The concept of using embedded or surface-bonded solid-state actuators to effect shape change in carbon fibre composite laminates continues to have technical merit and appeal. Conventional laminate design methods tend to lead to stiff structures, whilst it is easiest to impose a change of shape on a compliant structure. This presents a possible conflict of design and suggests that the useful performance of solid- state actuators will always be limited by the stiffness of the host laminate. One possible solution is to increase the in-plane work capacity of the actuators either by using improved materials such as phase change perovskites like PLZT or improved eletroding techniques such as inter-digitated electrodes (IDEs). In this study, the performance of several different actuator/laminate systems have been modelled to determine a baseline capability in pure bending. Four cases have been considered for different panel thicknesses and lay-up sequences. The materials performance and IDE design issues have also been addressed. Modelling indicates that even with conventional actuator materials, structural displacements can be produced which could provide useful shape change in applications such as missile roll control.

The paper presents the results obtained by using NASTRAN and ANSYS finite element codes to predict doming of the THUNDER piezoelectric actuators during the manufacturing process and subsequent straining due to an applied input voltage. To effectively use such devices in engineering applications, modeling and characterization are essential. Length, width, dome height, and thickness and important parameters for users of such devices. Therefore, finite element models were used to assess the effects of these parameters. NASTRAN and ANSYS used different methods for modeling piezoelectric effects. In NASTRAN, a thermal analogy was used to represent voltage at nodes as equivalent temperatures, while ANSYS processed the voltage directly using piezoelectric finite elements. The results of finite element models were validated by using the experimental results.

A new class of ferroelectric polymetric material was developed by using the copolymers of polyvinylidene fluoride and trifluoroethylene, P(VDF- TrFE). Copolymer samples were subjected to high-energy electron irradiation, and the material appeared to be converted into a relaxor ferroelectric. The polarization hysteresis loop near the dielectric peak temperature became nearly non-hysteric, but gradually broadened and evolved into a normal ferroelectric hysteresis loop as the temperature was reduced. The dielectric constant as a function of temperature showed a broad dispersive peak in frequency, wiht the dispersion obeying the Vogel-Fulcher law. The dielectric constant was greatly increased with electron bombardment and a massive electrostriction was observed. Accompanying these changes were the resulting large electrically induced strain and high elastic energy density. The cause of these changes is presently believed to be the breakup of the coherent macro-polar regions containing the all-trans molecular conformation into micro-polar regions with increasing amorphous-crystalline interface as the sample was exposed to electron irradiation.

In contrast to conventional lightweight material like aluminum or titanium, fiber composites offer the possibility to integrate functional elements directly into the material. Thus, multifunctional materials are developed which have the ability to serve more than the load-carrying function. As there is extensive work on the field of integration of thin piezoceramic platse and foils into carbon fiber reinforced polymeres, this will be focused on in this paper. First, the design of an active carbon fiber composite with integrated piezoceramic is shown. Different fiber layups and connecting methods to supply the piezoceramic are discussed. A sophisticated processing technology for active composite materials, the so-called DP-RTM (Differential Pressure - Resin Transfer Moulding), is presented. Various damage mechanisms may reduce or even destroy the sensing and actuaing capabilities of the piezoceramic material. Therefore the capability of high resolution non-destructive methods to evaluate manufacturing defects as well as defects resulting from mechanical overload is presented. Finally two applications are discussed in more detail to demonstrate the potential of the active composite material. Representing static applications an active composite plate is shown which has an infinite bending stiffness up to a certain load. A second active composite plate is used for active noise control.

Recent researches in aeronautics showed that fluidic actuator systems could offer possibilities for drag reduction and lift improvement. To this end many actuator types were designed. This paper deals with the design, fabrication and test of piezoelectric actuator in order to generate pulsated jets normal to a surface and control air flow separation. It is based on the flexural displacement of a rectangular metal plate clamped on one of its large edge. Piezoelectric patches cemented on the plate were used for driving into vibration the actuator. Experimental measurements show that pulsed flow velocities are adjustable from 1.5m/s to 35m/s through a 100x1mm² slit andwithin a 100 to 400 Hz frequency range. Prototype provides the jet performances classically required for active control flow.

