Homogenization of fibrous piezoelectric composites with general imperfect interfaces under anti-plane mechanical and in-plane electrical loadings

The present paper aims mainly to estimate the size-dependent effective properties of fibrous piezoelectric composites with general imperfect interfaces. The interface model used states that the displacement, traction, electric potential, and normal electric displacement all suffer jumps across an interface. In addition, it can degenerate into the well-known special ones by employing appropriate high-contrast interfacial parameters. To achieve our objective, an auxiliary inhomogeneity problem of a circular fiber embedded in an infinite cylindrical reference phase via general imperfect interface under anti-plane mechanical and in-plane electrical boundary conditions is analytically solved. This solution allows us to apply the well-known micromechanical schemes such as the dilute, Mori–Tanaka to obtain the closed-form expressions for the size-dependent overall properties of composites under consideration. Some numerical examples are provided for illustrating the features of the obtained general results.     

Fracture criterion for conductive cracks in soda-lime glass under combined mechanical and electrical loading

Fracture tests of electrically conductive cracks on pre-notched four-point bending soda-lime glass samples were conducted under combined mechanical and electrical loading. The experimental results show that the critical stress intensity factor at fracture is reduced if an electric field is applied, thereby indicating that the electric field makes contributions to the fracture of conductive cracks. Base on the charge-free zone (CFZ) model, the total local J-integral including the local mechanical and electrical J-integrals serves as a fracture criterion for conductive cracks in dielectric ceramics under combined mechanical and electrical loading. The experimental results confirm the fracture criterion deduced from the CFZ model.     

Study of mechanical behavior of circular FGM micro-plates under nonlinear electrostatic and mechanical shock loadings

This paper deals with the study of mechanical behavior of a circular functionally graded material (FGM) micro-plate subjected to a nonlinear electrostatic pressure and mechanical shock. It is assumed that the FGM micro-plate is made of metal and ceramic and that material properties are changed continuously along the plate thickness according to a typical function. The nonlinear equation of static deflection and dynamic motion is solved using a step-by-step linearization method and Galerkin-based reduced order model, respectively. In order to find the response of the FGM micro-plate to the electrostatic load and analyze stability of fixed points, static deflection, time history and phase portrait for different applied voltages and initial conditions are illustrated and the effects of different percentages of metal and ceramic constituent on the response of the system are investigated. In addition, effects of mechanical shocks characteristics (amplitudes and durations) on the stability of FGM micro-plate are studied.     

Buckling and postbuckling response of sandwich panels under non-uniform mechanical edge loadings

The buckling and postbuckling responses of cylindrical sandwich panels, subjected to non-uniform in-plane loadings are investigates in this paper by analytical method. A fourth and fifth order expansions are used respectively for the transverse and tangential displacement of the core to model the core compressibility effect. The stress distribution within the panels due to the applied non-uniform in-plane edge loadings are determined by prebuckling analysis. The governing partial differential equations describing the buckling and postbuckling behavior of cylindrical sandwich panels are derived using the principle of minimum total potential energy. Galerkin’s method is used to reduce the governing partial differential equations to a set of non-linear algebraic equations. Newton–Raphson method in conjunction with Riks approach is employed to solve the algebraic equations. Numerical results are presented for both flat and cylindrical sandwich panels subjected to various non-uniform in-plane edge loadings. The sandwich panels used in the present investigation are made up of isotropic and composite materials.     

不同加载条件下钢筋混凝土构件力学特性及影响规律研究

基于OpenSees中的BeamwithHingesElement单元,采用直接在截面层次上定义线弹性剪切恢复力模型,引入零长度单元利用粘结滑移材料模拟构件末端粘结滑移,考虑P-D效应引起的几何非线性,钢筋材料采用能精确模拟钢筋低周疲劳损伤与开裂现象的Mohle-Kunnath本构模型,分析构件在不同加载历史条件下的力学特性。研究构件在单调加载、非对称循环加载、对称循环加载条件下构件的力学特性,并进一步分析构件在不同循环周数条件下对称、非对称加载时构件的力学特性及钢筋材料应变随位移加载历史的变化特点,分析结果表明,钢筋混凝土框架柱在循环加载条件下的构件刚度退化、强度退化现象明显,构件的损伤程度随着剪跨比的降低和轴压比的增大逐渐加剧,构件随着循环周数的增加损伤程度更明显,钢筋材料应变随着循环周数的增加变化明显,随着轴压比增加和剪跨比的降低,构件刚度退化与强度退化随循环周数的增加更显著,钢筋应变同样有一定程度增加。     

