Abstract Carbon nanotubes (CNT) represent an effective filler to be incorporated into polymer matrices. Their physical properties allow them to exert a remarkable strengthening effect, while their nano-scale leaves the polymer weight unaltered. Exploiting their high strength-to-weight ratio, CNT/polymer nanocomposites appear to be the ideal materials to be shaped as wires and fibers. In this work, an ad-hoc innovative extrusion process is proposed to fabricate though and ultralong CNT/polymer nanocomposite wires. The process parameters are finely tuned to produce nanocomposite filaments exhibiting optimized mechanical properties. Optical analyses validate the morphological features of the fabricated filaments having an averaged diameter of 350 μm. Monotonic tensile tests are carried out to investigate the mechanical response of wires with CNTs content ranging from 1 wt% to 3 wt%. Young’s modulus and tensile strength registered increments of 47% and 43%, respectively, when comparing the 3 wt% CNT nanocomposite wires with the neat polymer wires. Finally, cyclic tensile tests are employed to investigate the change in damping capacity that accompanies the integration of CNTs into the polymer matrix. Such optimized CNTs nanocomposite wires can be easily integrated into several devices or assembled into ropes and yarns with multifunctional, improved properties.

Abstract Synchronized switch damping (SSD) technique utilizing piezoelectric elements is one of good methods of vibration control. This work develops numerical tools for dynamic analysis of structures with SSD dampers, and conducts experiments to verify the effectiveness. A cantilevered beam bonded with piezoelectric patches is considered as the host structure. Two types of SSD circuits are considered, one with no subsequent electric element (SSDS) and another with inductance (SSDI). Firstly, a lumped parameter electromechanical coupling model is employed, with parameters determined experimentally. Then, the frequency response curves of the nonlinear vibration systems are analyzed by the multi-harmonic balance method combined with alternating frequency-time techniques (MHBM/AFT). In order to verify the proposed method, an experimental study is performed. In the experiment SSD circuit is realized by an enhanced analog circuit which is more complex but also more stable than the original SSD circuits. The measured results are compared with those obtained by proposed numerical tools with good agreements. It is also shown that the modal frequencies and modal shapes of SSD systems are almost unchanged with the vibration amplitudes, which indicates that the nonlinear force generated by SSD has little influence on the characteristics of linear structure. It is verified both numerically and experimentally that SSDI damper can produce significant damping for multiple modes.

A new passive damper coupling the energy dissipative mechanisms of dry friction and piezoelectric shunting circuit is proposed. The idea is to embed the shunted piezoelectric materials to the dry friction dampers at appropriate positions, so that the elastic deformation of the dry friction dampers can be utilized to generate additional damping. Moreover, this provides a more practical way to install the piezoelectric dampers into realistic mechanical systems such as aero-engines. A five Degree-of-freedom (DOFs) lumped system model is introduced to demonstrate the feasibility of such an idea. The damping performance is revealed using the forced response results obtained by the Multi Harmonic Balance Method (MHBM). We show that the coupled damper significantly outperforms the standalone piezoelectric or dry friction dampers. The coupled damper is better than, at least equivalent to, the case where both piezoelectric and dry friction dampers are applied but in uncoupled manner. Eventually, the mechanism of the proposed damper is further explained from the perspective of vibrational mode and energy conversion.

