Transverse sensitivity of piezoelectric accelerometric transducers

Conclusions The transverse sensitivity of piezoelectric acceleration transducers is caused by the lack of coincidence between the transducer's sensitivity axis and the polarization vector of the piezoelectric plate. This fact should be taken into consideration both in the production of piezoelectric elements, and in the design of transducers and their manufacture.     

Flexible composite transducers

Flexible PZT/polymer composite transducers have been fabricated with a novel microstructural configuration. The concept of connectivity has been applied in the evaluation of the type of structure needed to optimize the properties of the composite. Properties of several kinds of piezoelectric transducers are compared.     

Transverse sensitivity in nonaligned piezoceramic vibration transducers

Summary It is necessary to take into account in measurements made with unaligned transducers the possibility of considerable nonlinear distortions due to the doubling of harmonic oscillating frequencies which are included in the spectrum of the vibration under consideration.     

Transverse shear modulus and strength of honeycomb cores

The transverse shear mechanical behavior and failure mechanism of aluminum alloy honeycomb cores are investigated by the single block shear test in this paper. The transverse shear deformation process of honeycomb cores may be approximately categorized into four stages, namely elastic deformation, plastic deformation, fracture of cell walls and debonding of honeycomb cores/facesheets. The elastic deformation of unit cell under transverse shear displacement is also investigated by the finite element method, and the result shows that the bending deformation of the cell walls is similar to that of the cantilever beam. In order to precisely predict the equivalent transverse shear modulus and strength, not only shear deformation but also bending deformation of cell walls should be considered. Therefore, in the present paper, the equivalent transverse shear modulus and strength are predicted by application of the cantilever beam theory and thin plate shear buckling theory in conjunction with simplifying assumption as to the displacement in the cores. It is concluded that the contribution of bending deformation of cell walls to equivalent transverse shear modulus and strength is obvious with the decreasing height of cell walls.     

A novel centresymmetric honeycomb composite structure

In this paper analytical and finite element (FE) simulations are carried out to calculate the in-plane Poisson’s ratio and Young’s moduli of a new centresymmetric honeycomb configuration under uniaxial loading. Opposite to similar re-entrant honeycomb structures studied in the past, the new re-entrant unit cell topology takes into account possible manufacturing constraints typical of production routes like Resin Transfer Moulding (RTM) or Rapid Prototyping (RP). The results obtained through the analytical and FE analysis show a significant decrease of the Poisson’s ratio for the internal cell angle between −20° and +20° compared to the classical re-entrant configurations exhibited in literature. The results also show that the presence of edge corners in the unit cell honeycomb configuration gives rise to a cellular structure with enhanced flexibility compared to the classical centresymmetric one. The results obtained by the analytical model show good agreement with the Gibson and Ashby rib-bending model when the honeycomb configuration reduces to the theoretical layout without modifications due to manufacturing constraints.     

Ballistic behaviors of injection-molded honeycomb composite

The purpose of this study is to explore a new type of structure design and injecting material selection of the composite sandwich bulletproof system. A re-entrant hexagonal honeycomb with the negative Poisson’s ratio effect was designed. Thermoplastic polyurethanes (TPU), polypropylene (PP) and polycarbonate (PC) were chosen to inject into the re-entrant honeycomb structure by injection molding to form the composite sandwich layers. The total bulletproof system was later constructed with the boron carbide ceramic plates and aluminum alloy plates. The mechanical behaviors and energy absorption characteristics of different composite sandwich layers were investigated by quasi-static and dynamic compression tests. The dynamic responses of different structured layers were analyzed via numerical simulations in ANSYS/LS-DYNA, which employed 7.62 mm projectiles. The results indicated that the injection-molded composite sandwich layer shows excellent compressive strength and energy absorption capacity. The bulletproof performance of the injection-molded system has been greatly improved comparing with the non-injected molded system. Compared with TPU and PP, the composite injected with PC presents optimal penetration resistance.     