Aluminum alloy matrix composite reinforced by continuous TiNi shape memory allow (SMA) fiber was fabricated by Spark Plasma Sintering (SPS) process of A1 alloy powder with 20 vol. % of the TiNi SMA fiber, and its microstructure and mechanical properties were examined. The A1 alloy powder with the TiNi fiber was readily consolidated into composite at temperatures between 633K and 873K. The relative packing density of the composite fabricated increased with increasing sintering temperature. Reaction occurred at the boundary between A1 alloy matrix and TiNi fiber and the interfacial reaction is considered to consist of three intermetallic phases, Ni₃Ti (next to TiNI), Ni₂Ti and Al₃Ni (next to A1 matrix). The tensile yield stress of the composite deformed in tension at 373K was higher by about 40MPa than at 293K.

The peculiar thermomechanical and functional properties of adaptive composites with embedded shape memory (SMA) wires are directly related to the reversible martensitic transformation in the SMA-wires. The gradual transformation and the related strain recovery of the prestrained SMA-wires during heating is hampered by the rigid matrix. The constraining matrix thus influences the transformational behaviour of the embedded SMA-wires. The effects on the transformation heat and on the transformation temperatures of the forward and reverse transformation have been quantified and explained.

Construction is recognised as a potential application for Smart Materials and Structures. However, currently there has been little real penetration of these technologies into this sector. This paper highlights the potential gains from using Smart technologies form the perspective of the technological push and contrasts this with its current poor focus on the real needs of the sector. A visual assessment technique for the selection of smart materials and the needs of the construction, or indeed any other, sector. It is shown that this technique is a powerful aid in assisting the technology transfer of smart materials into construction.

The nonlinear ceramics BaZr_xTi_1-xO₃ (x equals 0.3,0.35,0.4,0.45,.05) were investigated. The DC bias field was applied to the samples in order to investigate the dielectric constant behavior of these materials. The effects of temperature and frequency on the dielectric constant and dielectric loss were studied as well. The results show that BaZr_xTi_1-xO₃ (x equals 0.35-0.5) materials have strong nonlinearity. In addition, BaZrTi_1-xO₃ (x equals 0.35- 0.45) materials have suitable dielectric constant and low dielectric low, higher temperature-stability and better high-frequency properties, so that they can be used as the material of phase shifter for phased- array antennas. A phase shifter was fabricated using BaZr_0.35Ti_0.65O₃ ceramics and phase shift angle of about 66 degree at 2.45GHZ is obtained.

Discs and waveguide samples of polymeric mixed conductor nanocomposite materials comprising a conducting polymer and redox active switching agent in a polymer electrolyte have been prepared and studied. These novel materials have been shown to exhibit large, rapid and reversible changes in their microwave impedance when small d.c. electric fields are applied across them from the edges. The results of simultaneous cyclic voltammetry or potential square waves and microwave transmission measurements have shown that the changes are apparantly instantaneous with the application or removal of the applied field. Analysis of the microwave results has shown that the impedance of the materials changes by a factor of up to almost 50 with the imposition or removal of the fields. Nanocomposite materials having either poly(pyrrole) or poly(aniline) as the conducting polymer component and either silver/silver tetrafluoroborate or copper/copper(II) tetrafluoroborate as the redox active components have been investigated. The results of the nanocomposite materials are compared with those of microparticulate composities of similar composition. A new configuration of single layer tunable microwave absorber using only resistive control has been investigated and shown to exhibit wideband, low reflectivity performance combined with reduced thickness. A major advantage of the new topology is the requirement for only a 3:1 change in controllable resistance.

Thick soft and hard-PZT films have been fabricated on different geometries of Alumina substrates using a screen-printing technique. Three mechanical characterization tests were set up to determine Young's modulus of the films: dynamic, quasi-static and nanoindentation tests. A Young's modulus around 52+/- 7 Gpa was calculated. The piezoelectric coefficient d₃₁ was also investigated and is found to be close to -33pC/N. The fabricated films showed also good performances when used in damping control.

The feasibility of an integrated system for permanent detection and estimation of damaging impacts on composite plates has been evaluated. This system is based on the existence, during a damaging impact, of an intense acoustic emission in the high frequency range. This acoustic emission is registered by a network of piezoelectric sensors and is used to obtain estimations of the location of damaging impacts and estimations of damage areas. Experiments have been carried out with carbon-epoxy plates equipped with four small and thin disc-shaped piezoelectric sensors. Each plate has been impacted using a weight drop machine equipped with a Boeing window. The impact energy varies from 2 J to 6 J for 16 plies coupons and from 4 J to 8 J for 32 plies coupons. The procedure used to identify the damage as a noise source from the signals received by the sensors allows to localize them with an accuracy of a few centimeters. It is also possible to obtain a nearly linear relation between the amplitude of the identified noise source and the projected area of the damage. It is then possible to estimate the damage area in a plate from the amplitude of the noise source after a calibration of the sensors of a reference plate.

