Hybrid numerical scheme for time-evolving wave fields

Many problems in geophysics, acoustics, elasticity theory, cancer treatment, food process control and electrodynamics involve study of wave field synthesis (WFS) in some form or another. In the present work, modelling of wave propagation phenomena is studied as a static problem, using finite element method and treating time as an additional spatial dimension. In particular, WFS problems are analysed using discrete methods. It is shown that a fully finite element-based scheme is very natural and effective method for the solution of such problems. Distributed WFS in the context of two-dimensional problems is outlined and incorporation of any geometric or material non-linearities is shown to be straightforward. This has significant implications for problems in geophysics or biological media, where material inhomogeneities are quite prevalent. Numerical results are presented for several problems referring to media with material inhomogeneities and predefined absorption profiles. The method can be extended to three-dimensional problems involving anisotropic media properties in a relatively straightforward manner. Copyright © 2006 John Wiley & Sons, Ltd.     

Two-scale homogenization of electromechanically coupled boundary value problems

The contribution addresses a direct micro-macro transition procedure for electromechanically coupled boundary value problems. The two-scale homogenization approach is implemented into a so-called FE²-method which allows for the computation of macroscopic boundary value problems in consideration of microscopic representative volume elements. The resulting formulation is applicable to the computation of linear as well as nonlinear problems. In the present paper, linear piezoelectric as well as nonlinear electrostrictive material behavior are investigated, where the constitutive equations on the microscale are derived from suitable thermodynamic potentials. The proposed direct homogenization procedure can also be applied for the computation of effective elastic, piezoelectric, dielectric, and electrostrictive material properties.     

Fracture analysis of a nonhomogeneous coating/substrate system with an interface under thermal shock

The work is devoted to establish a model for the interface problem of a nonhomogeneous coating/substrate system. In the model, according to the distribution of material properties, three types of interface problems are considered: (i) The material properties and their derivatives are continuous on the interface; (ii) the material properties are continuous, but their derivatives are discontinuous on the interface; and (iii) the material properties as well as their derivatives are discontinuous on the interface. In order to solve the complex interface problems, a transient interaction energy integral method (IEIM) is developed in this paper. The transient thermal stress intensity factors are evaluated using the IEIM combined with the finite element method and the finite difference method. The influences of the interface discontinuity and the geometric parameters on the transient TSIFs are investigated. Particularly, the crack growth behavior with different interface discontinuities is discussed.     

Comparison study of MPM and SPH in modeling hypervelocity impact problems

Due to the high nonlinearities and extreme large deformation, the hypervelocity impact simulation is a challenging task for numerical methods. Meshfree particle methods, such as the smoothed particle hydrodynamics (SPH) and material point method (MPM), are promising for the simulation of hypervelocity impact problems. In this paper, the material point method is applied to the simulation of hypervelocity impact problems, and a three-dimensional MPM computer code, MPM3D, is developed. The Johnson–Cook material model and Mie–Grüneisen equation of state are implemented. Furthermore, the basic formulations of MPM are compared with SPH, and their performances are compared numerically by using MPM3D and LS-DYNA SPH module.     

Boundary-only element solutions of 2D and 3D nonlinear and nonhomogeneous elastic problems

This paper presents a robust boundary element method (BEM) that can be used to solve elastic problems with nonlinearly varying material parameters, such as the functionally graded material (FGM) and damage mechanics problems. The main feature of this method is that no internal cells are required to evaluate domain integrals appearing in the conventional integral equations derived for these problems, and very few internal points are needed to improve the computational accuracy. In addition, one of the basic field quantities used in the boundary integral equations is normalized by the material parameter. As a result, no gradients of the field quantities are involved in the integral equations. Another advantage of using the normalized quantities is that no material parameters are included in the boundary integrals, so that a unified equation form can be established for multi-region problems which have different material parameters. This is very efficient for solving composite structural problems.     

The conservative M-integral for thermal-elastic problems

In this investigation, the conservative M-integral is extended to treat thermal-elastic, mixed mode problems. With it, stress intensity factors are obtained for cracks in homogeneous, isotropic materials, as well as isotropic and anisotropic, bimaterials. Excellent agreement is found between results determined in this study and those found in the literature. In addition, new results are obtained for interface cracks for a wide range of material properties and for a delamination in a composite material.     