多级复合材料蜂窝结构的力学性能

根据多级结构设计思想,把高性能聚甲基丙烯酰亚胺（PMI）泡沫加入到单向碳纤维增强树脂复合材料之间,制备多级复合材料蜂窝结构。对多级复合材料蜂窝结构的平压性能进行了研究,包括多级复合材料蜂窝结构平压性能的理论预报和试验验证。研究了多级复合材料蜂窝结构平压性能随结构等效密度变化的关系。并对多级复合材料蜂窝结构的三点弯曲性能进行了研究,主要包括理论预报和试验验证。通过理论研究对结构的失效模式进行了预报,绘制了失效模式机制图,并通过三点弯曲试验对理论预报结果进行了验证。      

Neutron imaging inspections of composite honeycomb adhesive bonds

Numerous commercial and military aircraft, including the Canadian Forces CF188 Hornet, use composite honeycomb structures in the design of their flight control surfaces (FCS). These structures provide excellent strength to weight ratios, but are often susceptible to degradation from moisture ingress. Once inside the honeycomb structure moisture causes the structural adhesive bonds to weaken, which can lead to complete failure of the FCS in flight. There are two critical structural adhesive bonds: the node bond and the filet bond. The node bond is integral to the honeycomb portion of the composite core and is located between the honeycomb cells. The filet bond is the adhesive bond located between the skin and the core. In order to asses overall structural degradation and develop repair procedures, it is important to determine the degree of degradation in each type of bond. Neutron radiography and tomography of the adhesive bonds was conducted at the Royal Military College (RMC) and FRM-II. Honeycomb samples were manufactured from FCS with in-service water ingress. The radiographs and tomograms provided important information about the degree of degradation in the core as well as about which adhesive bonds are more susceptible. The information obtained from this study will help to develop repair techniques and assess the flight worthiness of FCS.     

Finite-element analysis of 1-3 composite transducers

The vibrational and electromechanical characteristics of a wide range of 1-3 composite structures, comprising ceramic pillars aligned within a polymer phase, are considered using finite-element analysis. The influence of pillar geometry, ceramic volume fraction, and pillar orientation is described in terms of overall transduction efficiency. It is shown that the finite-element method provides a versatile means of analysis and the results obtained permit a set of useful design guidelines to be developed. In general, a small pillar aspect ratio and a relatively high volume fraction provides the most satisfactory performance, in terms of electromechanical efficiency and uniformity of thickness dilation.     

On the dynamic behavior of honeycomb based composite solids

Summary The aim of this contribution is twofold. First, a dispersive model of periodic composite solids made of an isotropic matrix reinforced by a hexagonal system of slender fibres or by a honeycomb-like slender skeleton is formulated. Second, this model is applied to the analysis of vibration and wave propagation problems in the above honeycomb based composites. Contrary to the known homogenized models the main feature of the proposed model is that it describes the effect of cell size on the overall dynamic behavior of a composite solid. It is shown that on the macro-level the response of honeycomb based composites is isotropic. It is also proved that there exist dispersive dilatational-type and shear-type waves, which can propagate in these composites. Simple formulae for lower and higher free vibration frequencies are derived, and the existence of certain restrictons imposed on the physically allowable wave propagation speeds is shown.     

蜂窝夹层复合材料不确定性参数识别方法

提出蜂窝夹层复合材料不确定性参数识别方法。采用三明治夹芯板理论建立铝蜂窝夹层结构的初始有限元模型,其中芯层等效弹性参数由均匀化方法计算。据芯层结构及相对灵敏度分析选存在不确定性且对动态特性敏感性较大的面外剪切模量及面板厚度为待识别参数。对6块铝蜂窝复合材料板进行自由-自由边界条件下动态试验,获得试验模态参数的均值及标准差。据试验结果采用所提方法识别铝蜂窝夹层板不确定性参数。结果表明,对存在不确定性参数的铝蜂窝夹层复合材料用该方法能准确识别铝蜂窝夹层板不确定参数的均值及标准差。并建立具有准确统计意义的动力学模型。     

