Fiber optic sensor for the monitoring of mixed mode cracks in structures

Cracks in structures may open in mixed mode under general loading conditions. In other words, there are both opening and shearing displacements along the crack. In our former work, a distributed fiber optic sensor has been developed for the detection and monitoring of purely opening cracks (or Mode I cracks). The sensing principle is based on the bending loss of an optical fiber intersecting the opening crack at an angle other than 90°. In the present work, the sensing concept is extended to the monitoring of mixed mode cracks. To obtain both the opening and shear displacements at the crack, two independent measurements are required. Based on theoretical simulation, the feasibility of various sensing strategies, including the use of two wavelengths and two fibers, are first investigated to identify the most promising approach. A method to solve the inverse problem of obtaining the crack displacements from the measured signal losses is then derived. Experiments are performed under controlled crack opening/shearing history to verify the sensing approach. Good agreement between derived opening/shearing displacements and imposed displacements has been obtained.