热力载荷下多材料构件连接区的界面断裂分析

应用改进的虚裂纹闭合技术对热、力载荷作用下多材料构件连接区界面进行断裂分析。首先,通过对含橡胶夹层的复合材料层合板单腿弯曲（SLB）试件断裂分析,研究了在不同温度载荷作用下,橡胶夹层对试件能量释放率及其各型分量的影响。其次,对具有热流边界下,典型复合材料-橡胶-金属组成的多材料圆柱壳体连接裙结构进行了热力耦合断裂分析,结果表明裂纹总能量释放率随温度升高而增大。最后,针对该连接裙结构讨论了裂纹位置和橡胶层厚度对裂纹能量释放率的影响,指出适当增加橡胶层厚度可以降低裂纹能量释放率,但橡胶厚层度与界面韧性之间存在尺寸效应。     

Microstructural features of mechanical failure in thermal barrier coating systems under static loadings

Abstract

In order to clarify qualitatively and quantitatively the failure mechanism of plasma-sprayed thermal barrier coating (TBC) systems from the microstructural viewpoint, in situ observation of the mechanical failure behavior was conducted for TBC systems under the static loadings at ambient temperature; as the most fundamental aspect, by means of an optical microscopy. Several kinds of TBC systems were prepared by using different sorts of ceramic coating materials. Mechanical tensile loading or compressive loading was gradually applied to the plate shape of TBC specimen using a four-point bending test methodology. It was found that the tensile failure behavior of TBC systems depends strongly on the top-coat microstructures as well as heat treatment after the plasma spraying. The compressive failures were also found rather incidental and depended on the strength of top-coat at the interfacial region. Among different TBC systems, those with the finely segmented top-coat exhibited a good spalling resistance.     

Specificity of matrix cracking development in CFRP laminates under mechanical or thermal loadings

Fatigue tests at room temperature and thermal cycling experiments have been performed on carbon/epoxy laminates of cross-ply and complex stacking sequences. An ‘equivalent’ fatigue loading level has been evaluated in order to impose on the 90° plies the same amplitude of transverse stress σ₂₂ than in thermal cycling. A comparison of the matrix crack development throughout both types of tests has been undertaken: they have been found analogous, but with very far much faster kinetics under thermal cycling. Moreover, the fatigue test frequency has a significant influence on crack onset and development. However, it seems that the parameters ‘time’ and ‘transverse ply stress amplitude’ are not sufficient to completely explain the very fast cracking kinetics observed under thermal cycling.     

中应变率加载下云杉各向异性力学行为研究

采用高速加载INSTRON设备对云杉开展10⁰ s^-1~10² s^-1中应变率压缩实验,研究了材料沿顺纹、横纹径向、弦向、以及径(弦)切面内与顺纹呈15°、30°、45°、60°和75°夹角方向的力学性能。实验表明随着加载方向由顺纹向横纹径(弦)向变化,材料屈服强度逐渐减小,应力-应变曲线塑性流动段由\     

Effects of strain rate and mean strain on cyclic behavior of aluminum alloys under isothermal and thermo-mechanical fatigue loadings

In this paper, effects of strain rate and mean strain on the cyclic behavior and the lifetime of aluminum–silicon alloys are investigated under thermo-mechanical and isothermal fatigue loadings. To achieve these goals, low cycle fatigue tests are accomplished at evaluated temperatures under various strain rates (by changing the loading frequency) and different strain ratios (minimum to maximum strain). Thermo-mechanical fatigue experiments are performed in an out-of-phase condition where the temperature varies between 50 and 250 °C. Various heating/cooling rates are taken into account to assess the strain rate effect and different starting temperatures are considered to study the mean strain effect.     

