Showing papers in "Smart Materials and Structures in 2002"

Damage detection in composite materials using lamb wave methods

Abstract: Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for in situ damage detection of composite materials. Experimental results are presented for the application of Lamb wave techniques to quasi-isotropic graphite/epoxy test specimens containing representative damage modes, including delamination, transverse ply cracks and through-holes. Linear wave scans were performed on narrow laminated specimens and sandwich beams with various cores by monitoring the transmitted waves with piezoceramic sensors. Optimal actuator and sensor configurations were devised through experimentation, and various types of driving signal were explored. These experiments provided a procedure capable of easily and accurately determining the time of flight of a Lamb wave pulse between an actuator and sensor. Lamb wave techniques provide more information about damage presence and severity than previously tested methods (frequency response techniques), and provide the possibility of determining damage location due to their local response nature. These methods may prove suitable for structural health monitoring applications since they travel long distances and can be applied with conformable piezoelectric actuators and sensors that require little power.

An overview of vibration and seismic applications of NiTi shape memory alloy

Abstract: Shape memory alloys (SMAs) exhibit peculiar thermomechanical, thermoelectrical and thermochemical behaviors under mechanical, thermal, electrical and chemical conditions. Examples of these materials are Cu-based SMAs, NiTi SMAs, ferrous SMAs, shape memory ceramics and shape memory polymers. NiTi SMAs in particular, have unique thermomechanical behaviors such as shape memory effect and pseudoelasticity, which have made them attractive candidates for structural vibration control applications. Numerous studies have been conducted in modeling and applications of NiTi SMAs in structural vibration control. Several active, passive and hybrid energy absorption and vibration isolation devices have been developed utilizing NiTi SMAs. In this paper we present an overview of NiTi behaviors, modeling and applications as well as their limitations for structural vibration control and seismic isolation.

Rheological properties of magnetorheological fluids

Abstract: The effects of dispersed phase saturation magnetization and applied magnetic fields on the rheological properties of magnetorheological (MR) fluids are described. MR fluids based on two different grades of carbonyl iron powder with different average particle size, 7–9 μm (grade A) and 2 μm (grade B), were prepared. Vibrating sample magnetometer measurements showed that the saturation magnetization values were 2.03 and 1.89 T for grades A and B, respectively. Rheological measurements were conducted for 33 and 40 vol% grade A and grade B based MR fluids with a specially built double Couette strain rate controlled rheometer at flux densities ranging from 0.2 to ~0.8 T. The yield stresses of 33 and 40 vol% grade A were 100 ± 3 and 124 ± 3 kPa, respectively at 0.8 ± 0.1 T. The yield stress values of MR fluids based on finer particles (grade B) were consistently smaller. For example, the yield stresses for 33 and 40 vol% grade B based MR fluid were 80 ± 8 and 102 ± 2 kPa, respectively at 0.8 ± 0.1 T. The yield stresses at the flux density approaching magnetic saturation in particles (B ~ 0.8T) were found to increase quadratically with the saturation magnetization (μ0Ms) of the dispersed magnetic phase. This is in good agreement with the analytical models of uniformly saturated particle chains developed by Ginder and co-workers. The results presented here show that the decrease in yield stress for finer particle based MR fluids is due to the relatively smaller magnetization of the finer particles.

Damage detection and assessment in fibre-reinforced composite structures with embedded fibre optic sensors-review

Abstract: A state-of-the-art review is presented regarding the research and development of in situ fibre optic damage detection and assessment systems (FODDAS) embedded in fibre-reinforced composite structures. Representative individual fibre optic strain sensors and distributed sensor networks are briefly described. A major emphasis is placed on their capabilities for detecting damage, determining damage location and assessing the nature of damage, arising primarily from specific events such as impacts or quasi-static stress overloads. The main features of such systems as custom-built and structure-specific units with minimal human involvement are highlighted. Issues that could affect the validity of the performance of such strain sensors are discussed. Fracture and non-fracture of fibre optic sensors are identified as two fundamentally different approaches for damage detection and their primary features are discussed in relation to location determination and evaluation of the nature of damage. The major advantages and limitations of each approach are discussed. Directions and areas of potential future research in the development of related FODDAS are highlighted.

