0-3型压电复合材料覆盖层水下 吸声性能的理论研究

目前压电分流阻尼技术在振动和噪声领域的应用得到了广泛的关注. 本文尝试将压电分流阻尼技术应用于水下吸声领域, 以提高覆盖层的吸声性能. 将压电覆盖层厚度模态的机电方程和声波传播的传递矩阵相结合, 建立一维电声模型. 该模型可以用于分析多层压电和非压电水下吸声覆盖层的吸声性能. 采用该模型分析了0-3型压电复合材料覆盖层的水下吸声性能. 压电复合材料的参数是采用Furukawa的模型计算的. 研究结果表明, 采用合适的分流电阻, 负电容分流电路可以在较宽的频率范围显著提高覆盖层的吸声性能. 其原理可以从阻抗匹配的角度解释, 负电容分流电路可以调整压电覆盖层的表面声阻抗, 使之与水的特性声阻抗相匹配.     

A microstructure model for finite-element simulation of 3D rectangular braided composite under ballistic penetration

The non-delamination feature of 3D braided composites under transverse impact leads to their potential application in the field of ballistic impact protection. One of the effective ways to investigate the ballistic impact damage of the 3D braided composite is to simulate the penetration process by numerical method, such as finite element method. However the numerical simulations of ballistic impact damage are seldom conducted based on the microstructure level. This paper presents a microstructure model for simulating ballistic impact damage of 4-step 3D braided rectangular composite penetrated by a rigid steel projectile. The microstructure model is based on the same yarn spatial configuration with that of the braided composite and also on the assumptions of the braided yarns appear straight inside the braided preform, bending and then change to other directions only at the surface. The ballistic perforation of the braided composite specimen by a cylindrical-conically steel projectile has been simulated with finite element method. The comparisons between FEA and experimental results show the validity of the microstructure model, especially for the penetration resistance and impact damage of the composite. Compared with the other continuum models of the braided composite, the microstructure model can simulate impact damage more precisely. The velocity history and acceleration history of projectile, and impact damage development of the composite in FEM simulation indicate the different damage and energy absorption mechanisms of the braided composite compared with those of laminated composite.     

电泳辅助制备伪1-3陶瓷/聚合物压电复合材料

1-3压电复合材料的压电、介电及铁电性能要远远优于0-3压电复合材料.在制备传统的0-3复合材料过程中引入电泳技术,使得压电颗粒在聚合物基体中取向排列,制备得到伪1-3复合材料.实验结果表明:在制备PZT/环氧树脂0-3复合压电材料固化过程中,采用500 V/mm,4 kHz的电场对其进行电泳辅助取向,可使得颗粒呈现珍珠串状排列,得到伪1-3复合材料;其压电、介电、铁电性能均比原来的0-3复合材料有显著的提高.电泳辅助制备技术用于制备伪1-3复合压电材料具有操作简单、成本低廉、压电、介电、铁电性能显著提高等优点,在智能传感领域具有很好的实际应用前景.     

Manipulating sonic band gaps at will: vibrational density of states in three-dimensional acoustic metamaterial composites

Vibrational properties of elastic composites containing a mass-in-mass microstructure embedded in a solid matrix are numerically studied. Using a lattice model, we investigate the vibrational density of states in three-dimensional composite structures where resonant particles are randomly dispersed. By dispersing such particles in the system, a sonic band gap appears. It is confirmed that this band gap can be introduced in a desired frequency regime by changing the parameters of resonant particles and the frequency width of this band gap can be controlled by varying the concentration of the resonant particles to be dispersed. In addition, multiple sonic band gaps can be realized using different species of resonant particles. These results enable us to suggest an alternative method to fabricate devices that can inhibit the propagation of elastic waves with specific frequencies using acoustic metamaterials.     

