Showing papers in "Smart Materials and Structures in 2006"

A carbon nanotube strain sensor for structural health monitoring

Abstract: A carbon nanotube polymer material was used to form a piezoresistive strain sensor for structural health monitoring applications. The polymer improves the interfacial bonding between the nanotubes. Previous single walled carbon nanotube buckypaper sensors produced distorted strain measurements because the van der Waals attraction force allowed axial slipping of the smooth surfaces of the nanotubes. The polymer sensor uses larger multi-walled carbon nanotubes which improve the strain transfer, repeatability and linearity of the sensor. An electrical model of the nanotube strain sensor was derived based on electrochemical impedance spectroscopy and strain testing. The model is useful for designing nanotube sensor systems. A biomimetic artificial neuron was developed by extending the length of the sensor. The neuron is a long continuous strain sensor that has a low cost, is simple to install and is lightweight. The neuron has a low bandwidth and adequate strain sensitivity. The neuron sensor is particularly useful for detecting large strains and cracking, and can reduce the number of channels of data acquisition needed for the health monitoring of large structures.

Analysis of power output for piezoelectric energy harvesting systems

Abstract: Power harvesting refers to the practice of acquiring energy from the environment which would be otherwise wasted and converting it into usable electric energy. Much work has been done on studying the optimal AC power output, while little has considered the AC–DC output. This article investigates the optimal AC–DC power generation for a rectified piezoelectric device. In contrast with estimates based on various degrees of approximation in the recent literature, an analytic expression for the AC–DC power output is derived under steady-state operation. It shows that the harvested power depends on the input vibration characteristics (frequency and acceleration), the mass of the generator, the electrical load, the natural frequency, the mechanical damping ratio and the electromechanical coupling coefficient of the system. An effective power normalization scheme is provided to compare the relative performance and efficiency of devices. The theoretical predictions are validated and found to be in good agreement with both experimental observations and numerical simulations. Finally, several design guidelines are suggested for devices with large coupling coefficient and quality factor.

Resonance tuning of piezoelectric vibration energy scavenging generators using compressive axial preload

Abstract: Vibration energy scavenging, harvesting ambient vibrations in structures for conversion into usable electricity, provides a potential power source for emerging technologies including wireless sensor networks. Most vibration energy scavenging devices developed to date operate effectively at a single specific frequency dictated by the device's design. However, for this technology to be commercially viable, vibration energy scavengers that generate usable power across a range of driving frequencies must be developed. This paper details the design and testing of a tunable-resonance vibration energy scavenger which uses the novel approach of axially compressing a piezoelectric bimorph to lower its resonance frequency. It was determined that an axial preload can adjust the resonance frequency of a simply supported bimorph to 24% below its unloaded resonance frequency. The power output to a resistive load was found to be 65–90% of the nominal value at frequencies 19–24% below the unloaded resonance frequency. Prototypes were developed that produced 300–400 µW of power at driving frequencies between 200 and 250 Hz. Additionally, piezoelectric coupling coefficient values were increased using this method, with keff values rising as much as 25% from 0.37 to 0.46. Device damping increased 67% under preload, from 0.0265 to 0.0445, adversely affecting the power output at lower frequencies. A theoretical model modified to include the effects of preload on damping predicted power output to within 0–30% of values obtained experimentally. Optimal load resistance deviated significantly from theory, and merits further investigation.

Biomimetic self-healing of advanced composite structures using hollow glass fibres

Abstract: The use of functional repair components stored inside hollow reinforcing fibres is being considered as a self-repair system for future composite structures. The incorporation of a self-healing capability within a variety of materials, including fibre reinforced polymers (FRPs), has been investigated by a number of workers previously. This paper considers the placement of self-healing plies within an FRP to mitigate damage occurrence and restore mechanical strength. The flexural strength results indicate that the inclusion of hollow fibres results in an initial strength reduction of 16% from a baseline FRP laminate. However, the effect of impact damage on the performance of the baseline FRP laminate and the laminate containing the hollow fibre layers was comparable, with a flexural strength typically 72–74% of the undamaged state. Self-healing of the damage site saw the laminate recover 87% of the undamaged baseline FRP laminate's strength. This study provides clear evidence that an FRP laminate containing hollow fibre layers can successfully self-heal. This result suggests that biomimetic repair is now possible for advanced composite structures.

