Damage monitoring of the civil infrastructure is critically needed. This article provides the first report of impact damage self-sensing in cement-based materials. Cement mortar reinforced with short (5–7 mm) carbon fiber and in bulk or coating (5–10 mm thick) form is effective for sensing its own impact damage through DC/AC electrical resistance measurement, provided that the region of resistance measurement contains the point of impact. The mortar resistivity needs to be 10⁴–10⁵ cm, as provided by pitch-based fiber (15 µm diameter, unsized) at 0.5% or 1.0% by mass of cement, or type A PAN-based fiber (7 µm diameter, desized) at 0.5%. Due to the low mortar resistivity of 10³ cm, pitch-based fiber at 1.5% and type B PAN-based fiber (7 µm diameter, unsized) at 0.5% are less effective. Without fiber, there is no sensing ability. The surface resistance of the surface receiving the impact is an effective indicator of the damage, even for minor damage without cracking, inflicted by impact at 880 J. The oblique or longitudinal volume resistance is much less effective. The surface resistance increases abruptly upon impact, but it decreases abruptly upon impact after 5–40 impacts (number decreasing with increasing impact energy) have been inflicted.

The effect of electrode patterns on the actuator performance of electro-active paper (EAPap) is studied. A rectangular electrode pattern has been typically used for EAPap actuator. Since the electrical field can be concentrated at the edge of electrodes, the electrode patterns of EAPap can influence the actuator performance. Thus, a fishbone pattern electrode was designed and fabricated on EAPap materials and its actuation characteristics were compared with the rectangular electrode ones. Two EAPap materials, DCell, which is made by dissolving cellulose pulp with N,N-Dimethylacetamide and LiCl followed by curing process, and cellophane were used. Bending displacements and the resonance frequencies of EAPap actuators were investigated. Although the bending displacements of the fishbone patterned EAPap actuators were slightly lower than the rectangular patterned actuators, the resonance frequencies of the fishbone patterned actuators were higher than the rectangular patterned actuators. Electrical field concentration around the edge of fishbone pattern electrode might result in increased bending stiffness of the actuator and electrical power consumption. Electrical power consumption and electrode damage of the fishbone pattern electrode were addressed.

This article considers pyroelectric energy harvesting using piezoelectric materials such as PZT-5A, PMN-PT, PVDF, and thin-films. A model is developed to predict the power generation based on the temporal change in temperature of the material. The measured and predicted power measurements are compared, where the maximum power density of 0.36 µW/cm² is calculated with PMN-PT. The predicted peak power density under similar boundary conditions for thin-film lead scandium tantalate is over 125 µW/cm². The study concludes that the power density is highly dependent upon the surface area and the pyroelectric coefficient of the material, underlining the importance of maximizing these parameters.

The recent advent of highly durable engineering materials and the advancement of latest structural design theories have made possible the fabrication of more efficient engineering structures. However, the safety and reliability of these structures remains the primary challenge and concern for engineers. Especially, for those structures which involve human traffic and huge investments such as the aerospace structures and bridges. Therefore, there is a compelling need to have high-quality online structural health monitoring (SHM) of such structures. The development of a real-time, in-service, and smart material-based SHM method has recently attracted the interest of a large number of academic and industrial researchers. In the recent past, piezoceramic (PZT) transducer has evolved as an efficient smart material, which is usually employed in electro mechanical impedance (EMI) and guided ultrasonic wave propagation techniques. In EMI technique, a PZT transducer interact with the host structure to result in unique health signature, as an inverse function of structural impedance, when it is subjected to high-frequency structural excitations in the presence of electric field. Using the self-actuating and sensing capabilities of PZT transducers, the EMI models attempted to detect loadings on, and damages in, the structures to be monitored. This article reviews some of the advancements in the field of PZT-based SHM made over the past two decades in engineering structures. This article also provides an insight into the possible future work and improvements required for PZT-based EMI technique.

Storage and loss moduli of an MRF submitted to a magnetic field, as measured by a plate–plate magnetorheometer versus shear amplitude in oscillatory shear, exhibit a complex non-linear response. Above a critical amplitude, the storage modulus decreases strongly, while the loss modulus passes through a distinct maximum. The periodic shear stress signal changes from sinusoidal to saw-tooth like. A mechanical analogue, consisting of a Bingham element (friction element and damper arranged in parallel) and elastic spring in series, is capable to capture the measured behaviour. Model predictions for the time-dependent shear stress and resulting moduli at agitation frequency (fundamental mode) for various amplitudes are discussed and the determination of model parameters from the experimental data is addressed. The linear radial shear strain profile in plate–plate geometry causes a smearing of the moduli and shift to higher amplitudes by about a factor of 4/3. The conversion from physical to engineering parameters for an MR device is demonstrated for disk clutches using cone–plate and plate–plate geometry. Complications due to a non-homogeneous shear in the device are addressed. The elastic modulus of the MRF may be relevant for vibration damping and may cause some shift in the resonance frequency of the device.