Modeling of micromachined silicon–polymer 2-2 composite matching layers for 15 MHz ultrasound transducers

Silicon–polymer composites fabricated by micromachining technology offer attractive properties for use as matching layers in high frequency ultrasound transducers. Understanding of the acoustic behavior of such composites is essential for using them as one of the layers in a multilayered transducer structure. This paper presents analytical and finite element models of the acoustic properties of silicon–polymer composites in 2-2 connectivity. Analytical calculations based on partial wave solutions are applied to identify the resonance modes and estimate effective acoustic material properties. Finite Element Method (FEM) simulations were used to investigate the interaction between the composite and the surrounding load medium, either a fluid or a solid, with emphasis on the acoustic impedance of the composite. Composites with lateral periods of 20, 40 and 80 μm were fabricated and used as acoustic matching layers for air-backed transducers operating at 15 MHz. These composites were characterized acoustically, and the results were compared with analytical calculations. The analytical model shows that at low to medium silicon volume fraction, the first lateral resonance in the silicon–polymer 2-2 composite is defined by the composite period, and this lateral resonant frequency is at least 1.2 times higher than that of a piezo-composite with the same polymer filler. FEM simulations showed that the effective acoustic impedance of the silicon–polymer composite varies with frequency, and that it also depends on the load material, especially whether this is a fluid or a solid. The estimated longitudinal sound velocities of the 20 and 40 μm period composites match the results from analytical calculations within 2.7% and 2.6%, respectively. The effective acoustic impedances of the 20 and 40 μm period composites were found to be 10% and 26% lower than the values from the analytical calculations. This difference is explained by the shear stiffness in the solid, which tends to even out the surface displacements of the composites.     

三维材料微结构设计与数值模拟

为了研究材料细观尺度的力学性能与失效行为,达到对材料微结构的“性能导向型”设计与性能预测的目的,通过程序设计结合有限元数值模拟的方法实现多元多相异质体材料微观组织结构的计算机仿真、材料微结构的细观力学计算与虚拟失效分析.以材料微观组织结构计算机仿真软件ProDesign构造的多晶体材料与多晶体基复合材料微结构的代表性体积单元为基础,基于对商业有限元软件ABAQUS的二次开发,实现对材料微结构细观力学的数值计算,并根据数值模拟结果预测微结构的材料性能,识别“材料结构弱点”,评估异质体材料微结构内微裂纹的启裂 关键词： 材料微结构 数值模拟 各向异性 虚拟失效     

颗粒增强金属基复合材料变形强化中的应变梯度效应

 针对单轴压缩实验,根据颗粒增强金属基复合材料中颗粒和基体两相的局部变形协调条件,并通过简单的位错模型,确定出与变形协调相应的几何必需位错密度,进而导出一种颗粒强化-应变梯度律。从中可以清楚地看出,颗粒增强金属基复合材料的强化由材料的微结构特征几何参数l和基体应变梯度联合控制。对于颗粒含量一定的复合材料,颗粒越小,应变梯度越高,强化效果越好。这一结果揭示了,颗粒强化及尺寸效应主要是通过应变梯度效应来表现的。这也同时说明,应变梯度可能是控制材料变形与断裂的重要因素之一。     

Electromechanical response of 2-2 layered piezoelectric composites: A micromechanical model based on the asymptotic homogenization method

