Unusual one-way edge state in acoustic gyroscopic continuum

Unusual one-way edge states have been observed in composite structures composed of periodic lattices loaded with gyroscopes.Here, we provide a continuum-mechanics understanding to the one-way edge state by formulating surface state equations of acoustic gyroscopic mediums with Hermite mass density tensor. We discover that the unidirectional edge effect arises from nontrivial off-diagonal components of Hermite densities, which causes the symmetric breaking of surface wave propagation towards forward and backward directions. Theoretical predictions on the velocity and decay length of surface waves coincide excellently with numerical simulations. The unidirectional edge state in a two-interface gyroscopic medium is also analyzed.Due to the rotational symmetry in geometry, the unidirectional edge state on one interface is able to prevent itself from the coupling to surface waves on the other interface regardless of the slab thickness. With these anomalous effects, surface waves residing on gyroscopic mediums can flow around the edge defects without back-scatterings, or can be split into two beams of equal energy magnitudes. Our findings may make a bridge that would help to reach the design of non-reciprocal composite materials via an effective medium approach.     

Microstructural studies and analysis of residual stresses in Co–TiC composites obtained by the spontaneous crystallization method

Microstructural studies and analysis of internal stresses in Co–TiC composites prepared by the spontaneous crystallization method (high-temperature solution technique) have been performed. Metallographic observations and X-ray phase analysis of the obtained Co–TiC composite ingots were carried out. The microstructure of the Co–TiC interface was studied using transmission electron microscopy and high-resolution transmission electron microscopy. The Co–TiC composites were also subjected to numerical analysis by the finite-element method. The presented results of calculations include the analysis of the state of internal stresses generated in the composite during its cooling from the crystallization temperature to room temperature. On the basis of these results the internal stress distribution and its level were determined for the obtained Co–TiC composites.     

Influence of phase interface properties on mechanical characteristics of metal ceramic composites

The paper reports on theoretical study to elucidate the influence of geometric (width) and mechanical characteristics of phase interfaces on strength, ultimate strain, and fracture energy of metal ceramic composites. The study was performed by computer simulation with the movable cellular automaton method and a well-developed mesoscale structural composite model that takes explicit account of wide transition zones between reinforcing inclusions and the matrix. It is shown that the formation of relatively wide “ceramic inclusions-binder” interfaces with gradual variation in mechanical properties allows a considerable increase in the mechanical properties of the composite. Of great significance is not only the interface width but also the gradient of mechanical properties in the transition zone. The presence of defects and inclusions of nano- and atomic scales in interface regions can increase internal stresses in these regions, induce a steep gradient of mechanical properties in them, and hence decrease strain characteristics and fracture energy of the composite.     

Localized excitonic states in ZnS–ZnSe single quantum wells

We present low temperature photoluminescence spectra taken from an 11Å ZnSe quantum well in ZnS barriers. The samples are grown by the technique of photo-assisted vapour phase epitaxy (PAVPE) and the spectra show evidence for interface disorder. The observed dependences of the excitonic luminescence on excitation power and temperature are interpreted by a model involving excitonic localization below an exciton mobility edge. This mobility edge is measured for these samples to be 6 meV below the free exciton energy in the ideal quantum well.     

Surface deformation induced by a variation in surface stresses in anisotropic half-regions

The deformations and the stresses in anisotropic half-regions taking into account surface stresses originating from surface energy, which exists originally at surfaces and interfaces dividing phases, are analysed theoretically. In the present paper, the equilibrium equation of force considering surface stresses is used to calculate the inelastic deformation induced by a variation in surface stresses. The problem of varying surface stresses in a half-surface of a half-infinite anisotropic domain is analysed using the theory of elasticity. This problem is related to the occurrence of cracks in contaminated, oxidized or chemisorbed surfaces. Stress analysis on the basis of continuum mechanics is performed precisely under the boundary condition taking into account surface stresses. The Fourier transform technique is applied to perform the analysis, and the components of stress and displacement are expressed in an explicit form. The shear component of bulk stress attains infinity at the edge of discontinuity of the surface stresses, and the free surface deforms like an edge dislocation. This result suggests that cracking in a chemically contaminant surface is easier than in a clean surface.     