This study presents and examines the concept of flexible skins for morphing aircraft applications comprising of a cellular honeycomb core covered by a compliant face-sheet. The overall properties of the flexible skins are then largely governed by the characteristics of the cellular honeycomb core, which are in turn dependent on the cell parameters. The results of this study showed that the cellular cores could easily undergo global strains over 10 times greater than the virgin material of which they were built. The in-plane stiffness of the cellular cores is generally several orders of magnitude lower than the virgin material. Using cores that are thicker than isotropic sheet skins, the out-of-plane stiffness can be many times greater than the sheet skin for comparable mass (due to porosity of the cellular core). In general, honeycomb cores with positive cell angles (as opposed to auxetic cores) produce a higher out-of-plane stiffness. For cellular cores made from high-strain capable materials and undergoing large strains, geometric and material non-linearities need to be considered. When the cores are stretched along the principle axes they geometrically stiffen, thereby reducing the maximum global strains achievable. When material softening is considered, the forces required to deform the cellular core to large global strains are reduced.

Flow pumps are important tools in several engineering areas, such as in the fields of bioengineering and thermal management solutions for electronic devices. Nowadays, many of the new flow pump principles are based on the use of piezoelectric actuators, which present some advantages such as miniaturization potential and lower noise generation. In previous work, authors presented a study of a novel pump configuration based on placing an oscillating bimorph piezoelectric actuator in water to generate flow. It was concluded that this oscillatory behavior (such as fish swimming) yields vortex interaction, generating flow rate due to the action and reaction principle. Thus, following this idea the objective of this work is to explore this oscillatory principle by studying the interaction among generated vortex from two bimorph piezoelectric actuators oscillating inside the same pump channel, which is similar to the interaction of vortex generated by frontal fish and posterior ones when they swim together in a group formation. It is shown that parallel–series configurations of bimorph piezoelectric actuators inside the same pump channel provide higher flow rates and pressure for liquid pumping than simple parallel–series arrangements of corresponding single piezoelectric pumps, respectively. The scope of this work includes structural simulations of bimorph piezoelectric actuators, fluid flow simulations, and prototype construction for result validation.

Application of shape memory alloy (SMA) actuators is limited to low frequencies due to slow cooling time especially in the embedded conditions where heat transfer rate is the controlling factor. In this study, we investigate various active cooling techniques and effect of pre-stress to improve the response time of two commercially available SMAs: Flexinol from Dynalloy Inc. and Biometal fiber from Toki Corporation. Flexinol and Biometal fiber of equal length and diameter were found to exhibit different actuation behavior under pre-stress. Time domain force response of SMA actuators was found to be dependent upon the applied pre-stress, heating rate, and amplitude of applied electrical stimulus. Compared to Biometal fibers, time domain response of Flexinol was found to decrease significantly with increasing pre-stress indicating the difference in transformation behavior. Fluid flow and heat sinking were found to be suitable methods for improving the response time by reducing the cooling cycle from 1.6 s to 0.30–0.45 s. This is a significant improvement in the actuation capability of SMAs.

A novel magnetorheological (MR) damper, whose magnetic circuit contains two permanent magnets apart from an electromagnet, is described and tested. Without any consumption of electric energy, a medium damping force is achieved, which ensures an excellent fail-safe behavior in case of an electric power loss. This medium damping force can be increased or decreased, depending on the polarity of the current in the coil. The influence of the composition of the MR fluid in terms of the concentration of iron particles between 20 and 50 vol.% on the damping force and frequency dependency could be evaluated. Finally, the damping force after various storage times of the damper was studied and indicated a good redispersibility of the MR fluid.

With robots and users more commonly sharing space such as in the fields of medicine and home automation, the possibility of a physical collision has increased, even though many robots use actuators with high-ratio gear trains to minimize the effects of impact. We developed a 3-DOF manipulator having a smart flexible joint using an ER fluid and a sensor-equipped pneumatic cushion. Results of position control and collision experiments using the manipulator demonstrated its effectiveness.