Non-parametric damage detection and characterization using smart piezoceramic material

Abstract: The detection of damages by modal analysis and similar vibration techniques depends upon the knowledge and estimation of various modal parameters. In addition to the associated difficulties, such low-frequency dynamic response based techniques fail to detect incipient damages. Smart piezoelectric ceramic (PZT) transducers, which act both as actuators and sensors in a self-analyzing manner, are emerging to be effective in non-parametric health monitoring of structural systems. In this paper we present the results of an experimental study for the detection and characterization of damages using PZT transducers on aluminum specimens. The method of extracting the impedance characteristics of the PZT transducer, which is electromechanically coupled to the host structure, is adopted for damage detection. Three types of damage are simulated and assessed by the bonded PZT transducers for characterization. We present the effectiveness of PZT transducers in the detection and characterization of incipient damages without the need to know the modal parameters. The PZT transducers are found to have a significantly large sensing area for detecting even small incipient damages. The possibility of replicating the pristine state signatures of different transducers under similar conditions of bonding and geometrical location is also explored. For appropriate characterization of damages, a few statistical signature pattern recognition techniques are evaluated.

Optimizing layout of obstacles for enhanced mixing in microchannels

Abstract: To improve mixing, obstacles have been placed in the channel to try to disrupt flow and reduce the diffusion path. The disruption to flow velocity field alters the flow direction from one fluid to another. In this way, convection may occur to enhance the mixing. Ideally, properly designed geometric parameters, such as layout and number of obstacles, improve the mixing performance without increasing the pressure drop. In this work, CoventorWareTM, a commercial computational fluid dynamics tool for microfluidics was used to study the mixing of two liquids in a 'Y' channel. The results indicate that asymmetric layout of the obstacle has more effect on the mixing than the number of obstacles. Placing obstacles in the microchannels is a novel method for mixing in microfluidic devices, and the results can provide useful information in the design of these devices.

Resonant controllers for smart structures

Abstract: In this paper we propose a special type of colocated feedback controller for smart structures. The controller is a parallel combination of high-Q resonant circuits. Each of the resonant circuits is tuned to a pole (or the resonant frequency) of the smart structure. It is proven that the parallel combination of resonant controllers is stable with an infinite gain margin. Only one set of actuator–sensor can damp multiple resonant modes with the resonant controllers. Experimental results are presented to show the robustness of the proposed controller in damping multimode resonances.

Electro-active paper actuators

Abstract: In this paper, the actuation mechanism of electro-active paper (EAPap) actuators is addressed and the potential of the actuators is demonstrated. EAPap is a paper that produces large displacement with small force under an electrical excitation. EAPap is made with a chemically treated paper by constructing thin electrodes on both sides of the paper. When electrical voltage is applied on the electrodes the EAPap produces bending displacement. However, the displacement output has been unstable and degraded with timescale. To improve the bending performance of EAPap, different paper fibers - softwood, hardwood, bacteria cellulose, cellophane, carbon mixture paper, electrolyte containing paper and Korean traditional paper, in conjunction with additive chemicals, were tested. Two attempts were made to construct the electrodes: the direct use of aluminum foil and the gold sputtering technique. It was found that a cellophane paper exhibits a remarkable bending performance. When 2 MV m-1 excitation voltage was applied to the paper actuator, more than 3 mm tip displacement was observed from the 30 mm long paper beam. This is quite a low excitation voltage compared with that of other EAPs. Details of the experiments and results are addressed.