A generalization for a two-tier approach to damage identification based on structural performance and levels or magnitude of damage is presented. The two tiers are defined as health or damage monitoring and situation assessment. Damage monitoring involves the inspection of a structure for continual degradation caused by accumulated damage. Situation assessment results from a known incident with a high probably of damage. Initial work on damage monitoring of structural components examines the response of a flat plate as the first step in a series of analyses that will address more complex structures. Damage is included in the computational study in the form of damage to joints such as weld lines. Trends in local and global responses have been evaluated in order to develop an understanding of the implications of varying amounts of damage in the joints on structural response. Numerical based visualization techniques are used to isolate regions of mode shape variation with increasing damage. Implications and use of the developed techniques for monitoring and sensor placement requirements are noted.

Structures with a large number of embedded sensors are becoming more common, and this refined spatial information can be used to advantage in damage location and model validation. These sensors could be accelerometers, strain gauges, piezoceramic patches, PVDF film sensors, or optical fibre sensors. This approach requires that the sensors are function correctly, which on a smart structure operating in the field should be continuous and automatically monitored. This paper considers possible approaches to sensor validation, based on the assumption that a model of the structure is available. The aim is to make use of the natural data redundancy since there will more sensors than modes in the data. The validation approaches considered are based on hypothesis testing based on a number of techniques, such as modal filtering. The methods are demonstrated on simple examples that exercise their strengths and weaknesses.

The objective was to determine design changes that reduce the risk of damage in an embedded piezoceramic transducer. Finite element analyses were performed to calculate stresses in embedded piezoceramic transducers. The model consisted of elements representing the piezoceramic, the interconnectors and the conductive adhesive, and also included the cross-ply laminate. The parameters chosen in the study were the thickness of the interconnector, the material properties, as well as the length and thickness of the conductive adhesive. The stress state in the transducer was determined for different parameter combinations to find a design with low damage risk. For the parameters studied, the lowest risk for damage initiation was obtained for a transducer with a compliant adhesive was a small thickness, and an adhesive that covered the entire piezoceramic element from edge to edge. The strain at failure in the transducer was estimated, and the position for damage initiation in the transducer was determined. The findings from the finite element analysis were supported by the experimental results.

The health of a structure depends on both the homogeneously distributed degradation of its mechanical properties during its life cycle and the presence of localised defects such as cracks or delaminations. The proposed non-destructive health monitoring method allows to recover both kinds of information using ultrasonic waves. To avoid traditional techniques limitations, such as coupling reproducibility for instance, we propose here to integrate a piezoelectric element into the plate-like composite structure. The element dimensions are determined in order to uncouple the frequency ranges of the thickness and radial vibration modes. The thickness mode is used to monitor the homogeneous ageing of the structure through electrical impedance measurement. As for the radial vibrations, they are used to generate and detect Lamb waves, which have the advantage of propagating over long distances and offering specific sensitivity of various modes to different kinds of defects. The present work focuses on this last application and studies the ability of the proposed technique to detect and identify defects such as low speed impact-induced delaminations and cracks incomposite plate-like structures.

Quasi-isotropic samples of Hexcel T300/914 composite laminate have been impacted in an instrumented drop weight machine. The impact energy ranged from 5 to 8 Joules. Electrical potentials before and after the impact events were monitored using a network of probes mounted on top and bottom surfaces. Impact damage could be readily detected in all samples. A finite element analysis of the potential distribution within a sample, of identical lay-up to the one tested experimentally, has been carried out using the Abaqus 5.8-17 package. Current flow has been observed in each layer of the model. In the model, an artificial impact damage area (size 100x100mm) has been modelled as 13 square delaminations placed on top of each other, between every ply. Changes in the potential field have been found, depending on the location of the damage area. Comparison of calculated and experimentally derived potential measurements give broadly similar responses to creation of impact damage.

In composite materials, delaminations are discontinuities producing mode conversion processes generating various out-going modes. The Discrete Wavelet Transform allows isolating various propagation modes and extracting them in order to measure the time delay between the arrivals of the main burst and a specific out-going mode, for various propagation paths. This process permits, with a good accuracy, to localize a damage and to estimate its extension. An active health monitoring system composed of integrated disc-shaped, 100 (mu) m-thick and 5 mm-dia PZT transducers working sequentially as actuators and receives is presented. The diagnostic is based on multiresolution process by wavelet transform applied on recorded Lamb wave signals obtained before and after damage. The robustness and portability of this technique is demonstrated by the fact that, after validation in our laboratory it was successfully applied to data coming from an experiment conducted in an other Laboratory using its own Health Monitoring system.

