This paper presents a FEM model for the simulation of delamination in composite materials under fatigue loads using extended cohesive interface elements. A virtual fatigue damage variable is proposed to continually locate the crack tip elements with local element information only. The proposed method allows for a fully automatic identification of crack tip elements without any global information. Once crack tip elements are identified, the fatigue damage accumulation is determined by a modified Paris-law with a strain energy release rate correction method to reduce mesh sensitivity. The predicted delamination growth rates of different mesh sizes were compared with experimental results from open literature using a mode-I Double Cantilever Beam specimen and a mode-II 4-point End Notch Flexure specimen respectively. In all cases good agreement was obtained with weak mesh dependency.

Reconstruction of the nine stiffness coefficients of two different composites was implemented by inverting the measured Lamb waves phase velocities. The inversion was carried out through genetic algorithms (GAs) by minimizing the error between the calculated and the measured velocities. Forward problem of calculating phase velocities for a material properties individual in GAs was solved using a third-order shear deformation theory (ToSDT). Lamb wave propagation was visualized with a laser generation based imaging (LGBI) system which uses a laser source to generate ultrasonic waves. The experimental phase velocities were evaluated directly based on the position-time diagrams of Lamb waves. The nine stiffness coefficients were reconstructed using phase velocities of S0 mode and A0 mode measured along only three propagation directions. The reconstructed material properties were verified by comparing their corresponding theoretical velocities with the experimentally measured velocities.

Composites comprising TiO2@MWCNTs (shell@core) nanoparticles and poly(vinylidene fluoride) (PVDF) were prepared through a two-step method, including solution cast and mechanical rolling, inducing a highly oriented structure with both PVDF lamellar and TiO2@MWCNTs. A systematic study was performed to investigate the crystalline structure and piezoelectric properties of composites. The obtained results reveal that greatly enhanced breakdown strength and piezoelectric properties can be achieved in composites by the incorporation of highly aligned TiO2@MWCNTs. A maximum piezoelectric coefficient d33 of ∼41 pC/N can be achieved in composites at a loading level of 0.3wt.%, which is nearly double that of the pure PVDF.

0.55BaxSr1−xTiO3–0.45MgO composite ceramics (BST–MgO, x = 0.5–0.65) co-modified with CuO and MnO2 were prepared by conventional soild state reaction method. The microstructure, phase and morphology were investigated. The results showed the formation of the perovskite BST phase and MgO phase. The dielectric constant (εr) and tunability (T) of the ceramic samples increased with increasing Ba content. The dielectric loss (tan δ) and figure of merit achieved their minimum and maximum at x = 0.6, respectively. The obtained ceramic sample with x = 0.6 exhibited favorable properties, which are listed as follows: εr = 190, tan δ = 0.22 %, T = 16 %, Ec = 5.6 kV/cm and FOM = 72.7.

Dielectric elastomers have great potentials as flexible actuators in micro-electromechanical systems (MEMS) due to their large deformation, light weight, mechanical compliancy, and low cost. The low dielectric constant of these elastomers requires a rather high voltage electric field, which has greatly limited their applications. In this work, a diaphragm-type flexible microactuator comprising a hyperbranched aromatic polyamide functionalized graphene (HAPFG) filler embedded into the polyurethane (PU) dielectric elastomer matrix is described. The rational designed HAPFG sheets exhibits uniform dispersion in PU matrix and strong adhesion with the matrix by hydrogen-bond coupling. Consequently, the HAPFG–PU composites possess high dielectric performance and low loss modulus. The effect of hyperbranched aromatic polyamide functionalized graphene on high voltage electric field induced strain was experimentally investigated using the Fotonic sensor. The high electric field response of the composite was discussed by applying different kinds of alternating-current field. In addition, a comparison of the breakdown strength between the HAPFG–PU composite and the pure PU was carried out.