Comparison of low-frequency piezoelectric switching shunt techniques for structural damping

Abstract: In this paper, the performances of two novel low-frequency piezoelectric switching shunt techniques for structural damping are compared to the traditional passive tuned resonant shunt circuit technique. The first novel technique, state switching, is a semi-active variable stiffness technique in which bonded piezoelectric elements are switched from the short circuit to open circuit states. This technique changes the stiffness of the structure for two quarters of its vibration period, thus dissipating energy. The second novel technique, pulse switching, is a semi-active continuous switching technique in which a resistor/inductor shunt circuit is periodically connected to the bonded piezoelectric elements. This technique applies charges to the piezoelectric elements in a manner similar to the direct velocity feedback and bang-bang time optimal control techniques. A brief description of each of the damping techniques is given. Numerical simulations of the switching techniques are shown and compared to the resonant shunt damping technique. Finally, preliminary experimental results are presented for the resonant shunt, state switching, and pulse switching techniques on a simply supported beam.

Functionalization of carbon nanotubes by potassium permanganate assisted with phase transfer catalyst

Abstract: An improved process is presented to functionalize carbon nanotubes by potassium permanganate with the help of phase transfer catalyst (PTC). The PTC helps to extract potassium permanganate from the solid phase to an organic solvent phase and improves the efficiency of nanotube oxidation. The higher reaction efficiency as well as mild reaction conditions leads to a higher yield of functional nanotube preparation. X-ray photoelectron spectroscopy confirms the functional groups attached to the nanotube surface. A preliminary comparison is given of the potassium permanganate oxidation of nanotubes with and without PTC. This method is believed to be a potential economic method for large-scale functional nanotube preparation.

Real-time cure monitoring of smart composite materials using extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

Abstract: Real-time cure monitoring of composite materials is very important to improve the performance of advanced composite materials. It is very difficult to monitor the cure process online using conventional methods. Fiber optic sensors in smart composite materials provide a unique opportunity to monitor the cure process of composite materials in real time by using embedded sensors. In this paper, extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors are embedded in carbon/epoxy composite laminates and used to monitor the cure process simultaneously. Furthermore, measurements of residual strains of composite laminates during the cure have been performed. The results show that both EFPI and FBG sensors can be used to monitor the strain development of composite laminates with and without damage during cure. An excellent correlation between the EFPI and FBG sensors is presented.

Design and manufacture of a lightweight piezo-composite curved actuator

Abstract: In this paper we are concerned with the design, manufacture and performance test of a lightweight piezo-composite curved actuator (called LIPCA) using a top carbon fiber composite layer with near-zero coefficient of thermal expansion (CTE), a middle PZT ceramic wafer, and a bottom glass/epoxy layer with a high CTE. The main point of the design for LIPCA is to replace the heavy metal layers of THUNDER™ by lightweight fiber reinforced plastic layers without losing the capabilities for generating high force and large displacement. It is possible to save up to about 40% of the weight if we replace the metallic backing material by the light fiber composite layer. We can also have design flexibility by selecting the fiber direction and the size of prepreg layers. In addition to the lightweight advantage and design flexibility, the proposed device can be manufactured without adhesive layers when we use an epoxy resin prepreg system. Glass/epoxy prepregs, a ceramic wafer with electrode surfaces, and a carbon prepreg were simply stacked and cured at an elevated temperature (177 °C) after following an autoclave bagging process. We found that the manufactured composite laminate device had a sufficient curvature after being detached from a flat mould. An analysis method using the classical lamination theory is presented to predict the curvature of LIPCA after curing at an elevated temperature. The predicted curvatures are in quite good agreement with the experimental values. In order to investigate the merits of LIPCA, performance tests of both LIPCA and THUNDER™ have been conducted under the same boundary conditions. From the experimental actuation tests, it was observed that the developed actuator could generate larger actuation displacement than THUNDER™.