Development of an adaptive tuned vibration absorber with magnetorheological elastomer

Abstract: In this technical note we develop an adaptive tuned vibration absorber (ATVA) based on the unique characteristics of magnetorheological elastomers (MREs), whose modulus can be controlled by an applied magnetic field. The MRE used in the developed ATVA was prepared by curing a mixture of 704 silicon rubber, carbonyl iron particles and a small amount of silicone oil under a magnetic field. The ATVA works in shear mode and consists of an oscillator, smart spring elements with MREs, a magnet conductor and two coils. Natural frequencies of the ATVA under different magnetic fields were both theoretically analyzed and experimentally evaluated by employing a beam structure with two ends supported. The experimental results demonstrated that the natural frequency of the ATVA can be tuned from 55 to 82 Hz. The relative frequency change is as high as 147%. Furthermore, the absorption capacity of the developed ATVA can achieve as high as 60 dB, which was also experimentally justified.

Vibration of carbon nanotubes studied using nonlocal continuum mechanics

Abstract: A nonlocal continuum mechanics model is developed and applied to study the vibration of both single-walled nanotubes (SWNTs) and double-walled nanotubes (DWNTs) via elastic beam theories. The small-scale effects on vibration characteristics of carbon nanotubes are explicitly derived through a complete mechanics analysis. A qualitative validation study shows that the results based on nonlocal continuum mechanics are in agreement with the published experimental reports in this field. Numerical simulations are conducted to quantitatively show the small-scale effect on vibrations of both SWNTs and DWNTs with different lengths and diameters.

Performance monitoring of the Geumdang Bridge using a dense network of high-resolution wireless sensors

Abstract: As researchers continue to explore wireless sensors for use in structural monitoring systems, validation of field performance must be done using actual civil structures. In this study, a network of low-cost wireless sensors was installed in the Geumdang Bridge, Korea to monitor the bridge response to truck loading. Such installations allow researchers to quantify the accuracy and robustness of wireless monitoring systems within the complex environment encountered in the field. In total, 14 wireless sensors were installed in the concrete box girder span of the Geumdang Bridge to record acceleration responses to forced vibrations introduced by a calibrated truck. In order to enhance the resolution of the capacitive accelerometers interfaced to the wireless sensors, a signal conditioning circuit that amplifies and filters low-level accelerometer outputs is proposed. The performance of the complete wireless monitoring system is compared to a commercial tethered monitoring system that was installed in parallel. The performance of the wireless monitoring system is shown to be comparable to that of the tethered counterpart. Computational resources (e.g. microcontrollers) coupled with each wireless sensor allow the sensor to estimate modal parameters of the bridge such as modal frequencies and operational displacement shapes. This form of distributed processing of measurement data by a network of wireless sensors represents a new data management paradigm associated with wireless structural monitoring. (Some figures in this article are in colour only in the electronic version)

Concrete early-age strength monitoring using embedded piezoelectric transducers

Abstract: At early ages of concrete structures, strength monitoring is important to determine the structures' readiness for service. Piezoelectric-based strength monitoring methods provide an innovative experimental approach to conduct concrete strength monitoring at early ages. In this paper, piezoelectric transducers in the form of 'smart aggregates' are embedded into the concrete specimen during casting. Piezoceramic materials can be used as actuators to generate high frequency vibrating waves, which propagate within concrete structures; meanwhile, they can also be used as sensors to detect the waves. The smart aggregate is a one cubic inch, pre-cast concrete block with a wired, embedded PZT (lead zirconate titanate, a type of piezoceramic) patch. The strength development of concrete structures is monitored by observing the development of harmonic response amplitude from the embedded piezoelectric sensor at early ages. From experimental results, the amplitude of the harmonic response decreases with increasing concrete strength. The concrete strength increases at a fast rate during the first few days and at a decreasing rate after the first week. Concordantly, the amplitude of the harmonic response from the piezoelectric sensor drops rapidly for the first week and continues to drop slowly as hydration proceeds, matching the development of the concrete strength at early ages. Concrete is heterogeneous and anisotropic, which makes it difficult to analyze mathematically. Fuzzy logic has the advantage of conducting analysis without requiring a mathematical model. In this paper, a fuzzy logic system is trained to correlate the harmonic amplitude with the concrete strength based on the experimental data. The experimental results show that the concrete strength estimated by the trained fuzzy correlation system matches the experimental strength data. The proposed piezoelectric-based monitoring method has the potential to be applied to strength monitoring of concrete structures at early ages.

