An introduction to smart materials and structures

B S Thompson, M V Gandhi and S Kasiviswanathan*

Abstract - This paper presents an exposition on the embryonic eclectic field of smart materials and structures, prior to discussing how different classes of these innovative biomimetic materials will influence design practices pertaining to diverse commercial and industrial products in the machine tool, medical, aerospace, automotive and sporting goods industries.

Upon reviewing the history of the science of materials from its conception in the Paleolithic Period through the Stone Age, the Bronze Age and the Iron Age, to the current age of synthetic materials it is clearly evident that there has been a distinct evolution, from purely structural materials towards materials with sophisticated engineered microstructures as the scientific and technical prowess of Homo sapiens has matured. This trend is depicted in Fig. 1. During the past two centuries, materials science has witnessed the emergence of research and development thrusts focused not only upon the synthesis of new classes of functional materials but also new classes of structural materials such as the advanced polymeric fibrous composites.

Current industrial practices have capitalized on the ability to fabricate members in fibrous polymeric composite materials which offer the designer structural properties which are significantly superior to those of the traditional monolithic materials. The attractive features offered by these state-of.the- art materials have been responsible for the development of more sophisticated analytical techniques utilizing modern computational facilities that have enabled a new generation of products to be designed that exploit the characteristics of these advanced materials. It is clearly evident, therefore, that Homo sapiens have developed distinct design and manufacturing skills in order to synthesize, and optimally-tailor, the macrostructural properties of these synthetic materials.

The trend towards the synthesis of materials with a higher degree of sophistication at the macroscopic level, and the reduction in the weight of the materials employed in diverse product lines, has been clearly enunciated in the literature for a broad range of industries and this trend is schematically presented in Fig. 2 for a diverse range of materials[I]. This trend towards synthesizing materials with microstructures, or composite materials, has been fuelled by the desire to mimic biological materials. Thus, the fibre and matrix phases of fibrous polymeric composite materials, replicates the polysaccharides and lignin phases of wood, or the chitin fibres and proteinaceous matrix of insect cuticles[2].

Consider, for example, the design of a state-of-the-art robot arm in an advanced fibrous composite material. The designer has the ability to adjust the properties of the materials from which this structural member is manufactured by fabricating the member in several different types of fibres such as glass, graphite, or aramid, or in different matrices to create a hybrid design. In addition, stacking sequence, ply lay-up, fibre volume fraction, and the use of continuous or chopped fibres are all options for synthesizing a material with the desired impact resistance, strength, stiffness, damping properties, mass, or constitutive behaviour. This versatility, which can be

*Intell igent Materials and Structures Laboratory, Machinery Elastodynamics Laboratory, Michigan State Universi~, East Lansing, M148824-1326, USA. Tel'. ÷ 1 (517) 355 5131, Fax: + 1 (517) 353 1750

significant in the design process, is not available to the designer when fabricating components in commercial metals. By developing theories relating the macromechanical design variables (such as ply fibre volume fraction and fibre orientation, for example) to the global mass stiffness and damping properties of the member prior to incorporating these properties in an optimal design algorithm, then the basis exists for developing an optimal structural design for a prescribed set of operating conditions. This is the current state-of-the- art in the design and manufacture of parts fabricated in advanced composite materials[3,4].

Thus, an optimally-tailored machine element designed in a traditional advanced composite material is passive in the sense that it cannot actively respond to changes in the environment caused by changes in temperature, dynamic mechanical loading, or humidity, for example. It is clearly evident, therefore, that the elastodynamic response of the

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MATERIALS & DESIGN Vol. 13 No. 1 FEBRUARY 1992 0261 - 3069/92/010003 - 07 © 1992 Butterworth-Heinemann Ltd 3

Introduction to smart materials and structures

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machine element, and hence, the performance of the associated mechanical or robotic system is sub-optimal for all service conditions except the one for which the member was 'optimally designed'. In sharp contrast to this scenario, the new generations of intelligent materials will be able autonomously to adapt to these changing conditions in order to achieve continually an optimal performance.

