Table of contents

Wireless monitoring of the health of CFRP structures reduces the cost and time of inspections and can be usefully applied for continuous monitoring. In a previous study, we presented a wireless sensor for detection of internal delamination in a CFRP laminate. The method utilizes a simple electrical resistance change in CFRP and so monitors delamination at only one location. For monitoring of large-scale structures, however, many sensors have to be distributed to cover the structure. A major problem for using many sensors is time synchronization among sensors. To overcome the problem and enable strain/damage to be monitored at multiple locations with time synchronization, we develop a simple wireless strain/damage sensor that consists of a bridge circuit, voltage-controlled oscillator and amplifiers. Since the sensor does not need A/D conversion procedures or memory storing, there is no time delay. Each sensor has an original basic frequency that changes in accordance with the electrical resistance. The frequencies from the multiple sensors are transmitted to a receiver. Using a short-time maximum entropy method, the received waves are converted to multiple electrical resistance data. The proposed method is applied to CFRP laminates and oscillating frequencies are measured in real time. The results show that the system successfully measures applied strain and detects fiber breakage at multiple locations in CFRP laminates with time synchronization.

The recent advent of smart materials, such as piezoelectric materials, shape-memory alloys, and optical fibers, has added a new dimension to present structural health monitoring techniques. In particular, the electro-mechanical impedance (EMI) sensing technique utilizing piezoelectric materials has emerged as a potential tool for the implementation of a built-in monitoring system for damage detection of civil structures. However, there is little effort to apply this technique for concrete monitoring. In this study, an effort to extend the applicability of the EMI sensing technique is made for strength gain monitoring of early age concrete. PZT (piezoelectric lead zirconate titanate) patches are employed to sense the EMI signature of curing concrete. A series of experiments was conducted on concrete specimens to verify the applicability of the EMI sensing technique. The results show the excellent potential of the EMI sensing technique as a practical and reliable nondestructive method for strength gain monitoring.

In a previous study (Barham et al 2007 Acta Mech.191 1–19), the finite deformation of a circular magnetoelastic membrane in an axisymmetric dipole field was calculated by specializing the equations of three-dimensional magnetoelastic equilibrium. The predicted response was found to be similar to the classical limit-point instability occurring in analogous purely mechanical problems. A limit-point instability occurs under conditions corresponding to the incipient non-existence of equilibria. Under such conditions the body is necessarily on the verge of a dynamical state. In the present setting, this corresponds to the occurrence of a maximum in the equilibrium deflection of the membrane with respect to applied field strength and proximity of the field source. The earlier conjecture of a limit-point instability, advanced in Barham et al (2007 Acta Mech.191 1–19), is confirmed in the present work by using a variational method based on an adaptation of the energy criterion of elastic stability to the magnetoelastic setting.

The finite layer method is the most efficient numerical method for three-dimensional analysis of simply supported rectangular plates. Using this method, the three-dimensional analysis is transformed into one-dimensional analysis by virtue of the orthogonal properties of trigonometric interpolation functions. In the present study, the finite layer method is extended to the thermal buckling analysis of symmetrical cross-ply piezoelectric composite plates with full coupling between the thermal, electrical and mechanical fields. The pre-buckling state of the plate is assumed to be steady. Thus, the initial temperature distribution in the plate is independently determined based on the equation of heat conduction, and the associated thermal stresses are computed accordingly. The geometrical stiffness matrix is then formed in the same manner as in the elastic three-dimensional buckling analysis, and the critical temperature rise and buckling modes are obtained by solving the related matrix equations. Numerical examples are presented to verify the proposed method. The critical temperature rise is determined for both the adiabatic and isothermal buckling processes. The thermal buckling behaviours of some piezoelectric laminates and the effects of the thermo-electro-mechanical coupling are also investigated.

The ultrasonic technique is very effective for non-destructive measurement of fresh and hardened cementitious composites. In this study, the ultrasonic technique is further developed with embedded piezoelectric transducers for cement paste hydration monitoring. In this method, the transducers were pre-placed in a mold before casting the specimen. After the specimen was cast, the transducers were used to generate and receive ultrasonic waves. Wave velocity and amplitude were measured. From the velocity measurement, the hydration process of cement paste could be interpreted. The interaction between the transducers and the cement paste was analyzed. In addition, the dynamic modulus was calculated using the wave velocity. Since the embedded transducers had good coupling with the cement paste, reliable test results have been obtained. It is feasible to use the technique as an in situ technique to monitor the hydration process at an early age, as well as the health condition after maturing for a concrete structure.

