Showing papers in "Smart Materials and Structures in 1996"

Modeling and Control of Magnetorheological Dampers for Seismic Response Reduction

Abstract: Control of civil engineering structures for earthquake hazard mitigation represents a relatively new area of research that is growing rapidly. Control systems for these structures have unique requirements and constraints. For example, during a severe seismic event, the external power to a structure may be severed, rendering control schemes relying on large external power supplies ineffective. Magnetorheological (MR) dampers are a new class of devices that mesh well with the requirements and constraints of seismic applications, including having very low power requirements. This paper proposes a clipped-optimal control strategy based on acceleration feedback for controlling MR dampers to reduce structural responses due to seismic loads. A numerical example, employing a newly developed model that accurately portrays the salient characteristics of the MR dampers, is presented to illustrate the effectiveness of the approach.

A model of the behaviour of magnetorheological materials

Abstract: Magnetorheological materials are a class of smart materials whose rheological properties may be rapidly varied by application of a magnetic field These materials typically consist of micron-sized ferrous particles dispersed in a fluid or an elastomer A quasi-static, one-dimensional model is developed that examines the mechanical and magnetic properties of magnetorheological materials This model attempts to account for magnetic non-linearities and saturation by establishing a mechanism by which magnetic flux density is distributed within the composite material Experimental evidence of the viscoelastic behaviour and magnetic properties of magnetorheological fluids and elastomers suggests that the assumptions made in the model development are reasonable It is shown that the model is semi-empirical in that it must be fit to the experimental data by adjusting a parameter that accounts for unmodelled magnetic interactions

Thermomechanical properties in a thin film of shape memory polymer of polyurethane series

Abstract: The thermomechanical properties of a thin film of shape memory polymer of polyurethane series were investigated experimentally. Based on the experimental results, the dynamic mechanical properties, cyclic deformation properties at high temperature, thermomechanical cycling properties, creep and stress relaxation are discussed. The shape fixity with loading above the glass transition temperature followed by unloading below does not change under thermomechanical cycling. The residual strain is recovered in the vicinity of during the heating process. Several applications of the polymer are introduced.

Applications of electro-rheological fluids in vibration control: a survey

Abstract: Electro-rheological (ER) fluids are now regarded as one of the most versatile of the materials available for building smart structures and machines. In principle, ER fluids promise an elegant means of providing continuously variable forces for the control of mechanical vibrations. In practice, the development of industrial devices has been hampered by the unavailability of suitable ER fluids. Prompted by recent advances in ER fluid development this paper provides a comprehensive survey of ER devices for vibration control. The key modes of operation are identified and progress towards a unified approach to visualizing the macroscopic behaviour is summarized before presenting a comprehensive survey which includes contributions to the identification of ER fluid dynamics and the application of feedback control. The discussion of results includes some thoughts on future trends.

Fiber optic sensors in concrete structures: a review

Abstract: The overall deterioration of the national civil infrastructure due to aging and usage beyond the anticipated loads and lifetimes for which it was designed, combined with the increasing cost of maintenance and repair, has resulted in the need for improved techniques for non-destructive evaluation of the structural health of reinforced concrete. A recent review of the available statistics reveals that almost 40% of United States bridges are ‘structurally deficient’ or ‘functionally obsolete’ [1]. New reinforced concrete constructions would also benefit from in situ structural monitors which could detect a decrease in performance or imminent failure, thereby optimizing lifetimes without compromising safety. Finally, although modeling the behavior of some structures made from well-characterized materials is fairly accurate, the use of new materials, unusually complex designs, or variability in strength-related factors such as void fraction or moisture content can lead to unexpected structural weakening, damage or failure. The inadequacy of the nation’s highways, bridges, etc. prompted the initiation in 1993 of a National Science Foundation program, with the goal of developing new technologies aimed at ‘prolonging the life and enhancing the capacity of our existing and future civil infrastructure systems’ [2]. In response to the increased need, various techniques are being developed, and some of the most promising are based on the use of fiber optic sensors (FOS).