These trends in synthesizing the microstructure of structural materials have been complemented during the past 200 years or so by the development of a variety of functional materials, such as piezoelectric materials, or magnetostrictive materials, for example, in which the functional properties of the material, rather than their structural properties, are exploited in practice. Successes in this field have quite naturally spawned the subsequent development of multi-functional materials in which materials are characterized by several functional properties.

The research efforts focused on multi-functional materials typically exploit the notion of biomimetics because the molecular units of most biological systems possess multi- functional characteristics which ensure both economy and efficiency. Consider bone, for example, which not only ensures structural homeostasis, but also mineral homeostasis too. Consider too, the paradoxical characteristics of mollusc shells: these are very strong, tough, composite ceramic structures which feature several weak phases, and so are totally unsuitable as structural materials because of their brittle behaviour. The successful evolution of these multi-functional naturally-occurring materials during the millennia has resulted in this ingenious and awe-inspiring class of materials to be considered to be the ultimate class of intelligent materials.

Smart materials Homo sapiens have developed the ability to create a diverse range of structural materials, functional materials, and also poly-functional materials by employing theoretical, computational, and manufacturing techniques and by integrating and exploiting the knowledge-base in the different fields of science and engineering, These significant scientific

Fig 3 The principal ingredients of a premier class of smart materials

P a r t / S u b s y s t e m performance Applicat ion: speci f ica t ions

I Loading Helicopter rotors Stresses Aircraft wings Deflections Robot arms Fatigue life Machine tools Thermal environment Missile systems l~ectrical environment

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composite materials can be exploited to adapt intelligently to changes in service conditions and unstructured environments.

dissipation > Natural frequencies • Settling time • R e s o n a n c e s

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,F Optimal synthesis of smm-t ultra-advanced

composite structures and articulating systems

Fig 4 A decision-making algorithm for material selection

and technological capabilities are essential ingredients for the synthesis of different classes of smart materials/5, 6]. At the most sophisticated level, these smart materials are generally termed 'intelligent materials', because they typically feature a combination of sensors, actuators, and processors, as presented in Fig. 3, which will permit them to respond intelligently and autonomously to dynamically-changing

4 MATERIALS & DESIGN Vol. 13 No. 1 FEBRUARY 1992

environmental conditions by capitalizing on embedded innovative functions.

The applications for these new generations of smart materials will be diverse, but a common denominator for the deployment of these materials will probably be the unstructured environment in which the mechanical or structural system must operate. Thus, the uncertainty associated with the behaviour of the relevant external stimuli which govern the system response relative to prescribed design criteria will largely dictate the deployment of these smart materials. A generic decision-making algorithm concerning metal selection is presented in Fig. 4 from which it is evident that the necessity for synthesizing materials and structures with autonomous self-adapting, self-correcting characteristics is governed by the desire to achieve optimal performance at all times under variable service conditions and while operating in unstructured environments.

The material functions of structure, actuator and sensor are currently incorporated into smart structures in a discrete global sense. Thus, for example, a current generation smart structure could feature a load-bearing graphite-epoxy fibrous polymeric structural material in which are embedded piezoelectric discs for sensing purposes and a multitude of embedded shape- memory-alloy wires for actuation purposes. Research is currently being carried out on embedding these material functions of sensor, actuator, and structure at a much more local level. For example, carbon fibres may be coated with piezoelectric materials in order to synthesize a smart composite material which has distributed actuation and structural properties at length scales comparable to the diameter of the fibre. Similarly, electro-rheological fluids whose properties may be controlled by the imposition of an appropriate electric field, may be embedded within hollow fibres which have been employed in the structure for reinforcement or sensing, for example, in order to synthesize a self-adaptive material at the global level.