Future aircraft wing technology is rapidly moving toward flexible and morphing wing concepts capable to enhance aircraft wing performance in off-design conditions and to reduce operative maneuver and gust loads. However, due to the reduced stiffness, increased mass, and increased degree of freedom (DOF), such mechanical systems require advanced aeroelastic assessments since the early design phases; this appears crucial to properly drive the design of the underlying mechanisms since the conceptual phase by mitigating their impact on the whole aircraft aeroelastic stability. Preliminary investigations have shown that the combined use of adaptive flap tabs and morphing winglets significantly improves aircraft aerodynamic performance in climb and cruise conditions by the order of 6%. Additionally, by adapting span-wise lift distributions to reduce gust solicitations and alleviate wing root bending moment at critical flight conditions, significant weight savings can also be achieved. Within the scope of Clean Sky 2 Airgreen 2 project, flutter and divergence characteristics of a morphing wing design integrating adaptive winglets and flap tabs are discussed. Multi-parametric flutter analyses are carried out in compliance with CS-25 airworthiness requirements (paragraph 25.629, parts (a), (b), (c) and (d)) to investigate static and dynamic aeroelastic stability behavior of the aircraft. The proposed kinematic systems are characterized by movable surfaces, each with its own domain authority, sustained by a structural skeleton and completely integrated with EMA-based actuation systems. For that purpose, a sensitivity analysis was performed taking into account variations of the stiffness and inertial properties of the referred architectures. Such layouts were reduced to a stick-equivalent model which properties were evaluated through MSC-NASTRAN-based computations. The proprietary code SANDY 4.0 was used to generate the aero-structural model and to solve the aeroelastic stability equations by means of theoretical modes association in frequency domain. Analyses showed the presence of critical modal coupling mechanisms in nominal operative conditions as well as in case of system malfunctioning or failure. Design solutions to assure clearance from instabilities were then investigated. Trade-off flutter and divergence analyses were finally carried out to assess the robustness of the morphing architectures in terms of movable parts layout, mass balancing and actuators damping.

In the present article, we focus on the forced vibration and control analysis of functionally graded (FG) graphene-polymer composites bonded with piezoelectric layers considering strong electric fields. Different non-uniform gradient distributions of graphene platelets (GPLs) are assumed through the thickness direction. The Modified Halpin-Tsai micromechanics model is used to obtain the effective material properties of GPL/polymer composites. Electromechanical coupling of piezoelectric layers is described by two rotationally invariant non-linear constitutive relations. A four-node shell element considering transverse shear effect based on the Reissner-Mindlins hypothesis has been developed for forced vibration and control analysis of smart FG-GPL/composites using the principle of virtual work considering nonlinear material law for the piezoelectric layers. The developed element is verified and compared with the numerical results those available in the literature. Different configurations of FG-GPL composite shells have been analysed and discussed to compare in terms of settling time, first resonance frequency and absolute amplitude corresponding to first resonant frequency by carrying out time and frequency response analysis, and the effects of weight fraction of GPLs on vibration response of such shell structures are also discussed. The influence of electromechanical nonlinear constitutive relations is also presented and discussed by performing active control analysis on different FG-GPL composite shell structures. Moreover, the results show that the GPL distribution and weight-fraction of GPLs have a significant effect on the vibration and damping characteristics of the FG-GPL composite shell structures.

Magnetorheological fluid is a special smart fluid which can show different rheological properties under different magnetic flux densities due to its magnetically sensitive structure. This makes the fluid able to be manipulated and semi-actively controlled for various applications such as dampers, clutches and brakes. To provide an effective damping it is necessary to create an appropriate control algorithm. In order to design a structure with magnetorheological fluid and to get an idea for a control approach, the physics of the fluid motion has to be modelled. Computational Fluid Dynamics is an effective tool to model any fluid behaviour or any fluid involved structure. For magnetorheological devices, despite number of numerical models available in the literature, a befitting model is not yet presented. In this study a mapped rheological model is proposed and used in a magnetorheological damper simulation. The results are compared with other models and experimental data. It is shown that the new mapped model is effective and better than old approaches. It also showed a good agreement with the experimental data.

Impulsive energy provides a promising source for energy harvesting techniques due to their high amplitude and abundance in a living environment. The sensitivity to excitation of bistable energy harvesters makes them feasible for impulsive-type events. In this paper, a novel impulsively-excited bistable energy harvester with rotary structure and plectrum is proposed to achieve plucking-based frequency up-conversion. The input excitation is converted to plucking force on the bistable energy harvester, so as to help it go into the high-energy orbit. The piezoelectric and electromagnetic transduction mechanisms are combined by incorporating a coil to the structure in order to overcome the increase of damping introduced by the bistable configuration. As a result, high-energy output and broadband performance could be realized. Impact mechanics is employed to develop a comprehensive model, which could be used to analyze the nonlinear dynamics and predict the system responses under various plucking velocities and overlap lengths. Numerical simulation shows that the bistable energy harvester could experience large-amplitude oscillation under impulsive excitation and the hybrid configuration outperforms the standalone ones under high damping ratio and low coupling coefficient. The proposed design is targeted to be applied on the turnstile gates of the subway station. Less human effort would be needed when passengers pass the turnstile gate due to the snap-through motion of bistability.