Active vibration control of composite sandwich beams with piezoelectric extension-bending and shear actuators

Abstract: We have used quasi-static equations of piezoelectricity to derive a finite element formulation capable of modelling two different kinds of piezoelastically induced actuation in an adaptive composite sandwich beam. This formulation is made to couple certain piezoelectric constants to a transverse electric field to develop extension-bending actuation and shear-induced actuation. As an illustration, we present a sandwich model of three sublaminates: face/core/face. We develop a control scheme based on the linear quadratic regulator/independent modal space control (LQR/IMSC) method and use this to estimate the active stiffness and the active damping introduced by shear and extension-bending actuators. To assess the performance of each type of actuator, a dynamic response study is carried out in the modal domain. We observe that the shear actuator is more efficient in actively controlling the vibration than the extension-bending actuator for the same control effort.

Cure monitoring of composite laminates using fiber optic sensors

Abstract: In this paper, we present the simultaneous measurement of the strain and temperature during cures of various composite laminates using fiber Bragg grating/extrinsic Fabry-Perot interferometric (FBG/EFPI) hybrid sensors. The characteristic matrix of the hybrid sensor is derived analytically. For the fabrication of the three types of graphite/epoxy composite laminate, two FBG/EFPI hybrid sensors were embedded in each composite laminate in two mutually perpendicular directions. We performed the real-time measurement of fabrication strains and temperatures at two points within the composite laminates during the curing process in an autoclave. Through these experiments, FBG/EFPI sensors are proven to be a good choice for efficient smart monitoring of composite structures.

Thermal post-buckling and aeroelastic behaviour of shape memory alloy reinforced plates

Abstract: A novel concept is proposed: the use of shape memory alloy (SMA) to reduce panel thermal deflection and flutter responses. SMA has a unique ability to recover large pre-strains completely when the alloy is heated above the austenite finish temperature Af. The transformation austenite start temperature As for nitinol can be anywhere between -60 °F (-50 °C) and +340 °F (+170 °C) by varying the nickel content. During the recovery process, a large tensile recovery stress occurs if the SMA is restrained. The shape memory effect phenomenon is attributed to a change in crystal structure known as a reversible austenite to martensite phase transformation. This solid-solid phase transformation also gives a large increase in Young's modulus and yield stress. In this paper, a panel subject to the combined aerodynamic and thermal loading is investigated. A nonlinear finite element model based on the von Karman strain-displacement relation is utilized to study the effectiveness of an SMA-embedded panel on the flutter boundary, critical buckling temperature, post-buckling deflection and free vibration. The study is performed on an isotropic panel with embedded SMA. The aerodynamic model is based on the first-order quasi-steady piston theory. The dynamic pressure effect on the buckling and post-buckling behaviour of the panel is investigated by introducing the aerodynamic stiffness term, which changes the critical buckling temperature. Panels with SMA embedded in either the longer or shorter direction and either fully or partially embedded are investigated for post-buckling behaviour. Similarly, the influence of temperature elevation on the flutter boundary and vibration frequencies is investigated.

Harmonic analysis of a magnetorheological damper for vibration control

Abstract: 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. In particular, it has been found that magnetorheological (MR) fluids can be designed to be very effective vibration control actuators, which use MR fluids to produce controllable damping force. The objective of this paper is to study a single-degree-of-freedom (SDOF) isolation system with an MR fluid damper under harmonic excitations. A mathematical model of the MR fluid damper with experimental verification is adopted. The motion characteristics of the SDOF system with the MR damper are studied and compared with those of the system with a conventional viscous damper. The energy dissipated and equivalent damping coefficient of the MR damper in terms of input voltage, displacement amplitude and frequency are investigated. The relative displacement with respect to the base excitation is also quantified and compared with that of the conventional viscous damper through updating the equivalent damping coefficient with changing driving frequency. In addition, the transmissibility of the MR damper system with semi-active control is also discussed. The results of this study are valuable for understanding the characteristics of the MR damper to provide effective damping for the purpose of vibration isolation or suppression.

