Craig M. Lawrence

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.

Measurement of transverse strains with fiber Bragg gratings

In this paper, we present a method to measure two components of transverse strain in an optical fiber using a single Bragg grating written into high-birefringent, polarization- maintaining (PM) fiber. The reflected spectrum from this grating contains two peaks corresponding to the two orthogonal polarization modes of the fiber. If the axial strain and temperature in the fiber is known, then two components of transverse strain can be computed from the changes in wavelength of the two peaks. A Bragg grating written near 1300 nm in PM fiber was loaded in diametrical compression, and the changes in wavelength of the Bragg peaks were monitored using an optical spectrum analyzer. Transverse strains were computed from the changes in wavelength using available strain-optic coefficients for low-birefringent optical fiber. These strains are compared to finite element analysis predictions, and it is shown that the observed sensor response is greater than the response predicted by the low-birefringent analysis. A calibration factor is developed for the sensor to allow the determination of transverse strains from the measured wavelength shifts.

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.

Development of a three-axis strain and temperature fiber optic grating sensor

Development of a novel 3 axis strain and temperature fiber optic grating sensor is described. This sensor relies on dual overlaid fiber gratings written onto birefringent optical fiber resulting in four effective fiber gratings. By using dual overlaid phase masks fabrication costs for this sensor can be expected to be similar to that of a single fiber grating. This paper reports on early development efforts associated with the 3 axis strain and temperature sensor.

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.

Three-axis strain and temperature fiber optic grating sensor

For many applications it would be highly desirable to be able to measure all three axes of strain and temperature internal to composite materials. Conventional electrical strain gauges are undesirable to embed into composite materials because of their size, conductive nature, susceptibility to electromagnetic interference, incompatibility with the host material and temperature limitations. All of the tests done to date with single element fiber sensors have been limited to the measurement of strain in the in plane dimension. This paper describes an innovative fiber sensor based on dual overlaid fiber gratings on short lengths of birefringent polarization preserving fiber that allows three axes of strain and temperature to be measured at a single point.

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.

Multiparameter sensing with fiber Bragg gratings

This paper describes the design of a fiber optic sensor capable of sensing temperature and three independent components of strain simultaneously in a single, short gage length device. The sensor utilizes two fiber Bragg gratings at widely spaced wavelengths (1300 nm and 1550 nm) written at a single location in polarization maintaining optical fiber. When a broad-band light source is used to illuminate the gratings, the reflected spectrum will contain four peaks corresponding to the two polarization states for each of the two gratings. If the fiber is subjected to a change in temperature or strain, the resulting change in wavelength of the reflected peaks can be used to determine the magnitude and direction of the perturbation. In theory, the four peaks can be used to simultaneously determine the grating temperature, and three independent components of strain in the fiber.

Determination of the K-matrix for the multiparameter fiber grating sensor in AD072 fibercore fiber

A methodology is described for determining a relation (K- matrix) between wavelength shifts and (1) axial strain, (2) two transverse strains and (3) temperature change experienced by a multi-parameter Bragg grating sensor. The sensor is formed by writing gratings at two wavelengths in polarization maintaining fiber. The methodology is based on separate experimental calibrations of sensor response to transverse loading (diametral compression), axial loading and temperature changes. Strains produced in the core by the loadings or temperature changes used in the calibrations are determined by finite element analyses.

Modeling of the multiparameter Bragg grating sensor

Strains produced in the core of a multi-parameter Bragg grating sensor by transverse loading, axial loading or a uniform temperature change are computed by finite element analyses. The sensor is formed by writing gratings at two wavelengths in polarization maintaining fiber in which birefringence in the core is induced by an elliptical stress applying region. The finite element model accounts for the differences in geometry and mechanical properties of the different regions of the fiber. The computed strains are needed to determine the elements of a matrix that relates wavelength shifts of Bragg peaks to axial strain, two components of transverse strain and temperature change.