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Smart materials and intelligent structures comprise a wide ranging multidisciplinary activity embracing subjects ranging through polymer chemistry, materials research, sensor technology, signal processing techniques, novel mechanical and structural engineering and innovative approaches to control and actuation.

The distinction between smart and intelligent is not clear.  The word smart appears in one Japanese/(American) English dictionary as the Japanese for intelligent (EPSRC Newsline, September 1995)!

For our purpose here we will take smart to be the repeatable response to a specific stimulus or combination of stimuli.  It is thus a conditioned/automatic/second nature response to the situation and does not require any form of decision taking on the part of the material or structure.  The use of the word intelligent will be restricted to those situations where the system has a choice of responses and a decision has to be made in respect of which action to take.

A material will always act in a predictable way (within the statistical variation inherent in the properties), whereas a structure may act in a predictable way, an indeterminate way or according to some control system.  A material cannot make a decision as to how to respond, but must respond in a consistent manner, unless the properties have been changed by its history (by fracture, yielding, heat treatment etc).  It has no capability to decide which action to take and therefore can only be smart according to the above definitions.  A structure may be either smart or intelligent.

There are fundamentally three separate inter-acting parts to an intelligent structure.  These parts are embedded sensors > signal processing and control > actuator.  Following from the above definitions, we can now separate smart (eg. photochromic glass or low melting point wax in a fire sprinkler) from intelligent (eg. active suspensions) such that the former does not have a control system and the latter has all the three required components.

Note that some so-called materials have complex internal structures and can only be considered as a single material when the scale at which they are considered is large in relation to the scale of the microstructure.  This is especially apparent in composite materials where the structure can only be considered homogeneous at a scale somewhat larger than the unit cell of the fabric reinforcement.

• microdielectric electrodes        for resin cure or moisture content
• nitinol (Ni/Ti SMA) wires         for strain measurement
• optical fibre arrays                  for AE, cracking, cure or strain
• piezoelectric transducers          for acoustic emission detection

Key issues in signal processing and control are data fusion for large sensor arrays and control protocols (eg genetic algorithms or fuzzy logic or neural networks or knowledge based systems/artificial intelligence/expert systems).

• piezoelectric actuators              to convert electric control signals to movement
• magnetostrictive materials        to convert magnetic signals to strain
• electrorheological fluids            especially for active vibration damping
• shape memory materials           for temperature dependent reaction forces
          for example ADAPTive Composites with Embedded Shape Memory Alloy Wires
          including Video showing the shape change of a SMA-composite (Katholieke Universiteit Leuven)

Related technologies include MEMS (Micro electro mechanical systems) and biomimetics (lessons from nature for engineering).

Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. The electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS 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.  For more information on MEMS, see ...

• What is MEMS technology? (MEMS and Nanotechnology Clearinghouse)
• Sandeep Kumar Vashist, Review of Microcantilevers for Sensing Applications, Journal of Nanotechnology Online, 18 June 2007 (PDF format 319 KB).