Numerical simulation of the steady-state deformation of a smart hydrogel under an external electric field

Abstract: In this paper, we develop a numerical model based on a multiphasic theory to simulate the deformation response of a hydrogel strip immersed into an acidic solution under an external electric field. The deformation response consists of complicated mechano-electrochemical behaviours including mechanical effects (pressure and diffusive drag), chemical effects (concentration, chemical potential, osmotic pressure) and electric effects (electric field intensity, electric potential). The complexly coupled nonlinear governing equations are numerically solved using a recently developed meshless radial basis function method. We analyse the major factors which influence the swelling/shrinking behaviours of the hydrogel strip. The numerical results show good agreement with the experimental measurements.

Constructing input vectors to neural networks for structural damage identification

Abstract: This paper addresses construction of appropriate input vectors (input patterns) to neural networks for hierarchical identification of structural damage location and extent from measured modal properties. Hierarchical use of neural networks is feasible for damage detection of large-scale civil structures such as cable-supported bridges and tall buildings. The neural network is first trained using one-level damage samples to locate the position of damage. After the damage location is determined, the network is re-trained by an incremental weight update method using additional samples corresponding to different damage degrees but only at the identified location. The re-trained network offers an accurate evaluation of the damage extent. The input vectors have been designed to meet two requirements: (i) most parameters of the input vectors are arguably independent of damage extent and only depend on damage location; (ii) all parameters of the input vectors can be computed from several natural frequencies and a few incomplete modal vectors. The damage detection capacity of such constructed networks is experimentally verified on a steel frame with extent-unknown damage inflicted at its connections, and the applicability of the hierarchical identification strategy to cable-supported bridges is discussed.

Integrity of graphite/epoxy laminate embedded with piezoelectric sensor/actuator under monotonic and fatigue loads*

Abstract: This study investigated the integrity of the host graphite/epoxy laminate as well as of the embedded active PZT sensor/actuator under monotonic and fatigue loads. For this, graphite/epoxy (AS4/3501-6) laminates were fabricated where the commercially available piezoelectric device in the pre-packaged form was embedded. Two lay-ups were tested: [0/±45/90]S or [0/0/±45/0/0/90]S. The piezoelectric actuator/sensor was embedded in two ways: one method involved placing them into a cut-out area in the two middle 90° plies, and the second one involved insertion between the two middle 90° plies without any cut-out. Ultimate tensile strength and Young's modulus of the tested laminates were not affected due to insertion of the piezoelectric actuator/sensor using either of the embedding techniques. There was also no degradation in the fatigue strength/lives of the tested laminates due to insertion of the piezoelectric actuator/sensor. Furthermore, these were not affected by the embedding method (i.e. cut-out versus simple insertion method). Also, the integrity of the embedded piezoelectric actuator/sensor was preserved when mechanically fatigued or loaded monotonically to the maximum stress level equal to its operational design limit.

Nanometer thickness laser ablation for spatial control of cell attachment

Abstract: We demonstrate here a new method to control the location of cells on surfaces in two dimensions, which can be applied to a number of biomedical applications including diagnostic tests and tissue engineered medical devices. Two-dimensional control over cell attachment is achieved by generation of a spatially controlled surface chemistry that allows control over protein adsorption, a process which mediates cell attachment. Here, we describe the deposition of thin allylamine plasma polymer coatings on silicon wafer and perfluorinated poly(ethylene-co-propylene) substrates, followed by grafting of a protein resistant layer of poly(ethylene oxide). Spatially controlled patterning of the surface chemistry was achieved in a fast, one-step procedure by nanometer thickness controlled laser ablation using a 248 nm excimer laser. X-ray photoelectron spectroscopy and atomic force microscopy were used to confirm the production of surface chemistry patterns with a resolution of approximately 1 µm, which is significantly below the dimensions of a single mammalian cell. Subsequent adsorption of the extracellular matrix proteins collagen I and fibronectin followed by cell culture experiments using bovine corneal epithelial cells confirmed that cell attachment is controlled by the surface chemistry pattern. The method is an effective tool for use in a number of in vitro and in vivo applications.

