Abstract: Abstract Piezoelectric stack transducers in d33 mode have a much higher mechanical-to-electric energy conversion efficiency compared with d31 mode piezoelectric harvesters. However, multilayered piezoelectric stacks usually operate in off-resonance due to the higher stiffness and thereby have a lower power output under low-frequency excitations. This paper proposes to apply the dynamic magnetic pre-loading to a piezoelectric stack transducer to significantly increase the power output. The energy harvesting system consists of a multilayered piezoelectric stack with a compliant force amplification frame, a proof mass, and two magnets configured in attraction. The static force–displacement relationship of the magnets is identified from experiments and extended to a dynamic model capable of characterizing the dynamic magnetic interaction. An electromechanical model is developed based on the theoretical derivation and the experimentally identified parameters to predict the voltage outputs under different resistive loads. Approximate analytical solutions are derived by using the harmonic balance method and show good agreements with the numerical and experimental results. The performance of the system is examined and compared with that of the harvester without magnetic pre-loading. The influences of the distance between the two magnets and the electrical resistive loads on the power output are investigated. Results indicate the energy harvesting system with magnetic pre-loading can produce over thousand times more power than the system without magnetic pre-loading at the base excitation of 3 Hz and 0.5 m/s2, far below the resonance at 243 Hz PubDate: 2020-02-13

Abstract: Abstract The theory of plasticity as a special field of continuum mechanics deals with the irreversible, i.e. permanent, deformation of solids. Under the action of given loads or deformations, the state of the stresses and strains or the strain rates in these bodies is described. In this way, it complements the theory of elasticity for the reversible behavior of solids. In practice, it has been observed that many materials behave elastically up to a certain load (yield point), beyond that load, however, increasingly plastic or liquid-like. The combination of these two material properties is known as elastoplasticity. The classical elastoplastic material behavior is assumed to be time-independent or rate-independent. In contrast, we call a time- or rate-dependent behavior visco-elastoplastic and visco-plastic—if the elastic part of the deformation is neglected. In plasticity theory, because of the given loads the states of the state variables stress, strain and temperature as well as their changes are described. For this purpose, the observed phenomena are introduced and put into mathematical relationships. The constitutive relations describing the specific material behavior are finally embedded in the fundamental relations of continuum theory and physics. Historically, the theory of plasticity was introduced in order to better estimate the strength of constructions. An analysis based purely on elastic codes is not in a position to do this, and can occasionally even lead to incorrect interpretations. On the other hand, the entire field of forming techniques requires a theory for the description of plastic behavior. Starting from the classical description of plastic behavior with small deformations, the present review is intended to provide an insight into the state of the art when taking into account finite deformations. PubDate: 2020-02-12

Abstract: Fracture is a very common failure mode of the composite materials, which seriously affects the reliability and service-life of composite materials. Therefore, the study of the fracture behavior of the composite materials is of great significance and necessity, which demands an accurate and efficient numerical tool in general cases because of the complexity of the arising boundary-value or initial-boundary value problems. In this paper, a phase field model is adopted and applied for the numerical simulation of the crack nucleation and propagation in brittle linear elastic two-phase perforated/particulate composites under a quasi-static tensile loading. The phase field model can well describe the initiation, propagation and coalescence of the cracks without assuming the existence and the geometry of the initial cracks in advance. Its numerical implementation is realized within the framework of the finite element method (FEM). The accuracy and the efficiency of the present phase field model are verified by the available reference results in literature. In the numerical examples, we first study and discuss the influences of the hole/particle size on the crack propagation trajectory and the force–displacement curve. Then, the effects of the hole/particle shape on the crack initiation and propagation are investigated. Furthermore, numerical examples are presented and discussed to show the influences of the hole/particle location on the crack initiation and propagation characteristics. It will be demonstrated that the present phase field model is an efficient tool for the numerical simulation of the crack initiation and propagation problems in brittle two-phase composite materials, and the corresponding results may play an important role in predicting and preventing possible hazardous crack initiation and propagation in engineering applications. PubDate: 2020-02-12

