Optimum Design of Erosion-Stable Heatshield Composite Materials

The present paper describes the development of multilayered heatshield composite materials. These methods permit us to design an optimum composition of heatshield composite materials for aerospace equipment under different heat and mechanical influences. In accordance with the theory developed, an erosion-stable heatshield three-layered composite material was synthesized, consisting of the upper erosion-stable layer based on the fabric reinforcing filler, the middle high-temperature heatshield layer and the lower low-temperature heatshield layer. Thicknesses of the layers and compositions of the fillers are selected by the methods of optimum design. Specimens were manufactured and gas-dynamical testing was conducted on an erosion-stable composite revealing its very suitable properties and verifing the adequacy of the model developed for the optimum design of heatshield composites.     

Layerwise fundamental solutions and three-dimensional model for layered media

A hybrid method is presented for the analysis of layers, plates, and multilayered systems consisting of isotropic and linear elastic materials. The problem is formulated for the general case of a multilayered system using a total potential energy formulation and employing the layerwise laminate theory of Reddy. The developed boundary integral equation model is two-dimensional, displacement based and assumes piecewise continuous distribution of the displacement components through the system's thickness. A one-dimensional finite element model is used for the analysis of the multilayered system through its thickness, and integral Fourier transforms are used to obtain the exact solution for the in-plane problem. Explicit expressions are obtained for the fundamental solution of a typical infinite layer (element), which can be applied in a two-dimensional boundary integral equation model to analyze layered structures. This model describes the three-dimensional displacement field at arbitrary points either in the domain of the layered medium or on its boundary. The proposed method provides a simple, efficient, and versatile model for a three-dimensional analysis of thick plates or multilayered systems.Visiting Assistant ProfessorOscar S. Wyatt, Jr. Chair     

Microwave detection and depth determination of disbonds in low-permittivity and low-loss thick sandwich composites

Nondestructive evaluation (NDE) of disbonded low-permittivity and low-loss dielectric multilayered composite media is of considerable interest in many applications. The ability of microwaves to penetrate inside dielectric materials makes microwave NDE techniques very suitable for interrogating structures made of multilayered dielectric composites. Additionally, the sensitivity of microwaves to the presence of dissimilar layers in such materials allows for accurate detection of a disbonded layer. In a multilayered composite, a disbond may occur between any two (or more) layers. The potential of utilizing microwave NDE techniques for the detection and depth estimation of disbonds in a thick sandwich composite is investigated. This study utilizes a theoretical model developed for investigating the interaction of microwave radiation from an open-ended rectangular waveguide sensor with ann-layer dielectric composite medium. The influence of the standoff distance between the sensor and the medium and the operating frequency on the sensitivity of disbond detection and depth estimation is studied to obtain an optimum set of parameters for enhanced detection sensitivity. Results of the theoretical study are presented with a discussion on the optimization process for a thick sandwich composite composed of 13 dielectric layers.     

Thermoelastic solutions for multilayered electronic assemblies

The presence of dissimilar material systems and thermal gradients introduces thermal stresses in multilayered electronic assemblies and packages during fabrication and operation. The thermal stresses of multilayered electronic assemblies near free edges play an important role in determining their reliability. Therefore, it is important to provide designers a good estimate of stresses near free edges. According to the heat conduction mechanism of integrated circuits, the temperature field in the chip, adhesive layer and substrate is derived and solved when the chip works in a steady state. Taking the temperature field in the chip, adhesive layer and substrate as the heat source, we solve the thermal stress field in the chip, adhesive layer and substrate by using the technique of Fourier's series expansion. The effects of the geometric parameters of the chip, adhesive layer and substrate on thermal stresses are analyzed. From the analysis of thermal stresses in the chip–adhesive layer–substrate structure, it can be found that the stress concentration near free edges is more prominent. In the design of electronic packagings, the stress concentration near free edges which may lead to the failure or malfunction of electronic assemblies and packages should be taken into account in detail.     