Knitted Textiles made from Nickel-Titanium (NiTi) shape memory alloy wires are a new structural element with enhanced properties for a variety of applications. Potential advantages of this structural form include enhanced bending flexibility, tailorable in-plane, and through-thickness mechanical performance, and energy absorption and damping. Inspection of the knit pattern reveals a repeating cell structure of interlocking loops. Because of this repeating structure, knits can be evaluated as cellular structures that leverage their loop-based architecture for mechanical robustness and flexibility. The flexibility and robustness of the structure can be further enhanced by manufacturing with superelastic NiTi. The stiffness of superelastic NiTi, however, makes traditional knit manufacturing techniques inadequate, so knit manufacturing in this research is aided by shape setting the superelastic wire to a predefined pattern mimicking the natural curve of a strand within a knit fabric. This predefined shape-set geometry determines the outcome of the knit’s mechanical performance and tunes the mechanical properties. In this research, the impact of the shape setting process on the material itself is explored through axial loading tests to quantify the effect that heat treatment has on a knit sample. A means of continuously shape setting and feeding the wire into traditional knitting machines is described. These processes lend themselves to mass production and build upon previous textile manufacturing technologies. This research also proposes an empirical exploration of superelastic NiTi knit mechanical performance and several new techniques for manufacturing such knits with adjustable knit parameters. Displacement-controlled axial loading tests in the vertical (wale) direction determined the recoverability of each knit sample in the research and were iteratively increased until failure resulted. Knit samples showed recoverable axial strains of 65–140%, which could be moderately altered based on knit pattern and loop parameters. Furthermore, this research demonstrates that improving the density of the knit increases the stiffness of the knit without any loss in recoverable strains. These results highlight the potential of this unique structural architecture that could be used to design fabrics with adjustable mechanical properties, expanding the design space for aerospace structures, medical devices, and consumer products.

This article illustrates an approach to develop innovative smart materials based on carbon fiber composites. The proposed approach relies on the use of ultra-light strain sensors that are embedded into the composite and are adopted to monitor in real-time the actual material configuration. Such sensors are composed of electrospun PVDF fibers that exploit piezoelectricity to identify strain and thanks to their extreme lightweight can easily be embedded within the composite layers without affecting the structural integrity. On the other hand, the composite is equipped with a system of internal distributed heaters that can locally and globally vary the composite temperature. Since the adopted epoxy has a considerable temperature-dependent behaviour, it is possible to control its stiffness and thus to control the structural frequencies and damping. By coupling the sensing system with the control system, the structural properties are tuned to match prescribed working conditions, thus optimizing the performance of the proposed smart system. The proposed approach is investigated experimentally by manufacturing prototypes of the smart composite and by performing multiple tests to study the material response and evaluate the obtained performance.

The reduction of low-frequency noise transmission through thin-walled structures is a topic of research for many years now. Due to large wavelengths and the mass law, passive solutions usually gain low performance in the frequency range below 500 Hz. Active systems promised to fill the gap and to achieve significant reductions of transmitted sound. Nevertheless, experiments showed the outstanding performance of such specialized systems, but also demonstrated the computational and hardware effort of such solutions. The upcoming additive manufacturing technology enabled new multi-material designs of complex structures. Based on this technology, acoustic metamaterials emerged in the laboratories and in literature. Arrays of miniaturized locally resonant structures are able to change the noise transmission of thin walled structures beyond the limits of the given mass law in certain frequency bands. For future aircraft contra-rotating open rotor (CROR) engines are a promising technology to reduce their CO 2 footprint. Since the contribution of CROR engines to the cabin noise is higher than for jet engines, new strategies for the reduction of noise transmissions for frequency bands below 200 Hz are necessary. For the tonal noise of the CROR engines, acoustic metamaterials seem to be an appropriate solution. In this paper a 110 × 110 × 1 mm 3 thin-walled sample plate is presented. It is covered with a 5 × 5 array of multi-material resonant structures, which are printed as mass on a beam. The rubber-like beam material combines a low Young’s modulus with a high material damping, leading to a low eigenfrequency of the resonators. The design of the resonators using simulations and experimental data is shown. To explore the potential of the design, an acoustic test box is manufactured. Starting with all resonators unblocked the emitted sound intensity of the plate is measured. Sequential blocking of selected resonators proves the concept. Additional laser scanning vibrometer measurements give insights into the vibration behavior of single resonators.