Abstract: Abstract On the basis of finite element analysis, an eigenvalue problem is performed to examine the vibrational characteristics of a hetero-nanotube made of carbon (C) and boron nitride (BN) nanotubes in magnetic and thermal environment. By incorporating the assumption of nonlocal elasticity theory, the size-dependent behavior of the considered structure is also taken into account. The obtained results demonstrate that the onset of the divergence and flutter instabilities may be postponed by exploiting a hetero-nanotube rather than a uniform one composed of carbon nanotube. Moreover, it is exhibited that, in the presence of fluid flow, the mode shape configuration of nanotubes may be different from those of classical modes and therefore the later one should not be utilized in the dynamic analysis of fluid-conveying tubes. Finally, it is shown that, as the temperature decreases, the natural frequencies of the system decrease in high temperature conditions and increase for the case of room temperature. PubDate: 2020-01-24

Abstract: Abstract Galloping based piezoelectric energy harvester is a kind of micro-environmental energy harvesting device based on flow-induced vibrations. A novel tristable galloping-based piezoelectric energy harvester is constructed by introducing a nonlinear magnetic force on the traditional galloping-based piezoelectric energy harvester. Based on Euler–Bernoulli beam theory and Kirchhoff’s law, the corresponding aero-electromechanical model is proposed and validated by a series of wind tunnel experiments. The parametric study is performed to analyse the response of the tristable galloping-based piezoelectric energy harvester. Numerical results show that comparing with the galloping-based piezoelectric energy harvester, the mechanism of the tristable galloping-based piezoelectric energy harvester is more complex. With the increase of a wind speed, the vibration of the bluff body passes through three branches: intra-well oscillations, chaotic oscillations, and inter-well oscillations. The threshold wind speed of the presented harvester for efficiently harvesting energy is 1.0 m/s, which is decreased by 33% compared with the galloping-based piezoelectric energy harvester. The maximum output power of the presented harvester is 0.73 mW at 7.0 m/s wind speed, which is increased by 35.3%. Compared with the traditional galloping-based piezoelectric energy harvester, the presented tristable galloping-based piezoelectric energy harvester has a better energy harvesting performance from flow-induced vibrations. PubDate: 2020-01-23

Abstract: Abstract In this paper, a series of static/dynamic tensile tests are performed for glass fiber reinforced plastic (GFRP) composites. Using the combination of high-speed photography and digital image correlation (DIC) technology, true stress–strain curves in different directions and strain rates are obtained. We also obtained the dynamic failure strain of the material in different directions, which are used to accurately describe the dynamic tensile and failure behavior of the material. The experimental results show that there is a stiffness change point N in three directions under different strain rate (10−3 s−1, 10 s−1, 100 s−1) tensile conditions. The stiffness before and after N point is recorded as Einitial and Echanged respectively. The values of Echanged in weft direction and warp direction are about 30% to 50% of Einitial, while Echanged in tilt direction is only about 10% of Einitial. The fiber has the highest strength in the weft direction and the tilt direction has the lowest strength. With the combination of high-speed photography and DIC technology, the dynamic failure parameters of different directions under the strain rate of 100 s−1 are obtained. The dynamic failure strains in three directions are 0.245, 0.373 and 0.341, respectively. The parameters are verified by impact three-point bending test. These works can more accurately describe the dynamic mechanical behavior of glass fiber reinforced plastic (GFRP) composites and provide reference for the design of GFRP structures. PubDate: 2020-01-08

Abstract: Abstract In this paper, the nonlinear dynamic responses of a piezoelectric cantilever plate near the first-order and second-order natural frequencies under the action of electromechanical coupling are studied by experiments and finite element (FE) methods. The influence of different excitation frequencies on the dynamical characteristics of piezoelectric cantilever plates is analyzed with the fixed excitation amplitude. First, an experimental setup is built, including a carbon fiber cantilever plate attached to a macro fiber composite (MFC) sheet. Then, the electromechanical coupling excitations are subjected to the plate with different frequencies, which are chosen near the first and second-order natural frequencies of the plate. The piezoelectric cantilever plate has periodical motions under a lower frequency excitation, and the motions of the plate become more complex after another high frequency excitation added in the physical field. The experimental results show that the motion of the piezoelectric cantilever plate changes from stable to unstable with high–low coupled resonant frequencies. At last, the FE study is carried out to compare and verify the experimental results and the effects of isotropic and orthotropic materials on the accuracy of natural frequencies results are also compared. PubDate: 2020-01-02

