Stephen T. Kreger

High pressure sensing using fiber Bragg gratings written in birefringent side hole fiber

Pressure measurements are made by measuring Bragg grating peak splitting caused by transverse strain differences in the core of a single mode side hole fiber. The side holes and not the fiber exterior are pressurized, demonstrating the ability to use the side holes as a pressure conduit so that pressure measurements can be made in a thermally and mechanically stable environment. Geometrical and residual stress-based birefringence stemming from partial side hole collapse during the drawing process allows measurements to be made near atmospheric pressures. Peak fitting techniques are used to determine peak separation to sub-pm levels despite substantial peak overlap.

Fiber grating systems for traffic monitoring

Blue Road Research has designed, built, and installed fiber grating sensor systems onto bridges, and most recently into an asphalt and concrete highway test pad. The sensitivity levels of the fiber grating sensors are sufficiently high to enable detection of people standing on the bridge or highway. This paper briefly overviews the usage of these sensors for traffic monitoring.

Modeling and experimental strain measurements on a nonhomogeneous cylinder under transverse load

Optical fiber sensors have the capability of sensing environmental factors, including strain and temperature. In particular, optical fiber Bragg gratings have been used to create multi-parameter sensors capable of measuring axial and transverse strain in addition to temperature. These measurements are made by writing gratings into the optical fibers at two separated wavelengths, 1300 and 1550 nm, and monitoring the polarization sensitive output from the sensor. While there is an analytical model for determining the strain on a homogeneous cylinder under transverse load, these fiber optic sensors are not homogeneous as they consist of distinct regions within the fiber: core, cladding, and stress rods. We measure the strain on a multi-parameter fiber Bragg grating written at 1550nm under transverse load at 0, 15, 30, 45, 60, 75, and 90° and compare these values with an analytical solution accounting for internal stresses and transverse load.

Using multi-axis fiber grating strain sensors to measure transverse strain and transverse strain gradients in composite materials with complex weave structures

Multi-axis fiber grating strain sensors have been used to quantitatively measure axial strain and temperature; transverse strain and transverse strain gradients in composite weave structures. This paper overviews the multi-axis fiber optic grating strain sensors and how they can be applied to measuring multidimensional strain fields interior to composite parts with complex composite weave structures. Experimental results are given for the case of a bi-axially woven composite coupon as well as for an E-glass/epoxy composite sample.

Failure mechanisms of fiber optic sensors placed in composite materials

This paper provides an overview of considerations associated with placement and operation of fiber optic sensors placed in composite materials. Issues that are discussed include coatings placed on optical fibers and their relationship to the composite structure, orientation of optical fibers in the composite parts, methods of providing strain relief, and terminations. Examples are given associated with a series of examples from aerospace and civil structure applications.

Nondestructive evaluation of composite materials using multi-axis fiber grating stain sensors

Fiber optic sensors may be used to monitor strain and temperature in composite materials. These measurements can be useful in determining rate and degree of cure of composite. Multi-dimensional strain measurements enabled by fiber gratings written onto polarization maintaining optical fiber enable monitoring changes in transverse strain, transverse strain gradients, and shear strain internal to composites and adhesive joints. This paper provides a brief historical overview of the usage of fiber sensors to provide strain measurements in composite parts, leading eventually to multi-axis strain sensing.