Table of contents

The system architecture of a novel structural health monitoring system that is optimized for the continuous real-time monitoring of dispersed civil infrastructures is presented. The monitoring system is based on a highly efficient multithreaded software design that allows the system to acquire data from a large number of channels, monitor and condition this data, and distribute it, in real time, over the Internet to multiple remote locations. Bandwidth and latency issues that impact the operation of monitoring systems are discussed. The application of the monitoring system under discussion to a long span, flexible bridge in the metropolitan Los Angeles region is described. The bridge had previously been instrumented with 26 strong motion accelerometers. Sample 'quick analysis' results continuously provided by the monitoring system are presented and interpreted. System identification results, obtained through off-line batch processing, are presented for a data set from a recent earthquake that automatically triggered the recording capability of the system. It is shown that, using a time domain system identification approach, the bridge stiffness and damping matrices can be identified from the earthquake data set and subsequently used to determine the bridge modal properties, such as frequencies and damping ratios. In this approach the bridge is modeled as a multi-input/multi-output system with order compatible with the number of available sensors. Implementation issues requiring further investigation are presented and discussed.

It is well known that barely visible damage is often induced in composite structures subjected to out-of-plane impact, and the mechanical properties of the composites degrade markedly. The stiffened composite panels which are the representative structural elements of airplanes are characterized by impact damage different from that of coupon-level specimens. Therefore, the objective of this study is to apply small-diameter optical fiber sensors to stiffened composite panels for the detection of impact damage. In this study, both multi-mode optical fibers and fiber Bragg grating (FBG) sensors are used for detecting impact load and impact damage in stiffened composite panels. The fibers have polyimide coating with 40 µm cladding diameter which has no serious effect on the mechanical properties of composites. Impact tests are performed using stiffened composite panels with embedded optical fibers. The characteristics of impact damage are investigated. The impact load, the strain and the optical responses of the optical fibers are measured as a function of time. And we discuss the relationships among the optical responses, the impact load and the impact damage.

This paper reports on research work which is a comprehensive study involving the design of an extrinsic Fabry–Perot interferometric strain sensor (EFPI-SS), the manufacture of smart carbon/epoxy beams preconditioned with artificial delamination, and the testing of these beams in three-point bending. A loading applied to the beams was quasi-static in nature and this was selected in order to facilitate the accurate and reliable control of the beam deformations. The EFPI-SSs were validated for their sensing capacity and survivability via surface mounting and interior embedment. Artificial delaminations of two different sizes were embedded within the host beams to simulate damage. Their effect on the bending stiffness induced by quasi-static loading was examined using the embedded EFPI-SSs along with surface-mounted conventional strain gauges (SGs) on a comparative basis. A linear response was obtained from the EFPI-SSs up to a strain level of 0.5%, and this was in good agreement with interpolated strains as well as those from analytical prediction. It was shown that the embedded EFPI-SSs were able to differentiate the effect of the tensile delaminations on the bending stiffness when the level of strain was substantial. It was noted that the interior strain data from the EFPI-SSs in conjunction with the surface SG data were very useful for extrapolating the strain difference on both sides of the delaminations, so that the possibility of delamination propagation could be deduced.

This paper presents a case study of magnetorheological (MR) and electrorheological (ER) valve design within a constrained cylindrical volume. The primary purpose of this study is to establish general design guidelines for volume-constrained MR valves. Additionally, this study compares the performance of volume-constrained MR valves against similarly constrained ER valves. Starting from basic design guidelines for an MR valve, a method for constructing candidate volume-constrained valve geometries is presented. A magnetic FEM program is then used to evaluate the magnetic properties of the candidate valves. An optimized MR valve is chosen by evaluating non-dimensional parameters describing the candidate valves' damping performance. A derivation of the non-dimensional damping coefficient for valves with both active and passive volumes is presented to allow comparison of valves with differing proportions of active and passive volumes. The performance of the optimized MR valve is then compared to that of a geometrically similar ER valve using both analytical and numerical techniques. An analytical equation relating the damping performances of geometrically similar MR and ER valves in as a function of fluid yield stresses and relative active fluid volume, and numerical calculations are provided to calculate each valve's damping performance and to validate the analytical calculations.