A micromechanical model based on the asymptotic homogenization technique has been developed to predict the complete elastic, dielectric and piezoelectric properties of a general 2-2 layered piezoelectric composite where the constituent phases are elastically anisotropic and piezoelectrically active. Two classes of layered piezoelectric composites (i.e. longitudinally and transversely layered) are considered in two widely different ceramic- and polymer-based systems and their effective properties are obtained in the limits of both large-volume (i.e. bulk) and small-volume (i.e. thin-film) systems. It is demonstrated that: (i) in the bulk, ceramic–ceramic layered composite system, the elastic, piezoelectric, and dielectric properties of the composites vary linearly with volume fraction of the second phase, while in the bulk ceramic–polymer layered composite system, the corresponding properties vary non-linearly with volume fraction of the second phase; (ii) in the prismatic (thin-film) layered piezoelectric composite system, the non-vanishing, effective elastic, piezoelectric and dielectric properties vary linearly with the volume fraction of the second phase for both the longitudinally and transversely layered composite structures in the ceramic–ceramic and the ceramic–polymer composite systems; (iii) the ceramic–polymer piezoelectric layered composites that incorporate a low density polymeric phase with lower acoustic impedance generally exhibit enhanced piezoelectric coupling constants and lowered acoustic impedance; (iv) the longitudinally layered composites exhibit higher piezoelectric coupling constants and lower acoustic impedance compared to that of the transversely layered composites; and (v) the best combination of properties for applications such as hydrophones (i.e. the highest piezoelectric coupling constants and the lowest acoustic impedance) is obtained in the ceramic–polymer, longitudinally layered, thin-film, piezoelectric composites.     

Experimental and finite element modelling studies on single-layer and multi-layer 1-3 piezocomposite transducers

Finite element modelling (FEM) using ATILA code and experimental studies have been carried out on 1-3 piezocomposite transducers. FEM study was initially carried out on a piezocomposite infinite plate and then extended to transducers of finite size. The infinite-plate model results agree well with that of a simple analytical model and experiments. The acoustic performance of multi-layer finite-size piezocomposite transducers was also studied. Transducer stacks were fabricated with different number of layers. The transducer characteristics such as the electrical impedance, the transmitting voltage response (TVR) and the receiving sensitivity (RS) of the 1-3 piezocomposite transducers were evaluated as functions of frequency, ceramic volume fractions and the number of layers. TVR increases and RS decreases with increase in ceramic volume fractions. The model results are found to agree with the experimental data, especially when the number of layers is less.     

Preparation and characterization of nickel nano-Al2O3 composite coatings by sediment co-deposition

Ni-Al₂O₃ composite coatings were prepared by using sediment co-deposition (SCD) technique and conventional electroplating (CEP) technique from Watt's type electrolyte without any additives. The microstructure, hardness, and wear resistance of resulting composites were investigated. The results show that the incorporation of nano-Al₂O₃ particles changes the surface morphology of nickel matrix. The preferential orientation is modified from (2 0 0) plane to (1 1 1) plane. The microhardness of Ni-Al₂O₃ composite coatings in the SCD technique are higher than that of the CEP technique and pure Ni coating and increase with the increasing of the nano-Al₂O₃ particles concentration in plating solution. The wear rate of the Ni-Al₂O₃ composite coating fabricated via SCD technique with 10 g/l nano-Al₂O₃ particles in plating bath is approximately one order of magnitude lower than that of pure Ni coating. Wear resistance for SCD obtained composite coatings is superior to that obtained by the CEP technique. The wear mechanism of pure Ni and nickel nano-Al₂O₃ composite coatings are adhesive wear and abrasive wear, respectively.     

Characterization of suspensions of particles in water by an ultrasonic resonant cell

This paper presents a resonant technique to accurately measure phase-velocity and attenuation of longitudinal acoustic waves in suspensions of solid particles in water. The technique is based on exciting thickness resonances of a layer of fluid and analyzing its spectrum. To this end, a resonant cell to contain the fluid is described and used. Two different type of water suspensions are studied: titanium dioxide and alumina particles; particle volume fraction is in the range 0–0.18. Simultaneous determination of particle size distribution in the suspension by an optical method are also carried out. Finally, the experimental results are compared with theoretical predictions obtained from three different approaches.     