Electro-mechanical impedance modeling of active material systems

Abstract: An active material system may be generalized as an electro-mechanical network because of the incorporation of actuators (electrically driven) and sensors (that convert mechanical energy into electrical energy). An investigation of the coupled electrical and mechanical aspects of an active material system will help reveal some of its most important characteristics, in particular regarding energy conversion and consumption issues. The research performed in the area of the electro-mechanical impedance (EMI) modeling of active material systems is herein summarized. In this paper, a generic EMI model to describe the electro-mechanical network behavior (time domain and frequency domain) of active material systems will be discussed. The focus of the discussion will be on the methodology and basic components of the EMI modeling technique and its application to assist in the design of efficient active control structures. This paper will first introduce the basic concept of the EMI modeling and its general utility in the area of active material systems. The methodology of the EMI modeling technique will be illustrated using an example of PZT actuator-driven mechanical systems. The basic components of the EMI modeling, including the electro-mechanics of induced strain actuators, the dynamic analysis of active material systems, and the electrical power consumption and requirements, will be discussed. Finally, some applications of the EMI modeling approach, including the determination of the optimal actuator locations, modal analysis using collocated PZT actuator - sensors, and the prediction of radiated acoustic power, will be presented.

Three-part methylmethacrylate adhesive system as an internal delivery system for smart responsive concrete

Abstract: The goal of this research project is to expand the previous work on a timed adhesive release system for the repair of cracks in concrete. The performance criteria for the adhesive are that it must have a long shell life, be resistant to extreme temperatures, have a high viscosity so it can flow easily into cracks in concrete and it must be strong enough to repair the cracks. The method of encapsulating the adhesive must not degrade over time and it must not be costly. A three-part methylmethacrylate adhesive system was chosen. The adhesive was released from hollow glass tubes coated with a brittle breakable sealer. The experimental results showed the release of the three-part methylmethacrylate adhesive into the matrix created a bond that restored the lost strength due to the cracking and increased flexibility.

Analysis and testing of Bingham plastic behavior in semi-active electrorheological fluid dampers

Abstract: Electrorheological- (ER-) fluid-based dashpot dampers have smart capabilities because ER fluids undergo large changes in yield stress as electric field is applied. Our objective is the development and experimental validation of quasi-steady dashpot damper models, based on an idealized nonlinear Bingham plastic shear flow mechanism, for purposes of preliminary design and performance predictions. The data required for the Bingham plastic model is normally supplied by ER fluid suppliers, that is, plastic viscosity and dynamic yield stress as a function of applied field, as determined from a shear stress versus shear strain rate diagram. As force is applied to the dashpot damper, the ER fluid flows through an annulus between the concentric inner and outer electrodes. The idealized Bingham plastic shear flow mechanism predicts that three annular flow regions develop as a function of the local shear stress. In the central pre-yield or plug region, the local shear stress is less than the dynamic yield stress, so that the plug behaves like a rigid solid. The remaining two annular regions, adjacent to the electrodes, are in the post-yield condition and correspond to the shear stress exceeding the dynamic yield stress, so that the material flows. Equivalent viscous damping performance of an ER fluid dashpot damper is strongly coupled with the plug behavior. For a constant force, as the applied field increases, so does the plug thickness and equivalent viscous damping. For a constant applied field, as the force increases, the plug thickness and equivalent viscous damping both decrease. The passive and active or field-dependent damping behavior of an ER-fluid-based dashpot damper can be designed for a specific application using these quasi-steady Bingham plastic models.

Formulation of an adaptive sandwich beam

Abstract: A new adaptive sandwich structure is constructed using the shear mode of piezoelectric materials. Governing equations for the proposed beam and its surface-mounted counterpart are derived based on the variational principle. Static solutions of a cantilever sandwich beam and its corresponding surface-mounted beam are obtained based on the derived general formulations. The theoretical formulations are verified by finite element analysis. Furthermore, stress distributions of the two types of adaptive beams are also theoretically investigated. It is shown that the sandwich construction offers many advantages over the conventional actuation structure.

