The statistical second-order two-scale method for thermomechanical properties of statistically inhomogeneous materials

A statistical second-order two-scale (SSOTS) method is established in a constructive way for predicting the thermomechanical properties of statistically inhomogeneous materials. For this kind of composite materials, the complicated micro-characteristics of inclusions, including their shape, size, orientation, spatial distribution, volume fraction and/or material properties and so on, lead to changes of the macroscopic thermomechanical properties, such as stiffness, coefficient of thermal expansion and strength of material. In this paper, a statistical model at an arbitrary position of the composite material is defined to represent the microstructure of the statistically inhomogeneous media at first. And then, the statistical second-order two-scale analysis formulation is derived. Finally, the numerical results for some statistically inhomogeneous composites are calculated by SSOTS algorithm, and compared with the data by experimental and theoretical methods.     

Numerical Study on Mechanical Properties of Steel Fiber Reinforced Concrete by Statistical Second-order Two-scale Method

The present study aims to evaluate the mechanical properties of steel fiber reinforced concrete (SFRC) by the statistical second-order two-scale (SSOTS) method. At first, the representation for microstructure of SFRC is described by a concept of statistical screen. According to the microstructure representation, the SSOTS method is displayed in a concise way. This method is on the basis of asymptotic expansion homogenization and Monte Carlo method, and can calculate the local strain and stress field through the two-order displacement solution. As the classical homogenization method, the expression of homogenized elastic modulus is derived analytically. Then combined with the appropriate strength criterion and correspondence principle, the homogenized strength and viscoelastic properties of SFRC are obtained respectively. The validity of the SSOTS method is confirmed by the comparison between numerical results and the available experiment data. Results show that the SSOTS method is effective to evaluate the elastic, strength and viscoelastic properties of SFRC. In addition, the influence of distribution of steel fibers on the macroscopic mechanical properties of SFRC is discussed.     

A random field model for generating synthetic microstructures of functionally graded materials

This article presents a new level‐cut, inhomogeneous, filtered Poisson random field model for representing two‐phase microstructures of statistically inhomogeneous, functionally graded materials with fully penetrable embedded particles. The model involves an inhomogeneous, filtered Poisson random field comprising a sum of deterministic kernel functions that are scaled by random variables and a cut of the filtered Poisson field above a specified level. The resulting level‐cut field depends on the Poisson intensity, level, kernel functions, random scaling variables, and random rotation matrices. A reconstruction algorithm including model calibration and Monte Carlo simulation is presented for generating samples of two‐phase microstructures of statistically inhomogeneous media. Numerical examples demonstrate that the model developed is capable of producing a wide variety of two‐ and three‐dimensional microstructures of functionally graded composites containing particles of various sizes, shapes, densities, gradations, and orientations. An example involving finite element analyses of random microstructures, leading to statistics of effective properties of functionally graded composites, illustrates the usefulness of the proposed model. Copyright © 2008 John Wiley & Sons, Ltd.     

Nucleation and Crystallization Phenomena in Glass‐Ceramics

Glass‐ceramics are modern multiphase materials. Different types of glass‐ceramics can be fabricated and their properties tailored to meet specific requirements. It is possible, therefore, to produce glasses that combine desirable optical properties such as transparency and mechanical properties such as strength. Furthermore, this controlled development of glass‐ceramics also enables the fabrication of materials exhibiting properties that are unknown in conventional glasses and ceramics. In this article, the authors discuss nucleation and crystallization phenomena in the controlled development of various glass‐ceramics on the basis of their own work. The reaction mechanisms in the formation of glass‐ceramics and their microstructures are illustrated. In addition, the resulting properties of the glass‐ceramics and their particular applications are presented.     

Nanomechanical and Charge Transport Properties of Two‐Dimensional Atomic Sheets

The materials properties of graphene and other two‐dimensional atomic sheets are influenced by atomic‐scale defects, mechanical deformation, and microstructures. Thus, for graphene‐based applications, it is essential to uncover the roles of atomic‐scale defects and domain structures of two‐dimensional layers in charge transport properties. This review highlights recent studies of nanomechanical and charge transport properties of two‐dimensional atomic sheets, including graphene, MoS₂, and boron nitrides. Because of intrinsic structural differences, two‐dimensional atomic sheets give rise to unique nanomechanical properties, including a dependence on layer thickness and chemical modification that is in contrast to three‐dimensional continuum media. Mapping of local conductance and nanomechanical properties on a graphene layer can be used to image the domain and microstructures of two‐dimensional atomic layers. This paper also reviews recent experimental and theoretical findings on the role of bending, defects, and microstructures on nanomechanical and transport properties of graphene‐derived materials.     