Modelling of the transverse strength of fibre reinforced epoxy composite at low and high temperature

Process-induced thermal residual stresses and matrix failure of unidirectional carbon fibre reinforced composites (CFRP) have been investigated by finite element analysis (FEA). We used a partial discrete FEA model based on a unidirectional composite consisting of a microscopic area of fibres and matrix surrounded by a homogenised composite area. The FEA provided information about the stress state in the matrix and the fibre–matrix interface. The transverse strength of the composite was calculated regarding matrix failure and fibre matrix debonding. The influence of the temperature on the Young's modulus, the non-linear stress–strain behaviour and the strength of the matrix were investigated in detail. Following this approach it was possible to incorporate the resulting microresidual stresses on the transverse strength of the composite. Tensile tests of the neat resin and of the composite were performed in the temperature range of ?40°C to 60°C. The results of the FEA modelling are in good agreement with the experimental results of the transverse tests.     

Generation of rectangular prismatic dislocation loops in shells and cores of composite nanoparticles

A theoretical model has been proposed for describing the relaxation of misfit stresses in a spherically symmetric composite core-shell nanoparticle due to the generation and expansion of rectangular prismatic dislocation loops at the internal and external interfaces. The critical conditions of the formation of these loops have been calculated for nanoparticles consisting of a relatively massive core and a thin shell. It has been shown that the generation of dislocation loops is possible when the misfit of the lattice parameters of the core and shell of the nanoparticle exceeds a critical value that depends on the nanoparticle radius, the shell thickness, the loop formation position, and the shape of loops. This condition holds for a loop in the shell when the shell thickness either lies in a specific range of small values or (for a larger misfit) is less than a critical value. For the generation of loops in the core, the shell thickness should exceed a critical value. The dislocation loops elongated along the core-shell interface are formed more readily. As the shell thickness increases at a fixed nanoparticle radius, the energetically more favorable generation of a dislocation loop occurs first from the free surface into the bulk of the shell, then from the interface into the shell, and finally from the interface into the core of the nanoparticle.     

Interface response theory of discrete composite systems

A general theory of “interface responses” in discrete composite d-dimensional systems for operators with two-body interactions is presented. It is shown that the “interface responses” of all the internal and external interfaces of any composite system are the linear superposition of the responses to a coupling operator of all individual interfaces and of the responses to a cleavage operator of the corresponding ideal free surfaces of the same but non-interacting subsystems. The response function and its elements between two space points of the system are given by a new simple general equation as a function of these “interface responses” and of the bulk response functions of each subsystem contained in the complete real system. The present paper establishes this new general two-body theory of interface responses for surfaces, interfaces, adsorbates, membranes, superlattices, defects of any kind and dimension, ... and for the first time, to the knowledge of the author, for any d-dimensional composite system. The presentation of the theory is followed here by a few general applications.     

Stability analysis on the free surface phenomena of a magnetic fluid for general use

This paper presents an analysis for elucidating a variety of physical processes on the interface (free surface) of magnetic fluid. The present analysis is composed of the magnetic and the fluid analysis, both of which have no limitations concerning the interface elevation or its profile. The magnetic analysis provides rigorous interface magnetic field under arbitrary distributions of applied magnetic field. For the fluid analysis, the equation for interface motion includes all nonlinear effects. Physical quantities such as the interface magnetic field or the interface stresses, obtained first as the wavenumber components, facilitate confirming the relations with those by the conventional theoretical analyses. The nonlinear effect is formulated as the nonlinear mode coupling between the interface profile and the applied magnetic field. The stability of the horizontal interface profile is investigated by the dispersion relation, and summarized as the branch line. Furthermore, the balance among the spectral components of the interface stresses are shown, within the sufficient range of the wavenumber space.     