Delamination Prediction in Composite Beams with Built-In Piezoelectric Devices Using Modal Analysis and Neural Network

Abstract: The effect of prescribed delamination on natural frequencies of laminated composite beam specimens is examined both experimentally and theoretically. Delamination is of particular interest because it can cause catastrophic failure of the composite structure. One consequence of delamination in a composite structure is a change in its stiffness. This change in stiffness will degrade the modal frequencies of the composite structure. Modal testing of a perfect beam and beams with different delamination size is conducted using polyvinylidene fluoride film (PVDF) sensors and piezoceramic (PZT) patch with sine sweep actuation. Modal testing of beams is also conducted using PVDF sensors and instrumented hammer excitation. The results of piezoceramic patch excitation and instrumented hammer excitation are discussed. The experimental modal frequencies are compared with the results obtained using a simplified beam theory. Also, backpropagation neural network models are developed using the results from the beam theory and used to predict delamination size. The effect of learning rate and momentum rate on neural network performance are discussed. Modal frequencies can be easily and accurately obtained with PZT patch excitation and PVDF sensing. There is good agreement between modal frequencies from modal testing and those from the simplified beam theory. The neural network models developed successfully predict delamination size.

Vibration control of plates with active constrained layer damping

Abstract: Bending vibration of flat plates is controlled using patches of active constrained layer damping (ACLD) treatments. Each ACLD patch consists of a visco-elastic damping layer which is sandwiched between two piezo-electric layers. The first layer is directly bonded to the plate to sense its vibration and the second layer acts as an actuator to actively control the shear deformation of the visco-elastic damping layer according to the plate response. With such active/passive control capabilities the energy dissipation mechanism of the visco-elastic layer is enhanced and the damping characteristics of the plate vibration is improved. A finite element model is developed to analyze the dynamics and control of flat plates which are partially treated with multi-patches of ACLD treatments. The model is validated experimentally using an aluminum plate which is 0.05 cm thick, 25.0 cm long and 12.5 cm wide. The plate is treated with two ACLD patches, each of which is made of SOUNDCOAT (Dyad 606) visco-elastic layer sandwiched between two layers of AMP/polyvinylidene fluoride (PVDF) piezo-electric films. The piezo-electric axes of the patches are set at zero degrees relative to the plate longitudinal axis to control the bending mode. The effect of the gain of a proportional control action on the system performance is presented. Comparison between the theoretical predictions and the experimental results suggest the validity of the developed finite element model. Also, comparisons with the performance of conventional passive constrained layer damping clearly demonstrate the merits of the ACLD as an effective means for suppressing the vibration of flat plates.

Triboluminescent damage sensors

Abstract: A damage sensor for detecting damage within a structure such as aircraft wings or fuselage, or a bridge. The sensor comprises a small piece of a triboluminescent material connected via light guiding fibres or layers to one or more detectors. The sensor may be embedded within the structure or mounted on its surface. Impact of objects on the structure causes a physical damage to the triboluminescent material. Such damage causes light emission which is detected and recorded for later observation. The intensity of emission may be measured to give an indication of amount of damage received by the structure. Several different triboluminescent materials may be arranged in different location within the structure so that location of damage may be detected by a single detector sensitive to the different wavelengths of light emitted by the several materials. Light from the triboluminescent material may be detected directly by the detectors. Alternatively, material doped with suitable photo excitable dyes may be caused to photo excite, and the resultant light output detected.

