The Pennsylvania State University

ABSTRACT

In the past decade, two areas of research have progressed that are brought together in this research. Numerous optical fiber sensors have been developed and have been shown to be accurate for measurement of strain in concrete. Also important to this research is modal analysis, whereby the physical state of a structure, most commonly a bridge, is determined by monitoring its vibratory signature, i.e. resonant frequencies, mode shapes, or damping. Traditionally, modal analysis has been performed using very short gage length sensors, such as accelerometers, geophones, LVDTs, or electric strain gages, and has had only limited success. Most research has determined that modal analysis is possible, but that it is often difficult to detect changes in the vibratory signature until significant damage has occurred. Additionally, most of the more popular fiber optic sensors being developed have very short gage lengths, such as the Bragg Grating and Fabry-Perot sensors. A long gage length fiber optic sensor has been developed for modal analysis on civil structures. Since modal analysis involves monitoring global attributes of a structure, a long gage length sensor allows a much larger portion of a structure to affect the sensor output. While some local effects may be lost, global effects such as a reduction in stiffness due to cracks may be more pronounced with long gage length sensors. This study uses a long gage length fiber optic sensors on a steel and concrete beam to monitor physical changes of the beams. An impact load is used to provide energy to the beams, and the sensor output is analyzed to obtain the frequency response of the beams. Any changes in the frequency response that correspond to physical changes in the beams are noted as being a potential basis for a nondestructive sensing technique.