Dynamic characteristics of 2-2 piezoelectric composite transducers

Lateral resonant modes associated with specific lateral structures in piezoceramic-polymer composites are the main drawback that interferes with performance of ultrasonic transducers utilizing the composites as driving elements. This paper presents a field solution to characterize dynamic behavior of lateral modes in composite transducers with 2-2 connectivity. The solution is derived based on a concept of Lamb wave propagation along piezoceramic and polymer layers in the thickness direction of the composite transducers. The behavior of 2-2 composite is investigated over a broad spectrum ranging from quasi-static stage up to mode transition zones. Dispersion curves for various modes were calculated and compared with experimental results. A safe zone can be identified from the dispersion curves where the interference of lateral modes to thickness oscillation is negligible. Strong interactions between thickness modes and lateral modes are observed in the transition zones where two modes merge together in the frequency domain. Electromechanical thickness coupling coefficient, k_t, is calculated for the first time to include the effect of aspect ratio of the composite. Results show that the coupling coefficient, k_t, varies dramatically as a function of both ceramic volume fraction and the aspect ratio. As k_t reaches its maximum value, the vibration of the ceramic phase is completely decoupled from that of the polymer phase     

复合材料蜂窝夹芯结构低速冲击位置识别研究

进行了碳纤维增强复合材料蜂窝夹芯结构的低速冲击实验,采用一种基于应力波和免疫遗传算法的冲击载荷定位方法对蜂窝夹芯结构上的低速冲击载荷进行分析和定位。首先,通过一组事先确定冲击位置的低速冲击载荷产生的冲击应力波实验数据,使用小波变换方法对其在时频域进行分析,获得多个频率上冲击应力波在蜂窝夹芯结构中的传播速度;然后在此基础上,考虑蜂窝夹芯结构中应力波的各向异性特性,采用免疫遗传算法对未知的低速冲击载荷进行位置识别。实验研究结果表明了该方法的可行性和有效性     

Impact analysis of fiber reinforced polymer honeycomb composite sandwich beams

Large scale fiber reinforced polymer (FRP) composite structures have been used in highway bridge and building construction. Recent applications have demonstrated that FRP honeycomb sandwich panels can be effectively and economically applied for both new construction and rehabilitation and replacement of existing structures. This paper is concerned with impact analysis of an as-manufactured FRP honeycomb sandwich system with sinusoidal core geometry in the plane and extending vertically between face laminates. The analyses of the honeycomb structure and components including: (1) constituent materials and ply properties, (2) face laminates and core wall engineering properties, and (3) equivalent core material properties, are first introduced, and these properties for the face laminates and equivalent core are later used in dynamic analysis of sandwich beams. A higher-order impact sandwich beam theory by the authors [Yang MJ, Qiao P. Higher-order impact modeling of sandwich beams with flexible core. Int J Solids Struct 2005;42(20):5460–90] is adopted to carry out the free vibration and impact analyses of the FRP honeycomb sandwich system, from which the full elastic field (e.g., deformation and stress) under impact is predicted. The higher order vibration analysis of the FRP sandwich beams is conducted, and its accuracy is validated with the finite element Eigenvalue analysis using ABAQUS; while the predicted impact responses (e.g., contact force and central deflection) are compared with the finite element simulations by LS-DYNA. A parametric study with respect to projectile mass and velocity is performed, and the similar prediction trends with the linear solution are observed. Furthermore, the predicted stress fields are compared with the available strength data to predict the impact damage in the FRP sandwich system. The present impact analysis demonstrates the accuracy and capability of the higher order impact sandwich beam theory, and it can be used effectively in analysis, design applications and optimization of efficient FRP honeycomb composite sandwich structures for impact protection and mitigation.     

Transverse vibration of symmetrically laminated rectangular composite plates

A general numerical method for estimation of the vibrational response of symmetrically laminated rectangular composite plates is presented. This is based on the Rayleigh-Ritz method using the admissible 2-D orthogonal polynomials to derive the governing eigenvalue equation. The natural frequencies and mode shapes for the laminated plates are obtained by solving this governing eigenvalue equation. Several test problems are solved to demonstrate the accuracy and flexibility of the proposed method. The present results, where possible, are verified with those available values from the literature. The effects of the material, number of layers and fibre orientation upon the frequencies and mode shapes are discussed. This study may provide valuable information for researchers and engineers in design applications.     

Transverse tensile properties of an unbonded model composite

Models of unbonded composites have been made by drilling holes in aluminium alloy strip and plugging the holes with steel rods. These specimens, together with similar specimens left unplugged, have been tested in tension. Young's modulus, stress at the elastic limit and ultimate tensile stress have been measured. The results, particularly for the Young's modulus of the plugged strips, show a complex behaviour. The application of the results to real materials is discussed.     