A finite element for beams having segmented active constrained layers with frequency-dependent viscoelastics

Abstract: A finite element for planar beams with active constrained layer (ACL) damping treatments is presented. Features of this non-shear locking element include a time-domain viscoelastic material model, and the ability to readily accommodate segmented (i.e. non-continuous) constraining layers. These features are potentially important in active control applications: the frequency-dependent stiffness and damping of the viscoelastic material directly affects system modal frequencies and damping; the high local damping of the viscoelastic layer can result in complex vibration modes and differences in the relative phase of vibration between points; and segmentation, an effective means of increasing passive damping in long- wavelength vibration modes, affords multiple control inputs and improved performance in an active constrained layer application. The anelastic displacement fields (ADF) method is used to implement the viscoelastic material model, enabling the straightforward development of time-domain finite elements. The performance of the finite element is verified through several sample modal analyses, including proportional-derivative control based on discrete strain sensing. Because of phasing associated with mode shapes, control using a single continuous ACL can be destabilizing. A segmented ACL is more robust than the continuous treatment, in that the damping of modes at least up to the number of independent patches is increased by control action.

Shape and vibration mode sensing using a fiber optic Bragg grating array

Abstract: This paper discusses the use of wavelength division multiplexed fibre Bragg gratings for structural shape sensing and vibrational mode analysis. The gratings are surface attached to a cantilever beam and demodulated by a scanning fibre Fabry - Perot filter to obtain strain information at different locations along the structure. Two demodulation schemes are used, namely a single-sensor locked mode interrogation technique which permits vibrational analysis and a scanning multi-sensor approach for beam shape determination. Static beam deformation prediction is performed using a Rayleigh - Ritz type analysis with three optimized trial functions and the strain information obtained from the fibre Bragg grating sensors. The strain information is read into a PC which performs the shape modelling of the beam.

Finite-element modeling of a smart cantilever plate and comparison with experiments

Abstract: The behavior of a cantilever plate instrumented with a piezoelectric sensor and actuator is described using finite-element modeling. To demonstrate the accuracy of the numerical model, a parallel experimental study was carried out in the laboratory for the same geometric dimensions. The two results are compared and show excellent agreement, demonstrating that finite-element modeling is a good approach for optimized smart structure design. A three-dimensional finite-element formulation is employed in the piezoelectric material region and a small neighboring region of the plate structure on which it is mounted. Shell elements, approximated by many flat-shell elements, are used in modeling the remaining part of the plate structure. Transition elements that connect the three-dimensional solid elements to the flat-shell element are used. For the cantilever plate example, the electrical input admittance as well as the sensor response are found from the finite-element analysis and they are compared with experimental measurements. From this, the accuracy and efficiency of this approach is demonstrated. In contrast to many other modeling techniques used for smart structures which are approximate and hence limited, the finite-element model is applicable to complicated geometries.

Some design considerations for active and passive constrained layer damping treatments

Abstract: Active constrained layer damping treatments promise to be an effective means of vibration suppression in structures. Basically, the concept consists in either replacing or augmenting the constraining layer of a constrained viscoelastic material with piezoceramic actuators in an attempt to improve vibration suppression properties by capitalizing on both passive and active damping techniques. An important issue in such configurations is the concept that the actuation ability of the piezoceramic is reduced by the viscoelastic layer, rather than enhanced. On the other hand, an active constraining layer increases the shear in the viscoelastic and thus forms an effective means of enhancing the damping mechanism. Some design considerations for pure passive, pure active control, and active constrained layer damping are discussed here. Several authors have reported comparisons and formulations of active constrained layer damping techniques. The approach presented here differs in that it employs an energy principle for the equations of a beam with partial active/passive constrained layer damping treatments. To simulate a realistic design problem, the optimal sizing, length, and thickness of treatments subject to a total thickness restriction is studied for cases of active constrained layer, passive constrained layer, and pure active control. The results show that the active constrained layer damping treatment provides better vibration suppression than passive damping treatments, and it even out-performs pure active control for low-gain applications.