Modal analysis using fiber optic sensors and neural networks for prediction of composite beam delamination

Abstract: Extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensors and a neural network provided a health-monitoring capability for laminated glass/epoxy composite beams. The EFPI sensors experimentally determined the first five modal frequencies of the cantilever beams. The feedforward backpropagation neural network used these modal frequencies to predict the size and location of delaminations in the composite beams. Beam modal frequencies shift as a function of delamination size and location. Five beams with prescribed delaminations, as well as a `healthy' beam with no delaminations, were excited by a surface-mounted piezoelectric actuator at frequencies up to 1 kHz. All beams had an eight-ply symmetric glass/epoxy composite design, were fabricated simultaneously, and had length and width dimensions of 26.04 and 2.33 cm, respectively. The beams with flaws had different delamination sizes ranging from 1.27-6.35 cm long prescribed in the mid-plane, i.e. between the fourth and fifth plies. The neural network was trained using classical-beam theory and tested using the experimental EFPI data. The delamination size and location predictions resulting from the neural network simulation had an average error of 5.9 and 4.7%, respectively. Also, analytical classical-beam theory, finite element methods, and ceramic piezoelectric sensors validated the EFPI modal frequency measurements.

An experimental study of a semiactive magneto-rheological fluid variable damper for vibration suppression of truss structures

Abstract: The purpose of this paper is to demonstrate that vibrations of a truss structure can be suppressed nicely by a magneto-rheological (MR) fluid variable damper for semiactive vibration suppression. A variable MR fluid damper was designed and fabricated for this study. The principal characteristics of an MR damper were measured in dynamic tests, and a mathematical model of the damper was proposed. To investigate if the variable damper effectively suppresses the vibration of actual truss structures, semiactive vibration suppression experiments were performed using a cantilevered ten-bay truss beam. The experimental result has shown that the vibration was suppressed nicely by the variable MR damper, and that was compared with that of an electro-rheological (ER) damper investigated in previous research. The MR damper showed a higher performance than that of the ER damper.

Dynamic testing of inflatable structures using smart materials

Abstract: In this paper we present experimental investigations of the vibration testing of an inflated, thin-film torus using smart materials. Lightweight, inflatable structures are very attractive in satellite applications. However, the lightweight, flexible and highly damped nature of inflated structures poses difficulties in ground vibration testing. In this study, we show that polyvinylidene fluoride (PVDF) patches and recently developed macro-fiber composite actuators may be used as sensors and actuators in identifying modal parameters. Both smart materials can be integrated unobtrusively into the skin of a torus or space device forming an attractive testing arrangement. The addition of actuators and PVDF sensors to the torus does not significantly interfere with the suspension modes of a free–free boundary condition, and can be considered an integral part of the inflated structure. The results indicate the potential of using smart materials to measure and control the dynamic response of inflated structures.

General solution of a density functionally gradient piezoelectric cantilever and its applications

Abstract: We have used the plane strain theory of transversely isotropic bodies to study a piezoelectric cantilever. In order to find the general solution of a density functionally gradient piezoelectric cantilever, we have used the inverse method (i.e. the Airy stress function method). We have obtained the stress and induction functions in the form of polynomials as well as the general solution of the beam. Based on this general solution, we have deduced the solutions of the cantilever under different loading conditions. Furthermore, as applications of this general solution in engineering, we have studied the tip deflection and blocking force of a piezoelectric cantilever actuator. Finally, we have addressed a method to determine the density distribution profile for a given piezoelectric material.