This paper presents a theoretical study comparing sound transmission through different kinds of smart panels, comprising an aluminium plate, of dimensions 247 mm × 278 mm × 1 mm, with various kinds of embedded structural sensors and actuators. The analysis is focused on systems using simple single-channel feedback controllers, so that self-contained, compact and light sensor–controller–actuator devices can be built.

Initially, two idealized smart panels have been studied: first, a panel with a volume velocity sensor and a collocated uniform force actuator pair, connected via a single-channel fixed gain feedback control; and second, a panel with sixteen collocated point velocity sensors and point force actuators each controlled in a decentralized fashion by a single-channel fixed gain feedback control system.

The control effectiveness obtained with ideal sensor and actuator systems has also been contrasted with those of more realistic devices, in which the sensing system is made either of a large piezoelectric distributed film or an array of point velocity transducers, and the actuation is given either by a large piezoelectric film or an array of small piezoelectric patches. The control effectiveness, stability and robustness of each control configuration have been discussed. Also, some practical problems related to the construction of the smart panels have been briefly described.

The thermal post-buckling responses of shape memory alloy hybrid composite (SMAHC) shell panels are investigated using a finite element method formulated on the basis of the layerwise theory. The von Karman nonlinear displacement–strain relationships are applied to consider large deflections due to thermal loads. The cylindrical arc-length method is used to take account of the snapping phenomenon which is an unstable behavior observed in the shell panels. A nonlinear finite element procedure based on Brinson's model is developed to investigate the behaviors of shape memory alloy (SMA) wire. The results of numerical analysis show that the recovery stresses of SMA wires can enhance the stiffness of structure and the SMAHC shell panel exhibits superior behaviors of thermal post-buckling compared to the conventional composite panel. It is also shown that embedding SMA wires in a composite structure can prevent the unstable post-buckling behavior.

Two types of shape memory polyurethanes (PUs) with various hard segment contents (hsc) were designed and synthesized based on the position of crosslinks, either in the soft segment or in the hard segment, and were tested for tensile cyclic loading behavior as well as mechanical and dynamic mechanical properties. It was found that hard segment crosslinks gave much smaller shape fixability and much higher shape recoverability as compared with soft segment crosslinks at the same hsc. For soft segment crosslinks, shape recovery increased as the hsc decreased, and these results could possibly be interpreted in terms of dynamic mechanical properties of the PUs, which possibly were interpreted in terms of microphase separation and crosslink density.

Recently, magnetorheological (MR) dampers have emerged as a potential technology to implement semi-active control in structures and vehicle applications in order to efficiently suppress vibration. Perfect understanding about the dynamic characteristics of such dampers is necessary when implementing MR struts in application. One of the important factors to successfully attain desirable control performance is to have a damping force model which can accurately capture the inherent hysteresis behavior of MR dampers. Different models have been proposed to simulate the hysteresis phenomenon in such a kind of damper. The Bouc–Wen model has been extensively used to simulate the hysteresis behavior of MR dampers. However, considerable differences still exist between the simulation and experimental results. In this work, a methodology to find the characteristic parameters of the Bouc–Wen model in the attempt to better characterize the hysteresis phenomenon of MR dampers has been proposed. The methodology takes into consideration the effect of each individual term of the Bouc–Wen model over the hysteretic loop to estimate the appropriate values of the parameters. The Bouc–Wen model in which the new established characteristic parameters have been used has been validated against experimental data and an excellent agreement has been shown between the simulation and experimental results. Moreover, the findings pointed towards the fact that linear or exponential relationships exist between the estimated parameters and the current excitation. Considering this, a new model based on the Bouc–Wen model has been proposed in which the excitation current has been incorporated as a variable. This proposed modified Bouc–Wen model has also been validated against the experimental results and a good correlation has been found.