Mo‐Si‐B Alloys for Ultrahigh‐Temperature Structural Applications

A continuing quest in science is the development of materials capable of operating structurally at ever‐increasing temperatures. Indeed, the development of gas‐turbine engines for aircraft/aerospace, which has had a seminal impact on our ability to travel, has been controlled by the availability of materials capable of withstanding the higher‐temperature hostile environments encountered in these engines. Nickel‐base superalloys, particularly as single crystals, represent a crowning achievement here as they can operate in the combustors at ～1100 °C, with hot spots of ～1200 °C. As this represents ～90% of their melting temperature, if higher‐temperature engines are ever to be a reality, alternative materials must be utilized. One such class of materials is Mo‐Si‐B alloys; they have higher density but could operate several hundred degrees hotter. Here we describe the processing and structure versus mechanical properties of Mo‐Si‐B alloys and further document ways to optimize their nano/microstructures to achieve an appropriate balance of properties to realistically compete with Ni‐alloys for elevated‐temperature structural applications.     

Statistical Second-order Two-scale Method for Nonstationary Coupled Conduction-Radiation Heat Transfer Problem of Random Porous Materials

This paper develops a novel statistical second-order two-scale (SSOTS) method to predict the heat transfer performances of three-dimensional (3D) porous materials with random distribution. Firstly, the mesoscopic configuration for the structure with random distribution is briefly characterized Secondly, the SSOTS formulas for calculating effective thermal conductivity parameters, temperature field and heat flux densities are derived by means of construction way. Then, the algorithm procedure based on the SSOTS method is described in details. Finally, numerical results for porous materials with varying probability distribution models are calculated by SSOTS algorithm, and compared with the data by finite element method (FEM) in a very fine mesh and theoretical methods. They show that the SSOTS method is not only valid, but also accurate to predict the coupled heat transfer performances of random porous materials and demonstrate its potential applications in thermal engineering.     

Wave propagation analysis in inhomogeneous piezo‐composite layer by the thin‐layer method

The thin‐layer method (TLM) is used to study the propagation of waves in inhomogeneous piezo‐composite layered media caused by mechanical loading and electrical excitation. The element is formulated in the time‐wavenumber domain, which drastically reduces the cost of computation compared to the finite element (FE) method. Fourier series are used for the spatial representation of the unknown variables. The material properties are allowed to vary in the depthwise direction only. Both linear and exponential variations of elastic and electrical properties are considered. Several numerical examples are presented, which bring out the characteristics of wave propagation in anisotropic and inhomogeneous layered media. The element is useful for modelling ultrasonic transducers (UT) and one such example is given to show the effect of electric actuation in a composite material and the difference in the responses elicited for various ply‐angles. Further, an ultrasonic transducer composed of functionally graded piezoelectric materials (FGPM) is modelled and the effect of gradation on mechanical response is demonstrated. The effect of anisotropy and inhomogeneity is shown in the normal modes for both displacement and electric potential. The element is further utilized to estimate the piezoelectric properties from the measured response using non‐linear optimization, a strategy that is referred to as the pulse propagation technique (PPT). Copyright © 2005 John Wiley & Sons, Ltd.     

Solids Under Extreme Shear: Friction‐Mediated Subsurface Structural Transformations

Tribological contacts consume a significant amount of the world's primary energy due to friction and wear in different products from nanoelectromechanical systems to bearings, gears, and engines. The energy is largely dissipated in the material underneath the two surfaces sliding against each other. This subsurface material is thereby exposed to extreme amounts of shear deformation and often forms layered subsurface microstructures with reduced grain size. Herein, the elementary mechanisms for the formation of subsurface microstructures are elucidated by systematic model experiments and discrete dislocation dynamics simulations in dry frictional contacts. The simulations show how pre‐existing dislocations transform into prismatic dislocation structures under tribological loading. The stress field under a moving spherical contact and the crystallographic orientation are crucial for the formation of these prismatic structures. Experimentally, a localized dislocation structure at a depth of ≈100–150 nm is found already after the first loading pass. This dislocation structure is shown to be connected to the inhomogeneous stress field under the moving contact. The subsequent microstructural transformations and the mechanical properties of the surface layer are determined by this structure. These results hold promise at guiding material selection and alloy development for tribological loading, yielding materials tailored for specific tribological scenarios.     

Level‐cut inhomogeneous filtered Poisson field for two‐phase microstructures

Level‐cut homogeneous filtered Poisson fields developed in (J. Appl. Phys. 2003; 94 (6):3762–3770) to model two‐phase microstructures are defined, and their properties are briefly reviewed. Filtered Poisson fields are sums of randomly scaled and oriented kernels that are centered at the points of homogeneous Poisson fields. The cuts of these fields above specified thresholds are called level‐cut homogeneous filtered Poisson fields. It is shown that an arbitrary inhomogeneous Poisson field becomes homogeneous if observed in new coordinates, and that the mapping relating inhomogeneous and homogeneous Poisson fields can be constructed in a simple manner. This mapping and the model in (J. Appl. Phys. 2003; 94 (6): 3762–3770) provide an efficient algorithm for generating arbitrary inhomogeneous two‐phase microstructures. Developments in (Int. J. Numer. Meth. Engng 2008; DOI: 10.1002/nme.2340 ), using arguments essentially identical to those in (J. Appl. Phys. 2003; 94 (6):3762–3770) to define and generate inhomogeneous Poisson fields, overlook the natural extension of results in (J. Appl. Phys. 2003; 94 (6): 3762–3770) to these fields provided by the mapping constructed in this paper. Copyright © 2008 John Wiley & Sons, Ltd.     

