Transport properties in fibrous elastic rhombic composite with imperfect contact condition

In this contribution, effective elastic moduli are obtained by means of the asymptotic homogenization method (AHM), for oblique two-phase fibrous periodic composites with two models (spring and interphase) of imperfect contact conditions. This work is an extension of previous reported results, where only perfect contact for elastic or piezoelectric composites for square and hexagonal arrays were considered. The constituents of the composites exhibit transversely isotropic properties. A doubly periodic parallelogram array of cylindrical inclusions under longitudinal shear is considered. The behavior of the shear elastic coefficient for different geometry arrays related to the angle of the cell is studied. As validation of the present method, some numerical examples and comparisons with theoretical and experimental results verified that the present model is efficient for the analysis of composites with presence of imperfect interface and parallelogram cell. The effect of the arrangement of the cells on the shear effective property is observed. The present method can provide benchmark results for other numerical and approximate methods.     

Asymptotic buckling analysis of imperfect shallow spherical shells on non-linear elastic foundation

An asymptotic solution is formulated for non-linear buckling of elastically restrained imperfect shallow spherical shells continuously supported on a non-linear elastic foundation. The asymptotic iteration method is introduced to result in a cubic non-linear analytical relation between the external load and central transverse displacement (deflection) of the structures incorporating the effects of geometrical imperfection, edge-restraint coefficients, moduli of foundation and characteristic geometrical parameter. The resulting expression can be used easily to evaluate the effects of these factors on buckling behaviors. Numerical examples are given, and comparisons of the available results show validity of the method suggested in the present work.     

Homogenized properties of elastic–viscoplastic composites with periodic internal structures

A homogenization theory for elastic–viscoplastic composites with periodic internal structures is developed in rate and incremental forms by considering unit cells subjected to macroscopically uniform stress and strain. The theory enables us to incrementally compute the macroscopic as well as the microscopic stress and strain states in nonlinear time-dependent periodic composites. The theory is effective for problems in which the history of either macro-strain or macro-stress, or a combination of them, is prescribed as a function of time. As applications of the theory, transverse and off-axial deformation problems of metal matrix composites reinforced unidirectionally with continuous fibers are analyzed to discuss the effects of fiber arrangement and orientation on the macroscopic deformation behavior.     

Analysis of composite finite wedges under anti-plane shear

Problems of composite finite wedges under anti-plane shear applied on a circular arc are analyzed in this study. The considered conditions of radial edges are free–free, free–fixed, and fixed–fixed. A procedure that uses the finite Mellin transform and the Laplace transform is developed to solve these problems. Explicit solutions for displacement and stress fields are derived. Stress intensity factors (SIFs) of composite circular shafts with an interfacial edge crack are extracted from the derived stress fields, and the distributions for various loading angles are presented and discussed. It was found that if the loading angles are the same, free–free and fixed–fixed edge problems can be degenerated into single material problems. Uniform stresses were found along the interface in free–free and fixed–fixed edge problems. Solutions of a general loading case deduced from the derived results compare well with those obtained from finite element (FE) analyses.     

Natural frequencies of composite plates with random material properties using higher-order shear deformation theory

Composites are known to display a considerable amount of scatter in their material properties due to large number of parameters associated with the manufacturing and fabrication processes. In the present work, the material properties have been taken as random variables for accurate prediction of the system behavior. Higher order shear theory including rotatory inertia effects has been accounted for in the system dynamic equations. A first order perturbation technique has been employed to obtain the solution of the governing equations. An approach has been outlined for obtaining closed form expressions for the variances of eigen solutions. The effects of side to thickness ratio and variation in standard deviation of the material properties have been investigated for cross-ply symmetric and anti-symmetric laminates. The mean and standard deviations of the first five natural frequencies have been worked out for laminated rectangular plates with all edges simply supported. The higher order shear deformation theory results have been validated with Monte Carlo simulation results and compared with the results based on classical laminate and first order shear deformation theories.     