Abstract: Abstract In this paper, we study the dynamics of an idealized benchmark bicycle moving on a surface of revolution. We employ symbolic manipulations to derive the contact constraint equations from an ordered process, and apply the Lagrangian equations of the first type to establish the nonlinear differential algebraic equations (DAEs), leaving nine coupled differential equations, six contact equations, two holonomic constraint equations and four nonholonomic constraint equations. We then present a complete description of hands-free circular motions, in which the time-dependent variables are eliminated through a rotation transformation. We find that the circular motions, similar to those of the bicycle moving on a horizontal surface, nominally fall into four solution families, characterized by four curves varying with the angular speed of the front wheel. Then, we numerically investigate how the topological profiles of these curves change with the parameter of the revolution surface. Furthermore, we directly linearize the nonlinear DAEs, from which a reduced linearized system is obtained by removing the dependent coordinates and counting the symmetries arising from cyclic coordinates. The stability of the circular motion is then analyzed according to the eigenvalues of the Jacobian matrix of the reduced linearized system around the equilibrium position. We find that a stable circular motion exists only if the curvature of the revolution surface is very small and it is limited in small sections of solution families. Finally, based on the numerical simulation of the original nonlinear DAEs system, we show that the stable circular motion is not asymptotically stable. PubDate: 2019-12-14

Abstract: Abstract This work is dedicated to the experimental study of the shear properties of three-dimensional reinforced composites taking into account their structural features, in Iosipescu tests. Shear strains have been determined using Vic-3D non-contact three-dimensional digital optical system. The evolution of inhomogeneous strain fields on the surface of composite specimens of the structure under study has been analyzed. The variants of strain averaging in the specimen working area have been analyzed using Vic-3D tools. AMSY-6 acoustic emission system has been used to assess the structural integrity of composite materials under loading. PubDate: 2019-12-14

Abstract: Abstract Deep-sea mining, through which shattered coarse solid mineral resources are hydraulically collected and transported, is a promising solution for problems associated with resource exhaustion. This study reports some interesting phenomena observed in a series of upward pumping experiments conducted on a coarse sphere with a 2-cm radius. The sphere was hydraulically lifted using a vertical pipe with a 5-cm radius suspended above the sphere. Remarkably, the tangential motion benefited the collection. The discrete element method–computational fluid dynamics was used to investigate the collection mechanism; this method was used to simulate the collecting processes of the sphere under different initial motion conditions. The simulated results agreed well with the experimental observations. The vortices over the sphere induced by its motion coupled with the main stream mainly provide sufficient lift force to raise the sphere. PubDate: 2019-12-14

Abstract: Abstract Additive-manufacturing process has substantially promoted the development of lattice structure and makes it possible to fabricate complex lattice sandwich structures. In-plane compression load always appears in engineering, such as in the primary structure of spacecraft. In order to reveal potential engineering application in future, this paper focuses on the lattice sandwich plate that is fabricated by additive-manufacturing and subjected to in-plane compression. Firstly, five failure modes are proposed for lattice sandwich plate under in-plane compression, including Euler buckling, shear buckling, face sheet dimpling, face sheet wrinkling and face sheet crushing. Secondly, an optimization method is proposed to obtain the optimum sizes, including the panel thickness, the length of rod, the size of rod cross-section, the inclined angle of rod and the wideness ratio of cell. Then, the in-plane compression experiment is operated after measuring the geometrical imperfection of the specimen by the optical microscope. Numerical method is adopted to illustrate the effect on failure behaviors caused by the imperfections of struts, face sheets and global shape. By introducing these imperfection, numerical result can be well agreed with experimental result and explain the failure mechanisms mostly derived from the radius imperfection for the struts. PubDate: 2019-12-14

Abstract: Abstract The effective properties of composite materials have been predicted by various micromechanical schemes. For composite materials of constituents which are described by the classical governing equations of the local form, the conventional micromechanical schemes usually give effective properties of the local form. However, it is recognized that under general loading conditions, spatiotemporal nonlocal constitutive equations may better depict the macroscopic behavior of these materials. In this paper, we derive the thermo-elastic dynamic effective governing equations of a fibre-reinforced composite in a coupled spatiotemporal integral form. These coupled equations reduce to the spatial nonlocal peridynamic formulation when the microstructural inertial effects are neglected. For static deformation and steady-state heat conduction, we show that the integral formulation is superior at capturing the variations of the average displacement and temperature in regions of high gradients than the conventional micromechanical schemes. The approach can be applied to analogous multi-field coupled problems of composites. PubDate: 2019-12-11

