Static and dynamic strain measurements within composite materials using a localized Michelson fiber optic sensor

A localized, all-fiber Michelson fiber optic sensor (MFOS) is described for the monitoring of strain and vibration information inside a composite material structure. The fiber optic sensor is analogous to the bulk Michelson interferometer using a pair of mirror-ended optical fibers to create the optical path. The gauge length of the optical sensor is determined by the difference in the lengths of the paired fibers. Common mode rejection along the fibers is attained by mechanically coupling the two fibers. A desired gauge length can be attained by ensuring that one fiber is longer than the other by the appropriate amount, thus achieving localization. This fiber optic sensor was successfully embedded in Kevlar/epoxy and graphite/epoxy thermosets as well as graphite/PEEK thermoplastic in order to determine the embedded axial strain sensitivity of the sensor and to perform local strain measurements at the lamina level. A 16 ply cantilevered composite beam was interrogated using the MFOS to determine the through-thickness static strain distribution. Vibrational information was attained by the employment of a (3 x 3) coupler and a passive homodyne quadrature demodulation system.