A feasibility study of using smart materials for rotor control

Abstract: Rotor actuation in the rotating system promises a quantum jump in overall rotor craft performance. Smart material actuator technology for operation `on the blade' is now becoming available and has the potential to overcome the size, weight, and complexity issues of hydraulic and electric on-rotor actuation. The present paper is based on the results of a feasibility study to investigate the use of smart materials for primary and active control on the AH-64 helicopter. Based on the results of the study, it is seen that imbedded actuator concepts, i.e. pitch, twist, and camber control, are not practical at this time. Servoflap control, using hinged control surfaces driven by discrete actuators emerges as the most suitable candidate for smart material actuation. Preliminary data show that rotor control using smart materials might be feasible if a combination of smart materials is used and the rotor design is driven towards low control loads and motions.

Continuous carbon fibre epoxy-matrix composite as a sensor of its own strain

Abstract: Unidirectional continuous carbon fibre reinforced epoxy was found to be able to sense its own strain in the fibre direction, due to its longitudinal electrical resistance decreasing reversibly and its transverse resistance increasing reversibly upon longitudinal tension. The strain sensitivity (gauge factor) is from -35.7 to -37.6 and from +34.2 to +48.7 for the longitudinal and transverse resistances respectively. Both effects originate from resistivity changes associated with the increase in the degree of fibre alignment upon longitudinal tension. Either effect allows strain sensing. Slight irreversibility is associated with the resistance decreasing after the first strain cycle and stems from the decrease in the degree of neatness of the fibre arrangement.

Development of a piezoelectric servoflap for helicopter rotor control

Abstract: A servoflap that uses a piezoelectric bender to deflect a trailing edge flap for use on a helicopter rotor blade was designed, built, and tested. This servoflap design is an improvement over a design developed previously at MIT. The design utilizes a new flexure mechanism to connect the piezoelectric bender to the control surface. The efficiency of the bender was improved by tapering its thickness with length. Also, the authority of the actuator was increased by implementing a nonlinear circuit to control the applied electric field, allowing a greater range of actuator voltages. Experiments were carried out on a bench test article to determine the frequency response of the actuator, as well as hinge moment and displacement capabilities. Flap deflections of or more were demonstrated while operating under no-load conditions at frequencies up to 100 Hz. The data indicate that, if properly scaled, the actuator will produce flap deflections greater than at the 90% span location on a full-scale helicopter. In addition, the first mode of the actuator was at frequency of the target model rotor. Proper inertial scaling of this actuator could raise this modal frequency to greater than on an operational helicopter, which is adequate for most rotor control purposes. A linear state space model of the actuator was derived. Comparisons of this model with the experimental data highlighted a number of mild nonlinearities in the actuator's response. However, the agreement between the experiment and analysis indicate that the model is a valid tool for predicting actuator performance.

Smart materials and structures: what are they?

Abstract: There has been considerable discussion in the technical community on a number of questions concerned with smart materials and structures, such as what they are, whether smart materials can be considered a subset of smart structures, whether a smart structure and an intelligent structure are the same thing, etc. This discussion is both fueled and confused by the technical community due to the truly multidisciplinary nature of this new field. Smart materials and structures research involves so many technically diverse fields that it is quite common for one field to completely misunderstand the terminology and start of the art in other fields. In order to ascertain whether a consensus is emerging on a number of questions, the technical community was surveyed in a variety of ways including via the internet and by direct contact. The purpose of this survey was to better define the smart materials and structures field, its current status and its potential benefits. Results of the survey are presented and discussed. Finally, a formal definition of the field of smart materials and structures is proposed.