Large deformation of soft materials is harnessed to provide functions in the nascent field of soft machines. In this work, PEDOT:PSS noncovalent functionalized graphene–polyurethane dielectric elastomer composites (PEDOT:PSS–RGO@PU) have been synthesized as promising candidate materials for micro-actuator electromechanical applications. PEDOT:PSS conducting polymer chains not only reinforce the interaction between the polyurethane matrix and graphene sheets, but also prevent graphene sheets from aggregating and connecting, which are beneficial to forming microcapacitors in the matrix and suppressing the leakage current. The PEDOT:PSS–RGO@PU composite exhibits ultra high permittivity (350 at 1 kHz), low dielectric loss (∼0.2 at 1 kHz), low loss modulus (200 MPa), and low loss tangent (∼0.4), all being essential to create a high performance electric-induced strain material. The maximum thickness strain of 164% is significantly higher than reported values for polyurethane elastomers and nanocomposites.

•Functionalized graphene was prepared by using atom transfer radical polymerization.•Embedding of functionalized graphene to PU increased the electromechanical property.•3D electric field induced strain of dielectric elastomer film surface was measured.Poly(methyl methacrylate)-functionalized graphene/polyurethane (MG–PU) dielectric elastomer composites were synthesized. MG was prepared by atom transfer radical polymerization (ATRP) and used as conducting filler. The microstructure of MG and MG–PU was characterized by SEM, TEM and XRD. The thermal degradation, mechanical, dielectric and electromechanical properties were investigated. The three-dimensional (3D) electric field induced displacement and strain of dielectric elastomer surface was observed and measured for the first time. Comparing with pure PU elastomer, the dielectric properties and electric field induced strain performance of MG–PU dielectric elastomer composites were significantly improved. The 1.50 wt% MG–PU film had high relative permittivity and electric field induced strain of 28.21 and 32.8%, respectively.We report the novel poly(methyl methacrylate)-functionalized graphene/polyurethane dielectric elastomer composites with superior electric field induced strain. The three-dimensional (3D) electric field induced displacement and strain of dielectric elastomer film surface was observed and measured for the first time.

Fe3O4-reduced graphene oxide-polyaniline (Fe3O4–RGO–PANI) ternary electromagnetic wave absorbing materials were prepared by in situ polymerization of aniline monomer on the surface of Fe3O4–RGO nanocomposites. The morphology, structure and other physical properties of the nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, vibration sample magnetism, etc. The electromagnetic wave absorbing properties of composite materials were measured by using a vector network analyzer. The PANI–Fe3O4–RGO nanocomposites demonstrated that the maximum reflection loss was −36.5 dB at 7.4 GHz with a thickness of 4.5 mm and the absorption bandwidth with the reflection loss below −10 dB was up to 12.0 GHz with a thickness in the range of 2.5–5.0 mm, suggesting that the microwave absorption properties and the absorption bandwidth were greatly enhanced by coating with polyaniline (PANI). The strong absorption characteristics of PANI–Fe3O4–RGO ternary composites indicated their potential application as the electromagnetic wave absorbing material.

Poly(vinylidene fluoride) (PVDF) films containing multiwalled carbon nanotubes (MWCNTs) were prepared by solution casting method, followed by a rolling process. With the presence of MWCNTs (up to 0.3 wt%), the influence of rolling process on the crystal structure and the electrical properties of PVDF was studied. It is shown that rolling can induce phase transition as well as increase in the crystallinity of PVDF, leading to enhancement of ferroelectric and dielectric properties. In addition, MWCNTs may promote the phase transition of PVDF, while impeding the increase of crystallinity during the rolling process. With a higher breakdown field and a lower coercive electric field in comparison with the original films, the rolled films can be efficiently poled to obtain good piezoelectricity. Moreover, with the presence of MWCNTs, electric field-induced changes in crystallinity can be obviously observed in the poled MWCNTs/PVDF films. With the highest crystallinity of β-phase, the rolled films containing 0.075 wt% MWCNTs possess the highest piezoelectric coefficient d33 of ~33 pC/N while that of pure PVDF films is only ~22 pC/N.