In this work, the arising of stick-slip dissipation as well as the global mechanical response of carbon nanotube (CNT) nanocomposite films are tailored by exploiting a three-phase nanocomposite. The three phases are represented by the CNTs, a polymer coating localized on the CNTs surface and a hosting matrix. In particular, a polystyrene (PS) layer coats multi-walled carbon nanotubes (MWNTs) that are randomly dispersed in a polyimide (PI) matrix. The coating phase is strongly bonded to the CNTs outer sidewalls ensuring the effectiveness of the load transfer mechanism and reducing the material damping capacity. The coating phase can be thermally-activated to modify, and in particular, decrease the CNT-matrix interfacial shear strength (ISS) thus facilitating the stick-slip onset in the nanocomposite. The ISS decrease finds its roots in a partial degradation of the coating phase and, in particular, in the formation of voids. By weakening the CNT/polymer interfacial region, a significant enhancement in the material damping capacity is observed. An extensive experimental campaign consisting of monotonic and cyclic tensile tests proved the effectiveness of this novel multi-phase material design.

In the last years, researchers have presented concentrated and distributed parameter models of electromechanically coupled systems, leading to appropriate estimation of their electroelastic behavior. Equivalent electrical circuits have also been investigated and provide useful simulation tools to investigate the system behavior as well as to developed new energy harvesting or control circuits. In general, RLC (resistor, inductor and capacitor) circuits represent, respectively, the mass, damping and stiffness of single or multi-degree-of-freedom electromechanically coupled systems. In practice, however, the equivalent electrical representation of high-quality-factor systems demand equivalent circuits with extremely low internal resistance values. Furthermore, the assumption of an ideal transformer cannot be obtained in practice. This work presents a novel equivalent electrical circuit for linear and nonlinear electromechanically coupled systems. The effects of inductance, capacitance and electromechanical coupling are represented through operational-amplifier based sub-circuits of extremely low internal resistance. First, the linear behavior of a mass-spring-damping system is verified. Later, the behavior of a nonlinear electromechanically coupled system is investigated. In both cases, numerical results (Matlab-Simulink simulations) and experimental results (from breadboard implementations) will be verified against experimental results presented in the literature.

In this study, 3D printed magnetorheological (MR) elastomer has been characterized through a force vibration testing. The 3D printed MR elastomer is a composite consisting three different materials, magnetic particles and two different elastomers. The MR elastomers were printed layer-by-layer by encapsulating MR fluid within the polymeric elastomer and then allowed to cure at room temperature. The 3D printing allowed to print various patterns of magnetic particles within the elastomeric matrix. In the presence of an external magnetic field, both elastic and damping properties of the 3D printed MR elastomers were changed. Natural frequency, stiffness, damping ratio, damping coefficient, and shear modulus were increased with increasing magnetic field. For the single degree-of-freedoms system, shear mode MR elastomers suppressed the transmitted vibration amplitude up to 31.4% when the magnetic field was 550 mT. The results showed that the 3D printed MR elastomer could be used as a tunable spring element for vibration absorption or isolation applications. However, further optimization of the magnetic particles’ configurations should be performed to obtain the higher MR effect.

Magneto-rheological fluids (MRF) are commonly applied in MRF brakes and vibration damping. The apparent viscosity dependence with respect to the magnetic field has been addressed in detail in the state of the art. The aim of this paper is to experimentally study the vibration effects on the particle chain-like structures and, as a consequence, the shear stress variation applied to the fluid. Three vibration configurations have been applied to a ferromagnetic cylinder rotating between two magnetic poles filled with MRF a “Z-vibration” where the generated displacement is along the rotation axis of the shearing cylinder, a “θ-vibration”, tangential to the cylinder, and an “R-vibration”, normal to the cylinder surface. First we focus on the vibration mode characterisation in free air, and then when plunged in the fluid. In a second step, we measure the reactive torque generated on the clutch under different magnetic field intensities with different rotation speeds and vibration amplitudes. It appears that the “R-vibration” configuration is providing the most influence, up to 20% of torque reduction observed at moderate B field. The “Z-vibration” and the “θ-vibration” configurations respectively have less influence on the torque, nevertheless vibrations always tend to decrease the corresponding yield stress in the MRF.