Abstract: Abstract The converging Richtmyer–Meshkov (RM) instability on single- and dual-mode \(\hbox {N}_2\)/\(\hbox {SF}_6\) interfaces is studied by an upwind conservation element and solution element solver. An unperturbed case is first considered, and it is found that the shocked interface undergoes a long-term deceleration after a period of uniform motion. The evolution of single-mode interface at the early stage exhibits an evident nonlinearity, which can be reasonably predicted by the nonlinear model of Wang et al. (Phys Plasmas 22: 082702, 2015). During the deceleration stage, the perturbation amplitude drops quickly and even becomes a negative (phase inversion) before the reshock due to the Rayleigh–Taylor (RT) stabilization. After the reshock, the interface experiences a phase inversion again or does not, depending on the reshock time. The growth of the second-order harmonic in the deceleration stage clearly reveals the competition between the RT effect and the nonlinearity. For dual-mode interfaces, the growth of the first mode (wavenumber \(k_1\)) relies heavily on the second mode (wavenumber \(k_2\)) due to the mode coupling effect. Specifically, for cases where \(k_2\) is an even or odd multiple of \(k_1\), the growth of the first mode is inhibited or promoted depending on its initial amplitude sign and the phase difference between two basic waves, while for cases where \(k_2\) is a non-integer multiple of \(k_1\), the second mode has negligible influence on the first mode. Through a systematic study, signs of perturbation amplitudes of the generated \(k_2-k_1\) and \(k_2+k_1\) waves are obtained for all possible dual-mode configurations, which are reasonably predicted by a modified Haan model (Phys Fluids B 3: 2349–2355, 1991). PubDate: 2019-12-06

Abstract: Abstract Polymer electrolyte fuel cells (PEFCs) being employed in fuel cell electric vehicles (FCEVs) are promising power generators producing electric power from fuel stream via porous electrodes. Structure of carbon paper gas diffusion layers (GDLs) applying in the porous electrodes can greatly affect the PEFC performance, especially at the cathode side where electrochemical reaction is more sluggish. To discover the role of carbon paper GDL structure on the mass transfer properties, different cathode electrodes with dissimilar structural parameters are simulated via lattice Boltzmann method (LBM). 3D contours of oxygen and water vapor concentration through the GDL as well as the 2D contours of current density on the catalyst layer are illustrated and examined. The results indicate that the carbon fiber diameter has a negligible impact on the current density while the impact of carbon paper thickness and porosity is significant. In fact, increasing of carbon paper thickness or porosity leads to lack of cell performance. PubDate: 2019-12-04

Abstract: Abstract An experimental method for a single layer is extended to determine the elastic properties of nanostructured W/Cu multilayers on a flexible substrate. The strain difference between the W/Cu-polyimide-W/Cu composite and the uncoated substrate, measured by dual digital image correlation, allows us to extract the effective Young’s modulus of W/Cu multilayers (20 periods) equaling \(216 \pm 13\hbox { GPa}\). Finite element method is then performed, which agrees well with the experiment and classical rule of mixture (ROM) theory demonstrating that the extension to multilayers is effective and reliable. The numerical analysis also interestingly shows that the strain difference is linearly related to the thickness ratio (W/Cu), periods and sublayer thickness, respectively. In contrast to ROM theory, this approach could potentially be used for the evaluation of properties and design of emerging/unknown functional multilayers, whether or not they are crystalline or amorphous. PubDate: 2019-12-01

Abstract: Abstract Acute stress concentration plays an important role in plaque rupture and may cause stroke or myocardial infarction. Quantitative evaluation of the relation between in vivo plaque stress and variations in blood pressure and flow rates is valuable to optimize daily monitoring of the cardiovascular system for high-risk patients as well as to set a safe physical exercise intensity for better quality of life. In this study, we constructed an in vivo stress model for a human carotid bifurcation with atherosclerotic plaque, and analyzed the effects of blood pressure, flow rates, plaque stiffness, and stenosis on the elastic stress and fluid viscous stress around the plaque. According to the maximum values of the mechanical stress, we define a risk index to predict the risk level of plaque rupture under different exercise intensities. For a carotid bifurcation where the blood flow divides, the results suggest that the stenosis ratio determines the ratio of the contributions of elastic shear stress and viscous shear stress to plaque rupture. An increase of the plaque stiffness enhances the maximum elastic shear stress in the plaque, indicating that a high-stiffness plaque is more prone to rupture for given stenosis ratio. High stress co-localization at the shoulder of plaques agrees with the region of plaque injury in clinical observations. It is demonstrated that, due to the stress-shield effect, the rupture risk of a high-stiffness plaque tends to decrease under high-stenosis conditions, suggesting the existence of a specific stenosis corresponding to the maximum risk. This study may help to complement risk stratification of vulnerable plaques in clinical practice and provides a stenosis mechanical property-specific guide for blood pressure control in cardiovascular health management. PubDate: 2019-12-01