Interfacial microstructure and defect analysis in Cu(In,Ga)Se([sub]2)-based multilayered film by analytical transmission electron microscopy and focused ion beam

Interfacial microstructures of Cu(In,Ga)Se₂(CIGS)-based multilayered film are closely characterized by TEM (transmission electron microscopy), SEM (scanning electron microscopy) and FIB (focused ion beam). A cross-sectional TEM, energy dispersive X-ray spectroscopy and energy-filtered TEM reveal that a pronounced Cu diffusion occurs across the interface of the CdS/CIGS, which leads to a large amount of Cu rich in the CdS layer and a Cu-deficient sub-surface in the CIGS layer as well as a rough interfacial structure. TEM studies further reveal that the interface microstructures in the multilayered film are dissimilar, both ZnO/CdS and CdS/CIGS interfaces are strongly bonded whereas the CIGS/Mo interface is weakly bonded and interface separation occasionally occurs. Mo back contact layer shows a well adhesion to glass substrate.Detailed observation on defects in the CIGS-based multilayered film is carried out by 3D (3-dimensional) FIB and SEM techniques. Sequential 2D (2-demensional) cross-sectioning shows that dominant growth-defects in the CIGS and top SiO₂ layers are micro-scale crack, appearing as diversified morphologies. The micro-scale crack in the CIGS layer is possibly released by propagating into the adjacent layer while the crack in the SiO₂ layer is relieved usually by forming a small particle behind. It is noted that in the multilayered film the interface frequently acts as crack initiation sites due to distinct thermal expansion coefficients.     

Sputter deposition of multilayered structures for use in sputter depth profile calibration

The development of standards and methods for calibration and comparison of depth scales, sputter removal rates and sputter depth profiling by surface analytical techniques (e.g. AES, SIMS, RBS and X-ray absorption) requires well controlled deposition of multilayered structures. The plasma beam-sputter deposition technique was used to produce combinations of multilayered structures, consisting of metal and oxide layers on Si(100) substrates. Ni, Cr, Ag, Ta, Au, Cr₂O₃ and Ta₂O₅ films were combined and produced in three structural designs of standard reference materials (SRM's), to be used for sputter depth profiling calibration namely, single thin film types, periodically modulated multilayered thin film structures and multilayered ‘marker’ structures. Tetrode sputtering equipment (SPUTRON II¹ of Balzers) was found to be an appropriate apparatus for the deposition of the chosen materials on the small production scale. Four in situ interchangeable targets were used to make highly reproducible layers, having the required quality and especially minimal layer and inter-layer contamination. Characterization of some of the multilayered structures developed showed, that periodically modulated system Ni/Cr/Ni … with well defined repetitive profiles and interface depth resolution is primarily suited for use in sputter depth profiling calibration. This multilayered structure is issued by NBS, Washington as Standard Reference Material No 2135.     

Extended Sneddon and Muki solutions for multilayered elastic materials

This paper extends the Sneddon and Muki solutions to solve elastostatic problems in multilayered elastic materials. Using Hankel transforms, Sneddon and Muki developed general solutions of elastostatic problems in elastic materials occupying either halfspace or one layer regions. Based on the Sneddon and Muki solutions and a transfer matrix method, solutions are presented for elastostatic problems in multilayered elastic materials subject to both external and internal loading. A special technique is adopted to eliminate the ill-conditioned matrices associated with the conventional transfer matrix method for multilayered materials. The extended Sneddon and Muki solutions can be easily calculated using a personal computer. Numerical examples given in the paper show that the extended Sneddon and Muki solution generates results with high accuracy and efficiency. The extended Sneddon and Muki solution has wider applications to engineering science.     

Peeling Back the Layers: Controlled Erosion and Triggered Disassembly of Multilayered Polyelectrolyte Thin Films

Methods for the layer‐by‐layer deposition of oppositely charged polymers on surfaces can be used to assemble thin multilayered films using a broad range of natural, synthetic, and biologically relevant materials. These methods also permit precise, nanometer‐scale control over the compositions and internal structures of multicomponent assemblies. Provided that the individual components of these materials are selected or designed appropriately, these methods provide tantalizing new opportunities to design thin films and coatings that provide spatial, temporal, or active control over the release of one or several different agents from surfaces. The last two years have seen a significant increase in reports describing the development of new chemical, physical, and biomolecular approaches to the controlled erosion, triggered disassembly, or general deconstruction of multilayered polymer films. In this Progress Report, we highlight recent work from our laboratory and several other groups toward the design of ultrathin multilayered assemblies that i) permit broad, tunable, and sophisticated control over film erosion, and ii) provide new opportunities for the localized release of macromolecular therapeutics, such as DNA and proteins, from surfaces.     