激光辐照热障涂层中平面应变问题的热弹性变分分析

 针对激光辐照热障涂层材料的平面应变问题，提出热障涂层热弹性分析的基本方程，对定常温度场给出级数形式解析解，并用最小余能原理和变分法分析了结构的热弹性应力场，研究了最大应力和界面应力的分布特征，并就一些物理参数的影响进行了讨论。结果表明，热障涂层的主要破坏因素为表面拉伸应力，界面应力相对较小，但在自由边界有集中现象，剥落应力大于剪切应力，是导致涂层破坏的重要原因。涂层厚度增加会改变厚度方向上的应力分布，界面应力向中心集中。     

Effects of an inhomogeneous elliptic insert on the elastic field of an edge dislocation

The problem of an edge dislocation in the vicinity of an elliptical inhomogeneity is solved analytically in the form of infinite series. The inhomogeneous inclusion is assumed to be either perfectly bonded or sliding along the interface with the surrounding matrix. The problem is formulated in terms of Papkovich-Neuber displacement potentials and the results for the perfectly bonded and sliding interface are compared. The effects of the inclusion/matrix shear moduli ratio, inclusion/dislocation relative distance and inclusion aspects ratio on the interface stresses and the dislocation force are evaluated.     

Effects of shear stiffness,rotatory and axial inertia,and interface stiffness on free vibrations of a two-layer beam

The linear dynamics of a two-layer beam is considered. Each beam is modelled by the Timoshenko kinematics, and all inertia terms are considered. The interface allows normal and tangential detachments, which are linearly proportional to the transmitted stresses. The eigenvalue problem providing the natural frequencies is solved exactly.An extensive study of the natural frequencies and of the normal modes is performed with the aim of understanding how the mechanical parameters of the composite influence the vibration behaviour.The study of the convergence of the solution toward that of a simplified problem obtained by neglecting axial and rotational inertia, shear deformations and by considering interface perfect adherence in the normal direction, constitutes the second objective of the work. This permits to consciously assess when these mechanical terms can actually be neglected, contrarily to what is usually done in practice, where they are disregarded ‘a priori’, without adequate care.     

On the growth rate of bending induced edge cracks in acoustically excited panels

Parts of an aircraft structure may be made to vibrate as a result of acoustic waves generated by various aircraft noise sources impinging on the structure. The stresses associated with this acoustically induced vibration may be sufficiently large to result in fatigue failure of portions of the structure. If acoustically induced fatigue cracks occur in the stiffened skin structures widely used in aircraft construction they may initiate in the skin panels near the stiffener attachment points. The initiation and subsequent propagation of these cracks at the panel edges is primarily due to the bending stresses arising from the out-of-plane vibration of the individual skin panels.The emphasis of the work described in this paper is on examining the growth rate of edge cracks in acoustically excited panels. A single panel with an edge crack is considered and this structural element is modelled as a flat plate clamped on three edges and part of the fourth. The crack is represented by the unclamped part of the fourth edge. Fracture mechanics principles are used to predict the crack growth rates associated with the first two modes of vibration of the edge cracked panel. The crack tip stress intensity factors associated with these panel modes are estimated by a technique based on finding the nominal bending stresses at the crack tips. The nominal bending stresses are in turn found from mode shapes determined by the Rayleigh Principle. The validity of the various assumptions is assessed by comparing the predicted crack growth rates with measured growth rates in panels representative of those used in aircraft construction.     

Effect of local curvature of the coating-substrate interface on deformation and fracture of ceramic coatings under uniaxial tension

Scanning electron microscopy and atomic force microscopy have been applied to study the fracture of SiAlN coatings on Cu substrates under uniaxial tension. It is shown that coating spalling occurs in the zones of local curvature of the SiAlN-Cu interface which form due to dislocation glide in the substrate. Preliminary ion bombardment of the substrate suppresses dislocation-induced kinking at the coating-substrate interface and increases the adhesive strength of the coatings, thus preventing their edge delamination. At the same time, the wavy coatingsubstrate interface resulting from ion bombardment gives rise to normal stresses that lead to the buckling and spalling of the coatings in the zones of positive local curvature of the interface.     