This paper, the third in a series of four review papers to appear in this journal, presents a number of descriptions of various modeling and simulation techniques and, briefly, the associated experimental results in connection with ionic polymer–metal composites and, in general, ionic polymer–conductor composites, as soft biomimetic distributed sensors, actuators, transducers, and artificial muscles.

This paper discusses the mechanical characteristics of laminated piezoelectric actuators that are manufactured at an elevated temperature, to cure the adhesive bonding the layers together, and then cooled to a service temperature. The actuators are of the unimorph-type, which are composed of a layer or layers of passive materials and a layer of piezoelectric material. THUNDER (thin layer unimorph ferroelectric driver and sensor)-type actuators, which consist of layers of metal, adhesive, and piezoelectric material, are studied and investigated in detail to understand the thermal effects due to the elevated manufacturing temperature. Owing to the large out-of-plane deformations of THUNDER-type actuators as a result of cooling to the service temperature, inclusion of geometric nonlinearities in the kinematic relations is taken into consideration for prediction of the thermally induced deformations and residual stresses. The deformations and residual stresses are predicted by using a 23-term Rayleigh–Ritz model and a finite-element model using ABAQUS. The thermally induced deformations result in actuator shapes which can be dome-like or near-cylindrical. Which shapes actually occur depends on the geometry of the actuator. Actuation responses of the actuators caused by a quasi-static electric field applied to the piezoelectric layer are also studied with the Rayleigh–Ritz approach. It is shown that geometric nonlinearities play an important role in the actuation responses, and these nonlinearities can be controlled by the choice of actuator geometry.

The electrostrictive graft elastomer is a new type of electroactive polymer. Recently developed by NASA, it consists of flexible backbone chains, each with side chains, called grafts. Neighboring backbone grafts physically cross-link and form crystal units. The flexible backbone chain and the crystal graft unit consist of polarized monomers, which contain atoms with electric partial charges, generating dipole moments. When the elastomer is placed into an electric field, external rotating moments are applied to the dipole moment. This stimulates electrostrictive strain in the graft elastomer. In this paper, the deformation of the elastomer under the action of an electric field is explained by means of two dominant mechanisms: crystal graft unit rotation and backbone chain reorientation. A two-dimensional computational model is established to analyze the deformation.

An investigation of self-sensing and self-repairing bolted joints is presented. The goal of the self-sensing and self-repairing joint is to reduce the likelihood of failure due to self-loosening and to reduce the cost of maintenance of critical bolted joints. The concept combines piezoelectric-based health-monitoring techniques with shape memory alloy (SMA) actuators to restore tension in a loose bolt.

The objective of this study is to enhance the practicality of the self-healing bolted joint. One of the primary issues is the actuation of the SMA actuators. The relatively large mass of the shape memory washer and low resistance because of the washer's short length make resistive heating particularly difficult. A series of models was developed to assess the viability of resistive heating and provide an estimate for the power requirements for effective actuation. Modeling and experimental testing have shown that an external heater can be used to actuate a shape memory alloy actuator with conventional power sources. By making the shape memory alloy washer substantially easier to actuate, this method provides a convenient alternative to resistive heating, and aids the practical implementation of the concept of self-sensing, self-repairing joints.

In the present work, we have demonstrated a novel process for fabricating suspended micromechanical structures of P⁺ silicon and SiO₂ inside anisotropically etched rectangular cavities in (100) Si wafer. Using this new process, the moving parts are 'recessed' in the substrate and can be easily secured with a flat surface of any type of substrate as compared to non-recessed structures. We have used a standard spin coating process for photoresist application instead of the electrodeposition process reported elsewhere which requires an electroplating base. There is a common problem of thinning of photoresist on convex edges of the cavity during photolithography steps. As a result, silicon dioxide is etched away from these edges in buffered hydrofluoric acid. During subsequent anisotropic etching of silicon to release the microstructures, the unprotected convex edges are also etched, resulting in failure of the process. To overcome this problem, two different methods are used. Recessed microstructures have been successfully fabricated using this process. A prototype micro-relay is demonstrated using a P⁺ silicon recessed cantilever beam.

