Validation of Active Strain Measurements in Composites Using Mach-Zehnder Interferometry with Embedded Optical Fibers

This article reports the validation of active strain measurements in a vibrating composite lattice structure using Mach-Zehnder interferometry with embedded optical fibers. Optical fibers were used as internal strain sensors by embedding them in a highly flexible graphite/bismaleimide lattice structure to experimentally determine the dynamic response of the lattice. This is an extension of work previously reported on the natural response of a graphite/bismaleimide lattice structure with an optical fiber attached to the surface. The advanced composite lattice structure is a half-scale model of the alumi num grid used for active control studies at the Air Force Astronautics Laboratory. One- inch wide strips form the five-by-five grid geometry of the dynamically scaled composite lattice, which is 30 inches square overall with a nominal thickness of 0.037 inch. The lon gitudinal and transverse members employ the same seven-ply [0 ₃ /90] _s stacking sequence with respect to their length. At the intersections, the layers alternate. The scaled lattice structure was fabricated from G40-600/5245C graphite/bismaleimide prepreg tape. The paths of the embedded optical fibers were optimized for strain sensing of the first seven natural modes using a finite element model. Two optical fibers were embedded, one on each side of the neutral axis of the lattice, along "L"-shaped paths forming mirror images of each other. One of the optical fibers was spliced into the sensor arm of a modified Mach-Zehnder interferometer (which provides a voltage proportional to sensor elonga tion) to supply real-time integrated strain data. The lattice structure was cantilevered verti cally from a specially designed vise and vibrated at each of its first five natural frequencies by an oscillating magnetic field. In addition, many modes were excited simultaneously with the use of an impact hammer. The natural frequencies and modal amplitudes of the lattice structure were measured using the embedded optical fiber for both excitation methods. The experimental results were confirmed with noncontact proximity sensors. Results demonstrate that embedded optical fibers can be used as internal strain sensors to provide reliable indications of the dynamic response of a randomly vibrating flexible lat tice structure.