0.55Pb(Ni1/3Nb2/3)O3-0.135PbZrO3-0.315PbTiO3 (PNN-PZ-PT) ternary piezoelectric ceramics with excess 1.0 wt.% PbO were synthesized by the conventional solid-state reaction method at 1175–1300 °C for 2 h, respectively. The influence of sintering temperature (Ts) on microstructure, piezoelectric, dielectric, and ferroelectric properties were systematically investigated. The results of XRD and Raman scattering spectra demonstrated that a typical perovskite structure with mainly rhombohedral symmetry near the MPB region were obtained for all the samples. The tetragonal phase content was increased slightly with the increase of sintering temperature. In addition, with increasing Ts the average grain size increases while the density decreases were also found. The results of electrical measurements confirmed that piezoelectric constant, dielectric constant, remnant polarization were firstly increased and then decreased with the increase of sintering temperature. The optimum and remarkable enhanced electrical properties of d33 = 1070 pC/N, kp = 0.69, εr = 8710, tanδ = 0.026, Pr = 24.08 μC/cm2, and Ec = 3.2 kV/cm were obtained for the sample sintered at 1250 °C for 2 h. Meanwhile, the sample exhibits a typical relaxor ferroelectric behavior with the maximum dielectric constant εm =24541 at Curie temperature Tc = 113.3 °C at 1 kHz.

0.55Pb(Ni1/3Nb2/3)O3–0.45Pb(Zr0.3Ti0.7)O3(PNN–PZT) ceramics with different concentration of xFe2O3 doping (where x = 0.0, 0.8, 1.2, 1.6 mol%) were synthesized by the conventional solid state sintering technique. X-ray diffraction analysis reveals that all specimens are a pure perovskite phase without pyrochlore phase. The density and grain size of Fe-doped ceramics tend to increase slightly with increasing concentration of Fe2O3. Comparing with the undoped ceramics, the piezoelectric, ferroelectric and dielectric properties of the Fe-doped PNN–PZT specimens are significantly improved. Properties of the piezoelectric constant as high as d33 ~ 956 pC/N, the electromechanical coupling factor kp ~ 0.74, and the dielectric constant εr ~ 6095 are achieved for the specimen with 1.2 mol% Fe2O3 doping sintered at 1200 °C for 2 h.Highlights► The Fe2O3 doped PNN–PZT piezoelectric ceramics were successfully synthesized. ► The highest d33 value of 956 pC/N was obtained for the Fe-doped PNN–PZT ceramics. ► The proper iron additive is an effective approach to enhance PNN–PZT properties.

Lead-free (K0.4425Na0.52Li0.0375)(Nb0.93−xSb0.07Tax)O3 (abbreviated as KNLNSTx) piezoelectric ceramics with x = 0.045–0.075 have been prepared by an ordinary sintering technique with sintering temperature at 1,120 °C. The results of X-ray diffraction reveal that Ta5+ has diffused into the perovskite lattice to form a solid solution. The grain growth of the ceramics was inhibited by substituting Ta5+ for Nb5+. It has been shown that the substitution of Ta decreases Curie temperature TC and orthorhombic-tetragonal phase transition temperature TO-T. Besides, the dependence of the ceramics with different Ta content on the dielectric, piezoelectric and ferroelectric properties has been investigated. The results indicate that Ta substitution provides “soft” piezoelectric characteristics, owing to improvements in d33, kp and εr and a decease in Qm. For the ceramics with x = 0.06, the piezoelectric coefficient d33 becomes maximum at a value of 270 pC/N, while the other electrical properties remain reasonably high: dielectric constant εr = 1,577, planar mode electromechanical coupling factors kp = 0.4, Curie temperature TC = 252 °C and the remanent polarization Pr = 16.03 μC/cm2. These results show that (K, Na, Li) (Nb, Sb)O3 ceramics with small amount of Ta (x <8 mol%) are good lead-free piezoelectric ceramic.

