Crack sensing with a multimode fiber: experimental and theoretical studies

For many structures, failure is preceded by the formation and propagation of cracks. Crack monitoring is hence a useful technique to assess the condition of structures. Also, for laboratory specimens, the monitoring of cracking can provide useful information on the failure process. An important challenge in crack monitoring is that the location of cracks in structures may not be known in a priori. Conventional sensors or transducers that measure the average strain over a small region may easily 'miss' the crack. In this paper, a sensing concept based on the bending loss of an optical fiber is first introduced. With the use of multimode fibers, experiments are performed to obtain the signal loss versus crack opening relation. A theoretical model for power loss versus crack opening is then developed. The crack-induced curvature along the fiber is computed with a finite element analysis. Power loss in the curved fiber is then obtained with the ray tracing technique. The theoretical prediction is found to be in good agreement with experimentally measured power loss.