Effect of in-plane residual stresses on the post-deflection behaviour of a T-shaped deflected crack in layered systems

Post-deflection behaviour of a T-shaped crack in a bilayer material system under the presence of in-plane residual stress was investigated for a four-point bending configuration. Non-dimensional parameters for generalization of the residual stress state in a bilayer system were defined using globally measurable quantities such as curvatures and axial strains. It was predicted that the residual stress varying from tension at the outer edge to compression at the interface was beneficial to prevent kinking of the T-shaped crack out of the interface and therefore to toughening of the layered composites. However, it was expected that appreciable improvement in toughness properties was possible only when the intact layer is stiffer than the cracked layer. Further, incorporation of residual stresses higher than a certain level did not result in a further improvement in the toughness properties. Criteria for crack deflection-induced toughening of layered composites were derived from these predictions.     

无氧铜的准等熵压缩性

 利用递变冲击阻抗材料叠合而成的组合飞片，在二级轻气炮上对无氧铜进行了准等熵压缩性测量，加载时间约达1 μs。用激光速度干涉仪连续记录了不同厚度处无氧铜样品自由面速度随时间的变化过程，并通过拉格朗日波分析技术计算得到40 GPa下的准等熵的应力-应变曲线。结果表明：在低应力区，无氧铜的准等熵压缩线位于冲击绝热线之上；到32 GPa以上，准等熵压缩线才回落到冲击绝热线之下。这个现象与Barker、Chhabildas等对铝与钨的实测现象是一致的，它表明：在低应力区，材料的冲击强化效应与加载速率密切相关。     

Effect of long-range stresses on the structure of semiconductor heterosystems

The dependence between the misfit value of the interface layers f and the structural parameters of the misfit dislocations is obtained for arbitrary orientation of the interface of a semiconductor heterosystem subjected to the complete relaxation of misfit stresses. Such parameters are the distance D _i between neighboring dislocations of the ith family and projection of the edge component of the Burgers vector onto the interface (b _i ^e ). The number of families incorporated into the interface is determined by the orientation of the boundary and occurrence of the relaxation process. The role of specific cases of this expression for experimental and technological applications is discussed using the appearance of long-range shear and normal stresses in films with orientations of (001) and (111) as examples.     

Stress-driven phase transformation and the roughening of solid-solid interfaces

The application of stress to multiphase solid-liquid systems often results in morphological instabilities. Here we propose a solid-solid phase transformation model for roughening instability in the interface between two porous materials with different porosities under normal compression stresses. This instability is triggered by a finite jump in the free energy density across the interface, and it leads to the formation of fingerlike structures aligned with the principal direction of compaction. The model is proposed as an explanation for the roughening of stylolites-irregular interfaces associated with the compaction of sedimentary rocks that fluctuate about a plane perpendicular to the principal direction of compaction.     

Effects of residual stress and interfacial frictional shear stress on interfacial debonding at notch-tip in two-dimensional unidirectional composite

A calculation method based on the shear lag approach was presented to get an approximate estimate of influences of residual stresses and frictional shear stress at the debonded interface on the interfacial debonding behavior at the notch-tip along fiber direction in two-dimensional unidirectional double-edge-notched composites. With this method, the energy release rate for initiation and growth of debonding as a function of composite stress were calculated for some examples. The calculation results showed in outline how much the tensile and compressive residual stresses in the matrix and fiber along fiber direction, respectively, act to hasten the initiation and growth of the debonding when the final cut element in the notch is matrix, while they act to retard them when the final cut element is fiber, and how much the frictional shear stress at the debonded interface reduces the growth rate of the debonding.     

Investigation of interfacial stress transfer in a PBO/polypropylene microdroplet composite using synchrotron microfocus X-ray diffraction

The micromechanics of stress transfer in a PBO/polypropylene system were followed using a microdroplet model composite and a synchrotron microfocus X-ray diffraction technique. High quality X-ray diffraction patterns were obtained from both the PBO-HM fiber and the polypropylene matrix. Diffraction patterns were obtained from the fiber inside the PP droplet by subtraction of a PP diffraction pattern from that of the composite. It was found that there was good stress transfer at the fiber–matrix interface and that significant residual stresses occurred in the fiber inside the droplet due to the cooling and crystallization of the polypropylene. The maximum residual stress of 0.55 GPa measured corresponded to an axial fiber strain of 0.2% compressive strain, similar to the literature values for compressive fiber failure.