In a structural system with piezoelectric actuators, the damping effect can be achieved by properly switching the voltage on the actuators. Semi-active synchronized switch damping (SSD) approaches are typical switched-voltage control methods, which have recently been a topic of active research. In this study, the energy conversion of a SSD control system with an arbitrary switching frequency is investigated theoretically and validated numerically. First the general expression of the switched voltage on the piezoelectric actuator is derived. The results show that maximum voltage magnitude is obtained on the piezoelectric actuator when piezoelectric is switched at every odd number of displacement extrema. Next the average converted energy per vibration cycle is derived for an arbitrary switching frequency. The results show that the efficiency of energy conversion is reduced drastically even if the switching frequency is slightly deviated from the optimal frequency. Finally numerical simulation and experiments were carried out to validate the theoretical results.

In this work, microstructure characteristics, dielectric, piezoelectric and ferroelectric properties of lead-free (K0.4425Na0.52Li0.0375)(Nb0.87Ta0.06Sb0.07)O3 (KNLNST) doped with 1 mol% copper oxide (CuO) piezoelectric ceramics prepared by a conventional solid-state reaction route are investigated with an emphasis on the influence of sintering temperature. The introduction of CuO could significantly improve the sinterability of KNLNST ceramics. It is found that the tetragonality of the ceramics increases with raising sintering temperature. A dense microstructure with increased grains is developed, probably due to liquid-phase sintering. Both the piezoelectric constant d33 and planar electromechanical coupling kp increase with increasing relative density and grain size. The Curie temperature TC values increase slightly when the sintering temperature is increased. In addition, the KNLNST ceramics doped with 1 mol% CuO show obvious dielectric relaxor characteristics, and the relaxor behavior of ceramics is strengthened by increasing the sintering temperature. The improved piezoelectric and dielectric properties of d33 = 241 pC/N, kp = 0.437, dielectric loss tanδ = 0.0087, mechanical quality factor Qm = 138, dielectric constant εr = 1,304 can be obtained for specimens sintered at 1,080 °C for 3 h.

K0.5Na0.5NbO3–x ZnO (KNN–xZn) lead-free ceramics have been prepared using the conventional sintering technique and the effects of ZnO addition on the phase structure and piezoelectric properties of the ceramics have been studied. Our results reveal that a small amount of ZnO can improve the density of the ceramics effectively. Because of the high density and ZnO doping effects, the piezoelectric and dielectric properties of the ceramics are improved considerably. The good piezoelectric and dielectric properties of d33 = 114 pC/N, kp = 0.36, εr = 395, and Qm = 68 were obtained for the KNN ceramics doped with 1 mol% ZnO. Therefore, the KNN-1.0 mol%Zn ceramics is a good candidate for lead-free piezoelectric application.

Microstructure characteristics, phase transition, and electrical properties of (K0.4425Na0.52Li0.0375) (Nb0.8925Sb0.07Ta0.0375)O3 (KNLNST) lead-free piezoelectric ceramics prepared by normal sintering were investigated with an emphasis on the influence of sintering temperature. The microstructure and piezoelectric, ferroelectric, and dielectric properties were investigated, with a special emphasis on the influence of sintering temperature from 1,100 to 1,140 °C. Orthorhombic phases mainly exist in the ceramics sintered at 1,100–1,130 °C, whereas the tetragonal phase becomes dominant when sintering temperature is above 1,130 °C. Because of the existence of MPB-like transitional behavior, the piezoelectric coefficient (d33), electromechanical coupling coefficient (kp), and dielectric constant (ε) show peak values of 304pC/N, 0.48, and 1,909, respectively, which are obtained in the sample sintered at 1,120 °C, and its Curie temperature (TC) is about 271 °C.

Piezoelectric materials have become the most attractive functional materials for sensors and actuators in smart structures because they can directly convert mechanical energy to electrical energy and vise versa. They have excellent electromechanical coupling characteristics and excellent frequency response. In this article, some research activities on the applications of piezoelectric materials in smart structures, including semi-active vibration control based on synchronized switch damping using negative capacitance, energy harvesting using new electronic interfaces, structural health monitoring based on a new type of piezoelectric fibers with metal core, and active hysteresis control based on new modified Prandtl-Ishlinskii model at the Aeronautical Science Key Laboratory for Smart Materials and Structures, Nanjing University of Aeronautics and Astronautics are introduced.