In the future, the current methodology of large scale macroscopic and mesoscopic integration of structural, sensory, and actuator materials will be replaced by the integration of the microstructural properties at the atomic scale in order to synthesize somewhat more homogeneous substances as shown in Fig. 5. Typically, these techniques will be employed in regions with dimensions that are too large to be considered as at the inter-atomic level but too small to be considered as at the solid-state level. This concept has been referred to by several scientific terms such as micro- composite materials, mesoscopic materials, hybrid materials, structurally-controlled materials, and engineered materials. When this technology has been perfected, the materials scientist will be able to synthesize, design and create three- dimensional atomic arrangements which will render obsolete the categorization of materials into such groups as insulators, metals, polymeric materials and biomaterials, for example. The authors have the clear conviction that as the structural complexity of materials increases, the coupling between design, analysis, and manufacturing will become more and more inextricably intertwined. Therefore, the integration of actuators, sensors, processors, and structures for intelligent materials applications will mandate the evolution of sophisticated manufacturing-process-driven design, analysis, and synthesis methodologies. It is anticipated that, along with quantum leaps in manufacturing technologies, manufacturing- process-driven analysis and design technologies, packaging and environmental considerations will need to be addressed for the successful evolution of this class of materials.

In the context of intelligent materials there is considerable focus on sensors and actuators that are discrete materials with discrete functional properties. With the development of techniques for designing and manufacturing materials at the atomic level these terms will again become somewhat obsolete. After all, the basic unit of life in any biomaterial, the

B S Thompson et al

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Fig5 A future application of macroscopic ingenuity to the homogeneous microscopic substructure

cells, monolithically unite all of the structural, sensory and actuator functions in a truly integrated system.

The current generation of smart materials and structures incorporate one or more of the following features:

(1) Sensors which are either embedded within a structural material or else bonded to the surface of that material.

(2) Actuators which are embedded within a structural material or else bonded to the surface of the material. These actuators are typically excited by an external stimulus, such as an electric current, in order either to change their geometrical configuration or else change their stiffness and energy-dissipation properties in a controlled manner.

(3) Control capabilities which permit the behaviour of the material to respond to an external stimulus according to a prescribed functional relationship or control algorithm.

Materials with the above features are indeed worthy of being described by the adjective 'smart', as defined by Webster's Third International Dictionary of the English Language, which states that the meaning of 'smart' is 'having or showing mental alertness and quickness of perception, shrewd informed calculation, or contrived resourcefulness, marked by or suggesting brisk vigour, speedy effective activity, or spirited-liveliness'. Clearly, materials featuring control capabilities possess 'mental alertness', and some will certainly be 'informed' and 'resourceful' within specified limitations. Materials featuring sensing characteristics have the opportunity to demonstrate an 'informed' response along with 'quickness of perception'. Finally, materials featuring actuator functions possess characteristics of 'spirited-liveliness'. Thus 'smart' materials clearly exist today in the arsenal of weapons for deployment by the materials scientist, but 'intelligent' materials are an order of magnitude more sophisticated than smart materials. These capabilities have not been demonstrated at this time and they shall be the focus of much research and development during the coming decades.

MATERIALS & DESIGN Vol. 13 No. 1 FEBRUARY 1992 5

Introduction to smart materials and structures

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Upon again consulting the Webster's dictionary the definition of 'intelligent', is 'the available ability to use one's existing knowledge to meet new situations and to solve new problems, to learn, to forsee problems, to use symbols or relationships, to create new relationships, to think abstractly: ability to perceive one's environment, to deal with it symbolically, to deal with it effectively, to adjust to it, to work toward a goal: the degree of one's alertness, awareness, or acuity: ability to use with awareness the mechanism of reasoning whether conceived as a unified intellectual factor or as the aggregate of many intellectual factors or abilities, as intuitive or as analytic, as organismic, biological, physiological, psychological, or social in origin and nature: mental acuteness.'