A combined passive and active damping strategy is proposed to control vibration in structures using a combination of layers of ferro-magnetic (passive) damping and smart (active) magnetostrictive material (Terfenol-D). Two types of combined damping systems are considered viz., a noninteractive system and an interactive or hybrid system. Numerical investigations on a cantilever beam model are carried out to investigate various aspects in combined damping scenario. Using variational principle, a beam Finite Element is developed to study the dynamic characteristics of a beam containing both the passive and active damping layers. It is shown that the combined system could be used effectively to dampen the structural vibration over a wide frequency range. Comparisons with only passive and only active damping schemes are also made. The influence and the mode dependence of control gain in a hybrid system is clearly brought out.

As is well known, Lueders' lines are visible to the naked eye, and appear in carbon steel when yield point elongation occurs. Lueders' lines show that yield stress is bringing about failure in part of the structure where Lueders' lines appears. Therefore Lueders' lines may function as a visible smart sensor for failure locaation. By the use of Lueders' lines it is possible to discern with the naked eye the generation of a failure location on the member of a carbon steel strip. Observation by the naked eye was made of the failure zone corresponding to cyclic loading at a hole inserted in a structural carbon steel specimen. Investigation was undertaken of the utility of Lueders' lines for discerning clamping force at the moment of tightening a bolt. At the moment of tightening it was ascertained that no uniform axial stress was acting on the bearing surface.

The proposed technique is based on an intermittent switching of piezoelectric elements bonded on the structure to be d amped. As a result of the switching, the global losses coefficient of the structure is increased by a significant factor. From a physical point of view, the damping results from the energy dissipation due to the discharge of the piezoelement capacitance in the switch resistance. The switch has to be controlled and thus requires an electrical power about a few milliwatts for be activated. Consequently, the described approach is considered to be a semi-passive technique. For enhanced effects, the switching sequence has to be optimized. No tuning elements such as inductors or resistor¹ are required, consequently the switching method can operate at any frequency, in particular in the low frequency regime, and is inherently broadband. Transient or continuous vibrations are damped with a comparable efficiency. A theoretical model is proposed to interpret the experimental results, to give a comprehensive understanding of the underlying physics and to optimize the switching sequence. It is show that, unlike standard passive techniques, the added damping in non-newtonian but, indeed exhibits a dry friction behavior. Numerous experimental results are given for flexural damping of steel cantilever beam and aluminum plate. It is shown that the damping efficiency can be up to 20 dB for the steel beam configuration. Harmonic and transient regimes of the beams are considered and compared. The design of electronic switching board and power requirements of the micro-controller are discussed.

Semi-active control systems are becoming more popular because they offer both the reliability of passive systems and the versatility of active control systems without imposing heavy power demands. It has been found that magneto-rheological (MR) fluids can be designed to be very effective vibration control actuators. MR fluid damper is a semi-active control device that uses MR fluids to produce controllable damping force. The objective of this paper is to study a single-degree-of- freedom suspension system with a MR fluid damper for the purpose of vibration control. A mathematical model of MR fluid damper is adopted. The model is compared with experimental results for a prototype damper through finding suitable model parameters. In this study, a sliding mode controller is developed by considering loading uncertainty to result in a robust control system. Two kinds of excitations are inputted in order to investigate the performance of the suspension system. The vibration responses are evaluated in both time and frequency domains. Compared to the passive system, the acceleration of the sprung mass is significantly reduced for the system with a controlled MR damper. Under random excitation, the ability of the MR fluid damper to reduce both peak response and root-mean-square response is also shown.

Two control techniques are explored analytically for a dynamic vibration absorber with a magnetorheological fluid damper replacing the absorber dashpot. The approaches include skyhook control and an approximation to a linear quadratic optimal control. The approximate-optimal control, which attempts to match the magnetorheological damper force to an optimal control force based on a linear absorber, is shown to improve the dymanic absorber performance over a substantial range of excitation frequencies and force levels, while the skyhook approach is less successful. Detuning the absorber natural frequency below the optimal detuning for a linear absorber improves performance in the approximate- optimal case.