吸声材料对水下小平台上矢量传感器声学特性的影响研究

本文围绕粘贴黏弹性吸声材料方法对水下小平台上安装矢量传感器的指向性和测向的影响展开理论分析和实验研究.首先建立了吸声材料和平台结构组成的复合层结构的数学模型,对声波经过复合层结构的声学特性进行了分析,在此基础上利用有限元耦合边界元法对粘贴吸声材料前后的水下小平台上的矢量传感器的声学特性进行研究.通过理论计算和数值分析研究了吸声材料对矢量传感器的各通道的指向性的影响,并计算了覆盖吸声材料前后矢量传感器的测向精度.水池实验验证了分析结果的正确性.     

Acoustic modelling of exhaust devices with nonconforming finite element meshes and transfer matrices

Transfer matrices are commonly considered in the numerical modelling of the acoustic behaviour associated with exhaust devices in the breathing system of internal combustion engines, such as catalytic converters, particulate filters, perforated mufflers and charge air coolers. In a multidimensional finite element approach, a transfer matrix provides a relationship between the acoustic fields of the nodes located at both sides of a particular region. This approach can be useful, for example, when one-dimensional propagation takes place within the region substituted by the transfer matrix. As shown in recent investigations, the sound attenuation of catalytic converters can be properly predicted if the monolith is replaced by a plane wave four-pole matrix. The finite element discretization is retained for the inlet/outlet and tapered ducts, where multidimensional acoustic fields can exist. In this case, only plane waves are present within the capillary ducts, and three-dimensional propagation is possible in the rest of the catalyst subcomponents. Also, in the acoustic modelling of perforated mufflers using the finite element method, the central passage can be replaced by a transfer matrix relating the pressure difference between both sides of the perforated surface with the acoustic velocity through the perforations. The approaches in the literature that accommodate transfer matrices and finite element models consider conforming meshes at connecting interfaces, therefore leading to a straightforward evaluation of the coupling integrals. With a view to gaining flexibility during the mesh generation process, it is worth developing a more general procedure. This has to be valid for the connection of acoustic subdomains by transfer matrices when the discretizations are nonconforming at the connecting interfaces. In this work, an integration algorithm similar to those considered in the mortar finite element method, is implemented for nonmatching grids in combination with acoustic transfer matrices. A number of numerical test problems related to some relevant exhaust devices are then presented to assess the accuracy and convergence performance of the proposed procedure.     

Effect of the open roof on low frequency acoustic propagation in street canyons

This paper presents an experimental, numerical and analytical study of the open roof effect on acoustic propagation along a 3D urban canyon. The experimental study is led by means of a street scale model. The numerical results are performed with a 2D-Finite Difference in Time Domain approach adapted to take into account the acoustic radiation losses due to the street open roof. An analytical model, based on the modal decomposition of the pressure field in the street width mixed with a 2D image sources model including the reflection by the open roof, is also presented. Results are given for several frequencies in the low frequency domain. The comparison of these approaches shows a quite good agreement until f = 100 Hz at full scale. For higher frequency, experimental results show that the leakage, due to the street open roof, is not anymore uniformly distributed on all modes of the street. The notion of leaky modes must be introduced to model the acoustic propagation in a street canyon.     

铁电/铁磁三相颗粒复合材料的磁电性能计算

采用格林函数方法给出了三相复合材料的磁电系数的解析式，对稀土-铁合金/压电陶瓷/高分子（Terfenol-D/PZT/PVDF）三相颗粒复合材料的磁电系数进行了计算.计算结果给出了复合材料的磁电性能与材料显微结构的关系，包括三相颗粒复合材料的磁电性能随组分、颗粒的长径比、PZT颗粒的电极化方向以及外磁场的变化趋势，可为实验设计提供参考和指导.通过合理设计，三相磁电复合材料的性能可以达到数百mV/A.作为一种新的磁电复合材料，三相颗粒复合材料有望成为一种新型高性能易制备的磁电材料. 关键词： 磁电效应 复合材料 格林函数     

Propagation speed of combustion and invasion waves in stochastic simulations with competitive mixing

We consider the propagation speeds of steady waves simulated by particles with stochastic motions, properties and mixing (Pope particles). Conventional conservative mixing is replaced by competitive mixing simulating invasion processes or conditions in turbulent premixed flames under the flamelet regime. The effects of finite correlation times for particle velocity are considered and wave propagation speeds are determined for different limiting regimes. The results are validated by stochastic simulations. If the correlation time is short, the model corresponds to the KPP–Fisher equation, which is conventionally used to simulate invasion processes. If the parameters of the simulations are properly selected, the model under consideration is shown to be consistent with existing experimental evidence for propagation speeds of turbulent premixed flames.     