In the paper, a vibration damping system powered by harvested energy with implementation of the so-called SSDV (synchronized switch damping on voltage source) technique is designed and investigated. In the semi-passive approach, the piezoelectric element is intermittently switched from open-circuit to specific impedance synchronously with the structural vibration. Due to this switching procedure, a phase difference appears between the strain induced by vibration and the resulting voltage, thus creating energy dissipation. By supplying the energy collected from the piezoelectric materials to the switching circuit, a new low-power device using the SSDV technique is proposed. Compared with the original self-powered SSDI (synchronized switch damping on inductor), such a device can significantly improve its performance of vibration control. Its effectiveness in the single-mode resonant damping of a composite beam is validated by the experimental results.

The piezoelectric materials, as the most widely used functional materials in smart structures, have many outstanding advantages for sensors and actuators, especially in vibration control, because of their excellent mechanical-electrical coupling characteristics and frequency response characteristics. Semi-active vibration control based on state switching and pulse switching has been receiving much attention over the past decade because of several advantages. Compared with standard passive piezoelectric damping, these new semi-passive techniques offer higher robustness. Compared with active damping systems, their implementation does not require any sophisticated signal processing systems or any bulky power amplifier. In this review article, the principles of the semi-active control methods based on switched shunt circuit, including state-switched method, synchronized switch damping techniques, and active control theorybased switching techniques, and their recent developments are introduced. Moreover, the future directions of research in semi-active control are also summarized.

This paper investigates and compares the efficiencies of four different interfaces for vibration-based energy harvesting systems. Among those four circuits, two circuits adopt the synchronous switching technique, in which the circuit is switched synchronously with the vibration. In this study, a simple source-less trigger circuit used to control the synchronized switch is proposed and two interface circuits of energy harvesting systems are designed based on the trigger circuit. To validate the effectiveness of the proposed circuits, an experimental system was established and the power harvested by those circuits from a vibration beam was measured. Experimental results show that the two new circuits can increase the harvested power by factors 2.6 and 7, respectively, without consuming extra power in the circuits.

•A method for measuring bulk conductivity of CFRP by eddy current is proposed.•The effect of interlaminar interface on eddy current in CFRP laminate is studied.•Correlation between coil signal and conductivities of unidirectional CFRP is obtained.•Electric anisotropy is characterized by detecting fiber orientations with rotated coils.The purpose of this paper is to deal with the characterization of bulk electrical conductivity in multilayer carbon fiber reinforced polymer (CFRP) laminate by using eddy current method. The correlation between probe signals and conductivities in different directions of unidirectional CFRP is obtained with the aid of numerical code, which is based on finite element method in terms of magnetic vector potential. The effect of interlaminar interface on the bulk conductivity of CFRP laminate is investigated by visualizing the lift-off curves of probe signal in complex plane. It is found out that the cross-ply laminate has much better electrical conductance than unidirectional plate, and the bulk conductivity of CFRP laminate depends mainly on the number of current paths produced by the overlapped fibers at the interfaces between two angled layers. In addition the polar diagrams of T–R probe signals at multiple angular positions are depicted to characterize the anisotropy of CFRP laminate.Download full-size image

Morphing technology on aircrafts has found increased interest over the last decade because it is likely to enhance performance and efficiency over a wider range of flight conditions. The key technologies in morphing aircrafts include adaptive structures, deformable smart skin, driving actuators, flight dynamics and flight control. Among them, the deformable smart skin and light-weight driving actuators have been the main obstacles to the real-world implementation of the morphing aircraft. The difficulties in the smart skin rest on the contradiction between its ability to resist aerodynamic load in the normal direction and the in-plane flexibility for morphing functions. This contradiction cannot be solved by using sophisticated properties of the skin materials. It can only be solved by new skin structures, which give anisotropic stiffness for bending and in-plane deformation. In this article two skin structures for morphing aircrafts, the wavy skin structure and the honeycomb skin structure, are introduced. The actuators for morphing aircrafts are required to have large stroke, large output force, good compactness, and good controllability. In this article, piezoelectric-hydraulic actuators are introduced.