Intelligent materials will have the capability to select and execute specific functions intelligently in response to changes in environmental stimuli. For example, these innovative multifunctional materials may exhibit homeostatis, the tendency of an organism to maintain normal internal stability by coordinated responses of systems that autonomously compensate for environmental changes. This ability may be complemented by several other capabilities that are characteristic of intelligent systems, such as self-diagnosis, self-repair, self-multiplication, self-degradation, and self- learning. Furthermore, these features may be augmented by capabilities for anticipating future challenges and missions and the ability to recognize and discriminate. It is clearly evident, therefore, that all aspects of our lives will be significantly influenced by these developments as the evolution of intelligent materials impacts on design practices associated with the new generations of structural and mechanical systems for industries as diverse as automotive, aerospace, defence, biomedical, and advanced manufacturing.

The synthesis of materials with these significant capabilities

is extremely challenging and involves integration of diverse scientific and technological disciplines, as illustrated in Fig. 6. Nevertheless, research is currently being prosecuted from different premises in order to achieve this goal of creating materials and substances with the innate ability to respond autonomously in an intelligent manner to dynamically- changing environmental conditions.

The innovative functions that will characterize these new classes of intelligent materials will be similar to those associated with biological systems. This realization has motivated research efforts to scrutinize biomaterials and to develop a superior knowledge of their intrinsic behaviour in order to supply generic concepts to the design of intelligent materials. Thus, biological systems are generally regarded as a reference, or datum for measurement, with which to evaluate the properties of an intelligent material because these naturally occurring systems have evolved during the millennia and are, in some sense, optimal.

The field of smart materials is very diverse, involving the most sophisticated group of materials, sensing, actuation and control functions. However, while this class of sophisticated materials is somewhat embryonic, some of the less sophisticated classes of smart materials featuring only one or two of these functions at a global level are relatively mature, and they are the topic for discussion in the subsequent sections.

Health monitoring of smart materials The mimicking of biological systems is an ingredient of many smart materials technologies. The principal focus of this biomimetic philosophy is to identify and understand biological designs and processes exhibited by living organisms and apply them in practice to synthesize superior classes of materials. One of these technologies involves embedding sensors within a structural member in order to monitor the integrity and properties of the member, and this philosophy has resulted in the coining of the term 'health-monitoring', or the 'cradle-through-grave' approach[7, 8]. This philosophy of continual part monitoring from birth through death mimics the continual health-monitoring activities of Homo sapiens and it builds upon notions of biomimetics. In essence, human beings are often subjected to non-invasive techniques to monitor the status of their health prior to embarking on corrective courses of treatment. These medical practices often begin in the womb and continue throughout life in response to external stimuli of various forms. This idea is depicted in Fig. 7 where the sensing system is being employed to monitor the vital signs and record possible failure of the principal organs, such as the heart, liver and kidneys.

The analogous situations in engineering practice are depicted in Fig. 8, where in situ sensors are employed during the manufacture of the part and also throughout the service life of the part. Thus, as depicted in Fig. 9, the embedded sensing system would initially be employed to monitor the state of cure during the fabrication of smart components featuring fibrous polymeric composite materials. Subsequently, the embedded sensing system would be employed to monitor continuously a number of critical characteristics within the part during service, as illustrated by the dynamic stress characteristics of the robot arm shown in Fig. 8 resulting from the loading generated by the end- effector tooling. If these stresses exceed prescribed extrema, for example, then the mode of operation would be modified in order to restore the system characteristics to the prescribed work envelope. Finally, the embedded sensing system would be employed to monitor the structural integrity of the component. Also shown in Fig. 8, the embedded sensing system could be employed to monitor the structural integrity of structural members, such as aircraft wings, for example, by detecting cracks and monitoring the propagation of these defects. Such a capability will be an invaluable tool for

6 MATERIALS & DESIGN Vol. 13 No. 1 FEBRUARY 1992

B S Thompson et al

DEATH (critical monitoring) ' I

Fig 7 The health-monitoring of Homo-sapiens

inspection, and for maintenance personnel involved with parts that continuously operate in a fatigue environment such as the numerous aerospace and automotive systems.

