Method and Theory for a Multi-Gauge Distributed Fiber Optic Crack Sensor

A distributed sensor is developed for detection of cracks and measurement of crack widths in composite materials. The sensor consists of a number of individual segments on one line with gauge lengths designed according to the structural and materials requirements. Return loss measurements at the interfaces between adjoining segments provide the basis for crack detection and measurement. An optical time domain reflectometer (OTDR) is employed for interrogation of the sensor signal. The quasi-distributed sensors were embedded in reinforced concrete beams in order to evaluate their crack detection capability. Experiments involved both monotonic as well as fatigue loading. The sensors were embedded in the tension zone of the concrete beams in order to facilitate comparison of sensor response with visual crack observations. Experimental results in terms of crack detection capability and resolution are presented and discussed.