This paper presents the structural control results of shaking table tests for a steel frame structure in order to evaluate the performance of a number of proposed semi-active control algorithms using multiple magnetorheological (MR) dampers. The test structure is a six-story steel frame equipped with MR dampers. Four different cases of damper arrangement in the structure are selected for the control study. In experimental tests, the El Centro earthquake and Kobe earthquake ground motion data are used as excitations. Further, several decentralized sliding mode control algorithms are developed in this paper specifically for applications of MR dampers in building structures. Various control algorithms are used for the semi-active control studies, including the proposed decentralized sliding mode control (DSMC), LQR control, and passive-on and passive-off control. Each control algorithm is formulated specifically for the use of MR dampers installed in building structures. Additionally, each algorithm uses measurements of the device velocity and device drift for the determination of the control action to ensure that the algorithm can be implemented in a physical structure. The performance of each algorithm is evaluated based on the results of shaking table tests, and the advantages of each algorithm are compared and discussed. The reduction of story drifts and floor accelerations throughout the structure is examined.

We analyze active constrained layer damping (ACLD) of functionally graded (FG) shells under a thermal environment using vertically and obliquely reinforced 1–3 piezocomposites (PZCs) and investigate the performance of PZCs as materials for the constraining layer of the ACLD treatment. The shell's deformations are analyzed by using a modified first-order shear deformation theory and the finite element method. A temperature gradient is applied across the thickness of the shell. Both in-plane and out-of-plane actuations of the constraining layer of the ACLD treatment have been utilized. Particular emphasis has been placed on ascertaining the performance of patches when the orientation angle of the piezoelectric fibers in the constraining layer is varied in two mutually orthogonal vertical planes. It is found that the vertical actuation dominates over the in-plane actuation, and distributed actuators made of vertically and obliquely reinforced 1–3 PZCs have great potential for controlling the performance of FG shells under a thermal environment.

This paper considers the optimal placement of collocated piezoelectric actuator–sensor pairs on flexible beams using a model-based linear quadratic regulator (LQR) controller. A finite element method based on Euler–Bernoulli beam theory is used. The contributions of piezoelectric sensor and actuator patches to the mass and stiffness of the beam are considered. The LQR performance is taken as the objective for finding the optimal location of sensor–actuator pairs. The problem is formulated as a multi-input multi-output (MIMO) model control. The discrete optimal sensor and actuator location problem is formulated in the framework of a zero–one optimization problem which is solved using genetic algorithms (GAs). Classical control strategies like direct proportional feedback, constant gain negative velocity feedback and the LQR optimal control scheme are applied to study the control effectiveness. The study of the optimal location of actuators and sensors is carried out for different boundary conditions of beams like cantilever, simply supported and clamped boundary conditions.

This paper reports the study of asymmetric air-spaced cantilevers for vibration energy harvesting. Such novel structures increase the amplitude of the AC voltage generated, leading to a larger AC to DC energy conversion efficiency. The overall energy conversion efficiency is further increased by allowing the majority of the mechanical energy to be used for electricity generation. An analytical model for the bending of asymmetric air-spaced cantilevers is established by decomposing the cantilever deformation into pure bending and S-shape bending. This model is further verified by a discrete component model and finite element simulations. A criterion to determine the dominant bending mode is presented. Design optimization from an energy perspective has also been discussed. Finally, a prototype energy harvesting device based on asymmetric air-spaced cantilevers is constructed and tested. The results from these experiments show good agreement with the analytical model.

A series of tests have been conducted to determine the survivability and functionality of a piezoelectric-sensor-based active structural health monitoring (SHM) SMART Tape system under the operating conditions of typical liquid rocket engines such as cryogenic temperature and vibration loads. The performance of different piezoelectric sensors and a low temperature adhesive under cryogenic temperature was first investigated. The active SHM system for liquid rocket engines was exposed to flight vibration and shock environments on a simulated large booster LOX-H₂ engine propellant duct conditioned to cryogenic temperatures to evaluate the physical robustness of the built-in sensor network as well as operational survivability and functionality. Test results demonstrated that the developed SMART Tape system can withstand operational levels of vibration and shock energy on a representative rocket engine duct assembly, and is functional under the combined cryogenic temperature and vibration environment.