The finite element method was used to study the performance of hollow active fiber composites (HAFC) and the effect of the fiber damage on this performance. The finite element model was developed for the simulation of the PZT-5H hollow fiber–epoxy matrix composite with single and dual electrode ‘bus’ systems. The simulations were performed for the actuation and sensing functions of the composites with healthy and broken fibers. The results of the healthy HAFC models were compared to the experimental data and numerical results found in the literature. The effect of broken fibers in the composite structure on both the actuation and sensing performance was studied. The results demonstrate that a gap in the fiber leads to actuation performance loss. The loss in the performance is directly related to the location of the fiber and its proximity to the electric potential source. The results also demonstrate that this loss could be minimized by applying a dual electrode system at both ends of the composite. In the sensing application utilizing HAFC, it was shown that these types of composites have several advantages over solid active fiber composites.

The resistance to flow of magnetorheological (MR) fluids is greatly increased by the application of a magnetic field. At present, most devices exploiting this MR fluid behavior are pistons executing straight-line motion. The MR fluids in these devices are subjected to shear flow, and are modeled by either the Bingham plastic or Herschel–Buckley models, both 1D, phenomenological continuum-level forms relating strain rate, magnetic field magnitude, and stress magnitude, and fit to continuum-level empirical measurements. We employ a multiscale model of the MR fluid introduced in an earlier paper, which integrates nanoscale behavior over a mesoscale volume to deduce continuum properties. This approach replaces many of the phenomenological features of the Bingham plastic and Herschel–Buckley models with first principles, and isolates those few phenomenological features that remain into a single scalar term. The model is compared to the Bingham plastic and Herschel–Buckley models, assessing each model’s ability to capture the experimentally measured mechanical response of a particular MR fluid-based damper to specified magnetic fields. The result of this comparison is that, our model possesses the flexibility to best match the measured behavior of the MR fluid device observed in our experiments, with fewer required experimental measurements.

Lord Corporation has developed glycol-based magnetorheological (MR) fluids for use in applications such as engine mounts and bushings, in which the MR fluid will contact rubber or other oil-sensitive elastomers. Prototype MR fluid engine mounts were designed with either a simple MR valve or an MR valve in combination with an inertia track. The mounts were filled with glycol MR fluids containing either 22% or 36% iron by volume, and the mounts’ dynamic responses were measured. The mounts displayed large changes in dynamic stiffness and damping that were controllable by changing the strength of the magnetic field.

Most current studies of guided-wave-based damage detection have been conducted on thin plate-like structures. This article presents a study of damage identification based on activated ultrasonic waves in a thick steel beam. The diagnosis procedure, with key parameters such as excitation frequency and cycle number of the diagnostic waveform, is elaborated in relation to beam dimension as well as pulse-echo/pitch-catch configurations of PZT active sensors attached to the beam. Finite element simulation was conducted to characterize wave propagation in the beam, and the signals of wave propagation were experimentally measured; the results show good agreement with outcomes of the simulation. To aid damage identification, the group velocity of the guided wave was calculated using the envelope of the signal, which was obtained by Hilbert transform. The results for damage location and severity assessment demonstrate that the guided-wave-based damage identification approach can also be applied to certain thick structures for the purpose of structural health monitoring.

Cable-supported bridges rely on the use of steel cables to support the bridge deck and load on it. Cable tension forces are monitored during construction to assist the alignment of cables and to ensure no cables are overloaded. Given that the cables are critical load carrying elements, it is prudent to routinely monitor the levels of cable tensions during operation. With current measurement methods being costly and labor-intensive, this article proposes an automated and low-cost wireless sensor system for continuous monitoring of the cable tension based on the vibration signature of the cable. A vibration-based tension force estimation method using a peak picking algorithm is explored by embedding it in the computational core of a wireless sensor. Welch’s method to average Fourier spectra from the segments of a long time history signal is employed to remove the non-stationarity of a short-duration acceleration record, which is a limit of the memory-constrained wireless sensor. A series of laboratory tests are conducted on a slender braided steel cable with a variety of cable sags and tension forces. Excellent agreements have been found between the actual tensions and those estimated by the present wireless system.

Based on the new conception of natural frequency vector (NFV) and natural frequency vector assurance criterion (NFVAC), a new damage detection method using the natural frequencies as damage feature is described in detail. For a specific structure, a series of damage states can be simulated based on the finite element model of the intact structure. Then, the NFVs of these damaged structures can be calculated and used to build the damage feature database of the structure. At last, the NFVAC, which is defined by the relationship of NFVs in the damage feature database and the NFV measured on the actual structure are used as damage index. An 8-story shear frame model is adopted as an example to verify the feasibility and validity of the proposed method. Both numerical simulations and model experiments for damage detection are performed, the results of damage detection for several damage cases demonstrate that using NFVAC as damage index, the damage location can be identified correctly and the damage extents can be estimated as well.