In this presentation we consider artificial (photonic) crystals formed by dielectric or metallic inclusions arranged in the nodes of a regular three-dimensional lattice with parallelepipedal elementary cell of the general kind. The background medium is an isotropic dielectric. Oblique propagation of plane electromagnetic waves in such a structure is under consideration. A simple analytical theory of plane-wave propagation which takes into account full-wave electromagnetic interactions of all inclusions is developed. The dipole model of interactions and the local- field approach are used. However, our interaction model takes into account the phase shift of the wave not only over a cell but also over the scatterer volume (using high-frequency polarizability). The layer- layer interactions are considered using the Floquet representation of the field produced by periodically polarized layers, including evanescent modes in the model of interactions of adjacent layers. The dispersion equation is obtained form the condition of polarization periodicity, which results from the geometry of the problem and solved numerically. As a simple illustrative example, numerical simulations for lattices of lossless dielectric spheres have been made. The present theory gives an analytical model for the effective propagation constant, which can be universally applied in a very wide frequency rage from the quasi-static regime to the Bragg reflection region (photonic band-gap).

Glass fibers of high silica content, such as common optical lightguides, exhibit a nonlinear elastic response, becoming stiffer with increasing tensile stress. The nonlinear behavior may be of importance in certain sensing application, in particular stress and load monitoring, and in constructing correct models for these optical fiber-based sensor systems. The objectives of the current study is to investigate the nonlinear constitutive behavior of high-silica optical fibers and to determine the implication on mechanical modeling and the likely implications on sensor function. In this study, the nonlinear elastic tensile properties of an optical fiber are studied from an experimental as well as from an analytical standpoint. Critical tensile experiments were carried out on optical fibers. A nonlinear constitutive model based on elasticity theory is introduced to describe the material behavior, and its ramification on the stress analysis is investigated. Macroscopically, the elastic behavior may be accurately modeled using a quadratic function of strain from the onset to failure. However, in the normal operating range of embedded optical fiber sensors, i.e. less than 1% strain, the nonlinear effects small enough that they may in most cases effectively be ignored.

This paper reports on a comparative study carried out for different type of sensors which are able to monitor in-situ curing of thermosets resin. In fact, the thermoset matrix composites are increasingly used as structural parts in complex technological structures. So, monitoring the matrix microstructural evolution and the development of internal stresses during the cure is now of the utmost importance. Taking in account their inhomogeneous structures and processing methods, mesoscopic sized sensors may be embedded into the composite materials. Until now different characterisation techniques have been applied independently in order to monitor essentially the cure process: Frequency dependent dielectric measurements provide a sensitive in situ sensor able to give access to the electrical conductivity and complex permitivity of the surrounding medium. The conductivity parameter related to the ionic mobility is linked to the polymerisation advancement. The ultrasonic waves are generally used for global characterisation of mechanical properties.For in situ applications, a piezoelectric element is embedded in the structure during its processing and the rheological properties of such a material-system can be monitored. Refractive index measurement is carried out with a fibre-optic sensor. This optical parameter allows to determine the density of the thermoset resin during the cure process and to access the extent of cure. The multidetection measurement seems to be a powerful tool to understand the chemorheological mechanisms occurring during the thermosets resin cure process.

In this paper, we present our work on the fiber Bragg grating (FBG) sensors for structural health monitoring in 5m long concrete structures. Two sets of sensors were securely fastened onto the surfaces of the top and bottom reinforced bars respectively before concrete was poured in. Another set of the sensors was mounted onto the slab surface. These sensors were then monitored to observe the strain experienced at different locations within concrete slab. Loading and unloading cycle tests and failure test were performed on the completed structure. From the results obtained using the FBG sensors, we were able to correlate t he load-strain behavior of the slabs to the failure state as observed on the slab surface. These data are useful in determining the maximum allowable load before failure sets in. At the same time, we made comparisons of the data obtained using our FBG sensors with those obtained with electrical strain gauges. The two sets of data show a similar trend during the loading and unloading tests as well as during the failure tests.

We report on the use of a frequency-modulated continuous wave (FMCW) technique for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity for a broadband source by a linear swept-frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in frequency-domain and demodulated using a tunable optical filter. The potential and limitation of the technique are discussed. A 3-sensor FMCW multiplexed FBG array of parallel topology and a 6-sensor hybrid FMCW/WDM system were experimentally demonstrated with -30 dB crosstalk between sensors and a 2(mu) (epsilon) resolution in terms of root-mean- square strain value.