This paper describes a new class of flight control actuators using post-buckled precompressed (PBP) piezoelectric elements to provide much improved actuator performance. These PBP actuator elements are modeled using basic large deflection Euler-beam estimations accounting for laminated plate effects. The deflection estimations are then coupled to a high rotation kinematic model which translates PBP beam bending to stabilator deflections. A test article using PZT-5H piezoceramic sheets built into an active bender element was fitted with an elastic band which induced much improved deflection levels. Statically the bender element was capable of producing unloaded end rotations on the order of ± 2.6°. With axial compression, the end deflections were shown to increase nearly four-fold. The PBP element was then fitted with a graphite–epoxy aeroshell which was designed to pitch around a tubular stainless steel main spar. Quasi-static bench testing showed excellent correlation between theory and experiment through ± 25° of pitch deflection. Finally, wind tunnel testing was conducted at airspeeds up to 120 kts (62 m s⁻¹, 202 ft s⁻¹). Testing showed that deflections up to ± 20° could be maintained at even the highest flight speed. The stabilator showed no flutter or divergence tendencies at all flight speeds. At higher deflection levels, it was shown that a slight degradation deflection was induced by nose-down pitching moments generated by separated flow conditions induced by extremely high angles of attack.

A Lamb wave is a special type of elastic wave that is widely employed in structural health monitoring systems for damage detection. Recently, piezoelectric (piezo) patches have become popular for Lamb wave excitation and sensing because one piezo patch can serve as both the actuator and the sensor. All published work has assumed that the Lamb wave displacement field generated by a piezo patch actuator is axi-symmetric. However, we observed that piezo sensors placed at equal distances from the piezo patch actuator displayed different responses. In order to understand this phenomenon, we used a laser vibrometer to measure the full-field displacements around a circular piezo actuator noncontactly. The displacement fields excited by the piezo patch actuator are found to be directional, and this directionality is also frequency dependent, indicating that the out-of-plane bending dynamics of the piezo actuator may play an important role in the Lamb wave displacement fields. A simulation model that incorporates the bending deformation of the piezo patch into the calculations of the Lamb wave generation is then developed. The agreement between the simulated and measured displacement fields confirmed that the directionality of the Lamb wave displacement fields is governed by the bending deformation of the piezo patch actuator.

Micro-lenses are produced on two mirror-coated fiber ends in an extrinsic Fabry–Perot interferometer (EFPI) by curing epoxy droplets to obtain a high-finesse resonator. The high-finesse resonator is easy to construct, and can be used as a highly sensitive sensor at a low cost. The experimental results show that a temperature resolution of 0.025 °C and a strain resolution of 0.0625 µε can be achieved with such an EFPI.

Wavelet analysis has been extensively used in damage detection due to its inherent merits over traditional Fourier transforms, and it has been applied to identify abnormality from vibration mode shapes in structural damage identification. However, most related studies have only demonstrated its ability to identify the abnormality of retrieved mode shapes with a relatively higher signal-to-noise ratio, and its incapability of identifying slight abnormality usually corrupted by noise is still a challenge. In this paper, a new technique (so-called 'integrated wavelet transform (IWT)') of taking synergistic advantages of the stationary wavelet transform (SWT) and the continuous wavelet transform (CWT) is proposed to improve the robustness of abnormality analysis of mode shapes in damage detection. Two progressive wavelet analysis steps are considered, in which SWT-based multiresolution analysis (MRA) is first employed to refine the retrieved mode shapes, followed by CWT-based multiscale analysis (MSA) to magnify the effect of slight abnormality. The SWT-MRA is utilized to separate the multicomponent modal signal, eliminate random noise and regular interferences, and thus extract purer damage information, while the CWT-MSA is employed to smoothen, differentiate or suppress polynomials of mode shapes to magnify the effect of abnormality. The choice of the optimal mother wavelet in damage detection is also elaborately addressed. The proposed methodology of the IWT is evaluated using the mode shape data from the numerical finite element analysis and experimental testing of a cantilever beam with a through-width crack. The methodology presented provides a robust and viable technique to identify minor damage in a relatively lower signal-to-noise ratio environment.

This work presents a performance analysis of multimodal passive vibration control of a sandwich beam using shear piezoelectric materials, embedded in a sandwich beam core, connected to independent resistive shunt circuits. Shear piezoelectric actuators were recently shown to be more interesting for higher frequencies and stiffer structures. In particular, for shunted damping, it was shown that equivalent material loss factors of up to 31% can be achieved by optimizing the shunt circuit. In the present work, special attention is given to the design of multimodal vibration control through independent shunted shear piezoelectric sensors. In particular, a parametric analysis is performed to evaluate optimal configurations for a set of modes to be damped. Then, a methodology to evaluate the modal damping resulting from each shunted piezoelectric sensor is presented using the modal strain energy method. Results show that modal damping factors of 1%–2% can be obtained for three selected vibration modes.