A 'health management' capability is achievable in practice by synthesizing materials with a network of embedded sensors and data-links. These sensory and data-transmission systems must typically be small, lightweight, possess geometrical flexibility, operate with low power consumption, feature a wide bandwidth, and be able to withstand the manufacturing environment imposed on the host structural material without suffering performance degradation, and they must not adversely affect the structural integrity of the host material. The field of photonics, or opto-electronics, which exploits the ability of light beams to be transmitted through optical fibres with a diameter of a human hair or smaller, has a significant role to play in this endeavour.

Figure 10 presents a schematic diagram of a smart struc- ture featuring an embedded fibre-optic sensing system. The light source generates an optical signal which is introduced into the optical network embedded in the structure by an optical interface unit. This signal is subsequently modified as it is transmitted through the structure, and these modifi- cations, which typically manifest themselves as changes of wavelength, phase, frequency, intensity, polarization, or modal distribution, are then evaluated to provide information on the state, or health, of the structure.

Fibrous polymeric composite laminates are typically processed in an autoclave where the part is simultaneously subjected to both pressure and temperature to cure the matrix system surrounding the fibres. Currently, this manufacturing process is undertaken using an open-loop control philosophy

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Fig 8 The health-monitoring of parts utilizing embedded sensors

where the heat and pressure profiles of the autoclave are prescribed by the manufacturer of the resin. Consequently, if the curing process is incomplete during manufacture, then the resulting mechanical properties of the part, such as the strength and the stiffness characteristics, will not comply with the design specification and the part will typically be scrapped.

A closed-loop philosophy for the manufacture of autoclave parts may be invoked by embedding fibre-optic sensors involving a network of thin optical fibres in the composite part. This processing philosophy enables the state of cure throughout the part to be continually assessed by a microcomputer-based control system connected to the autoclave controller prior to tailoring the characteristics of the external stimuli imposed on a localized region of the part, by microwave heating, for example, in order to achieve the desired state of cure. Figure 9 presents a schematic diagram of the system. This embryonic technology has broad ramifications for the fabrication of parts with complex shapes, and also high-performance composite parts where quality is of paramount importance. The technology would naturally impact all of the diverse industrial and commercial machines and structures that currently exploit the superior properties of composite materials.

The cost of installing a passive smart structure featuring a structurally integrated microsensing system must be carefully weighed relative to the cost of part failure and subsequent loss of performance of the overall system containing the smart structural part. For example, consider the airframe structure of a commercial jetliner which is being subjected to a variety of unstructured external dynamic stimuli that precipitate fatigue failures with the consequential loss

MATERIALS & DESIGN Vol. 13 No. 1 FEBRUARY 1992 7

Introduction to smart materials and structures

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Commercial and industrial applications The insatiable demand for new generations of industrial, military, commercial, medical, automotive and aerospace products have fuelled research and development activities focused on advanced materials. This situation has been further stimulated by the intellectual curiosity of Homo sapiens in synthesizing new classes of biomimetic materials and, of course, global competition by the principal industrial nations is also a parameter in the equation governing the rate of technological progress.

By integrating the knowledge-bases associated with advanced materials, information technology and biotechnology, these three megatechnologies are facilitating the creation of a new generation of biomimetic materials and structures with inherent brains, nervous systems and actuation systems which are currently a mere skeleton compared with the anatomy perceived in the not-too-distant future. This closing section is focused upon the commercial and industrial applications of this embryonic eclectic field. A vigorous research thrust has been prosecuted on adaptive materials with the ability to change, in real time, the mass-distribution [M(t)], the stiffness [K(t)], and the energy dissipation characteristics [C(t)] for vibration-control purposes. Thus, the research has focused upon developing adaptive materials with controllable properties. The elastodynamic behaviour of these materials is governed by the equation:

Iol + to1 Iol + lul = Iol (i) where [LJI is the displacement vector and [Q} is the load

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vector. This work has permitted engineers to synthesize systems with controllable amplitudes of vibration, natural frequencies, resonances, and transient response characteristics. Other work has focused on actively changing the geometries of structures, so that initially straight members can develop a curved shape in a controlled manner upon command, for example. The generic research on intelligent materials and structures will profoundly impact the next generation of products that must operate under variable service conditions in unstructured environments.