Single fiber model of particle retention in an acoustically driven porous mesh

A method for the capture of small particles (tens of microns in diameter) from a continuously flowing suspension has recently been reported. This technique relies on a standing acoustic wave resonating in a rectangular chamber filled with a high-porosity mesh. Particles are retained in this chamber via a complex interaction between the acoustic field and the porous mesh. Although the mesh has a pore size two orders of magnitude larger than the particle diameter, collection efficiencies of 90% have been measured. A mathematical model has been developed to understand the experimentally observed phenomena and to be able to predict filtration performance. By examining a small region (a single fiber) of the porous mesh, the model has duplicated several experimental events such as the focusing of particles near an element of the mesh and the levitation of particles within pores. The single-fiber analysis forms the basis of modeling the overall performance of the particle filtration system.     

Plasma sprayed Al2O3/FeCrAl composite coatings for electromagnetic wave absorption application

Al₂O₃/FeCrAl composite coatings were fabricated by atmosphere plasma spraying technique. Microstructure and dielectric properties in the frequency range from 8.2 to 12.4 GHz were investigated. The microstructure of composite coatings shows a uniform dispersion of metal particles with litter pores and microcracks in the composite coatings. The relaxation polarization and interfacial polarization in the coatings would contribute to enhance ?′ with rising FeCrAl content, and the associated loss could be considered as a dominating factor enhancing ?″. By calculating the microwave-absorption as a single-layer absorber, for the composite coatings with 41 wt.% FeCrAl content, the reflection loss values exceeding −10 dB are achieved in the frequency range of 9.1-10.6 GHz when the coating thickness is 1.3 mm.     

On the macroscopic response,microstructure evolution,and macroscopic stability of short-fibre-reinforced elastomers at finite strains: I – Analytical results

This paper presents a homogenization-based constitutive model for the mechanical behaviour of particle-reinforced elastomers with random microstructures subjected to finite deformations. The model is based on a recently improved version of the tangent second-order (TSO) method (Avazmohammadi and Ponte Castañeda, J. Elasticity 112 (2013) p.139–183) for two-phase, hyperelastic composites and is able to directly account for the shape, orientation, and concentration of the particles. After a brief summary of the TSO homogenization method, we describe its application to composites consisting of an incompressible rubber reinforced by aligned, spheroidal, rigid particles, undergoing generally non-aligned, three-dimensional loadings. While the results are valid for finite particle concentrations, in the dilute limit they can be viewed as providing a generalization of Eshelby’s results in linear elasticity. In particular, we provide analytical estimates for the overall response and microstructure evolution of the particle-reinforced composites with generalized neo-Hookean matrix phases under non-aligned loadings. For the special case of aligned pure shear and axisymmetric shear loadings, we give closed-form expressions for the effective stored-energy function of the composites with neo-Hookean matrix behaviour. Moreover, we investigate the possible development of “macroscopic” (shear band-type) instabilities in the homogenized behaviour of the composite at sufficiently large deformations. These instabilities whose wavelengths are much larger than the typical size of the microstructure are detected by making use of the loss of strong ellipticity condition for the effective stored-energy function of the composites. The analytical results presented in this paper will be complemented in Part II (Avazmohammadi and Ponte Castaneda, Phil. Mag. (2014)) of this work by specific applications for several representative microstructures and loading configurations.