This work describes damage detection in a foam core composite wing (1320 mm x 152.4 mm x 13.4 mm) following a series of low energy impacts. Thirteen impacts (6–8 J deposited energy) were applied at adjacent locations approximately 1/4 of the way out from the wing center. Following every one or two impacts, the wing was tested using static tip deflection and dynamic vibrational excitation. Static and dynamic strains were measured using eight fiber Bragg grating sensors. Dynamic acceleration was also monitored using three conventional accelerometers. The estimated bicoherence was used to detect the presence of damage-induced non-linearity in time-series data recorded from each sensor. Receiver operating characteristic (ROC) curves were constructed for each sensor based on 15 or more dynamic measurements made for each damage case. The ROC curves provide a quantitative, statistical approach to evaluating the damage detection capabilities of the various sensors.

An experimental study was conducted to sense interlaminar delamination in carbon-fiber composites utilizing inherent material piezoresistivity. Mode I and II interlaminar fracture tests were carried out on double cantilever beam and end-notched-flexure specimens following ASTM standards. The traditional DC-sourcing two-point probe technique was employed to measure the through-thickness electrical resistance change. For comparison, optical marker method and acoustic emission technique were also applied to detect interlaminar crack growth. The investigation demonstrates the application potential of the self-sensing capabilities of carbon-fiber composites for structural health monitoring.

A modally selective, variable-length anisotropic piezocomposite transducer is designed for guided wave (GW) structural health monitoring applications. The transducer dimensions needed to maximize individual modes are selected based on 3D elasticity models for GW excitation by finite dimensional transducers. This theory is used to determine these transducer dimensions as a function of the wave phase velocity, and normalized by the substrate thickness. The design and fabrication of the transducer are subsequently described, and a set of experimental tests is conducted in pristine isotropic structures to characterize the actuation and sensing performance of the device. It is shown that the transducer dimensions can be tailored to obtain specific symmetric to antisymmetric mode transmission and sensing ratios.

This article introduces some of the experimental and analytical work in damage detection applied to polymeric composite T-joints used in maritime structures. Two methods of damage detection are discussed – A statistics-based outlier technique and one using artificial neural networks (ANNs). The SHM using ANNs system was found to be capable of not only detecting the presence of multiple delaminations in a composite structure, but also capable of determining the location and extent of all the delaminations present in the T-joint structure, regardless of the load (angle and magnitude) acting on the structure. The system developed, relies on the examination of the strain distribution of the structure under operational loading. Finally, on testing the SHM system developed with strain signatures of composite T-joint structures, subjected to variable loading, embedded with all possible damage configurations (including multiple damage scenarios), an overall damage (location and extent) prediction accuracy of 88.32% was achieved. These results are presented and discussed in detail in this article.

We propose a novel approach for optimal actuator and sensor placement for active sensing-based structural health monitoring. Of particular interest is the optimization of actuator–sensor arrays making use of ultrasonic wave propagation for detecting damage in thin plate-like structures. Using a detection theory framework, we establish the optimum configuration as the one that minimizes Bayes risk. The detector incorporates a statistical model of the active sensing process that accounts for both reflection and attenuation features, implements pulse-echo and pitch-catch actuation schemes, and takes into account line-of-site. The optimization space was searched using a genetic algorithm with a time-varying mutation rate. For verification, we densely instrumented a concave-shaped plate and applied artificial, reversible damage to a large number of randomly generated locations, acquiring active sensing data for each location. We then used the algorithm to predict optimal subsets of the dense array. The predicted optimal arrangements proved to be among the top performers when compared to large sets of randomly generated arrangements.

An inverse analysis based on the artificial neural network technique is introduced for effective identification of crack damage in aluminum plates. The concepts of digital damage fingerprints and damage parameter database, which are prerequisites for neural network developing and training, are presented. Parameterized modeling for finite element analysis and an information mapping approach are applied to constitute the damage parameter database cost-effectively. The generalization performance of the neural network is examined by a process of ‘leave-one-out’ cross-validation and diverse factors are discussed, based on which the optimization of the neural network architecture is evaluated. The capability of this inverse approach is assessed by two crack cases from experiments, with good accuracy obtained in damage parameters (central position, size, and orientation).