Effect of uniaxial stress on the electromechanical properties in ferroelectric thin films under combined loadings

The electromechanical properties of ferroelectric thin films under an alternating electric field and a static uniaxial compressive stress are investigated using the modified planar four-state Potts model. To implement the electromechanical properties and the coupling of the electrical and mechanical response, the mechanical energy density as well as the energy due to anisotropic switching between a-domain and c-domain are incorporated in the Hamiltonian. Besides, there are two contributions to the strain at each cell: eigenstrain and elastic strain. Our simulation results show that the longitudinal strain-electric field butterfly loop shifts downward along strain axis and that for the transverse strain shifts upward as the stress magnitude is increased. Moreover, the polarization-electric field hysteresis loop becomes a double-loop under a large compressive stress. The piezoelectric coefficient increases with the stress magnitude and reaches a maximum value at a critical stress level. It then gradually decreases to a small value at large stress magnitudes. Our results qualitatively agree with experimental ones.     

Interfacial fracture of piezoelectric multilayer actuators under mechanical and electrical loading

The elastic stress and strain fields in a plate of finite thickness containing an elliptical hole are systematically investigated using the 3D finite element method. It is found that the stress and strain concentrations are different in the plate of finite thickness even if the plate is in an elastic state. The relation between the stress and the strain concentration factors depends on Poisson’s ratio, the hole’s geometric configuration and the plate thickness. The stress concentration factor is equal to the strain concentration factor only at the notch root of the plate surface. The stress (or strain) concentration factor at the notch root of the plate surface decreases rapidly with increasing thickness and becomes lower than the stress and strain concentration factors corresponding to the plane stress state or at the notch root of the mid plane. It is too low to reflect the overall stress concentration as the thickness increases or as the b/a ratio decreases. The maximum stress concentration factor occurs on the mid plane only when the plate is thinner than the transition thickness of the stress concentration factor. When the plate is thicker than the transition thickness of the stress concentration factor, the distance between the location of the maximum stress concentration factor and plate surface tends to be constant with increasing thickness for the plate with a given  b/a ratio. The differences between the maximum value and the surface value of the stress and strain concentration factors increase rapidly and tend to their respective constant values with increasing plate thickness. The smaller the b/a ratio, the larger these differences. The difference of the stress concentration factor is larger than that of the strain concentration factor in the same plate.     

Thermo-hygro-mechanical behavior of early-age concrete deck in composite bridge under environmental loadings. Part 2: strain and stress

Restraint of volume change in concrete accompanies the development of stress in concrete structures. When concrete is placed in the field, the stress development becomes very complicated at early ages due to the hydration and environmental interaction. This study investigates quantitatively the effects of various factors on the stress variation of early-age concrete decks in composite bridges under environmental loading. The test members were made to exhibit the early-age behavior of a composite bridge. The effects of parameters related to thermal and drying shrinkage stress were analyzed through a numerical model and compared with measured data. The risk of transverse cracking in early-age concrete decks was evaluated in the numerical parametric study. The present study provides better understanding of the behavior of early-age composite bridge under environmental loading, which can be efficiently utilized to reduce the risk of cracking at early ages.     

Formability of AA 2219-O sheet under quasi-static,electromagnetic dynamic,and mechanical dynamic tensile loadings

The mechanism by which electromagnetic forming(EMF)enhances the formability of metals is unclear owing to the coupling effect of multi-physics fields.In the present work,the associated formability improvement mechanisms were qualitatively categorized and illustrated.This was realized by com-paring the formability of fully annealed 2219 aluminum alloy(AA 2219-O)sheet under quasi-static(QS),electromagnetic dynamic(EM),and mechanical dynamic(MD)tensile loadings.It was found that the forming limit of AA 2219-O sheet under EM tensile loading was significantly(45.4％)higher than that under QS tensile loading,and was marginally(3.7％-4.3％)higher than that under MD tensile loading.In addition,the forming limit of AA 2219-O sheet demonstrated a negative dependency on the strain rate within the range of the dynamic tensile tests conducted.The deformation conditions common to EM and MD tensile loadings were responsible for the significant formability improvement compared with QS tensile loading.In particular,the inertial effect was dominant.The different deformation conditions that distinguish EM tensile loading from MD tensile loading resulted in the marginal improvement in formability.This was caused by the absence of a sustaining contact force at the later deformation stage and the lower strain rate.The body force exerted little influence on the formability improvement,and the thermal effect under the two dynamic tensile loadings was negligible.     