Comparison of prediction on effective elastic property and shape optimization of truss material with periodic microstructure

This paper compares the representative volume element (RVE) method based on Dirichlet and Neumann boundary conditions with the homogenization method for predicting the effective elastic property of truss material with periodic microstructure. Numerical experiments show that, with increase of the number of the unit cell, n, the results of RVE method under the Dirichlet and Neumann boundary conditions converge towards those obtained with homogenization method from the above and below sides, respectively. For some specific types of the unit cell, RVE method gives the same results as those obtained with homogenization method, even if only one unit cell is included. For RVE method, a simple criterion for judging the existence of scale effects is whether the equilibrium of the boundary nodal forces is guaranteed under the Dirichlet boundary conditions, or whether the deformation compatibility at the unit cell boundaries is satisfied under the Neumann boundary conditions. We also discover that for a specific type of truss material, whose unit cell has no characteristic displacement defined in homogenization method, the volume average of members’ properties in the unit cell gives the exact prediction of the effective elastic properties. Finally, shape optimization technique is applied to find the optimal geometric shape of the unit cell for truss material with the maximum and minimum shear stiffness, and the numerical singularity involved is discussed as well.     

Modelling of curved interfaces in composite shells

Contact laws for thin curved interfaces in elastic multistructures have been established, within the framework of thin shell mechanics. Limit contact laws for the interface are obtained in a deductive manner, using an asymptotic expansion of the solution vs. the interface thickness. The limit solution is shown to depend on the ratio between the thickness of the shell and the radius of curvature of the mean plane of the shell. Different behaviours are evidenced, according to the scaling of the mechanical properties of the intermediate shell and the relative amount of curvature vs. thickness. The cases of very weakly curved shells, weakly curved shells, and strongly curved shells are highlighted. The effect of curvature on stress generation is evidenced, and the stress distribution is represented in some specific cases.     

Postbuckling of shear deformable cross-ply laminated cylindrical shells under combined external pressure and axial compression

A postbuckling analysis is presented for a shear deformable cross-ply laminated cylindrical shell of finite length subjected to combined loading of external pressure and axial compression. The governing equations are based on Reddy's higher order shear deformation shell theory with von Kármán–Donnell type of kinematic nonlinearity. The nonlinear prebuckling deformations and initial geometric imperfections of the shell are both taken into account. A boundary layer theory of shell buckling, which includes the effects of nonlinear prebuckling deformations, large deflections in the postbuckling range, and initial geometric imperfections of the shell, is extended to the case of shear deformable laminated cylindrical shells under combined loading cases. A singular perturbation technique is employed to determine interactive buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling response of perfect and imperfect, unstiffened or stiffened, moderately thick, antisymmetric and symmetric cross-ply laminated cylindrical shells for different values of load-proportional parameters.     

拉-拉疲劳载荷下复合材料迟滞回能分析研究

复合材料的疲劳失效是一个能量平衡的过程,分析了该过程中各种能量组份的特征及其相互间的关联.在对常幅拉-拉疲劳载荷下的复合材料疲劳过程研究后发现,迟滞回能的瞬态响应与循环次数和循环应变变程有关,且循环应变变程随循环次数线性增长,由此建立了拉-拉疲劳载荷下复合材料迟滞回能的计算分析模型,通过对玻璃纤维层合板的疲劳试验表明,模型计算值与试验值吻合较好.     

Effect of tool wear on delamination in drilling composite materials

Among all machining operations, drilling using twist drill is the most frequently applied for secondary machining of composite materials owing to the need for structure joining. Delamination is mostly considered as the principal failure model in drilling of composite materials. Drill wear is a serious concern in hole-making industry, as it is necessary to prevent damage of cutting tools, machine tools and workpieces. The industrial experience shows the worn drill causes more delamination. This paper presents a comprehensive analysis of delamination caused by the drill wear for twist drill in drilling carbon fiber-reinforced composite materials. The critical thrust force at the onset of delamination for worn drill is predicted and compared with that of ideal drill. The experimental results demonstrate that though the critical thrust force is higher with increasing wear ratio, the delamination becomes more liable to occur because the actual thrust force increases to larger extent, as the thrust factor (Z) illustrates. Compared to sharp drill, the worn twist drill allows for lower feed rate below which the delamination damage can be avoided.     

