Subsurface damage detection using a novel fiber optic sensing technique

Abstract

The detection of subsurface damage is important in ensuring the safety and timely repair of structures. Existing methods for non-destructive evaluation of structural elements tend to be either expensive or unreliable for monitoring large scale systems. The novel fiber optic based technique developed in this research overcomes many of the limitations of traditional non-destructive evaluation methods by providing an interferometric sensing technique coupled with a simple mechanical test. The method is based on monitoring the phase change in an integral interferometric fiber optic sensor caused by moving a mechanical load over the damaged structure. The method has been shown to unambiguously detect both the position and size of damage. The theoretical and experimental validation of the proposed method is presented for the case of open cracks in which the faces are not allowed to come into contact. The effect of damage position and damage size on sensor performance for two typical structural elements is also presented. A closed loop fiberoptic interferometer with modulated load is shown to overcome the traditional problems of environmental drift such as material creep, temperature and ambient noise. This interferometric technique is also shown to be one of the few fiber-optic based techniques that have adequate sensitivity for integral damage detection. Many traditional non-destructive evaluation methods tend to be insensitive in detecting closed cracks. Thus the closed crack problem represents a special challenge for structural damage monitoring. A fast iterative based boundary element method has been developed to solve this problem. This method is used to show the theoretical feasibility of detecting closed cracks with the developed novel sensing method.