Tomas Valis

Fiber optic Fabry-Perot strain gauge

A local all-fiber Fabry-Perot interferometric strain sensor is reported. The device makes use of a simple semireflective fusion splice technique and is operated in reflection mode. Five sensors of gauge lengths of approximately 10 mm were fabricated and adhered to the surface of a cantilever beam. The phase-strain sensitivity, Delta phi / Delta L, was measured to be 2.24+or-0.07*10/sup 7/ m/sup -1/ ( lambda =632.8 nm) and found to be symmetric with respect to tension and compression.<<ETX>>

Composite-material-embedded fiber optic Fabry-Perot strain rosette

A fiber-optic strain rosette is embedded in Kevlar/epoxy. The individual arms of the rosette are fiber Fabry-Perot interferometers operated in reflection-mode with gauge (i.e., cavity) lengths of approximately 5 mm. Procedures for manufacturing the cavities, and bending the fibers, to form a strain rosette are described. Experimental results showing 2D interlaminar strain-tensor measurement are presented. The sensor is also tested as a surface adhered device.

Passive-quadrature demodulated localized-Michelson fiber-optic strain sensor embedded in composite materials

A strain sensor embedded in composite materials that is intrinsic, all fiber, local, and phase demodulated is described. It is the combination of these necessary elements that represents an advance in the state of the art. Sensor localization is achieved by using a pair of mirror-ended optical fibers of different lengths that are mechanically coupled up until the desired gauge length for common-mode suppression has been reached. This fiber-optic sensor has been embedded in both thermoset (Kevlar/epoxy and graphite/epoxy) and thermoplastic (graphite/PEEK) composite materials in order to make local strain measurements at the laminar level. The all-fibre system uses a 3*3 coupler for phase demodulation. Parameters such as strain sensitivity, transverse strain sensitivity, failure strain, and frequency response are discussed, along with applications. >

Fiber optic sensors for smart structures

Abstract An overview is presented of our research towards the development of structurally integrated fiber optic sensors for Smart Structures. This includes the development of the first full-scale fiber optic damage assessment test system in the form of a composite aircraft leading edge and the fabrication, characterization and evaluation of the first fiber optic strain rosette. This optical strain rosette was shown to be capable of mapping the strain tensor from within composite materials.

Fiber-optic Fabry-Perot strain rosettes

The concept of a fiber-optic strain rosette is introduced. A mathematical framework describing how surface adhered optical fibers can be configured to measure the state of strain is presented. Details of the design, fabrication, and testing of fiber-optic strain rosettes based on Fabry-Perot interferometers are given. The performance of these devices indicates that they are viable alternatives to resistive-foil rosettes in situations where the benefits of using optical fiber for strain measurement are significant.

Structurally Integrated Fiber Optic Strain Rosette

A fiber optic analogue of the electrical strain rosette is proposed and shown to be necessary if two-dimensional strain fields are to be mapped using fiber optic sensors. The first fiber optic strain rosette has been constructed and preliminary testing demonstrates its ability to determine principal strain magnitudes and axis orientation. Polarimetric transduction was used for this device; the strain sensing region along each arm was localized using 45° fusion splices. This ensured that the strain rosette was lead-in and lead-out insensitive. Possible non-polarimetric designs for optical strain rosettes are also suggested.

Development of a fiber Fabry-Perot strain gauge

The intrinsic Fiber Fabry-Perot (FFP) sensor is being considered for strain sensing in smart structure applications. The current art of FFP sensor fabrication as published in the literature is reviewed. A pseudo-heterodyne demodulation technique is presented and performance of the system detailed. The issue of thermally-induced apparent strain as it relates to fiber-optic strain sensors will also be introduced.

Localized Fiber Optic Strain Sensors Embedded In Composite Materials

A phase sensitive fiber optic strain gauge that is both spatially local and unidirectional is described. The design consists of a path balanced lead-in/out Michelson interferometer using a single-mode directional coupler. It is tested as a surface adhered device in both uniaxial and rosette configurations. The device is integrated between plies of graphite/PEEK thermoplastic to provide lamina strain measurements. Performance parameters gauge factor, transverse sensitivity, and failure strainmare evaluated. The utility of such a strain gauge is discussed in the general context of smart structures and skins.

Distributed fiber optic sensing based on counterpropagating waves

The spatially and temporally resolved birefringence of a single-mode optical fiber can be ascertained using backward stimulated Raman scattering. The magnitude of the birefringence is determined from the optical power exchanged between two counterpropagating light pulses. The degree to which a signal pulse is amplified by a pump pulse is governed by their relative states of polarization when they overlap. A novel normalization procedure is proposed that eliminates many of the unknowns. An example of how this technique could be used to evaluate a distributed strain field is provided.

Thermal apparent-strain sensitivity of surface-adhered, fiber-optic strain gauges

We have derived, based on established practice in experimental mechanics, an equation for calculating the thermal apparent-strain sensitivity of phase-modulated, surfaceadhered, adhered, fiber-optic strain sensors. This formulation permits the thermal performance of fiber-optic strain gauges to be compared with conventional resistive gauges. This sensitivity for commonly used fiber-optic sensors is summarized.