Thomas E. Bennett

A fiber optic sensor for transverse strain measurement

A fiber optic sensor capable of measuring two independent components of transverse strain is described. The sensor consists of a single Bragg grating written into high-birefringent, polarization-maintaining optical fiber. When light from a broadband source is used to illuminate the sensor, the spectra of light reflected from the Bragg grating contain two peaks corresponding to the two orthogonal polarization modes of the fiber. Two independent components of transverse strain in the core of the fiber can be computed from the changes in wavelength of the two peaks if axial strain and temperature changes in the fiber are zero or known. Experiments were performed to determine the response of the sensor by loading an uncoated sensor in diametral compression over a range of fiber orientations relative to the loading. The results of these experiments were used with a finite element model to determine a calibration matrix relating the transverse strain in the sensor to the wavelength shifts of the Bragg peaks. The performance of the sensor was then verified by measuring the transverse strains produced by loading the fiber in a V-groove fixture.

Multidimensional strain field measurements using fiber optic grating sensors

Fiber optic grating sensors written into polarization preserving optical fiber may be used to monitor multidimensional strain fields in composite materials. This paper provides an overview of the characterization and test of multiaxis fiber grating sensors formed by writing 1300 and 1550 nm fiber gratings into polarization preserving optical fiber. A discussion of the usage of these multiaxis fiber grating sensors to measure two and three dimensional strain fields will be made. A brief review of practical applications of the technology to measure shear strain, transverse strain gradients as well as axial and traverse strain will be made with emphasis on aerospace and civil structure applications.

An Embedded Fiber Optic Sensor Method for Determining Residual Stresses in Fiber-Reinforced Composite Materials

A method to determine process-induced residual stress in fiber-reinforced composite materials using strain measurements from embedded fiber optic sensors is presented. This method allows non-destructive, real-time determination of residual macrostress in these materials and may be useful for both process monitoring and control. Extrinsic Fabry-Perot interferometer strain sensors were embedded in Hercules AS4/3501-6 graphite/epoxy composite specimens prior to cure. The specimens were cured in a press, and the internal strains and temperatures developed during processing were monitored and recorded. Residual macrostresses were computed from these measurements using a viscoelastic model of the material. The results compare favorably with analytical predictions, previous experimental measurements from a destructive technique, and with measurements of warpage of a non-symmetric laminate.

Determination of process-induced residual stress in composite materials using embedded fiber optic sensors

A method to determine process-induced residual stress in composite materials using strain measurements from embedded fiber optic sensors is presented. This method allows non- destructive, real-time determination of residual macrostress in composite materials and may be useful for both process monitoring and control. Extrinsic Fabry-Perot interferometer strain sensors were embedded in Hercules AS4/3501-6 graphite/epoxy composite specimens prior to cure. The specimens were cured in a press, and the internal strains and temperatures developed during processing were monitored and recorded. Residual macrostresses were computed using these measurements and a viscoelastic model of the material. The results compare favorably with previous analytical predictions and experimental measurements from a destructive technique.

Measurement of process-induced strains in composite materials using embedded fiber optic sensors

This paper presents the results of experiments to measure the internal strains and temperatures that are generated in graphite/epoxy composite specimens during processing using embedded fiber optic strain sensors and thermocouples. Measurements of strain and temperature, combined with a computational model, offer the potential for non-destructive, real-time determination of residual stress in composites, and may be useful for process monitoring and control. Extrinsic Fabry-Perot interferometer, Bragg grating strain sensors, and thermocouples were embedded in graphite/epoxy composite laminates prior to cure. The specimens were cured in a press, and the internal strains and temperatures developed during processing were monitored and recorded. The results are compared with expected values, and limitations of the experimental technique are discussed.

Fiber optic sensors at Sandia National Laboratories

Sandia National Laboratories in Livermore, Calif. and the Mechanical Engineering Department of Stanford University are involved in fiber optic sensor research and development for manufacturing process monitoring, smart materials, and other applications. Projects at Sandia and Stanford involving both embedded and surface mounted fiber optic strain and temperature sensors have demonstrated the desirability of this technology. This paper presents an overview of the fiber optic sensing capabilities at Sandia and a summary of the projects currently in progress.