Fiber‐Optic Bragg Grating Sensor for Nondestructive Evaluation of Composite Beams

Extensive testing of control specimens and large‐scale models of high‐strength concrete composite beams, 3,048‐mm (10‐ft) length, reinforced with pre‐stressed prisms, have resulted in deformation readings that are consistent with those obtained from classical measuring methods such as electric strain gauges and linear voltage differential transformers. The present paper provides the research findings of fiber‐optic Bragg gratings as strain/stress sensors for monitoring the critical sections of a series of this type of composite beams. The optical fibers used in this study had a numerical aperture of approximately 0.22 and a V‐value (reduced frequency) of 2.2 at 1,300‐nm operating wavelength. The theoretical strain coefficient of the optical fiber had a typical value of 0.762, which agreed quite well with the calibration test performed. A successful packaging of the fiber‐optic Bragg grating was used and modeled. Good bonding of sensor to the structure was shown to be imperative for maximum transfer of load‐induced strain. The average strains in the main tension reinforcement embedded in the concrete beams, the plane section assumption of the strain distributions across the critical section depth, the flexural stiffness of the beams, and the crack widths at service loads are evaluated.