Further reading

1. DM Addington and DL Schodek  Smart materials and technologies for the architecture and design professions, Architectural Press (Elsevier Science), Oxford, 2005. ISBN 0-7506-6225-5. UOP Library 721.044ADD. UOP Library
2. Peter R Ciriscioli and George S Springer, Smart autoclave cure of composites, Technomic Publishing AG, Basel CH, 1990.  ISBN 0-87762-802-5. UOP Library
3. Brian Culshaw, Smart structures and materials, Artech House, Boston, 1996. ISBN 0890066817.  UoP Library 620.11CUL. UOP Library
4. MV Gandhi and BS Thompson, Smart materials and structures, Chapman & Hall, London, 1992.  ISBN 0-412-37010-7. UOP Library
5. SK Ghosh, Self-healing materials: fundamentals, design, strategies and applications, Wiley, 2008.  ISBN 978-2-527-31829-2.
6. J Hu, Shape memory polymers and textiles, Woodhead Publishing, Cambridge, April 2007. ISBN-13: 978-1-84569-047-2.
7. K Otsuka and CM Wayman, Shape memory materials, Cambridge University Press, Cambridge, 1998/99.  ISBN 0-521-44487-x.  UoP 620.1632 SHA. UOP Library UOP Library
8. Jasprit Singh, Smart Materials - Fundamentals and Applications, Cambridge University Press, 2005, ISBN13=9780-521-85027-8.
9. V Srinivasan and DM McFarland, Smart structures: analysis and design, Cambridge University Press, Cambridge, 2000.  ISBN 0-521-65977-9. UOP Library
10. Xiaoming Tao, Smart fibres, fabrics and clothing, Woodhead Publishing, Cambridge, 2001.  ISBN 1-85573-546-6. UOP Library
11. Eric Udd, Fiber optic smart structures, Wiley, New York/Chichester, 1995. UoP library 621.3692FIB. UOP Library
12. S van der Zwaag, Self Healing Materials, Springer, 2007, ISBN 978-1-4020-6249-0.
13. K Worden, W A Bullough and J Haywood, Smart technologies, World Scientific Publishing Co Pte Ltd, Singapore, 2003.  ISBN 981-02-4776-1. UOP Library
    Book review from January 2005 Materials World (with references)
14. Smart Materials and Structures (journal), Institute of Physics e-resource
15. Basudam Adhikari and Sarmishtha Majumdar - Polymers in sensor applications, Progress in Polymer Science, 2004, 29(7), 699-766.
16. J Wang and G Meng - Magnetorheological fluid devices: principles, characteristics and applications in mechanical engineering, Proceedings of the Institution of Mechanical Engineers L: Journal of Materials: Design & Applications, 2001, 215(3), 165-174.
17. S Black, Megayacht composite masts get "smart", High-Performance Composites, January 2007, 15(1), 44-46.
18. D Roach, "Smart" Aircraft Structures: a Future Necessity - health monitoring of aircraft structures using distributed in situ sensor systems, High-Performance Composites, January 2007, 15(1), 28-30.
19. ZG Wei, R Sandström and S Miyazaki, Shape-memory materials and hybrid composites for smart systems: Part I Shape-memory materials, Journal of Materials Science, 1 August 1998, 33(15), 3743-3762.
20. ZG Wei, R Sandstrom and S Miyazaki, Shape memory materials and hybrid composites for smart systems: Part II Shape-memory hybrid composites, Journal of Materials Science, 1 August 1998, 33(15), 3763-3783.

• Fibre optics give the inside story, Advanced Composites Engineering, Winter 1987, 2(4), 17.
• Technical File 155: Electroactive polymers, Engineering, May 1987, 227(5), i-iii.
• Technical File 163: Electrorheological fluids, Engineering, February 1988, 228(2), i-iv.
• MA Hamstad and GP Sendeckyj Acoustic emission technology for smart structures, Journal of Acoustic Emission, 1993, 11(1), 33-41.
• J Lancaster Composites get smart, Advanced Composites Engineering, March 1990, 5(2), 21-22.
• RP Main Fibre optic sensors: future light, Sensor Review, July 1985, 5(3), 133-139.
• PH Miles Technical File 179: shape memory materials, Engineering, July/August 1989, 229(7), 29-30.
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• L McD Schetky Shape-memory alloys, Scientific American, November 1979, 241(5), 68-76.
• RM Measures, M le Blanc, K Liu, S Ferguson, T Valis, D Hogg, R Turner and K McEwen, Fibre optic sensors for smart structures, Optics and Lasers in Engineering, 1992, 16(2-3), 127-152.
• J Summerscales Embedded optical sensors in fibre reinforced plastics, International Journal of Optical Sensors, July 1986, 1(4), 287-298.
• RD Turner, T Valis, WD Hogg and RM Measures Fibre optic strain sensors for smart structures, Journal of Intelligent Materials Systems and Structures, January 1990, 1(.), 26-49.
• GG Wallace Intelligent polymer systems - concepts, approaches, present uses and potential applications, Materials Forum, 1992, 16(2), 111-115.

Advanced Composites Manufacturing Centre School of Engineering Faculty of Technology University of Plymouth Plymouth  PL4 8AA  United Kingdom