Analytical frequency response functions for contact of multilayered materials

Multilayer coatings are often seen in surface engineering for surface modifications. Optimal design of the multilayered materials requires the understanding of their mechanical behaviors based on deformation and stress analyses. The frequency response functions (FRFs) of the displacement and stress fields in multilayered materials under unit normal and shear loadings are the analytical cores for solving the contact of such materials. The authors have successfully derived these functions by utilizing the Papkovich–Neuber potentials and appropriate boundary conditions. Two matrix equations containing unknown coefficients in the FRFs are established by following the structure rules, and then the closed-form FRFs written in a recurrence format are established. A fast numerical semi-analytical model based on the derived FRFs is further developed for investigating the elastic contact of multilayered materials with any desired material design.     

A Crack Emanating from a Wedge In Dissimilar Anisotropic Materials Under Antiplane Shear

A crack in a composite wedge consisting of two dissimilar anisotropic materials under concentrated antiplane loads is analyzed. The problem of a crack in an isotropic composite wedge is solved first by using the Mellin transform and the Wiener-Hopf method. Using a linear transformation of the original composite wedge into an isotropic composite wedge consisting of dissimilar isotropic materials, the antiplane displacement and stresses for the anisotropic composite wedge are obtained from the solution for the transformed wedge. The stress intensity factor for the crack in the original anisotropic composite wedge is obtained from the solution of the crack in the transformed wedge. Special attention is given to the asymptotic problem of a wedge crack in an anisotropic bimaterial. Numerical computations are carried out to obtain the energy release rate for various apex angles and anisotropic parameters as a function of crack angle.     

Microstructural scale effects in the nonlinear elastic response of bio-inspired wavy multilayers undergoing finite deformation

Materials with wavy microstructures span disciplinary boundaries, yet relatively little systematic work has been done to characterize their thermo-mechanical response. Motivated by recent work on the elastic–plastic response of wavy periodic multilayers, and the discovered layer thickness effect in the post-yield domain, we extend our finite-volume based homogenization theory in order to investigate the effect of microstructural refinement, as well as geometric and material parameters, in this class of periodic materials in the finite-deformation domain. Micromechanical analysis of a model wavy multilayered system which mimics certain biological tissues quantifies the importance of layer thickness, and hence the microstructural bending stiffness, on the stiffening stress–stretch response. The role of the matrix phase in the unfolding process is also highlighted. The results provide insight into the design of materials intended to mimic the response of a certain class of biological tissues with stiffening characteristics such as chordae tendineae.     

A Review on Dissimilar Friction Stir Welding of Copper to Aluminum: Process,Properties, and Variants

Copper and aluminum materials are extensively used in different industries because of its great conductivities and corrosion resistant nature. It is important to join dissimilar materials such as copper and aluminum to permit maximum use of the special properties of both the materials. The joining of dissimilar materials is one of the most advanced topics, which researchers have found from last few years. Friction stir welding (FSW) technology is feasible to join dissimilar materials because of its solid state nature. Present article provides a comprehensive insight on dissimilar copper to aluminum materials joined by FSW technology. FSW parameters such as tool design, tool pin offset, rotational speed, welding speed, tool tilt angle, and position of workpiece material in fixture for dissimilar Cu–Al system are summarized in the present review article. Additionally, welding defects, microstructure, and intermetallic compound generation for Cu–Al FSW system have been also discussed in this article. Furthermore, the new developments and future scope of dissimilar Cu–Al FSW system have been addressed.     

Effect of interface layer consisting of nanosprings on stress field near interface edge

The purpose of this study is to investigate the effect of an interface layer consisting of discretely arrayed nano-sized elements on stress intensified fields. A material where an interface layer consisting of Ta₂O₅ helical nanoelements (nanosprings) is inserted between dissimilar components is prepared and two types of crack initiation experiments, which possess radically different stress conditions, are carried out. The finite element analyses indicate that the stress fields in the components with and without the interface layer are completely different, and it is experimentally clarified that the fracture mechanics concept cannot be applied to the crack initiation at the dissimilar interface edge with the interface layer. The stress distributions at the crack initiation reveal that the crack initiation is governed by the apparent stress of the nanospring, σ′, at the edge. This signifies that the interface layer eliminates the stress singular field at the interface edge. The criterion of the crack initiation is evaluated as .     