Performance assessment and validation of piezoelectric active-sensors in structural health monitoring

Abstract: A sensor diagnostics and validation process that performs in situ monitoring of the operational status of piezoelectric (PZT) active-sensors in structural health monitoring (SHM) applications is presented. Both degradation of the mechanical/electrical properties of a PZT transducer and the bonding defects between a PZT patch and a host structure could be identified by the proposed process. This study also includes the investigation into the effects of the sensor/structure bonding defects on high-frequency SHM techniques, including Lamb wave propagations and impedance methods. It has been found that the effects are significant, modifying the phase and amplitude of propagated waves and changing the measured impedance spectrum. These changes could lead to false indications on the structural conditions without an efficient sensor-diagnostic process. The feasibility of the proposed sensor diagnostics procedure is then demonstrated by analytical studies and experimental examples, where the functionality of the surface-mounted piezoelectric sensors was continuously deteriorated. The proposed process can provide a metric that can be used to determine the sensor functionality over a long period of service time or after an extreme loading event. Further, the proposed method can be useful if one needs to check the operational status of a sensing network right after its installation.

The temperature stability of guided wave structural health monitoring systems

Abstract: It is desirable for any structural health monitoring (SHM) system to have maximum sensitivity with minimum sensor density. The structural health monitoring system described here is based on the excitation and reception of guided waves using piezoelectric elements as sensors. One of the main challenges faced is that in all but the most simple structures the wave interactions become too complex for the time domain signals to be interpreted directly. One approach to overcoming this complexity is to subtract a baseline reference signal from the measured system when it is known to be defect free. This strategy enables changes in the structure to be identified. Two key issues must be addressed to allow this paradigm to become a reality. First, the system must be sufficiently sensitive to small reflections from defects such as cracking. Second, it must be able to distinguish between benign changes and those due to structural defects. In this paper the baseline subtraction approach is used to detect defects in a simple rectangular plate. The system is shown to work well in the short term, and good sensitivity to defects is demonstrated. The performance degrades over the medium to long term. The principal reason for this degradation is shown to be the effect of change in temperature of the system. These effects are quantified and strategies for overcoming them are discussed.

Feasibility of structural monitoring with vibration powered sensors

Abstract: Wireless sensors and sensor networks are beginning to be used to monitor structures. In general, the longevity, and hence the efficacy, of these sensors are severely limited by their stored power. The ability to convert abundant ambient energy into electric power would eliminate the problem of drained electrical supply, and would allow indefinite monitoring. This paper focuses on vibration in civil engineering structures as a source of ambient energy; the key question is can sufficient energy be produced from vibrations? Earthquake, wind and traffic loads are used as realistic sources of vibration. The theoretical maximum energy levels that can be extracted from these dynamic loads are computed. The same dynamic loads are applied to a piezoelectric generator; the energy is measured experimentally and computed using a mathematical model. The collected energy levels are compared to the energy requirements of various electronic subsystems in a wireless sensor. For a 5 cm3 sensor node (the volume of a typical concrete stone), it is found that only extreme events such as earthquakes can provide sufficient energy to power wireless sensors consisting of modern electronic chips. The results show that the optimal generated electrical power increases approximately linearly with increasing sensor mass. With current technology, it would be possible to self-power a sensor node with a mass between 100 and 1000 g for a bridge under traffic load. Lowering the energy consumption of electronic components is an ongoing research effort. It is likely that, as electronics becomes more efficient in the future, it will be possible to power a wireless sensor node by harvesting vibrations from a volume generator smaller than 5 cm3.

