Dynamic Strain Sensing of a Composite Lattice with an Integrated Optical Fiber

Abstract

A fiber-optic sensor was used to investigate the modal response of a highly-flexible graphite/bismaleimide lattice structure. The natural frequencies and modal amplitudes were determined based on strain readings taken from the structurally- integrated fiber-optic sensor. The advanced composite lattice structure to which the sensor was attached is a half-scale model of the aluminum grid currently being used for active control studies at the Air Force Astronautics Laboratory. The dynamically-scaled compos ite lattice structure, which employs 2.604-cm (1.025-inches) wide strips to form a five-by- five grid, is 76.52-cm (30.125-inches) square overall with a nominal thickness of 0.094 cm (0.037 inches). The longitudinal and transverse members employ the same seven-ply [O3/90] s stacking sequence with reference to their local coordinate system, except at the intersections, where the layers are alternated. The scaled lattice structure was fabricated from G40-600/5245C graphite/bismaleimide prepreg tape. The first seven modes from an ANSYS finite element model based on eight-node quadrilateral shell elements were used to optimize the path of the integral fiber-optic sensor. To form this sensor, an optical fiber was bonded to the surface along an "L"-shaped path and spliced into the sensor arm of a modified Mach-Zehnder interferometer, which provides a voltage proportional to sensor elongation to supply real-time integrated strain data. The structure was cantilevered verti cally from a specially-designed vise, and the natural frequencies and modal amplitudes of the lattice structure were measured using the integral fiber-optic strain sensor. The ex perimental results were confirmed with noncontact proximity sensors. The results demon strate that integral fiber-optic strain sensors can provide reliable indications of the dynamic response of a flexible lattice structure.