Fiber-optic crack-tip opening displacement sensor for concrete

The development of a fiber-optic sensor for embedment in cementitious composites and measurement of displacements associated with the opening of microcracks are described. The sensor can be employed as a transducer for measurement of crack-tip opening displacements during fracture tests. A polarization-maintaining optical fiber is used as the sensing element, and the transduction mechanism is similar to those of other sensors based on polarimetric-type based fibers. However the deformation resolving power of the sensor is increased by way of increasing the effective length of the optical fiber This is done by winding the optical fiber into a coil. A direct relationship between the number of loops in the coil and sensitivity of measurements is obtained. A calibration procedure is developed by which the optical signal is converted to displacements. The sensor was employed in a series of fracture tests. Experiments involved embedment of the optical fiber in concrete beams. Specimens were center-edge-notched and the fiber coil sensors were embedded at the tip of the notch. This arrangement allowed for direct measurement of displacements associated with the opening of microcracks at the crack tip. Experimental results are presented, and crack-tip opening displacement (CTOD) results are compared with crack-opening displacements measured by conventional transducers at the crack mouth (COD). (C) 1997 Elsevier Science Limited.