A fiber optic system for providing permanent downhole casing strain and shape monitoring is described. The optical
sensing technique, deployment strategy, as well as lab and field test results are presented. The system is based on optical
frequency domain reflectometry for reading out densely packed arrays of Bragg grating sensors. The feasibility of the
approach is established and quantitative measurements of casing strain and shape are presented.
We report test results using optical fiber sensor to measure dynamic strain and temperature on ceramic-matrix composite (CMC) specimens at temperatures up to 600 degrees C. For strain sensing we are employing extrinsic Fabry-Perot interferometric strain gages fabricated with gold-coated optical fibers and attached to the CMC specimens using high- temperature ceramic adhesive. For temperature measurements, specially fabricated Bragg and long-period grating sensors are being employed.
Optical fiber sensing techniques are ideal for applications where high-temperature, electromagnetic interference, or vibration cause traditional electrical sensors to become unreliable. Gold-coated, silica-based optical fibers can withstand temperatures up to 900 degrees C and sapphire fibers can be employed for temperatures as high as 2000 degrees C. We present dynamic strain and temperature measurements of ceramic matrix composite specimens using extrinsic Fabry-Perot interferometric fiber optic strain and temperature sensors. The extremely low-mass and rugged construction of the sensors will allow them to survive high- cycle, high-temperature fatigue testing.
KEYWORDS: Sensors, Fiber optics sensors, Fiber optics, Light emitting diodes, Digital signal processing, Data acquisition, Demodulation, Optical fibers, Signal processing, Control systems
Optical fiber pressure sensors have been developed for use on a structurally-adaptive `smart wing'; further details of the design, fabrication and testing of the smart wing concept are presented in companion papers. This paper describes the design, construction, and performance of the pressure sensor and a combined optical and electronic signal processing system implemented to permit the measurement of a large number of sensors distributed over the control surfaces of a wing. Optical fiber pressure sensors were implemented due to anticipated large electromagnetic interference signals within the operational environment. The sensors utilized the principle of the extrinsic Fabry-Perot interferometer (EFPI) already developed for the measurement of strain and temperature. Here, the cavity is created inside a micromachined hollow-core tube with a silicon diaphragm at one end. The operation of the sensor is similar to that of the EFPI strain gage also discussed in several papers at this conference. The limitations placed upon the performance of the digital signal processing system were determined by the required pressure range of the sensors and the cycle time of the control system used to adaptively modify the shape of the wing. Sensor calibration and the results of testing performed are detailed.
Optical fiber sensors can be used to monitor many parameters such as strain on modern high-temperature alloys during cyclic loading and pressure in composite manufacturing processes. Presented are the application and operation of several fiber optic sensors based on the extrinsic Fabry- Perot interferometer technique. The results from strain measurements demonstrated that the fiber optic strain sensors are able to withstand extreme temperature [ambient to 2070 degree(s)F (1132 degree(s)C)] while maintaining a high level of performance. The capabilities of the fiber optic sensors make it possible to monitor many material property changes and measureands.
A fiber optic optical time domain reflectometer-based (OTDR) sensing system is described for detecting the extent and/or location of damage incurred by a composite material following an impact event. Two multi-mode fibers are embedded within a graphite composite to form a 'checkerboard' fiber mesh with 0.5 inch spacing. The measurement system consists of a PC- based virtual instrument, a high resolution optical time domain reflectometer, and two fiber optic sensors. Using the OTDR and processing algorithms, the location of impact can be determined within the composite panel by scanning the length of each fiber from both ends. The OTDR scanning process yields four reference points which can be used to determine the location of the impact referenced to a coordinate axes designated within the composite panel.