When a long-period fibre grating (LPG) is bent, each resonant attenuation band of the transmission spectrum is observed to split into two. The wavelength separation of the split attenuation bands increases significantly with increasing bend curvature. Based on the recent observation of this novel effect¹, a high sensitivity structural bend sensor has been developed. The LPGs were bonded to the surface of a steel plate using an unsaturated polyester adhesive, and their bend sensing characteristics examined. Pairs of concave and convex testing jigs with a bending curvature ranging from 0.1 to 2m¹ were used to apply accurate bending curvature to the steel plate. The LPGs responses to bending in air and in the polyester adhesive are compared.

We present the results of our experiment for the characterization of mechanical vibrations and temperature gradients of a carbon fiber board. The sensor system is a multichannel interferometer with sensing optical fibers embedded onto the plate under test. We investigate the simultaneous measurement of dynamic gradients of temperature and propagation of vibrations applying a synchronous sampling at the frequency of vibrations. The signals resulting from this technique provides the information of the optical phase due to temperature changes and the relative amplitude of vibrations. We measure the optical phase difference between four separated fiber-optic channels relative to a common reference path. The results are the calibration of the sensor and the spatial propagation of vibrations and temperature gradient.

The simultaneous detection of in-plane and out-of-plane ultrasound displacements is crucial for the interpretation of Lamb wave mode pattern and their interaction with defects within the plate. In this report we present our preliminary experiments for the simultaneous measurement of these two components. A two-channel fibre optic interferometer system has been built which enable the measurements of in-plane and out-of-plane displacements separately and simultaneously. One channel is a Michelson and the other a modified Michelson interferometer or a Mach-Zehnder interferometer. The Michelson interferometer allowed direct measurement of the absolute out-of-plane displacement while the modified Michelson interferometer measured the in-plane displacement. The Mach-Zehnder interferometer measured both components. This two-channel fibre optic interferometer allows a directly calibrated measurement of the two components of displacement simultaneously and offers a great insight into ultrasonic flaw interrogation in plate-like materials.

A comparative study of piezoelectric materials is used to measure the angular acceleration with a mechanical transducer. The piezoelectric materials used in this transducer were either a pre-polarised PVDF file, a pre-polarized PTCa plate, a pre-polarized PZT plate or a polymer/ceramic piezoelectric composite films of (PTCa:P(VDF/TrFE)50:50vol%). The transducer consists of a circular plate with two masses connected mechanically and electrically between them. Between the masses and the plate (on each side of it) is glued a piece of piezoelectric material metalized and polarized. The main advantages of this type of transducers are their low cost compared with other sensors and that there are no moving parts. All experimental results are in good agreement with the theoretical ones, however the best results are obtained when a PZT transducer is used.

We will present an overview of the different polymer/liquid crystal micro-composites and their uses for the control of the optical flux. The preparation method is the key element of this generic family of materials. There is a great number of pertinent parameters (relative concentration of constituents, irradiation conditions, ...) Acting on the morphology and therefore on the final properties. Materials with very different electro-optical properties can be obtained. We will discuss the case where the spatial repartition of heterogeneity is uniform in the sample: light occlusion films (with direct or reverse mode, selective reflection films, films with optical bistability). The cholesteric films will be particularly emphasized. We will focus on the relation between the preparation, the electro-optical properties and the colorimetric properties of the films. Next, the effect of introducing a gradient in some parameters (as irradiation, ...) influencing the material will be discussed. The use of these electrically controllable materials in adaptive optical materials for some applications such as optical components, smart windows, laser protection... will be reviewed.

The spectacular growth in MOEMS interest is highlighted by the involvement of R&D centres and industrial companies. A lot of application fields offer large opportunities for Micro-Opto-Electro- Mechanical Systems (MOEMS): optical communications (switches, cross- connect matrix, DWDM systems ...), digital image processing, adaptive optics... but also industrial maintenance, environment, medicine,... After general ideas on MEMS and MOEMS, this paper presents the main application fields for MOEMS and a few outstanding devices are presented to illustrate the recent developments. Then the work of LETI in MOEMS is presented. Some devices are fabrication with MEMS technologies such as tunable Fabry-Perot interferometers for DWDM telecommunications or 2D micro-scanners for obstacle detection. But a more specific technology has also been developed by LETI, resulting in devices made of silica such as 1D micro-scanners for obstacle detection. Moreover some devices are constituted of micro-mechanical structures combined with Integrated Optics: micro-switches for protection applications and network reconfiguration in optical communications, micro-vibration sensor for surveillance of rotating machines in electrical generators.