Porous silicon capping by CVD diamond

In the last few years important research efforts were put concerning the optical properties of porous silicon (PS). Some interesting devices can be made using this new material (ex: L.E.D.s and micro cavities). However, a set of unsolved problems is keeping some distance between PS and its applications. Among these problems are the chemical instability and mechanical weakness.In this work, we attempted a CVD diamond protective layer deposition on PS. The optical properties of PS were preserved, as confirmed by photoluminescence (PL). The diamond film adhesion was checked by microhardness testing showing no crack propagation with a load of 200 gf. Nanoindentation measurements with 0.1 gf on the resulting surface showed a ten fold increase in hardness with respect to the uncoated material.     

Optimal design of acoustic material from tire fluff

Waste tires cause both health and environmental problems and this has forced governments to develop laws for recycling. There are several different recycling processes for end-of-life tires in which steel is recovered and rubber is reused. However, the reuse of other parts of the waste tires is currently not possible. These other parts mainly consist of textiles that are separated from the tire during the recycling process – this material is usually known as ‘fluff’. In this study a procedure for the design of materials with superior acoustic properties using this material was developed. The main component of the acoustic material is textile waste from mechanically fragmented tires obtained through recycling activity. The design process is based on several parameters: acoustic absorption coefficient measurement by the impedance tube method, acoustic modelling, mechanical characterization of the material and, finally, finite element modelling of the end acoustic product. In this work it was also established that the mechanical and acoustic properties need to be opposite to obtain the desired characteristics in the materials under investigation. In order to produce a competitive material formed completely from textile waste from recycled tires, a solution formed by two layers is proposed. This approach was used to provide a self-supporting material with a high acoustic absorption coefficient for use in acoustic ceiling tiles.     

LATIN: A new view and an extension to wave propagation in nonlinear media

The LATIN (acronym of LArge Time INcrement) method was originally devised as a non‐incremental procedure for the solution of quasi‐static problems in continuum mechanics with material nonlinearity. In contrast to standard incremental methods like Newton and modified Newton, LATIN is an iterative procedure applied to the entire loading path. In each LATIN iteration, two problems are solved: a local problem, which is nonlinear but algebraic and miniature, and a global problem, which involves the entire loading process but is linear. The convergence of these iterations, which has been shown to occur for a large class of nonlinear problems, provides an approximate solution to the original problem. In this paper, the LATIN method is presented from a different viewpoint, taking advantage of the causality principle. In this new view, LATIN is an incremental method, and the LATIN iterations are performed within each load step, similarly to the way that Newton iterations are performed. The advantages of the new approach are discussed. In addition, LATIN is extended for the solution of time‐dependent wave problems. As a relatively simple model for illustrating the new formulation, lateral wave propagation in a flat membrane made of a nonlinear material is considered. Numerical examples demonstrate the performance of the scheme, in conjunction with finite element discretization in space and the Newmark trapezoidal algorithm in time. Copyright © 2017 John Wiley & Sons, Ltd.     

Macroporous hydroxyapatite as alloplastic material for dental applications

In the dentistry field, synthetic hydroxyapatite can be conveniently used as semiabsorbable alloplastic material to solve a number of clinical problems. The hydroxyapatite structure influences bone ingrowth as well as its resorption. In particular, pore size must exceed 100 μm to allow new bone ingrowth. Because of its brittleness, highly porous hydroxyapatite is difficult to handle without causing damage, and, therefore, its mechanical performance has to be improved placing the material on a dense hydroxyapatite substrate. Dense/porous hydroxyapatite laminates can be obtained by slip casting technology. The method starts with the preparation of a ceramic powder–binder–solvent system. This slurry produces, after solvent evaporation, a soft green tape, that is successively baked at 300 °C to remove all organic component, and finally sintered at high temperature (1200 °C). The material porosity cannot be significantly modified by changes in the slurry composition and sintering temperature; therefore, the macroporous hydroxyapatite layer must be obtained using a porous hydroxyapatite powder. The powder is prepared by grinding of green tape pieces and the resulting coarse material is successively baked at 300 °C. Such material is mixed with a polymer solution and cast on a green tape substrate. Layer fractures are not observed in both film bulks and interface, because during sintering the consolidation of two layers happens simultaneously. In the resulting material, the first layer consisted of macroporous hydroxyapatite with high osteoconductive properties, and the second layer was a dense hydroxyapatite substrate able to improve the laminate mechanical properties. © 1999 Kluwer Academic Publishers     