A comparison of embedded sensor Lamb wave ultrasonic tomography approaches for material loss detection

Abstract: Computerized tomography (CT) algorithms have been used mainly in the medical field but their powerful capabilities are being exploited more and more in industrial applications. This paper demonstrates that the technology is capable of detecting material loss on real aircraft components using embedded piezoelectric sensors on hidden surfaces. The work is novel in more than one respect. Firstly, it demonstrates that Lamb wave ultrasonic tomography can be used to accurately map material loss on an exposed aircraft surface with sensors embedded on the structure's hidden surface. Hidden, in this case, refers to the surface that is not exposed to the atmosphere—the underneath of an aircraft wing, for example. Secondly, it compares tomographic images generated by fan-beam back projection and the signal difference coefficient methods, showing clearly that the latter are more sensitive to material loss.

Application of time series analysis for bridge monitoring

Abstract: Despite recent considerable advances in structural health monitoring (SHM) of civil infrastructure, converting large amounts of data from SHM systems into usable information and knowledge remains a great challenge. This paper addresses the problem through the analysis of time histories of static strain data recorded by an SHM system installed in a major bridge structure and operating continuously for a long time. The reported study formulates a vector seasonal autoregressive integrated moving average (ARIMA) model for the recorded strain signals. The coefficients of the ARIMA model are allowed to vary with time and are identified using an adaptive Kalman filter. The proposed method has been used for analysis of the signals recorded during the construction and service life of the bridge. By observing various changes in the ARIMA model coefficients, unusual events as well as structural change or damage sustained by the structure can be revealed.

Development of an MR-brake-based haptic device

Abstract: This paper describes the design, testing and modelling of a magneto-rheological (MR) fluid brake as well as its application in a haptic device. The MR device, in disc shape, is composed of a rotary shaft and plate, an electromagnetic coil, MR fluids, and casings. The working principle of the actuator is discussed and the transmitted torque equation employed by using the Bingham plastic model. The optimal dimensions of the actuator were obtained by finite-element analysis using the COSMOSEMS package. Following manufacturing and fabrication of the actuator prototype, the steady-state performance of the MR actuator was measured using a force gauge. The experimental results show that the actuator exhibits hysteresis behaviour. A sub-hysteresis model was then proposed and the model parameters were identified. Example applications of this actuator in virtual reality are demonstrated.

Dynamic properties of sandwich structures with integrated shear-thickening fluids

Abstract: The integration of shear-thickening fluids (STFs) into composite structures has been investigated with the aim of tuning part stiffness and damping capacity under dynamic deformation. Results from oscillatory rheological measurements for a STF based on concentrated fused silica in polypropylene glycol were correlated with results from vibrating beam tests on model sandwich structures containing layers of the same STF sandwiched between polyvinyl chloride (PVC) beams. Above a critical amplitude, the relative motion of the PVC beams provoked shear thickening of the silica suspensions, and the vibration and damping properties were significantly modified. These changes were related to the rheological response of the STF through analytical calculations of strains in the STF layers, an approach that was verified experimentally by replacing the STF with a slow-curing epoxy resin. The potential for integrating STFs into structures exposed to dynamic flexural deformation, with the aim of controlling their vibrational response, has thus been demonstrated.