In the present study, an attempt has been made to utilize the pH responsive nature of chitosan to produce fibers which can respond to an applied electric voltage. Single walled carbon nanotubes (SWCNTs) have been used for reinforcing the chitosan fibers. But the SWCNTs are associated with their own challenges of aggregation which have been solved in this study by suitable functionalization. Electron microscopy and Raman spectroscopy was used to investigate the dispersions. The functionalized SWCNTs showed improved dispersion in chitosan. The SWCNT/chitosan composite fibers were successfully solution spun and heat-set to give stable fibers of 50 ± 2 µm. The composite fibers exhibited a significant increase in the mechanical properties. The tenacity of the composite fibers reinforced with functionalized SWCNTs increased from 96 to 226 MPa with an increase in SWCNT content from 0 to 0.2 wt%. The strong interaction of carboxylic functional groups of functionalized SWCNTs with the chitosan matrix may be responsible for the improved dispersion and considerable enhancement of the mechanical properties. Further, the stability of the fibers towards the pH switching environment has also been investigated, along with the response to an applied electric voltage. Composite fibers prepared using 0.1 wt% of functionalized carbon nanotubes were observed to exhibit a stable response with high magnitude of strain and high strain rate.

This paper focuses on the development of fully hydrolyzed polyacrylamide (PAAM) hydrogel for applications in biomimetics. We present an analysis of the motion of actuators based on PAAM hydrogel in order to obtain the elementary background needed for the design of actuating devices based on this material, which has a high compatibility with living tissues. The gel properties are investigated, the electroactivity of the hydrogel is shown and a qualitative–quantitative study demonstrating the basics of motion of such actuators is presented.

Driven by the need to reduce the installation cost and maintenance cost of structural health monitoring (SHM) systems, wireless sensor networks (WSNs) are becoming increasingly popular. Perfect time synchronization amongst the wireless sensors is a key factor enabling the use of low-cost, low-power WSNs for structural health monitoring applications based on output-only modal analysis of structures. In this paper we present a theoretical framework for analysis of the impact created by time delays in the measured system response on the reconstruction of mode shapes using the popular frequency domain decomposition (FDD) technique. This methodology directly estimates the change in mode shape values based on sensor synchronicity. We confirm the proposed theoretical model by experimental validation in modal identification experiments performed on an aluminum beam. The experimental validation was performed using a wireless intelligent sensor and actuator network (WISAN) which allows for close time synchronization between sensors (0.6–10 µs in the tested configuration) and guarantees lossless data delivery under normal conditions. The experimental results closely match theoretical predictions and show that even very small delays in output response impact the mode shapes.

Recent advances in sensor technology promote using large sensor networks to efficiently and economically monitor, identify and quantify damage in structures. In structural health monitoring (SHM) systems, the effectiveness and reliability of the sensor network are crucial to determine the optimal number and locations of sensors in SHM systems. Here, we suggest a probabilistic approach for identifying the optimal number and locations of sensors for SHM. We demonstrate a methodology to establish the probability distribution function that identifies the optimal sensor locations such that damage detection is enhanced. The approach is based on using the weights of a neural network trained from simulations using a priori knowledge about damage locations and damage severities to generate a normalized probability distribution function for optimal sensor allocation. We also demonstrate that the optimal sensor network can be related to the highest probability of detection (POD). The redundancy of the proposed sensor network is examined using a 'leave one sensor out' analysis. A prestressed concrete bridge is selected as a case study to demonstrate the effectiveness of the proposed method. The results show that the proposed approach can provide a robust design for sensor networks that are more efficient than a uniform distribution of sensors on a structure.

Piezoelectric patches are usually externally bonded to host structures as sensors or actuators. The performance and integrity of this type of smart structure are determined by the interface stresses within the adhesive layer. To accurately evaluate these interface stresses, a novel analytical model is developed in this study. This new model treats the adhesive layer as two normal spring layers interconnected by a shear spring layer. The peel stresses along the top and bottom surfaces of the adhesive layer are assumed to be different. An interface deformable beam theory is used to describe the deformation of the piezoelectric patch and the host beam. Unlike existing elementary beam theories, this new beam theory captures the deformation of adherends induced by interface stresses by using two interface compliances. Closed-form solutions of interface stresses with enhanced accuracy have been successfully obtained by the new model. The new solutions not only satisfy both the equilibrium condition of the adhesive layer and zero-shear stress boundary condition at the free edge, but also correctly give two different peel stress distributions along the interface between the piezoelectric patch and adhesive layer and the interface between the adhesive layer and the host beam. A numerical example of the sensing charge output suggests that the present model is applicable to the cases of a thick adhesive layer or thick host beams, while the existing model is not.