A bi‐value coding parameterization scheme for the discrete optimal orientation design of the composite laminate

The discrete optimal orientation design of the composite laminate can be treated as a material selection problem dealt with by using the concept of continuous topology optimization method. In this work, a new bi‐value coding parameterization (BCP) scheme of closed form is proposed to this aim. The basic idea of the BCP scheme is to ‘code’ each material phase using integer values of +1 and –1 so that each available material phase has one unique ‘code’ consisting of +1 and/or –1 assigned to design variables. Theoretical and numerical comparisons between the proposed BCP scheme and existing schemes show that the BCP has the advantage of an evident reduction of the number of design variables in logarithmic form. The benefit is particularly remarkable when the number of candidate materials becomes important in large‐scale problems. Numerical tests with up to 36 candidate material orientations are illustrated for the first time to indicate the reliability and efficiency of the BCP scheme in solving this kind of problem. It proves that the BCP is an interesting and valuable scheme to achieve the optimal orientations for large‐scale design problems. Besides, a four‐layer laminate example is tested to demonstrate that the proposed BCP scheme can easily be extended to multilayer problems. Copyright © 2012 John Wiley & Sons, Ltd.     

Refined h-adaptive finite element procedure for large deformation geotechnical problems

Adaptive finite element procedures automatically refine, coarsen, or relocate elements in a finite element mesh to obtain a solution with a specified accuracy. Although a significant amount of research has been devoted to adaptive finite element analysis, this method has not been widely applied to nonlinear geotechnical problems due to their complexity. In this paper, the h-adaptive finite element technique is employed to solve some complex geotechnical problems involving material nonlinearity and large deformations. The key components of h-adaptivity including robust mesh generation algorithms, error estimators and remapping procedures are discussed. This paper includes a brief literature review as well as formulation and implementation details of the h-adaptive technique. Finally, the method is used to solve some classical geotechnical problems and results are provided to illustrate the performance of the method.     

Total quality control for a surface mount technology process for the manufacture of printed circuit board assemblies

Surface mount technology is used widely nowadays in the manufacture of printed circuit board assemblies in the electronics industry. The occurrence of defective products when this technology is used is mostly caused by technological problems, but sometimes it is also caused by management problems or human errors. When technological problems are being tackled, human interactions will always be involved. This is because the quality of a product is related to the quality of the material, the design of the product and the manufacturing process, and as tasks are subdivided, problems occurring in production cannot be solved completely by the sole effort of a single individual. Hence improvement of product quality involes man, machine, and material. This article explains how quality problems arising from surface mount technology are tackled by team effort in an organization via the implementation of a process-oriented total quality control system.     

A simple and efficient preconditioning scheme for heaviside enriched XFEM

The extended finite element method (XFEM) is an approach for solving problems with non-smooth solutions, which arise from geometric features such as cracks, holes, and material inclusions. In the XFEM, the approximate solution is locally enriched to capture the discontinuities without requiring a mesh which conforms to the geometric features. One drawback of the XFEM is that an ill-conditioned system of equations results when the ratio of volumes on either side of the interface in an element is small. Such interface configurations are often unavoidable, in particular for moving interface problems on fixed meshes. In general, the ill-conditioning reduces the performance of iterative linear solvers and impedes the convergence of solvers for nonlinear problems. This paper studies the XFEM with a Heaviside enrichment strategy for solving problems with stationary and moving material interfaces. A generalized formulation of the XFEM is combined with the level set method to implicitly define the embedded interface geometry. In order to avoid the ill-conditioning, a simple and efficient scheme based on a geometric preconditioner and constraining degrees of freedom to zero for small intersections is proposed. The geometric preconditioner is computed from the nodal basis functions, and therefore may be constructed prior to building the system of equations. This feature and the low-cost of constructing the preconditioning matrix makes it well suited for nonlinear problems with fixed and moving interfaces. It is shown by numerical examples that the proposed preconditioning scheme performs well for discontinuous problems and \(C^0\) -continuous problems with both the stabilized Lagrange and Nitsche methods for enforcing the continuity constraint at the interface. Numerical examples are presented which compare the condition number and solution error with and without the proposed preconditioning scheme. The results suggest that the proposed preconditioning scheme leads to condition numbers similar to that of a body-fitted mesh using the traditional finite element method without loss of solution accuracy.     