PZT-based active damage detection techniques for steel bridge components

Abstract: This paper presents the results of experimental studies on piezoelectric lead-zirconate–titanate (PZT)-based active damage detection techniques for nondestructive evaluations (NDE) of steel bridge components. PZT patches offer special features suitable for real-time in situ health monitoring systems for large and complex steel structures, because they are small, light, cheap, and useful as built-in sensor systems. Both impedance and Lamb wave methods are considered for damage detection of lab-size steel bridge members. Several damage-sensitive features are extracted: root mean square deviations (RMSD) in the impedances and wavelet coefficients (WC) of Lamb waves, and the times of flight (TOF) of Lamb waves. Advanced signal processing and pattern recognition techniques such as continuous wavelet transform (CWT) and support vector machine (SVM) are used in the current system. Firstly, PZT patches were used in conjunction with the impedance and Lamb waves to detect the presence and growth of artificial cracks on a 1/8 scale model for a vertical truss member of Seongsu Bridge, Seoul, Korea, which collapsed in 1994. The RMSD in the impedances and WC of Lamb waves were found to be good damage indicators. Secondly, two PZT patches were used to detect damage on a bolt-jointed steel plate, which was simulated by removing bolts. The correlation of the Lamb wave transmission data with the damage classified by in and out of the wave path was investigated by using the TOF and WC obtained from the Lamb wave signals. The SVM was implemented to enhance the damage identification capability of the current system. The results from the experiments showed the validity of the proposed methods.

Smart polymer fibers with shape memory effect

Abstract: In this study, a series of smart polymer fibers with a shape memory effect were developed. Firstly, a set of shape memory polyurethanes with varying hard-segment content were synthesized. Then, the solutions of the shape memory polyurethanes were spun into fibers through wet spinning. The thin films of the polyurethanes were considered to represent the nature of the polyurethanes. Differential scanning calorimetry tests were performed on both the thin films and the fibers to compare their thermal properties. Wide angle x-ray diffraction and small angle x-ray scattering techniques were applied to investigate the structure of the thin films and the fibers, and the structure change taking place in the spinning process was therefore revealed. The spinning process resulted in the polyurethane molecules being partially oriented in the direction of the fiber axis. The molecular orientation prompted the aggregation of the hard segments and the formation of hard-segment microdomains. The mechanical properties of the fibers were examined through tensile tests. The shape memory effect of the thin films and the fibers was investigated through a series of thermomechanical cyclic tensile tests. It was found that the fibers showed less shape fixity but more shape recovery compared with the thin films. Further investigations revealed that the recovery stress of the fibers was higher than that of the thin films. The smart fibers may exert the recovery force of shape memory polymers to an extreme extent in the direction of the fiber axis and therefore provide a possibility for producing high-performance actuators.

The mechanical behavior of smart magnet–hydrogel composites

Abstract: Micron sized magnetic particles (Fe3O4) were dispersed in a polyvinyl alcohol (PVA) hydrogel. This multiferroic ferrogel combines the elastic properties of PVA gel and the magnetic properties of Fe3O4 particles. The response of the ferrogel (with Fe3O4 concentration in the range of 1–10 wt%) to the application of a static magnetic field (up to a maximum of 40 mT) was investigated. The results showed that the extent of deflection depended strongly on the Fe3O4 content and the magnetic field strength. For each Fe3O4 concentration there existed a threshold value of magnetic field strength before large deflection occurred. This implied that the ferrogel system can be used as an 'on–off' type transducer. The threshold value decreases with an increase in Fe3O4 content. Finally, two approaches were used to evaluate this ferrogel system for artificial muscle or soft actuator applications.

A comprehensive analysis of the response time of MR dampers

Abstract: The primary purpose of this paper is to provide a comprehensive review on the response time of magnetorheological (MR) dampers. Rapid response time is desired for all real-time control applications. In reviewing the literature, a detailed description of the response time of semi-active dampers is seldom given. Furthermore, the methods of computing the response time are not discussed in detail. The authors intend to develop a method for the definition and the experimental determination of the response time of MR dampers. Furthermore, parameters affecting the response time of MR dampers are investigated. Specifically, the effect of operating current, piston velocity, and system compliance are addressed. Because the response time is often limited, not by the response of the fluid itself, but by the limitations of the driving electronics and the inductance of the electromagnet, the response time of the driving electronics is considered as well. The authors define the response time as the time required to transition from the initial state to 95% of the final state. Using a triangle wave to maintain constant velocity across the damper, various operating currents ranging from 0.5 to 2 A were applied and the resulting force was recorded. The results show that, for a given velocity, the response time decreases as the operating current increases. Results for the driving electronics show the opposite trend: as current increases, response time increases. To evaluate the effect of piston velocity on response time, velocities ranging from 0.1 to 4 in s−1 were tested. The results show that the response time decreases exponentially as the velocity increases, converging on some final value. Further analysis revealed that this result is an artifact of the compliance in the system. To confirm this, a series of tests was conducted in which the compliance of the system was artificially altered. The results of the compliance study indicate that compliance has a significant effect on the response time of the damper.