Induced strain actuation of composite beams and rotor blades with embedded piezoceramic elements

Abstract: The objective of this research is to develop a dynamically-scaled (Froude scale) helicopter rotor blade with embedded piezoceramic elements as sensors and actuators to control blade vibrations. A 6 ft diameter 2-bladed bearingless rotor model was built where each blade is embedded with banks of piezoelectric actuators at degree angles with respect to the beam axis on the top and bottom surfaces. A twist distribution along the blade span is achieved through in-phase excitation of the top and bottom actuators at equal potentials, while a bending distribution is achieved through out-of-phase excitation. In order to fix design variables and to optimize blade performance, a uniform strain beam theory is formulated to analytically predict the static bending and torsional response of composite rectangular beams with embedded piezoelectric actuators. Parameters such as bond thicknesses, actuator skew angle and actuator spacing are investigated by experiments and then validated by theory. The static bending and torsional response of the rotor blades is experimentally measured and correlated with theory. Dynamic torsional and bending responses are experimentally determined for frequencies from 2 - 120 Hz to assess the viability of a vibration reduction system based on piezo-actuation of blade twist. To assess the performance of the piezo-actuators in rotation, hover tests were conducted where accelerometers embedded in the blades were used to resolve the tip twist amplitudes. Although the magnitudes of blade twist attained in this experiment were small, it is expected that future models can be built with improved performance.

A piezoresistive carbon filament polymer-matrix composite strain sensor

Abstract: The relationship between strain and the fractional increase in electrical resistance of piezoresistive polyether-sulfone-matrix composite strain sensors was found to be much more linear and less noisy when the electrically conducting filler was 0.1 m diameter carbon filaments rather than the conventionally used 10 m diameter carbon fibers. For the fiber composite, the non-linearity manifested itself as increasing reversibly with increasing compressive strain - an effect opposite to and occurring on top of piezoresistivity. This effect was absent from the filament composite. Furthermore, the percolation threshold was lower for the filament composite than for the fiber composite. For both filament and fiber composites, became more negative as cycling progressed up to 10 cycles and then stabilized though the effect was more significant for the latter.

Bending and torsion models of beams with induced-strain actuators

Abstract: This paper develops one-dimensional pure bending, coupled bending and extension, and combined bending, extension and torsion models of isotropic beams with induced-strain actuation. A finite thickness adhesive layer between the crystal and beam is included to incorporate shear lag effects. Experimental tests evaluate the accuracy and limitations of the models. The bending and coupled bending and extension models show acceptable correlation with static test results whereas the combined extension, bearing, torsion model predicts the system behavior poorly and needs refinement.

Thermomechanical properties due to martensitic and R-phase transformations of TiNi shape memory alloy subjected to cyclic loadings

Abstract: The thermomechanical properties of the shape memory effect and superelasticity due to the martensitic transformation and the R-phase transformation of a TiNi shape memory alloy were investigated experimentally. The transformation line, recovery stress and fatigue property due to both transformations were discussed for cyclic deformation. The thermomechanical properties due to the R-phase transformation were excellent for deformation with high cycles.

Smart aperture antennas

Abstract: Recent studies have shown that reflector surface adaptation can achieve performance characteristics of the order of phase array antennas without their complexity and cost. This study develops a class of antennas capable of variable directivity (beam steering) and power density (beam shaping). The actuation for these antennas is employed by attaching polyvinylidene fluoride (PVDF) film to a metallized Mylar substrate. A voltage drop across the material will cause the material to expand or contract. This movement causes a moment to be developed in the structure which causes the structure to change shape. Several studies of flexible structures with PVDF films have shown that cylindrical antennas can achieve significant deflections and thereby offer beneficial changes to radiation patterns emanating from aperture antennas. In this study, relatively large curved actuators are modelled and a deflection - force relationship is developed. This relationship is then employed in simulations where the far-field radiation patterns of an aperture antenna are manipulated.

Dynamic modelling of an ER vibration damper for vehicle suspension applications

Abstract: In this paper, the authors describe the development of a mathematical model of a controllable vibration damper intended for eventual application to ground-vehicle suspension systems. The damper under investigation employs electro-rheological (ER) fluid as the working medium which enables a continuously variable damping force to be provided in response to an electrical control signal. There are some difficulties inherent in characterizing the ER damper's behaviour which the present study attempts to overcome. The paper begins by describing a novel form of non-dimensionalization which drastically reduces the number of variables required to characterize the quasi-steady behaviour of the ER fluid. The construction of the ER damper is described and, on the basis of physical reasoning, it is shown how a dynamic model can be derived by taking account of ER fluid inertia and compressibility. A recently developed iterative scheme is introduced in order to solve the resulting non-linear equations of motion. The paper concludes with a case study involving the application of the ER damper to controlling the lateral vibrations of a rail vehicle.