A unified stochastic framework for robust topology optimization of continuum and truss-like structures

In this article, a unified framework is introduced for robust structural topology optimization for 2D and 3D continuum and truss problems. The uncertain material parameters are modelled using a spatially correlated random field which is discretized using the Karhunen–Loève expansion. The spectral stochastic finite element method is used, with a polynomial chaos expansion to propagate uncertainties in the material characteristics to the response quantities. In continuum structures, either 2D or 3D random fields are modelled across the structural domain, while representation of the material uncertainties in linear truss elements is achieved by expanding 1D random fields along the length of the elements. Several examples demonstrate the method on both 2D and 3D continuum and truss structures, showing that this common framework provides an interesting insight into robustness versus optimality for the test problems considered.     

Identifying inventory problems in the aerospace industry using the theory of constraints

In the aerospace industry, where each component is time-critical, many operation management tools are applied to improve material management efficiency. By using the theory of constraints (TOCs), this study diagnoses the undesirable effects (UDEs) of material management systems and identifies root problems or conflicting material management actions. In the second step, via UDEs, this study constructs a current reality tree (CRT) to identify a company's material management objectives and requirements, and the actions it takes to meet these objectives. In addition, the UDEs and CRT are applied to redesign the future reality tree (FRT) to develop strategies and eliminate the problems in the case company. By eliminating the conflicting inventory management activities of different departments, inventory management performance for the entire system is improved.     

Implicit dynamic analysis using an isogeometric Reissner–Mindlin shell formulation

In isogeometric analysis, identical basis functions are used for geometrical representation and analysis. In this work, non‐uniform rational basis splines basis functions are applied in an isoparametric approach. An isogeometric Reissner–Mindlin shell formulation for implicit dynamic calculations using the Galerkin method is presented. A consistent as well as a lumped matrix formulation is implemented. The suitability of the developed shell formulation for natural frequency analysis is demonstrated by a numerical example. In a second set of examples, transient problems of plane and curved geometries undergoing large deformations in combination with nonlinear material behavior are investigated. Via a zero‐thickness stress algorithm for arbitrary material models, a J₂‐plasticity constitutive law is implemented. In the numerical examples, the effectiveness, robustness, and superior accuracy of a continuous interpolation method of the shell director vector is compared with experimental results and alternative numerical approaches. Copyright © 2016 John Wiley & Sons, Ltd.     

锂离子电池富锂锰正极材料的最新进展

富锂锰正极材料(Li-rich manganese cathode material,LMCM)因高比容量(>250 mAh·g^-1)、低成本等优势,被视为最具前景的下一代锂离子电池正极材料。然而,该正极材料在循环过程中存在不可逆的结构转变等问题,造成首次不可逆容量损失高、倍率性能差、能量衰减和电压衰减等问题。通过体掺杂、表面包覆和结构优化设计等策略可一定程度上改善LMCM存在的以上问题。本文重点介绍LMCM存在的问题及改性研究工作,分析LMCM存在的问题及起因,详细阐述目前主要改性方法的研究现状,并讨论各种改性方法的优缺点及今后的重点研究方向。此外,本文还分析目前LMCM材料产业化进展和面临的主要挑战。由于自身存在的问题和配套材料发展缓慢,目前仅在部分企业实现小批量生产。     

Methods for calculating stress intensity factors in anisotropic materials: Part I—z = 0 is a symmetric plane

The problem of a crack in general anisotropic material under LEFM conditions is presented. In Part I, three methods are presented for calculating stress intensity factors for various anisotropic materials in which z = 0 is a plane of symmetry. All of the methods employ the displacement field obtained by means of the finite element method. The first one is known as displacement extrapolation and requires the values of the crack face displacements. The other two are conservative integrals based upon the J-integral. One employs symmetric and asymmetric fields to separate the mode I and II stress intensity factors. The second is the M-integral which also allows for calculation of K_I and K_II separately.All of these methods were originally presented for isotopic materials. Displacement extrapolation and the M-integral are extended for orthotropic and monoclinic materials, whereas the J_I- and J_II-integrals are only extended for orthotropic material in which the crack and material directions coincide. Results are obtained by these methods for several problems appearing in the literature. Good to excellent agreement is found in comparison to published values. New results are obtained for several problems.In Part II, the M-integral is extended for more general anisotropies. In these cases, three-dimensional problems must be solved, requiring a three-dimensional M-integral.