grating characteristics, for example, strain and/or temperature, results in a shift in the reflected Bragg wavelength. The outer fiber diameter can vary between 80 and 250 txm, with gauge lengths from 1 nun to approximately 100 mm. The disadvantages with this approach at present are that costly and bulky instrumentation is needed to extract the information such as strain or temperature. However, the main advantage of these sensors is the potential of significant multiplexing, using wavelength division multiplexing (WDM), with the potential of hundreds of discrete sensing elements on the one optical fiber.

A.4 Electrorheological Fluids

In this system, microscopic hydrophilic non-conducting particles such as silica are suspended in a non-conducting hydrophobic carrier fluid. When a high voltage is applied to this suspension, the particles form a tight columnar arrangement. The viscosity of the fluid then increases dramatically to form a pseudo solid able to transmit forces.

Tubes or layers carrying these fluids could be used in smart structures, for example, to develop high damping properties.

A.5 Magnetostrictive Materials

Magnetostrictive materials develop large mechanical deformations when subjected to an external magnetic field. This phenomenon (as with normal magnetism) is attributed to the rotations of small magnetic domains that are randomly orientated when the material is not exposed to a magnetic field. The orientation of these small domains by the imposition of a magnetic field results in the development of a strain field. Terbium-iron alloys are typical magnetostrictive materials.

Magnetostrictive materials typically can develop strains an order of magnitude greater than the current generation of piezoelectric ceramic materials. Generally, magnetostrictive materials generate a greater force response than piezoelectric materials (when subjected to compressive loads) but have significantly greater power requirements.

A.6 Micro-Electro-Mechanical Systems

Micro-electro-mechanical systems (MEMS) are the integration of mechanical devices, such as sensors and actuators, and electronics on a common silicon substrate through microfabrication technology. MEMS fabrication technology is based on techniques developed by and used in the electronics industry. Whereas the electronics are fabricated using integrated circuit (IC) process sequences [e.g., complementary metal-oxide semiconductor (CMOS), Bipolar, or BICMOS

578 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES

processes], the micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices, and can range in size from micrometers to millimetres,tl The most common MEMS process is based on the use of masks (resists) and chemical etching techniques to produce three-dimensional and free-standing structures with integrated electronics. Because the fabrication process is currently limited to a few materials, with silicon predominating, the challenge is to design MEMS in more durable materials that can be fabricated as part of (or be compatible with) standard fabricating techniques for integrated circuits. Even though a significant number of MEMS fabrication processes have evolved from the lithography-based batch-fabricated world of microelectronics, some MEMS processes have evolved from other engineering activities and include microgrinding, electro-discharge machining (EDM), hot embossing, laser machining, and sol-gel techniques. These nonlithographic processes are extended to high-volume applications through their use is in creating master molds, for parts replication.

A.7 Comparison of Actuators

The preceding sections describe actuator systems that have been incorporated into composite structures for smart structure applications. A comparison of some

Table A.2 A Comparison of Smart Actuator Properties

Source: Hollerbacket al. "A ComparativeAnalysisof ActuatorTechnologiesfor Robotics."

mechanical and physical properties of various actuator materials is illustrated in Tables A.2 and A.3 The application of these actuators to dynamic applications for composite structures will depend on their relative weight, bandwidth, hysteresis, and environmental stability. The actuator performance is also an issue and is given by actuator authority, which is characterized by the maximum amount of work (stress strain product). Another significant parameter that needs to be considered is the robustness or damage tolerance of the actuator system.

References

1Tresler, J. F., "Piezoelectric Composite Sensors," Comprehensive Composite Materials, edited by A. Kelly and C. Zweben, Elsevier, 2000, Sec. 5.24.

2Roytburd, J., Slutsker, J., and Wuttig, M., "Smart Composites with Shape Memory Alloys," Comprehensive Composite Materials, edited by A. Kelly and C. Zweben, Elsevier, 2000, Sec. 5.23.

3Wayman, C. M., and Duerig, T. W., "Engineering Aspects of Shape Memory Alloys,"

An Introduction to Martensite and Shape Memory, Butterworth-Heinemann, Boston, 1990. 4Kelly Tsoi, A., Stalmans, R., and Schrooten, J., "Transformation Behaviour of Constrained Shape Memory Alloys," Acta Materialia, Vol. 50, Sept. 2002, pp. 3535-

3544.

5Michie, C., "Optical Fibre Sensors for Advanced Composite Materials," Comprehensive Composite Materials, edited by A. Kelly and C. Zweben, Elsevier, 2000, Sec. 5.21.

6Rogers, A., "Distributed Optical-fibre Sensing," Meas. Sci. Techno, Vol. 10, 1999, pp. R75-R99.

7Mrad, N., "Optical Fiber Sensor Technology: Introduction and Evaluation and Application," The Encyclopedia of Smart Materials, Vol. 2, John Wiley and Sons, 2002, pp. 715-737.

SHill, K. O., Fujii, F., Johnson, D. C., and Kawasaki, B., "Photosensitivity on Optical Fibre Waveguides: Applicatgion to Reflection Filter Fabrication" Applied Physics Letters, Vol. 32, 1978, pp. 647-649.

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9Rao, Y. J., Ribeiro, A. B. L., Jackson, D. A., Zhang L., and Bennion, I., "Combined Spatialand Time-division-multiplexing for Fibre Grating Sensors with Driftcompensated Phase-sensitive Detection," Optics Letters, 1995, pp. 2149-2151.

l°Roa, Y. J., "In-fibre Bragg Grating Sensors," Meas. Sci. Technol, Vol. 8, 1997, pp. 355-375.

l lTrimmer, W., Micromechanics and MEMS, IEEE Number PC4390, IEEE Press, New York, 1997.