Abstract: Abstract Shape memory polymers (SMPs) usually have a one-way shape memory effect. In this paper, an easy-operating method to realize a two-way shape memory effect was demonstrated in a ring-shaped bilayer structure where the two layers are SMPs with different thermal transition temperatures. By designing specific thermomechanical processes, the mismatched deformation between the two layers leads to a morphology change of ring-shaped bilayer structures from a smooth ring to a gear-like buckling shape under cooling and a reversible recovery to the smooth shape under heating. Such a morphology change is ascribed to occurrence and recovery of thermoelastic buckling. This method was validated by finite element simulation. We experimentally investigated the influence of pre-strain on buckling, and it was found that both the buckling occurrence and recovery temperature vary with pre-strain. Furthermore, considering a ring-shaped SMP–SMP bilayer structure, finite element analysis was conducted to study the influence of film thickness and modulus ratio of two layers on buckling behavior. The results showed that the critical buckling wavelength was greatly influenced by film thickness and modulus ratio. We made a theoretical analysis that accorded well with the numerical results. PubDate: 2019-12-01

Abstract: Abstract Pipe-in-pipe (PIP) structures are widely used in offshore oil and gas pipelines to settle thermal insulation issues. A PIP structure system usually consists of two concentric pipes and one softer layer for thermal insulation consideration. The total response of the system is related to the dynamics of both pipes and the interactions between these two concentric pipes. In the current work, a theoretical model for flow-induced vibrations of a PIP structure system is proposed and analyzed in the presence of an internal axial flow and an external cross flow. The interactions between the two pipes are modeled by a linear distributed damper, a linear distributed spring and a nonlinear distributed spring along the pipe length. The unsteady hydrodynamic forces due to cross flow are modeled by two distributed van der Pol wake oscillators. The nonlinear partial differential equations for the two pipes and the wake are further discretized by the aid of Galerkin’s technique, resulting in a set of ordinary differential equations. These ordinary differential equations are further numerically solved by using a fourth-order Runge–Kutta integration algorithm. Phase portraits, bifurcation diagrams, an Argand diagram and oscillation shape diagrams are plotted, showing the existence of a lock-in phenomenon and figure-of-eight trajectory. The PIP system subjected to cross flow displays some interesting dynamical behaviors different from that of a single-pipe structure. PubDate: 2019-12-01

Abstract: Abstract Generalized Kelvin–Voigt and Maxwell models using Prony series are some of the most well-known models to characterize the behavior of polymers. The simulation software for viscoelastic materials generally implement only some material models. Therefore, for the practice of the engineer, it is very useful to have formulas that establish the equivalence between different models. Although the existence of these relationships is a well-established fact, moving from one model to another involves a relatively long process. This article presents a development of the relationships between generalized Kelvin–Voigt and Maxwell models using the aforementioned series and their respective relaxation and creep coefficients for one and two summations. The relationship between the singular points (maximums, minimums and inflexion points) is also included. PubDate: 2019-10-15

Abstract: Abstract Vertical-axis wind turbines (VAWTs) have been widely used in urban environments, which contain dust and experience strong turbulence. However, airfoils for VAWTs in urban environments have received considerably less research attention than those for horizontal-axis wind turbines (HAWTs). In this study, the sensitivity of a new VAWT airfoil developed at the Lanzhou University of Technology (LUT) to roughness was investigated via a wind tunnel experiment. The results show that the LUT airfoil is less sensitive to roughness at a roughness height of < 0.35 mm. Moreover, the drag bucket of the LUT airfoil decreases with increasing roughness height. Furthermore, the loads on the LUT airfoil during dynamic stall were studied at different turbulence intensities using a numerical method at a tip-speed ratio of 2. Before the stall, the turbulence intensity did not considerably affect the normal or tangential force coefficients of the LUT airfoil. However, after the stall, the normal force coefficient varied obviously at low turbulence intensity. Moreover, as the turbulence intensity increased, the normal and tangential force coefficients decreased rapidly, particularly in the downwind region of the VAWT. PubDate: 2019-10-09