In this paper, the buckling behavior of functionally graded material (FGM) rectangular plates subjected to pin loads, partial uniform loads and parabolic loads is studied using the mesh-free method based on the radial basis function. The proposed mesh-free method approximates displacements based on scattered nodes, thus it can avoid the disadvantages that arise in the finite element method (FEM) from the use of elements. Variational forms of the system equations for the calculation of non-uniform prebuckling stress distribution and buckling loads of the plate are established. Two-step solution procedures are implemented. First the non-uniform prebuckling stresses are obtained based on a two-dimensional (2D) elastic plane stress problem. Then buckling loads of plates with the predetermined non-uniform prebuckling stresses are calculated based on Mindlin's plate assumption. Selected numerical examples are presented to validate the proposed mesh-free method.

A meshfree model based on the first-order shear deformation theory is presented for the shape and vibration control of laminated composite plates with integrated piezoelectric sensors and actuators. A point interpolation method using radial basis functions (RPIM) is employed to construct shape functions for mechanical and electrical variables, which possess the delta function property and show linear reproduction behavior. The method shows a high convergence rate equivalent to that of the second-order finite elements approach. Comparing, one sees that a very simple nodal topology can be used for the field representation and no element continuity is required. A constant displacement and velocity feedback control algorithm is used for the active control of the static deflection as well as the dynamic response of plates through closed loop control. Numerical results for the static deformation, vibration modes and dynamic responses are in good agreement with those from the finite element method.

The behavior of composite panel embedded Macro Fiber Composite (MFC) actuators subject to combined aerodynamic, thermal and piezoelectric loads is investigated. This study shows how the aero-thermal large deflection of the composite panel can be suppressed using MFC actuators; however, the excessive actuation of the MFC can cause snap-through phenomena. In addition, the characteristics of the vibration, thermal postbuckling and flutter of the composite panel embedded MFC actuators are investigated. The panel is modeled by the nonlinear finite element method based on the first-order shear deformation plate theory. The von Karman strain–displacement relation is used to account for the large deflection of the panel, and the cylindrical arclength method is adopted to simulate the snap-through behavior of the panel. The aerodynamic load model is based on the first-order quasi-steady piston theory. The numerical results show that MFC actuators can improve the performance of the panel; however, snap-through behavior can occur when excessive actuation of the MFC is applied to suppress the aero-thermal large deflection of the panel.

The effect of damping of underwater acoustic signals by composites with carbon fiber and molecular sieves of various pore sizes in Conathane EN-1556 based matrix is presented in this paper. The measurements were made in a water-filled pulse tube set-up for a frequency range of 4–12 kHz. The results show that molecular sieves of pore size 3 Å are a better selection than others. Also, molecular sieves in the form of pellets gave good underwater acoustic damping and processability. With 11% mass loading of 3 Å pellets in Conathane EN-1556 RhoC rubber containing 0.25% volume of carbon fiber, an echo reduction of 17–24 dB was obtained for a frequency range of 7–12 kHz.

In tubular nanocomposites prepared from carbon nanotubes and polyaniline, carbon nanotubes can improve the structural ordering, compactness, delocalization of charges and carrier mobility. Such properties are very much critical in the case of organic semiconducting materials for electronics applications. In this paper we describe the synthesis of a nanotubular composite of polyaniline and carbon nanotube using p-phenylenediamine functionalized multiwalled carbon nanotubes. This functionalization helped to disperse the nanotubes well in acidic solution. The in situ polymerization of aniline in the presence of these well dispersed nanotubes gave rise to a new tubular composite of carbon nanotubes having a uniform encapsulation of doped polyaniline. It also improved the structural order, molecular packing and delocalization of the charges in the material.

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