Fabrication and performance of high-frequency composite transducers with triangular-pillar geometry

A single-element, 40-MHz, 3-mm diameter transducer was fabricated with a geometric focus at 9 mm. The transducer was based on a piezo-composite substrate with triangular-shaped composite pillars. The 2-way bandwidth of 50% and impedance magnitude were in agreement with that predicted using finite-element modeling. A one-way radiation pattern was collected using a needle hydrophone. The one-way -3 dB beamwidth at the geometric focus was measured to be 120 mum and the -3 dB depth of field was 2.5 mm. This is in good agreement with the theoretical predictions of 112.5 mum and 2.4 mm. The triangular-pillar composite transducer was then compared with a transducer with square composite pillars with similar volume fraction of active ceramic. A 9.5 dB reduction in the amplitude of the secondary resonance was found for the triangular-pillar composite as well as a 30% gain in the 2-way pulse bandwidth. A 256-element 30-MHz linear array was fabricated as a preliminary investigation into the use of the triangular pillar as the substrate in a high-frequency linear array transducer. In vivo images were generated with both the single-element and linear-array transducers.     

明胶鸟弹撞击复合材料蜂窝夹芯板试验

开展明胶鸟弹撞击复合材料蜂窝夹芯板试验,研究夹芯结构在软体高速冲击下的损伤形式,分析相关因素对结构动态响应结果的影响。通过CT扫描对复合材料蜂窝夹芯板内部进行检测可知,面板出现分层、基体开裂、纤维断裂、凹陷、向胞内屈曲等损伤形式,蜂窝芯出现芯材压溃、与面板脱粘的损伤形式;分析复合材料蜂窝夹芯板后面板的动态变形过程及撞击中心处位移-时间数据可知,复合材料蜂窝夹芯板在撞击过程中出现由全局弯曲变形主导和局部变形主导的两种变形模式;通过对比不同工况下的复合材料蜂窝夹芯板损伤程度可知,复合材料蜂窝夹芯板损伤程度随鸟弹撞击速度的增加而增大;蜂窝芯高度为10 mm的复合材料蜂窝夹芯板较蜂窝芯高度为5 mm的复合材料蜂窝夹芯板的损伤程度大;初始动能较大的球形鸟弹较圆柱形鸟弹对复合材料蜂窝夹芯板造成的冲击损伤程度更大。      

湿热环境对复合材料蜂窝板振动特性的影响

为了研究复合材料蜂窝板在湿热环境下的振动特性,针对由碳纤维/双马来酰亚胺复合材料层合板和Nomex芯层复合而成的蜂窝板进行了不同温湿度下固有频率的数值分析。基于分段剪切变形理论,分别考虑复合材料蜂窝薄板和厚板两种情况,利用湿度与温度的等效性,求解了复合材料蜂窝板的振动特征方程。利用有限元软件ABAQUS,建立了四端固支的复合材料蜂窝板精细化模型。分别讨论了温度、湿度、温湿度联合作用对复合材料蜂窝薄板和厚板固有频率的影响。结果表明:相比于温度的升高,复合材料蜂窝板固有频率对吸湿量的增加更为敏感;相同的湿热环境下,复合材料蜂窝厚板结构的固有频率比薄板结构大,且阶次越高,固有频率上升的幅度越大;温湿度的联合作用比它们单独作用的叠加对复合材料蜂窝板固有频率的影响更大,且在复合材料蜂窝薄板中更加明显。      

A theoretical approach to the deformation of honeycomb based composite materials

Honeycomb sheet is already widely used as a core in aeronautical sandwich construction. An alternative application is to use it as the reinforcing element for composites in which the cells of the honeycomb are filled with various materials. This paper presents the results of a study of such a composite, in which a low modulus infill is used. The work covers, in simple terms, the elastic properties of the unfilled sheet and the composite under in-plane direct loading and out-of-plane bending. The plastic deformation characteristics under in-plane direct loading are also considered. Specimen experimental results are presented, which show that the simple analytical approach used is clearly justifiable.