Figure 11 presents experimental results on the frequency response of a beam containing an embedded electro- rheological fluid domain. By controlling the voltage imposed upon the fluid, the global dynamic response of the smart beam can be actively tuned to provide a desired response[9, 10]. Thus, natural frequencies can be changed, the damping characteristics can be changed, mode shapes can be changed, and resonances can be changed. These capabilities can be exploited in many engineering applications. Thus, automobile suspension leaf-springs embodying this tech- nology would permit the suspension characteristics of the

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8 MATERIALS & DESIGN Vol. 13 No. 1 F E B R U A R Y 1992

B S Thompson et al.

vehicle to be actively controlled in order to ensure passenger comfort on diverse road surfaces. Robot arms employed for repetitive tasks would be able to increase the damping proper- ties of the arm in order to minimize the settling-time upon com- pletion of a manoeuvre. Robotic devices subjected to dynamic excitations at the end effectors, which cause resonance con- ditions in structural members, would be able to change the natural frequencies in order to avoid this state of resonance. Other robotic applications have involved shape memory alloys in the design of robot grippers while other work has involved piezoelectric materials to undertake sensing tasks which repli- cate the characteristics of the human dermis and epidermis[11].

Smart materials possessing the innate ability to change their inherent mass, stiffness and damping properties have considerable utility in medicine. This is evident from the number of devices featuring shape memory materials that have been employed in blood clot filters, prostheses, devices for the treatment of scoliosis, and in various pins, nails and plates employed by orthopaedic surgeons. Other devices that would benefit from the ability to change the properties of the materials from which they are fabricated include colonoscopes and catheters, which must typically function in an unstructured environment within the tubular members of the human body.

The diverse range of products marketed by the sporting goods industry will also benefit from smart materials technologies. Anglers will be able to change the stiffness and energy dissipation characteristics of their fishing rods in order to enhance the pleasure associated with the task of attempting to catch a particular species or size of cold-blooded vertebrate animal, or to enhance the angler's casting technique. Rackets employed for tennis or squash, for example, could feature adaptive materials in order to enhance player performance under a variety of different playing conditions.

Structures which must operate autonomously in space are also ideal candidates for the incorporation of several types of smart structural systems because of the variable service conditions and the nature of the unstructured environment in which they must operate. The deployment of large space structures such as platforms, telescopes or solar arrays from the confinement of the payload envelope of the launch vehicle and the payload constraints imposed by launch vehicles mandate that these large space structures be lightweight, and a consequence of this is that they are somewhat flexible. Thus, once the spacecraft is on station in orbit, engineers are then confronted with the tasks of accurately controlling the shape of the structure and also vibration control.

Vibration control situations would typically occur as a result of meteor impact, and in the case of an orbiting laboratory, as a consequence of imperfect docking between a space shuttle which may be returning with supplies, or an astronaut may be exercising on a tread mill. These classes of transient and dynamic responses could be controlled by hybrid schemes of actuators operating in concert with a network of sensors throughout the structure. Other classes of space structures involve geometrical control scenarios. Consider the proposed NASA large deployable reflector programme in which the surface contour of a 20 m diameter paraboloidal reflector must be accurately maintained to within a few micrometres when in the observation mode. This is a challenging task when considered in the context of manu- facturing errors, creep of the structure, thermal gradients and furthermore the telescope must typically be in the observation mode for 20-minute time intervals. Structural members incorporating embedded piezoelectric materials have been proposed to control the critical geometry of the reflector.