This study is at attempt to explore the nonlinear behavior of bistable composite laminates for vibration energy harvesting. Asymmetric four-ply [0/90/0/90] carbon-fiber plate with two cylindrical stable equilibria supported at its center and free at all boundaries is used for the experimental testing. Macro-fiber composite (MFC) patches are attached to the plate to transform the mechanical vibration energy into electrical energy. The mechanical bistable property of the plate makes it possible to snap from one stable equilibrium state to the other. This snapthrough motion is highly nonlinear and associated with large-amplitude vibrations. The experimental tests aim at exploiting the nonlinearity due to the snapthrough motion to enhance the energy extraction. First, the resonant frequencies and damping of the plate are identified. A primary-resonance excitations of the first mode are carried out using two schemes: amplitude sweep and frequency sweep. In the first case, amplitude sweep, the excitation frequency is kept fixed at the resonant frequency and the amplitude of excitation is increased. The time history and FFT of the response as well as the output voltage are measured and reported. In the second case, frequency sweep, the excitation frequency is varied around the resonant frequency while the excitation amplitude is kept fixed. In both cases, the response shows a small-amplitude single-well vibrations at low excitation amplitudes and chaotic and periodic snapthrough motion as the amplitude and frequency of excitation are varied. The snapthrough motion has been found to greatly enhance the energy extraction capability. This study can serve as a motive for more testing and modeling efforts in order to understand the complex nonlinear behavior of bistable composite laminates and exploit it for vibration energy harvesting.

This paper presents a simulation-based study to investigate the damping properties of a novel piezocomposite, consisting of piezoelectric fiber and epoxy reinforced with randomly orientated double walled carbon nanotubes (DWCNT), termed as piezoelectric fiber nano reinforced composite (PFNRC). Authors have observed that the past research dealt with the effect of aligned single walled carbon nanotubes (CNT) on active damping of piezoelectric composite in extension mode (e 13 and e 33 ). It is known from the past research that DWCNT inclusions improve the passive damping of a composite. Therefore, the authors use DWCNT inclusions to study the active-passive damping of the piezoelectric composite, in this article. The random orientation of the DWCNT is considered to replicate the physical composite as it known that aligning CNTs in a single direction is not feasible due to fabrication constraints. A multistep homogenization method involving Method of Cells (MOC) is employed to obtain effective properties of PFNRC. A modified 3D-MOC is used to obtain the effective properties of epoxy matrix with DWCNT inclusions (DWCNT-epoxy), considering the effect of nano particle agglomeration. A 2D-MOC is then implemented with long fiber PZT as the active material and DWCNT-epoxy as the matrix. This procedure is followed for computing the effective material properties of extension (e 33 ) as well as shear (e 15 ) mode of PFNRC, when DWCNT inclusions are added into the epoxy matrix at different weight percentages. The constitutive equations are derived with the help of Maple and simulated in MATLAB. These results are used to compare the active-passive damping performance of the composites using a single degree of freedom damping model, employing Newmark’s numerical integration method. The active damping performance of the composites is evaluated by varying the displacement and velocity gains in a negative feedback system. The main focus of the study is to find the most efficient operating mode of the proposed composite for damping of structural vibrations.

Limitations on neuron firing rates restrict the frequency bandwidth of many biological sensory systems. The cochlea overcomes these limitations to hear high frequency sounds through its tonotopic structure and non-synchronous sampling. The cochlea’s tapering basilar membrane serves as a filter bank decomposing an applied sound into its frequency components. Auditory hair cells produce neural impulses at the peaks in the local basilar membrane oscillations resulting in an event-driven, sub-Nyquist rate sampling strategy. These two effects extend the human hearing range to about 20 kHz despite maximum neuron firing rates of just a few hundred hertz. Inspired by this, this paper presents a concept demonstration of an accelerometer and signal compression strategy for high-rate impact events using a similar filter bank approach. A series of clamped-clamped beams will serve as analog analysis filters much like the cochlea’s basilar membrane. This paper focuses on the design and simulation of such a beam array and how the natural frequencies and damping ratios of the beams’ first modes affect measuring a broadband impact excitation.