c-Axis zig-zag ZnO film ultrasonic transducers for designing longitudinal and shear wave resonant frequencies and modes

A method for designing frequencies and modes in ultrasonic transducers above the very-high-frequency (VHF) range is required for ultrasonic non-destructive evaluation and acoustic mass sensors. To obtain the desired longitudinal and shear wave conversion loss characteristics in the transducer, we propose the use of a c-axis zig-zag structure consisting of multilayered c-axis 23° tilted ZnO piezoelectric films. In this structure, every layer has the same thickness, and the c-axis tilt directions in odd and even layers are symmetric with respect to the film surface normal. c-axis zig-zag crystal growth was achieved by using a SiO(2) low-temperature buffer layer. The frequency characteristics of the multilayered transducer were predicted using a transmission line model based on Mason's equivalent circuit. We experimentally demonstrated two types of transducers: those exciting longitudinal and shear waves simultaneously at the same frequency, and those exciting shear waves with suppressed longitudinal waves.     

Multimaterial multijoint topology optimization

In this paper, a methodology that solves multimaterial topology optimization problems while also optimizing the quantity and type of joints between dissimilar materials is proposed. Multimaterial topology optimization has become a popular design optimization technique since the enhanced design freedom typically leads to superior solutions; however, the conventional assumption that all elements are perfectly fused together as a single piece limits the usefulness of the approach since the mutual dependency between optimal multimaterial geometry and optimal joint design is not properly accounted for. The proposed methodology uses an effective decomposition approach to both determine the optimal topology of a structure using multiple materials and the optimal joint design using multiple joint types. By decomposing the problem into two smaller subproblems, gradient‐based optimization techniques can be used and large models that cannot be solved with nongradient approaches can be solved. Moreover, since the joining interfaces are interpreted directly from multimaterial topology optimization results, the shape of the joining interfaces and the quantity of joints connecting dissimilar materials do not need to be defined a priori. Three numerical examples, which demonstrate how the methodology optimizes the geometry of a multimaterial structure for both compliance and cost of joining, are presented.     

聚偏氟乙烯/聚甲基丙烯酸甲酯交替多层材料的热稳定性

利用微层共挤出技术制得不同层数(2,16,64层)的聚偏氟乙烯(PVDF)/聚甲基丙烯酸甲酯(PMMA)交替多层材料,通过偏光显微镜、垂直燃烧测试、热失重分析、红外光谱分析、微型量热测试研究了层数的变化对体系热分解和热释放行为的影响。结果表明,PVDF层与PMMA层沿层状样品的厚度方向交替排列,层结构明显,层界面清晰,随着层数的增加,层界面数增加,材料的垂直燃烧行为几乎不变,但表现出更高的热稳定性;高层数样品中热稳定性优异的PVDF层对易热解的PMMA层保护作用增强,且在热释放过程中,更多的层界面为PVDF炭层的形成提供了更丰富的空间,使材料的热释放速率减小,总热释放降低。     

铁磁层/导电层/铁磁层多层膜中巨磁阻抗效应理论*

采用经典电磁理论,对铁磁层／导电层／铁磁层（M／C／M）多层膜中出现的巨磁阻抗效应进行了理论分析。对于单轴横向磁各向异性多层膜,理论计算结果表明：高频阻抗在某一外加磁场（近似等于等效各向异性场）下出现最大值,铁磁层和导电层电阻率相关较大的多层膜中将出现较强的巨磁阻抗效应。多层膜在1MHz附近即可出现远大于单层膜的阻抗变化比。多层膜理论计算与实验结果能够较好地符合。     

Plasmonic Metamaterial Cloaking at Optical Frequencies

In this paper, we present the design of cylindrical and spherical electromagnetic cloaks working at visible frequencies. The cloak design is based on the employment of layered structures consisting of alternating plasmonic and nonplasmonic materials, and exhibiting the collective behavior of an effective epsilon-near-zero material at optical frequencies. The design of a cylindrical cloak to hide cylindrical objects is first presented. Two alternative layouts are proposed, and both magnetic and nonmagnetic objects are considered. Then, the design of spherical cloaks is also presented. The full-wave simulations presented throughout the paper confirm the validity of the proposed setup, and show how this technique can be used to reduce the observability of cylindrical and spherical objects. The effect of the losses is also considered.