Unit cell models of piezoelectric fiber composites for numerical and analytical calculation of effective properties

Abstract: Numerical unit cell models of 1-3 periodic composites made of piezoceramic unidirectional cylindrical fibers embedded in a soft non-piezoelectric matrix are developed. The unit cell is used for prediction of the effective coefficients of the periodic transversely isotropic piezoelectric cylindrical fiber composite. Special emphasis is placed on a formulation of the boundary conditions that allows the simulation of all modes of the overall deformation arising from any arbitrary combination of mechanical and electrical loading. The numerical approach is based on the finite element method and it allows extension to composites with arbitrary geometrical inclusion configurations, providing a powerful tool for fast calculation of their effective properties. For verification, the effective coefficients are evaluated for square and hexagonal arrangements of unidirectional piezoelectric cylindrical fiber composites. The results obtained from the numerical technique are compared with those obtained by means of the analytical asymptotic homogenization method for different volume fractions. Furthermore, the results are compared with other analytical and numerical methods reported in the literature.

Health monitoring and rehabilitation of a concrete structure using intelligent materials

Abstract: This paper presents the concept of an intelligent reinforced concrete structure (IRCS) and its application in structural health monitoring and rehabilitation. The IRCS has multiple functions which include self-rehabilitation, self-vibration damping, and self-structural health monitoring. These functions are enabled by two types of intelligent (smart) materials: shape memory alloys (SMAs) and piezoceramics. In this research, Nitinol type SMA and PZT (lead zirconate titanate) type piezoceramics are used. The proposed concrete structure is reinforced by martensite Nitinol cables using the method of post-tensioning. The martensite SMA significantly increases the concrete's damping property and its ability to handle large impact. In the presence of cracks due to explosions or earthquakes, by electrically heating the SMA cables, the SMA cables contract and close up the cracks. In this research, PZT patches are embedded in the concrete structure to detect possible cracks inside the concrete structure. The wavelet packet analysis method is then applied as a signal-processing tool to analyze the sensor signals. A damage index is defined to describe the damage severity for health monitoring purposes. In addition, by monitoring the electric resistance change of the SMA cables, the crack width can be estimated. To demonstrate this concept, a concrete beam specimen with reinforced SMA cables and with embedded PZT patches is fabricated. Experiments demonstrate that the IRC has the ability of self-sensing and self-rehabilitation. Three-point bending tests were conducted. During the loading process, a crack opens up to 0.47 inches. Upon removal of the load and heating the SMA cables, the crack closes up. The damage index formed by wavelet packet analysis of the PZT sensor data predicts and confirms the onset and severity of the crack during the loading. Also during the loading, the electrical resistance value of the SMA cable changes by up to 27% and this phenomenon is used to monitor the crack width.

An experimental comparison between several active composite actuators for power generation

Abstract: The use of piezoelectric materials for power harvesting has attracted significant interest over the past few years. The majority of research on this subject has sought to quantify the amount of energy generated in power harvesting applications, or to develop methods of improving the amount of energy generated. Usually, a monolithic piezoelectric material with a traditional electrode pattern and poled through its thickness is used for power harvesting. However, in recent years several companies and research institutions have begun to develop and market a broad range of piezoelectric composite sensor/actuator packages, each conceived for specific operational advantages and characteristics. Commonly, these devices are employed in control and vibration suppression applications, and their potential for use in power-harvesting systems remains largely unknown. Two frequently implemented design techniques for improving the performance of such actuators are the use of interdigitated electrodes and piezofibers. This paper seeks to experimentally quantify the differences in performance in power-harvesting applications between several of these new actuators and to identify the reasons for their relative performance characteristics. A special focus on the structural and compositional differences between each actuator is incorporated in the discussion of the effectiveness of each actuator as a power-harvesting device.