In the design of aerospace materials, developmental testing is conducted to characterize the behavior of the material under severe environmental conditions of high stress, temperature and vibration. Such materials are designed to withstand these environmental extremities without undergoing mechanical failure. Of importance is the performance of the materials at elevated temperatures, particularly the yield strength, which must be known so that the component can be designed to operate below such limits. Currently, instrumentation used in high-temperature tests must rely on long throw instrumentation, such as a knife-edged extensometer, to probe the surface of the specimen. This often results in reduced accuracy and limits the flexibility available for material evaluation. In this paper, we present the development of a fiber optic strain gage which was used for creep measurement of a stainless steel specimen (Type 304) heated to a temperature of 1400 degree(s)F (760 degree(s)C) during tensile loading tests. Surface attached fiber optic strain gages designed for a .3% dynamic range were completely immersed in a 1400 degree(s)F (760 degree(s)C) furnace throughout the duration of the test. The yield strength of the steel specimen evaluated with optical strain sensors agreed well with the manufacturing specifications. In addition, strain measured with the optical fiber strain sensor showed a difference of less than 0.01% compared to a ceramic knife-edged extensometer.
The commercialization of optical fiber sensors in smart structure applications largely depends on the development of a multi-sensor system, capable of simultaneously monitoring different optical fiber sensors. Efficient multi-sensor systems can be realized through multiplexing arrays of sensors within a system, reducing the number of input/output information channels between the support electronics and optical sensors. We demonstrate a sensor system, comprised of four multiplexed extrinsic Fabry-Perot interferometers, that uses a code division multiplexing technique to address each sensor. Qualitative measurements of strain and temperature, obtained with this multi-measurand multiplexing system, and an evaluation of crosstalk between sensors is presented.
Inherent immunity to electromagnetic interference, light weight, and high data capacity, make optically based control systems attractive candidates to replace their electronic counterparts in many mobile platform applications. Additionally, size differences between optical fibers and metal conductors afford optical fiber data busses a reduced probability of sustaining a direct hit from ballistic or maintenance induced impacts. Drawing on techniques used in smart structure technology, this size reduction can be exploited by integrating the optical data bus with the airframe structure to achieve enhanced protection from external stresses. However, due to the brittle nature of optical fibers, a near impact of extreme thermal stresses may be more likely to induce catastrophic damage to an optical data bus. We demonstrate a technique which reduces the static and dynamic induced strain transferred to a structurally integrated optical fiber. Experimental data shows that this method eliminates nearly all fiber strain due to static loading, and greatly reduces the induced strain from a ballistic impact. The analysis shows that relatively small reductions in a constant stress, applied to an optical fiber, results in sizable increases in the predicted lifetime of the optical fiber.
This paper reports the performance of low-profile multi-fiber connectors between sensor-embedded composite panels. The interconnection of such composite panels has been cited as a major limitation to optical fiber sensor- embedded `smart materials and structures' during the past fourteen years. Typically the leads of optical fiber sensors embedded in composites have been brought out of the material either at the sides through protective tubing or through the surface via standard connectors in recessed depressions in the material surface. The first of these options suffers from a lack of mechanical robustness, while the second serves to locally weaken the material and to expose the fiber sensor channel to the external environment. We report successful connector embedment at the panel edge with singlemode connector losses of less than 1.1 dB, and connectorized fiber sensors accessed through the fiber connector.
Optical fibers are attractive candidates for sensing applications in near-term smart materials and structures, due to their inherent immunity to electromagnetic interference and ground loops, their capability for distributed and multiplexed operation, and their high sensitivity and dynamic range. These same attributes also render optical fibers attractive for avionics busses for fly-by-light systems in advanced aircraft. The integration of such optical fibers with metal and composite aircraft and aerospace materials, however, remains a limiting factor in their successful use in such applications. This paper first details methods for the practical integration of optical fiber waveguides and cable assemblies onto and into materials and structures. Physical properties of the optical fiber and coatings which affect the survivability of the fiber are then considered. Mechanisms for the transfer of the strain from matrix to fiber for sensor and data bus fibers integrated with composite structural elements are evaluated for their influence on fiber survivability, in applications where strain or impact is imparted to the assembly.
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