复合材料参数在超声检测中的影响

针对十六层碳纤维增强型复合材料进行超声波探伤的研究,提出一种判定复合材料分层缺陷位置的简便方法,经实验验证可行,并得出结论:碳纤维复合材料的弹性模量对超声波的声速与入射范围有重要影响,这将直接决定缺陷位置判定的准确性。     

The shear buckling behaviour of thin composite plates with particular reference to the effects of bend–twist coupling

The effect of bend–twist coupling on the shear buckling behaviour of laminated composite plates is examined in this paper using a finite strip procedure. The complex buckled shapes which are associated with shear loading are duly accounted for in the analysis approach through the multi-term facility of the strip formulation employed and, of course, through the appropriate level of structural modelling. The degree of bend–twist coupling in the laminated composite plates is varied by changing the level of anisotropy in the plies and by altering the lay-up configuration of the plies in the laminated stack. Symmetric laminates of a balanced and unbalanced nature are given consideration. It is shown that, for a given degree of anisotropy in the plies of a laminate and for a given laminate thickness, the stacking sequence of the plies significantly alters the degree of bend–twist coupling. The shear buckling performance of composite plates having the same dimensions and being made from the same material are therefore shown in the paper to be quite different. The preclusion of the bend–twist coupling coefficients in the solution procedure of the finite strip method allows the shear buckling orthotropic solution to be determined. Comparisons between the coupled and orthotropic solutions are shown in the paper to be markedly different with respect to critical shear performance level and also buckled mode shape. For square plates or plates with a moderate aspect ratio the influence of bend–twist coupling on buckled mode shape is shown in the paper to be noticeable through increased distortion. For the larger aspect ratio plates it is shown that the presence of bend–twist coupling can cause a complete change in the mode shape from a symmetric to an antisymmetric nature or vice versa. Amplitude modulation is shown in the paper to be clearly evident in the shear buckling mode shapes of long plates.     

Improvement in tool life of electroplated diamond tools by Ni-based carbon nanotube composite coatings

Ni-based CNT composite (Ni-CNT) coatings were applied for improving the grain bonding strength of electroplated diamond tools. Further, since it is extremely important for the coatings to have good surface roughness for applications to micro-electroplated diamond tools, ultrasonic-vibration stirring was employed for improving the surface roughness of the coatings. The effects of the amount of CNTs in the plating bath on the hardness and grain bonding strength of Ni-CNT coatings were evaluated. Finally, we evaluated the tool life of microelectroplated diamond tools comprising Ni-CNT coatings as the topcoat layers by conducting experiments involving side machining and hole-drilling. The Vickers hardness of Ni-CNT coatings electroplated in a bath that contained more than 1-g/l CNTs exceeded 500 HV. The grain bonding strength can be improved by codepositing CNTs in Ni coatings and was almost 1.3 times in our estimations. In the case of the side machining of glass plates, electroplated diamond tools composed of the Ni-CNT coatings had a tool life approximately eight times longer than that of normal tools. Moreover, micro electroplated diamond tools also had a longer tool life for drilling holes in fused quartz glass. This represents the first experimental verification of the efficacy of Ni-CNT coatings in improving the grain bonding strength. In this manner, we have demonstrated that Ni-CNT coatings are extremely effective for improving the grain bonding strength of electroplated diamond tools.     

A generalized self-consistent estimate for the effective elastic moduli of fiber-reinforced composite materials with multiple transversely isotropic inclusions

The effective elastic properties of a fiber-reinforced composite material with multiple transversely isotropic inclusions are estimated by the use of a generalized self-consistent method, which considers strong interactions between the inclusion and matrix as well as among inclusions. The accuracy of this method is established by comparing to the closed-form analytic solutions by Christensen when the matrix and inclusion are isotropic. Furthermore, current predictions from the generalized self-consistent method for a composite with multiple inclusions correspond well with the numerical results from finite element analysis. The generalized self-consistent method can be particularly useful in establishing micromechanics models of natural biological composite materials such as cortical bone to examine the dependence of the elastic properties of cortical bone on its porosity.     

A mesomodel for the numerical simulation of the multiphasic behavior of materials under anisothermal loading (application to two low-carbon steels)

The determination of residual stresses induced by welding or heat treatment operations requires the use of complex models taking into account thermal, metallurgical and mechanical phenomena. In this paper, we propose a mechanical model in which each phase can follow its own constitutive law. This model also takes into account phase transformation plasticity, which is treated independently of the behavior of each phase. This model has been implemented into the French FEM code Castem 2000. The interest of the proposed method is that it allows one to mix any type of nonlinear behavior using Taylor homogenization hypothesis. There is no need to develop a theory to get the equations of the homogenized material law. Two numerical examples demonstrate the efficiency and the flexibility of this approach. The results obtained are compared to experimental values for a typical welding situation and a high-temperature response. This comparison seems to indicate that viscous effects in the materials have a significative influence on the residual stresses produced by welding.     