Variable performance characteristics in a multifunctional structure may be achieved by identifying suitable material candidates, and spatially varying, or grading, their material properties along the structures. Additive manufacturing (e.g. 3D printing) offers various possibilities to fabricate/manufacture such graded structures. The material properties of multifunctional composite structures, such as beams or plates, are often graded along their thickness (laminate/sandwich) or distributed in a material matrix (fibers/nanoparticles). In recent years, it has been demonstrated that by tailoring the materials in other directions (axially/radially), superior mechanical behavior and structural stability can be realized. In this research, the modeling and analyses of axially graded polymeric beams to maximize their vibration performance for a large bandwidth of frequencies and damping is presented. Polymeric materials have frequency and temperature dependent viscoelastic properties (complex modulus, glass transition temperature etc.) which can be leveraged for different applications. The goal is to spatially combine these materials such that desired longitudinal vibration characteristics (natural frequencies, damping and modes) can be achieved. To this end, the modeling for the free and forced vibration of beams with spatially varying properties, is carried out by a piecewise uniform continuous model. The spectral characteristics (natural frequency, damping ratios, and frequency response functions) of the axially graded beams are computed by solving associated transcendental eigenvalues problems. The parametric studies included the grading of polymers which are regularly used for additive manufacturing, such as ABS, PLA, etc. These results demonstrate that the response of the system can be manipulated by axial grading and optimal design/fabrication (3D printing) of multifunctional smart structures may be developed for vibration control applications.

The present study concerns with the performance of a skid landing gear (SLG) system of a rotorcraft impacting the ground at a vertical sink rate of 5.0 m/s. The impact attitude is per chapter 527 of the Airworthiness Manual (AWM) of Transport Canada Civil Aviation and FAR Part 27 of the U.S. Federal Aviation Regulation. A single degree of freedom helicopter model is investigated under two rotor lift factors 0.67 and 1.0. Three Configurations are evaluated: a) A conventional SLG; b) SLG equipped with a passive viscous damper and c) SLG incorporated with a magnetorheological energy absorber. The non-dimensional solutions of the helicopter model show that the passive damper system could reduce the maximum acceleration experienced by the helicopter occupants by 21% and 19.8% in comparison to the undamped system for the above rotor lift factors, respectively. However, the passive damper fails to constrain the non-dimensional energy absorption stroke of the damper within the given 18 cm maximum stroke and a bottoming out of the damper piston was noticed. Therefore, the alternative and successful choice was to employ a magnetorheological energy absorber (MREA). To improve the MREA controllability and to resettle the payload with no oscillations, i.e. in one cycle, two different Bingham numbers for compression stroke and rebound stroke were defined in the non-dimensional solution. Several simulations were conducted for different values of Bingham numbers. Among these numerical simulation results, the solution that implemented the optimum Bingham numbers was found to be the only one feasible solution. In this case the MREA with optimum Bingham number for compression could utilize the full energy absorption stroke to attain soft landing. In the rebound stroke, the generated optimal on-state damping force successfully controls the bounce of the payload until the payload settles down to its original equilibrium position with no oscillations.

Large loads due to fluid-structure interaction can lead to high bending stresses and fatigue failure in wings and wind turbine blades. A solution for the mentioned problem is using a bistable composite laminate for load alleviation. A bistable composite laminate is capable of attaining two statically stable shapes, and it can be designed to alleviate a critical load, such as wind gust, by snapping from one stable position to the other. Piezocomposite actuators can be used to reverse the snap-through and bring back the structure to its original optimal aerodynamic shape, after the gust load is alleviated. However, there will always be a limit on the size of the piezocomposite actuator used; hence, severe force and energy constraints exist to achieve the snap-through. In this context, this paper focuses on the minimum required actuation energy for performing snap-through of a bistable structure. The paper shows how the required energy for cross-well transfer varies as a function of damping ratio and frequency ratio at specific harmonic force amplitude when the system is externally disturbed with a band-limited noise signal. A band-limited noise signal is chosen to model external/ambient disturbances. This paper uses the Duffing-Holmes equation as a one-degree-of-freedom representative model of a bistable structure. This equation is numerically solved to calculate the required energy for cross-well oscillation under different system and forcing conditions. Various non-dimensional parameters are used to highlight interesting phenomena. It is found that the domain of low energy regions decreases by increasing the level of noise.