Large-scale synthesis of multi-walled carbon nanotubes by microwave CVD

Abstract: Carbon nanotubes (CNTs) are an interesting class of nanostructures which can be thought of as arising from the folding of a layer of graphite (a graphene sheet) to form a hollow cylinder composed of carbon hexagons. However, practical applications are still limited by the intricate process of synthesis and the inability of present day methods to provide large-scale production of CNTs. Moreover, high-quality nanotubes with functionalization capability with polymers are desired for polymer microelectromechanical systems (MEMS), nanodvices and BioMEMS. In this paper, an innovative CVD approach using microwave energy was developed for the large-scale production of multi-wall carbon nanotubes (MWNTs). Straight and helical CNTs were obtained when acetylene created by a microwave field was decomposed over the cobalt catalyst at 700 °C. Scanning electron microscopy of microwave-driven MWNTs revealed their homogenous nature. High-resolution electron microscopy showed that typically the MWNT has 26 layers. The average diameter of the tubes observed was 20-30 nm. Electron microscope observations showed a higher yield of nanotubes obtained from microwave CVD than the thermal filament CVD method.

Identification of delamination in composite beams using spectral estimation and a genetic algorithm

Abstract: An efficient strategy for identification of delamination in composite beams and connected structures is presented A spectral finite-element model consisting of a damaged spectral element is used for model-based prediction of the damaged structural response in the frequency domain A genetic algorithm (GA) specially tailored for damage identification is derived and is integrated with finite-element code for automation For best application of the GA, sensitivities of various objective functions with respect to delamination parameters are studied and important conclusions are presented Model-based simulations of increasing complexity illustrate some of the attractive features of the strategy in terms of accuracy as well as computational cost This shows the possibility of using such strategies for the development of smart structural health monitoring softwares and systems

Actuation performance of embedded piezoceramic transducer in mechanically loaded composites

Abstract: The performance of embedded piezoceramic transducers (PZTs) used as Lamb wave generators was investigated in this paper. The composite specimens with a PZT embedded in the mid-plane were subjected to tensile and compressive monotonic loading as well as tension–compression fatigue loading. Both the static and fatigue tests revealed the large working range of embedded PZTs, despite the presence of damage observed by microscopy. The Lamb wave response remained unchanged after a large number of fatigue cycles; around 50 000–100 000 cycles at strain levels of ± 0.20%. At larger numbers of cycles, the changes in terms of amplitude and frequency in the Lamb wave response were believed to be associated with increasing matrix cracking in the specimens.

NARX-based technique for the modelling of magneto-rheological damping devices

Abstract: This paper presents a methodology for identifying variable-structure nonlinear models of magneto-rheological dampers (MRD) and similar devices. Its peculiarity with respect to the mainstream literature is to be especially conceived for obtaining models that are structurally simple, easy to estimate and well suited for model-based control. This goal is pursued by adopting linear-in-the-parameters NARX models, for which an identification method is developed based on the minimization of the simulation error. This method is capable of selecting the model structure together with the parameters, thus it does not require a priori structural information. A set of validation tests is reported, with the aim of demonstrating the technique's efficiency by comparing it to a widely accepted MRD modelling approach.

On the repair of a cracked beam with a piezoelectric patch

Abstract: In this paper, the repair of a cracked beam under an external load, employing the electro-mechanical characteristic of the piezoelectric material to induce a local moment, is presented Conceptually, an external voltage is applied to actuate a piezoelectric patch bonded on the beam to effect closure of the crack so that the singularity induced by the crack tip may be eliminated Using a simply supported beam as an illustration, the actuation voltage required is obtained from an equation containing variables such as the applied force, and the geometric and material properties of the beam and the piezoelectric material It is shown mathematically and numerically that the voltage required (a) decreases linearly as the distance between the force and the location of the crack is increased, (b) varies quadratically with the position of the crack along the length of the beam, and (c) decreases hyperbolically as the ratio of the thickness of the piezoelectric patch to that of the beam increases

A resistance-based damage location sensor for carbon-fibre composites

Abstract: A simple resistance-based sensor is presented, consisting of wire contacts embedded at the edge of a carbon-fibre composite. The sensor proved capable of detecting barely visible damage, induced by a falling-dart impact tester, and accurately providing the damage location. It also proved possible to follow the progression of damage with increasing impact energies. The analysis of the results was straightforward and required no complex calculations. The incorporation of the sensor into practical composite manufacturing routines is also straightforward.