Aircraft continually operate in unstructured environments because of uncertainties in the weather conditions, turbulence, temperatures, payloads, and the duration of the flights. Several smart structures programmes have been initiated for both commercial aircraft and also military aeroplanes. The focus of these 'smart skins' programmes,

so named because of the monocoque design of these structural systems, are dependent upon the specific appli- cation. The commercial aircraft programmes focus primarily upon monitoring the health and flight worthiness of aircraft. Arrays of sensors throughout the wings, control surfaces and the fuselage will monitor the structural properties in the context of fatigue cracks and incipient failures. Other types of sensors will monitor the ice build-up on wings and control surfaces, which can adversely affect aerodynamic perform- ance at take-off and also the controllability of the machine.

While the above programmes are largely sensor-based, other efforts are focused on the synthesis of smart structures featuring both sensors and actuators. Some of these programmes are directed towards the design of aircraft with non-articulating control surfaces where the appropriate regions at the edges of the relevant aerofoil sections would deform in order to provide the aerodynamic control required. Another programme is focused on the development of smart wings whose dynamical response can be automatically adjusted in order to provide smooth flight by tailoring the stiffness and damping properties in discrete sections of the wings. Similar programmes have been initiated for helicopter blades. It is therefore clearly evident that smart materials technologies have a pivotal role to play in the evolution of a diverse range of products in many fields of engineering, as humankind endeavours to replicate the wonders of biologically-synthesized naturally-occurring materials.

Conclusions An exposition on the field of smart materials and structures has been presented. The salient features of this embryonic biomimetic eclectic discipline have been highlighted and the ramifications on engineering practice have been extolled prior to presenting illustrative examples in machine tool, aerospace and automotive applications.

Acknowledgements The authors would like to acknowledge the partial support of this work by the US Army Research Office under contracts DAAL03-88-K-0163 and DAAL03-89-G0091, and also the State of Michigan Department of Commerce Research Excellence and Economic Development Fund.

References 1 Forester, T (ed.) The Materials Revolution. MIT Press, Cambridge,

MA, USA (1988) 2 Vincent, J F V, Structural Biomaterials Princetown University Press

(1991) 3 Liao, D X, Sung, C K and Thompson, B S The design of flexible

robotic manipulators with optimal arm geometrics and fabricated from composite laminates with optimal material processing In. J. Robotics Res. 6 (3) (1987) 116-130

4 Gandhi, M V, Thompson, B S and Fischer, F Manufacturing process-driven design methodologies for components fabricated in composite m~tedals. Materials and Design 11 (5) (1990) 235-242

5 Gandhi, M V and Thompson, B S Smart Materials and Structures: The Impending Revolution, Technomic Publishing Co., Lancaster, PA, (1990)

6 Gandhi, M V and Thompson, B S Smart Materials and Structures Chapman and Hall, London (1992) (in press)

7 Mazur, G J, Sendeckyj, G P and Stevens, D M Air force smart structures/skins program overview SPIE 986 Fiber Optic Smart Structures and Skins (1988) 19-29

8 Turner, R D, Vails, T, Hogg, W D and Measures, R M Fiber optic strain sensors for smart structures J. of Intelligent Material Systems and Structures 1 (1) (1990) 26-49

9 Gandhi, M V, Thompson, B S and Choi, S B A new generation of innovative ultra-advanced intelligent composite materials featuring electro-rheological fluids: an experimental investigation. J. Composite Mat. 23 (11) (1989) 1232-1255

10 Gandhi, M V, Thompson, B S and Choi, S B A note on an experimental investigation of a slider-crank mechanism featuring a smart dynamically-tunable connecting-rod incorporating an electrc- rheological fluid. Sound and Vibration 135 (3) (1989) 511-515

11 Vogel, S Smart skin Discover, April (1990) 26

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