Detection of bolt loosening in C–C composite thermal protection panels: I. Diagnostic principle

Abstract: A concept demonstrator of the structural health monitoring (SHM) system was developed to autonomously detect the degradation of the mechanical integrity of the standoff carbon–carbon (C–C) thermal protection system (TPS) panels. This system enables us to identify the location of the loosened bolts, as well as to predict the torque levels of those bolts accordingly. In the process of building the proposed SHM prototype, efforts have been focused primarily on developing a trustworthy diagnostic scheme and a responsive sensor suite. In part I of the study, an attenuation-based diagnostic method was proposed to assess the fastener integrity by observing the attenuation patterns of the resultant sensor signals. The attenuation-based method is based on the damping phenomena of ultrasonic waves across the bolted joints. The major advantage of the attenuation-based method over the conventional diagnostic methods is its local sensing capability of loosened brackets. The method can further discriminate the two major failure modes within a bracket: panel-joint loosening and bracket-joint loosening. The theoretical explanation of the attenuation-based method is performed using micro-contact theory and structural/internal damping principles, followed by parametric model studies and appropriate hypothesis testing.

A model for ionic polymer metal composites as sensors

Abstract: This paper introduces a comprehensive model of sensors based on ionic polymer metal composites (IPMCs) working in air. Significant quantities ruling the sensing properties of IPMC-based sensors are taken into account and the dynamics of the sensors are modelled. A large amount of experimental evidence is given for the excellent agreement between estimations obtained using the proposed model and the observed signals. Furthermore, the effect of sensor scaling is investigated, giving interesting support to the activities involved in the design of sensing devices based on these novel materials. We observed that the need for a wet environment is not a key issue for IPMC-based sensors to work well. This fact allows us to put IPMC-based sensors in a totally different light to the corresponding actuators, showing that sensors do not suffer from the same drawbacks.

A new dynamic hysteresis model for magnetorheological dampers

Abstract: Semi-actively controlled magnetorheological (MR) fluid dampers offer rapid variation in damping properties in a reliable fail-safe manner using very low power requirements. Their characteristics make them ideal for semi-active control in structures and vehicle applications in order to efficiently suppress vibration. To take advantage of their exceptional characteristics, a high fidelity model is required for control design and analysis. Perfect understanding of the dynamic characteristics of such dampers is necessary when implementing MR struts in applications. Different models have been proposed to simulate the hysteresis phenomenon of MR dampers. 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. Moreover, the characteristic parameters in the traditional Bouc–Wen model are not functions of the frequency, amplitude and current excitations; therefore, the estimated parameters can characterize the behavior of the tested MR damper under specific excitation conditions and must be re-evaluated if a different combination of excitation parameters is desired. This can be extremely cumbersome and computationally expensive. In this work, a new hysteresis model based on the Bouc–Wen model has been developed to better characterize the hysteresis phenomenon of the MR damper. The proposed model incorporates the frequency, amplitude and current excitation as variables and thus enables us to predict efficiently and accurately the hysteresis force for changing excitation conditions. The proposed modified Bouc–Wen model has been validated against the experimental results through graphical and quantitative analysis in time, displacement and velocity domains and an excellent correlation has been found.

