actuators, bringing seriousconsequences. Therefore, it is of great significance to distinguish the fatigued FBGthrough some characteristics.In this dissertation, the strain sensing principle of the FBG is introduced; then, thecomposition and mechanical transfer process of FBG strain sensor are analyzed indetails. What’s more, it is found that the FBG is a part which is most prone to fatigue inthe FBG strain sensing network.Four kinds of FBG peak detection algorithms are introduced, and they areemployed to analyze influences of spectral quality on the sensor’s sensing properties.The distorted spectra are divided into three types, and their effects on the accuracy ofcenter wavelength detection are discussed respectively. Correspondingly, threeevaluation indexes, side-mode suppression ratio, peak to valley ratio, and3dBmbandwidth, are proposed to characterize the fatigued FBG.In addition, a fatigue failure mechanism of FBG is established based on crackspropagation theory. Through a detailed analysis of the stress source in the FBG, and therelationship between stress fatigue stage and cracked state, the FBG stress fatigueprocess is divided into three stages: crack formation stage, the micro crack propagationstage and FBG fracture stage. Then, the reflection spectra of the FBG with variouscracks states are simulated by using the transfer matrix method. The simulation resultsshow that stress fatigue induces the distortion of spectra of FBG and that the simulateddistortions are consistent with the actual cases. The fatigue failure mechanismtheoretically proves that side-mode suppression ratio, peak to valley ratio and3dBmbandwidth can be used as the fatigue failure characteristics of the FBG.On the other hand, stress fatigue test devices are designed to accelerate the fatigueprocess of FBG, and a series of fatigue tests are completed. Combining the theoreticalanalysis and experimental results, it is found that side-mode suppression ratio and3dB bandwidth can be used to characterize the fatigued FBG effectively. And then the fibergrating stress fatigue failure characteristics are proposed: if the side-mode suppressionratio is less than5dBm and the bandwidth is greater than or equal to0.3nm, the fibergrating is likely to failure completely; if the side-mode suppression ratio is greater thanor equal to5dBm, less than or equal to10dBm, historical data and the change ofbandwidth increases, the FBG is very likely not reliable; if the side-mode suppressionratio is greater than or equal to10dBm, and the bandwidth remains unchanged, theFBG is likely works normally. This study provides a criterion to identify whether thein-service FBG fails or not.