The development of a fibre optic sensor for embedding in cementitious composites and the measurement of displacements associated with the opening of microcracks are described. The sensor can be employed as a transducer for the measurement of crack-tip opening displacements during fracture tests. A white light fibre optic interferometer was used to sense strain in a fibre embedded in a specimen concrete beam. A direct relationship between the fibre deformation and the optical path variation of the fibre sensor has been obtained. A calibration procedure was developed by which the optical signals were converted into displacements. The sensor was employed in a series of fracture tests and the experiments involved embedment of the optical fibre in concrete beams. Specimens were centre-edge-notched and the fibre optic sensor of the gauge length L was embedded at the tip of the notch. This arrangement allowed for the 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 (CODs) measured by conventional transducers at the crack mouth.
ABSTRACT--This paper deals with circumferential strain measurement for concrete in uniaxial compression by using a fiber optic sensing system. A fiber optic sensing system was employed to measure the elongation of the optical fiber for a cylindrical concrete specimen in uniaxial compression by using white light interferometry. A theoretical model describing the relationship between the elongation of the fiber glass and the circumferential strain of cylindrical concrete specimen in uniaxial compression is proposed. Some tests were performed to verify the measurement method. The results indicate that the proposed method is valid.
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