Modeling and testing of PZT and PVDF piezoelectric wafer active sensors

Abstract: Piezoelectric wafer active sensors (PWAS) used in structural health monitoring (SHM) applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive and inexpensive. They achieve direct transduction between electric and elastic wave energies. PWAS are charge mode sensors and can be used as both transmitters and receivers. The focus of this paper is to find a suitable in situ piezoelectric active sensor for sending and receiving Lamb waves to be used in the SHM of structures with a curved surface. Current SHM technology uses brittle piezoceramic (PZT) wafer active sensors. Since piezoceramics are brittle, this approach could only be used on flat surfaces. The motivation of our research was to explore the use of flexible piezoelectric materials, e.g. piezoelastic polymers such as PVDF. However, PVDF stiffness is orders of magnitude lower than the PZT stiffness, and hence PVDF Lamb wave transmitters are much weaker than PZT transmitters. Thus, our research proceeded in two main directions: (a) to model and understand how piezoelectric material properties affect the behaviour of piezoelectric wafer active sensors; and (b) to perform experiments to test the capabilities of the flexible PVDF PWAS in comparison with those of stiffer but brittle PZT PWAS. We have shown that, with appropriate signal amplification, PVDF PWAS can perform the same Lamb wave transmission and reception functions currently performed by PZT PWAS. The experimental results of PZT-PWAS and PVDF-PWAS have been compared with a conventional strain gauge. The theoretical and experimental results in this study gave a basic demonstration of the piezoelectricity of PZT-PWAS and PVDF-PWAS.

A new fabrication method for IPMC actuators and application to artificial fingers

Abstract: IPMC (ionic polymer metal composite), a kind of ionic electroactive polymer (EAP), has been used for various applications because it has light weight and can make large bending deformation under low driving voltage. In the present work, thick IPMC films were fabricated by hot-pressing several thin IPMC films and the actuating performance was evaluated. Displacement and maximum load with applied voltage were measured using a displacement measuring system, a load cell and a multimeter. Several cycles of Pt electroless-plating were performed on the IPMC films to improve the actuating performance. Then, SEM (scanning electron microscopy) micrographs and EDS (energy dispersive spectrometer) profiles of the IPMC specimen were examined. To demonstrate the feasibility of IPMC films for medical or robotic applications, the developed IPMC actuators were applied to artificial fingers and tested.

Structural identification and damage diagnosis using self-sensing piezo-impedance transducers

Abstract: The use of smart materials, such as lead zirconate titanate (PZT), has accelerated developments in the fields of structural identification and automated structural health monitoring (SHM). One such technique that has made much progress is the electro-mechanical impedance (EMI) technique, which employs self-sensing piezo-impedance transducers. In this technique, a PZT patch is surface bonded to the structure to be monitored and its corresponding electro-mechanical admittance signature is used for damage detection. This paper introduces a new method for identifying structures from the measured admittance signatures in terms of equivalent structural parameters, whereby the identified parameters are used for damage characterization. The new method has been applied to a truss, a beam and a concrete cube, and found to be able to successfully perform structural identification and damage diagnosis. In addition, several advantages have been ascertained in comparison with the conventional, non-parametric statistical methods.

Crack identification in aluminium plates using Lamb wave signals of a PZT sensor network

Abstract: With an integrated active piezoelectric sensor network, a Lamb wave-based crack identification technique for aluminium plates was developed. Experimental results showed that the propagation of Lamb waves in aluminium plate-like structures is considerably complicated due to wave dispersion, material attenuation, boundary reflection, etc. In order to eliminate the diverse interference, a wavelet transform technique was applied to purify the acquired Lamb wave signals, and the characteristics of Lamb wave signals were extracted from the wave energy spectrum. A correlation function was further established, which helped identify the crack position based on a triangulation approach with the aid of a nonlinear least-squares optimization algorithm. Such an approach provides satisfactory results in locating the crack position in aluminium plates with cracks of 5 and 20 mm in length.

Dielectric properties of mixed Li–Ni–Cd ferrites

Abstract: Ferrites with the general formula Li0.5Ni0.75−x/2Cdx/2Fe2O4 (where x = 0, 0.1, 0.3, 0.5, 0.7 and 0.9) were prepared by the standard double sintering ceramic method. X-ray diffraction analysis confirmed the single phase spinel structure of the samples. The variation of saturation magnetization (Ms) was studied as a function of Cd content. The dielectric constant (e') and dielectric loss tangent (tanδ) were measured at room temperature as a function of frequency in the range 100 Hz–1 MHz. These parameters decrease with increasing frequency for all of the samples. The compositional variation of e' and ρDC show an inverse trend of variation with each other.