Investigation into the microstructure and mechanical properties of diffusion bonded TiAl alloys

TiAl alloys are potential candidates for replacing conventional Ti-alloys in gas turbine applications in the relatively lower temperature sections, owing to their low density and excellent high temperature properties. However, their intolerable ambient temperature brittleness hinders their use in such applications. Recently, TiAl alloys with some room temperature ductility were developed through alloy development programmes using special production routes such as powder metallurgy. However, the room temperature brittleness of these alloys could not be overcome. Sound joining of these alloys is a fundamental prerequisite for their successful integration into high temperature aerospace applications. It has been well demonstrated that diffusion bonding, a commonly used joining technology in conventional Ti-alloys, can successfully be used in joining of TiAl alloys both in as-cast or special-rolled conditions. In this study, diffusion bondability of a recently developed C containing TiAl alloy with a duplex microstructure using bonding parameters in the range of commercially available equipments was studied. Microstructural investigations in the joint area of the bonds were conducted to observe the presence of any weld defect. Additionally, the mechanical behaviour of the bonds was determined by shear testing to find out the optimum bonding parameters. Furthermore, the effect of post-bond heat treatment on the mechanical properties was investigated.     

结构尺寸对RC柱剪扭复合受力抗震性能影响分析

在地震水平作用下,曲线梁桥的桥墩、不规则钢筋混凝土(RC)框架结构的边柱、角柱等受力构件,由于上部结构的重心偏心,使构件处于剪扭复合受力状态,而产生脆性破坏,脆性破坏往往会加剧钢筋混凝土构件的尺寸效应行为。为探究剪扭荷载相互作用下钢筋混凝土柱的抗震性能和尺寸效应,建立了不同扭弯比的钢筋混凝土柱剪扭复合受力三维细观数值模型,模拟分析了扭弯比对RC柱剪扭复合受力抗震性能和尺寸效应的影响。研究结果表明:在剪扭复合荷载作用下,钢筋混凝土柱破坏更具脆性,承载能力降低;扭弯比的增大导致构件变形能力,延性能力和耗能能力下降;扭矩的存在,在一定范围内增强了钢筋混凝土柱抗剪强度的尺寸效应。对比并修正了中国规范提出的剪‐扭承载力相关方程,保证了预测结果的安全度。     

Ductile crack growth and constraint in pipelines subject to combined loadings

An important failure mode of offshore pipelines is ductile fracture of the pipe wall triggered by a hypothetical welding defect. In this study, pipelines having an external part-through semi-circumferential crack of various sizes, subject to combined internal pressure and inelastic bending are considered. This is done to assess the response of pipelines during both their installation and operational conditions. Detailed 3D nonlinear finite element (FE) models of pipelines are developed. A row of elements ahead of the initial crack front are modeled using a voided plasticity material model, which enables simulation of crack growth and the subsequent fracture failure mode (denoted by the critical curvature, κ_crit). After discussing the typical response characteristics of such pipelines, the FE model is used to parametrically investigate the influence of varying pipe and crack dimensions, and also the internal pressure levels, on κ_crit. In the second part of this paper, the crack tip constraint ahead of a growing crack in such pipes is evaluated and systematically compared to the crack tip constraint of both the traditionally used deeply cracked Single Edge Notch Bend (SENB) specimens and the constraint-matched Single Edge Notch Tensile (SENT) specimens. This is achieved by comparing the crack resistance curves (R-curves) along with stress triaxiality and equivalent plastic strain fields evaluated ahead of a growing crack of the three systems. The results present grounds for justification of usage of SENT specimens in fracture assessment of such pipes as an alternative to the traditional overly conservative SENB specimens.