Heat activated shape memory polymers (SMPs) are increasingly being utilized in ambitious, large deformation designs. These designs may display unexpected or even undesirable performance if the evolution of the SMP's mechanical properties as a function of deformation is neglected. Yet, despite the broadening use of SMPs in complex load bearing structures, there has been little research completed to characterize how the material properties change upon application of large strain. The following is an experimental investigation into the strain induced anisotropic properties of the SMP Veriflex^®. It is found that under large uniaxial strain the SMP's stiffness in the transverse direction can be reduced as much as 86%, while the toughness in the axial direction may increase by an order of magnitude in some cases. A generalized analysis suggests that this trend should be expected for any SMP.

Wireless structural health monitoring (SHM) systems have emerged as a promising technology for robust and cost-effective structural monitoring. However, the applications of wireless sensors on active diagnosis for structural health monitoring (SHM) have not been extensively investigated. Due to limited energy sources, battery-powered wireless sensors can only perform limited functions and are expected to operate at a low duty cycle. Conventional designs are not suitable for sensing high frequency signals, e.g. in the ultrasonic frequency range. More importantly, algorithms to detect structural damage with a vast amount of data usually require considerable processing and communication time and result in unaffordable power consumption for wireless sensors. In this study, an energy-efficient wireless sensor for supporting high frequency signals and a distributed damage localization algorithm for plate-like structures are proposed, discussed and validated to supplement recent advances made for active sensing-based SHM. First, the power consumption of a wireless sensor is discussed and identified. Then the design of a wireless sensor for active diagnosis using piezoelectric sensors is introduced. The newly developed wireless sensor utilizes an optimized combination of field programmable gate array (FPGA) and conventional microcontroller to address the tradeoff between power consumption and speed requirement. The proposed damage localization algorithm, based on an energy decay model, enables wireless sensors to be practically used in active diagnosis. The power consumption for data communication can be minimized while the power budget for data processing can still be affordable for a battery-powered wireless sensor. The Levenberg–Marquardt method is employed in a mains-powered sensor node or PC to locate damage. Experimental results and discussion on the improvement of power efficiency are given.

Smart dampers with on-demand controllable damping curves are key components for the semi-active vibration control of structures. For current smart dampers either utilizing friction or viscosity to dissipate mechanical energy, potential thermal problems are their major drawbacks. Novel colloidal dampers have recently been developed with low heat generation and high damping efficiency; they are, however, passive, and have no on-demand controllable damping capability. In this paper, we propose a smart colloidal damper by employing water-based ferrofluids in damping media. We observe that the corresponding damping hysteresis loops can be affected by applied magnetic fields significantly and rapidly. We further retrieve the instant stiffness and the damping coefficient of the smart colloidal damper from the measured hysteresis loops. It is shown that negative stiffness and a negative damping coefficient may occur during the operation of the smart colloidal dampers.

Concrete structures have been used widely in civil infrastructural systems. Due to the complex nature of the microstructure, nondestructive testing (NDT) of concrete inherently imposes many challenges, which can cause severe limitations on the resolution and the sensitivity of the signals observed. In this study, a numerical simulation based on the finite element (FE) model is first performed to investigate surface wave generation and reception using piezoelectric actuators/sensors, especially in relatively higher frequency cases. The results provide a basic understanding of some features of the microstructure effects on the surface wave propagation. Experimental testing is then conducted to validate the numerical simulation. The group velocity dispersion curves of the surface waves, which are very useful for future damage detection in concrete materials, are obtained from both numerical and experimental results. The good agreement shows the great potential and feasibility of using piezoelectric actuators/sensors to generate and receive surface waves for quantitative damage detection in concrete structures.

We present an experimental investigation on a novel approach to produce micro-sized protrusive features atop a thermo-responsive shape memory polymer (SMP). This approach includes three steps, namely, indenting atop an SMP sample (using a Berkovich indenter in this study), polishing and then heating it for full shape recovery. Apart from ordinary samples, some SMP samples are pre-stretched in the in-plane direction or pre-compressed in the out-of-plane direction.

The relationships between the height/shape of protrusion and the depth of indent are obtained for all indents in samples with/without pre-straining. Intrinsic relationships among the indentation depth, polishing depth and height/shape of protrusion are further revealed quantitatively in a dimensionless manner. The influence of pre-straining is discussed.

In a recent paper, Ajitsaria et al (2007 Smart Mater. Struct.16 447–54) presented a mathematical formulation for the modeling and analysis of a bimorph piezoelectric cantilever beam for voltage generation. Their motivation was the recent increasing trend in using the piezoelectric effect to harvest electrical energy from ambient vibrations. This comment addresses the modeling errors and numerous undefined and missing terms in the mentioned work.