计算机模拟方法在晶粒长大过程研究中的进展

综述了晶粒长大过程模拟在国内外的研究进展,阐述了该研究领域主要采用的3种模拟方法（蒙特卡罗方法、元胞自动机法、扩散界面场变量模型方法）,提出了该研究领域目前存在的问题及今后的研究方向。     

Postbuckling of 3D braided composite cylindrical shells under combined external pressure and axial compression in thermal environments

A postbuckling analysis is presented for a three-dimensional (3D) braided composite cylindrical shell of finite length subjected to combined loading of external pressure and axial compression in thermal environments. Based on a micro–macro-mechanical model, a 3D braided composite may be a cell system and the geometry of each cell is highly dependent on its position in the cross-section of the cylindrical shell. The material properties of epoxy are expressed as a linear function of temperature. The governing equations are based on a higher order shear deformation shell theory with a von Kármán–Donnell-type kinematic nonlinearity and includes thermal effects. A singular perturbation technique is employed to determine interactive buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling behavior of perfect and imperfect, braided composite cylindrical shells with different values of shell geometric parameter and of fiber volume fraction under combined loading conditions. The results show that the shell has lower buckling loads and postbuckling paths when the temperature-dependent properties are taken into account. The effects of temperature rise, fiber volume fraction, shell geometric parameter, load-proportional parameter, as well as initial geometric imperfections are studied.     

Elastodynamic response of a crack perpendicular to the graded interfacial zone in bonded dissimilar materials under antiplane shear impact

A solution is given for the elastodynamic problem of a crack perpendicular to the graded interfacial zone in bonded materials under the action of antiplane shear impact. The interfacial zone is modeled as a nonhomogeneous interlayer with the power-law variations of its shear modulus and mass density between the two dissimilar, homogeneous half-planes. Laplace and Fourier integral transforms are employed to reduce the transient problem to the solution of a Cauchy-type singular integral equation in the Laplace transform domain. Via the numerical inversion of the Laplace transforms, the values of the dynamic stress intensity factors are obtained as a function of time. As a result, the influences of material and geometric parameters of the bonded media on the overshoot characteristics of the dynamic stress intensities are discussed. A comparison is also made with the corresponding elastostatic solutions, addressing the inertia effect on the dynamic load transfer to the crack tips for various combinations of the physical properties.     

[学科发展]极端工况下机械表面界面损伤与防护研究进展

中国科学院宁波材料技术与工程研究所海洋新材料与应用技术重点实验室,宁波,315201 分析了重大工程实施中极端环境的典型特点,简要介绍了机械表面界面科学的国内外研究现状与发展趋势,指出了相关研究的共性科学问题;针对国家重大战略需求和国际研究前沿,提出了极端工况下机械表面界面科学领域的研究发展方向;以重点方向——功能防护涂层研究为例,探讨了涂层材料与技术的发展趋势并评述了相关研究进展。     

Molecular dynamics simulation of dislocation nucleation and motion at γ/γ′ interface in Ni-based superalloy

Molecular dynamics simulations are conducted on the dislocation behavior at the apices and edges of cuboidal Ni₃Al precipitate in a pure Ni matrix, or the idealized γ/γ′ microstructure in a Ni-based superalloy. A tensile simulation of the [001] direction is implemented with a periodic cell that has eight cubic precipitates in order to investigate the nucleation site of dislocation in the idealized microstructure with no defects other than the γ/γ′ interfaces. The effect of residual internal stresses on the stability of the interfaces is also discussed. Other simulations are conducted on the behavior of edge dislocations nucleated from a free surface and proceeding in the γ matrix toward γ′ precipitates under shear force. Dislocation pinning at γ′ precipitates, bowing-out in the γ channel, pile-up and nucleation of superdislocation in the γ′ precipitate are simulated and inspected in detail. Discussions on the size of the γ/γ′ microstructure and the sharpness of the edge of the γ′ precipitate are also presented.