We report on results from a field test involving pavement embedded fiber Bragg grating pressure transducers. Data from the test is compared to theoretically predicted stress distribution in soil as obtained from the Boussinesq equation.
In many situations, it is desirable to measure the load acting in a specific direction by measuring the strain induced by Poisson effects in a direction perpendicular to the load direction. For this to be possible, a fixed relationship between the strains in both directions must be known. This can be useful, for example, when the geometry is such that there is not sufficient room to locate a strain gauge parallel to the load direction but a gauge can be placed in a transverse plane. In this paper, we investigate the use of a fiber Bragg grating in such an arrangement with the fiber embedded within the host material. The investigation is done by theoretical, numerical and experimental approaches and we concentrate on two aspects: (1) the non-uniform strain transfer, particular in axial strains, due to shear-lag effects, and (2) the effect of induced birefringence in the optical fiber due to a load cross to its axis. The results of these approaches indicate that the strains of an embedded fiber sensor subjected to transverse loads are dependent on the location of the embedded sensor and the material properties of the host material. The results also show that when the Young's modulus of the host material is much less than the modulus of the embedded sensor, the Bragg spectrum broadening due to induced birefringence is not significant. However, a lower host Young's modulus also results in longer sections on non-uniform axial strain near the ingress and egress sections of the optical fiber. These two factors must be balanced if we desire to use conventional methods of Bragg grating interrogation that measure only the central wavelength of the Bragg grating's spectrum. In the case investigated (Host Young's modulus of 4.83 GPa) full strain build-up requires approximately 4 mm of fiber length at each end. Likewise, the transverse stress coupling into the fiber modifies its wavelength-shift-to-axial-strain- coefficient by about 6%.
A soil pressure transducer by using fiber Bragg grating (FBG) sensors associated with a circular diaphragm is developed. The FBG based transducers can be used for pavement performance study and weigh-in-motion measurement. We consider three methods of bonding the FBG to the diaphragm: (1) radially, (2) radially, inside a glass capillary, and (3) circumferentially. The investigation of strain-gradient induced spectral broadening in FBG-based transducers is conducted since spectral broadening can have adverse effects on the sensor interrogations. We derive analytical closed form results for describing measurand-induced strain gradients in circular geometry transducers, which allow us to experimentally demonstrate novel FBG bonding approaches that eliminate spectral broadening. In addition, Bragg spectral broadening analysis using T-matrix calculation is also conducted to validate some of the experimental results. Two prototypes of soil pressure transducers are field tested at the Cold Region Research Engineering Laboratory (CRREL). The buried pressure transducers are impact-tested by use of a Falling-Weight- Deflectometer (FWD), and detected by NRL-developed FBG interrogation device. Lastly, we use the Boussinesq equation to verify the soil stress measured by the buried transducers.
We report on the instrumentation of a high-speed air-cushion catamaran (Surface Effect Ship) with more than 50 fiber optic Bragg grating strain gauges, as well as conventional resistive strain gauges, accelerometers, a Motion Reference Unit and Global Positioning System. A bow mounted wave radar was used to characterize the sea-state in order to estimate the wave loads on the hull. The relatively large number of strain gauges enabled us to determine the global deformation modes of the hull as well as local stress concentrations. This instrumentation was installed on a new Norwegian naval vessel and employed during sea-keeping tests in smooth and rough seas off the Norwegian coast. The measurements enable a detailed characterization of the vessel's dynamic response to wave loading and comparison with Finite Element Analysis modeling of the ship. The experimental results provide invaluable information for the subsequent development of a system for health monitoring of the structure. We present the instrumentation layout and selected results.
This paper describes the use of the first and second optical return paths in a moderate to high finesse Fabry-Perot sensor to measure the absolute phase in extrinsic Fabry- Perot interferometric (EFPI) sensors. Path-matched differential interferometry (PMDI) using high finesse EFPI sensors, a low finesse Fabry-Perot read-out interferometer, and a broadband light source consisting of amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA) is used to illustrate the idea. The first and second multiple paths in the Fabry-Perot read-out sensor are used to provide two distinct path-match conditions from the same scanning Fabry-Perot read-out interferometer. The difference in fringe numbers between the centers of two orders of interference fringe packets formed by the distinct path-match conditions makes possible a simple method of measuring the cavity length of EFPI sensors, which in turn can be used to measure absolute phase and the corresponding strain. Sensor cavity length measurement using the multiple return paths in the high finesse Fabry-Perot sensor is compared with measurements made using the modulation transfer function found using an optical spectrum analyzer. Then the multiple return path technique is then used to make strain measurements on a cantilever beam. Comparisons with resistance strain gate measurements are favorable. Characterization tests indicate that the proposed technique has a cavity length measurement resolution on the order of 1.1. micrometer, which translates to a strain resolution of 28 (mu) (epsilon) for a 4 cm gage length sensor.
This paper mainly describes a methodology of finding appropriate optical fiber sensors and associated potential demodulation techniques that have the capability to measure impact induced high-strain rate events in graphite/epoxy panels. The capacity of the sensor's dynamic range has to accommodate the average failure strain of about 9,000 (mu) (epsilon) and center frequency of about 40 Hz for a typical low velocity impact event for [90<SUB>2</SUB>/0<SUB>4</SUB>/OF(0)/0<SUB>4</SUB>/90<SUB>2</SUB>] and [(45/-45)<SUB>3</SUB>/OF(0)/(-45/45)<SUB>3</SUB>] layups. Three potential demodulators are investigated. These include the synthetic heterodyne with differential-cross-multiplier, single channel phase tracker, and sin((phi) )/cos((phi) ) based analog phase tracker all using a 10 KHz sinusoidal modulation. The maximum sensor lengths for different sensor types have been calculated based on the Fourier spectrum of a typical impact event and maximum detectable phase vs. frequency of these demodulators. As a result, a localized and reflective version of a In-Line Fiber Etalon sensor with a maximum cavity length of 73 micrometers for 1.3 micrometers wave-length of a light source is selected for impact testing. The strain responses from three different demodulators are comparable to that of a surface mounted resistance strain gage.
Standard Michelson interferometric optical fiber sensors are used to make strain measurements on a carbon/carbon composite beam at temperatures as high as 800 degree(s)C. Copper, titanium, aluminum, and combinations of the three are investigated as sputtered end mirrors for the Michelson sensor, and it is found that the copper mirrors maintained the highest reflectivity in the 100 degree(s)C to 1000 degree(s)C temperature range. All measurements indicate that the optical fibers become more brittle with prolonged exposure to high temperatures.
A process which uses electroplating methods has been developed to fabricate metal coated optical fiber sensors. The elastic-plastic characteristics of the metal coatings have been exploited to develop a sensor capable of `remembering' low velocity impact damage. These sensors have been investigated under uniaxial tension testing of unembedded sensors and under low velocity impact of graphite/epoxy specimens with embedded sensors using both Michelson and polarimetric optical arrangements. The tests show that coating properties alter the optical fiber sensor performance and that the permanent deformation in the coating can be used to monitor composite delamination/impact damage.