Closed loop finite-element modeling of active/passive damping in structural vibration control

Abstract: A three-dimensional (3D) finite-element closed loop model has been developed to predict the effects of active/passive damping on a vibrating structure. The example used is a cantilever structure containing a viscoelastic material (VEM) layer sandwiched between a piezoelectric actuator and the base structure. This hybrid arrangement is called an active constrained layer damper (ACLD). A piezoelectric sensor is also embedded in the structure. The finite-element analysis includes a control algorithm to close the loop between the sensor and the actuator. The parametric study considers different types of control as well as geometric parameters related to the ACLD. Comparisons are made between active constrained layer and passive constrained layer, and active damping only. The results obtained here reiterate that ACLD is better for vibration suppression than either the purely passive or active system and provides higher structural damping with less control gain when compared to the purely active system. This is the first attempt at a detailed 3D finite-element model that makes no approximations about the piezoelectric devices and includes closed loop modeling.

Efficient power amplifiers for piezoelectric applications

Abstract: The principal obstacle to greater utilization of piezoelectric actuators in aerospace applications is the extreme inefficiency and heat rejection requirements of the driving electronics. The purpose of this investigation is to take a critical look at how amplifiers for piezoelectric systems are designed and to look for potential areas for improvement. A dimensional analysis of a piezoelectric actuator is performed that indicates that power consumption in an unloaded actuator is extremely low, placing the blame for the exorbitant power demands squarely on the driving electronics. Several strategies for power savings in piezoelectric driving electronics are presented including pulse width modulation, discrete charge control, and a hybrid charge-recovery strategy.

In situ cure monitoring of epoxy resins using optical fibre sensors

Abstract: This paper describes a comparative study of in situ cure monitoring by three methods: (i) evanescent wave spectroscopy; (ii) refractive index change; and (iii) near-infrared spectroscopy. The cure characteristics of an epoxy/amine reaction were followed in real-time during the crosslinking reaction via the above-mentioned techniques. The evanescent wave spectroscopy technique was based on monitoring the characteristic infrared absorption bands of the resin system to compute the concentration of the amine hardener as a function of cure time. Good correlation was obtained between the evanescent wave spectroscopy data and a conventional method of studying cure reactions, i.e. infrared spectroscopy. During the cure reaction, the refractive index of the resin system increases as a function of the crosslink density. This increase in the refractive index was monitored using two optical fibre techniques. In the first case, a declad region of the optical fibre was immersed in the resin system and in the second method an optical fibre reflectometer was used to track the changes in the refractive index. Once again, good correlation was obtained between the optical fibre techniques and infrared spectroscopy cure data. The results obtained from the optical fibre sensor experiments were used to model the cure kinetics of the resin system. The cure kinetic models were found to predict the cure reaction up to approximately 60% of the reaction.

Active control of interior noise in a three-dimensional enclosure

Abstract: Analytical and experimental studies undertaken for controlling noise in the interior of a three-dimensional enclosure with a flexible wall are presented. The rigid walls are constructed from acrylic material, and the flexible wall, which is clamped along all four edges, is constructed from aluminium material. Noise generated by an external speaker is transmitted into the enclosure through the flexible boundary and active control is realized by using lead zirconate titanate (PZT) piezoelectric actuators bonded to the flexible boundary. Condenser microphones are used for noise measurements. For harmonic external disturbances, optimization analyses are carried out in the frequency domain to determine the optimal voltage inputs to the piezoelectric actuators for global and local noise control. In the associated experiments, analog feedforward control is implemented by using acoustic error signals for different panel and cavity controlled modes.