多孔材料辐射-传热耦合性能的统计二阶双尺度计算

对多孔材料辐射-传热耦合计算的数学模型, 即Rosseland方程, 给出了一种统计的二阶双尺度分析方法, 并针对典型问题进行了数值模拟。建立了考虑辐射项的统计二阶双尺度计算公式, 给出了统计意义下热流密度极值的预测算法, 并通过与理论解的比较对算法进行了验证, 利用本文中方法研究了孔洞体分比和空间分布状态对陶瓷多孔材料热传导系数、 辐射传导系数和热流密度极值的影响。结果表明: 孔洞体积分数的增加将导致有效热传导系数下降; 热流密度极值随孔洞体积分数的增加而变大, 并且在高温时辐射的作用明显增大; 数值试验表明, 使用统计二阶双尺度方法及其有限元算法预测孔洞随机分布复合材料结构的热性能是有效的。     

Corrosion Behaviors of Long‐Period Stacking Ordered Structure in Mg Alloys Used in Biomaterials: A Review

The long‐period stacking ordered (LPSO) phases have distinctive microstructures and significant effect on the promotion of mechanical properties of Mg alloys, which have received considerable attention not only as industrial materials but also as biodegradable implant materials recently. By now, numerous researchers devote to study the effects of the microstructures of LPSO phases on the mechanical properties of Mg alloys. But a few of them reveal the relationship between LPSO phases and corrosion behaviors of Mg alloys. Therefore, the knowledge of characteristics of LPSO phases and their effects on biocorrosion behaviors is essential. In this review, the current understanding about the structure, growth, transformation, and deformation of LPSO phases in Mg alloys are summarized. The recent developments of biocorrosion behaviors of Mg alloys are reviewed. The information on the immersion and corrosion mechanisms of Mg alloys are provided. The role of LPSO structures on corrosion behaviors of Mg alloys is intensively analyzed. Based on the current understandings, some problems are pointed out and suggestions for further research of Mg alloys with LPSO structures using as biomedical materials are provided.

     

Macro‐ and Nanomechanical Properties and Strain Rate Sensitivity of Accumulative Roll Bonded and Equal Channel Angular Pressed Ultrafine‐Grained Materials

Several processes of severe plastic deformation are suitable for the production of materials with ultrafine‐grained microstructures which are known to exhibit high strength and often good ductility as well as strain rate sensitive behavior. The most promising ones are equal channel angular pressing (ECAP) for bulk material and accumulative roll bonding (ARB) for the production of sheet material. In order to evaluate the influence of the process on these mechanical properties and the strain rate sensitivity, tensile tests, and nanoindentation tests were performed on material produced up to similar effective plastic strains of ε_ARB = 6.4 and ε_ECAP = 6.3. It could be shown that the macroscopic strength is slightly higher for ARB than for ECAP material and vice versa in nanoindentation. Independent of the testing method, the strain rate sensitivities and activation volumes are similar for both materials. Thus, both processes performed up to similar effective plastic strains lead to comparable improvements in the mechanical properties. Additionally it could be shown, that this comparison allows the identification of the dominant deformation mechanism which is responsible for the observed strain rate sensitivity.     

Tailoring the Mechanical Properties of 3D Microstructures Using Visible Light Post‐Manufacturing

The photochemistry of anthracene, a new class of photoresist for direct laser writing, is used to enable visible‐light‐gated control over the mechanical properties of 3D microstructures post‐manufacturing. The mechanical and viscoelastic properties (hardness, complex elastic modulus, and loss factor) of the microstructures are measured over the course of irradiation via dynamic mechanical analysis on the nanoscale. Irradiation of the microstructures leads to a strong hardening and stiffening effect due to the generation of additional crosslinks through the photodimerization of the anthracene functionalities. A relationship between the loss of fluorescence—a consequence of the photodimerization—and changes in the mechanical properties is established. The fluorescence thus serves as a proxy read‐out for the mechanical properties. These photoresponsive microstructures can potentially be used as “mechanical blank slates”: their mechanical properties can be readily adjusted using visible light to serve the demands of different applications and read out using their fluorescence.     

Effect of SiC‐Reinforcement and Equal‐Channel Angular Pressing on Microstructure and Mechanical Properties of AA2017

This article deals with powder metallurgical production and modification of properties of a composite material based on an age‐hardenable Al–Cu alloy. The main objective is to improve the mechanical properties by particle reinforcement and equal‐channel angular pressing (ECAP). Our approach makes use of four hardening mechanisms: precipitation hardening, particle reinforcement, strain‐hardening, and grain boundary hardening associated with an ultrafine‐grained microstructure produced by ECAP. The main processing steps are high‐energy ball milling, hot‐isostatic pressing, extrusion, heat treatment, and a single ECAP pass. Microstructures are analyzed by optical microscopy, scanning electron microscopy, and scanning transmission electron microscopy. The mechanical properties are characterized by hardness measurements and quasi‐static tensile testing. Our experimental results show that the proposed processing route results in a nearly homogeneous distribution of SiC particles in the matrix. The combination of particle reinforcement and ECAP leads to an improvement of ultimate tensile strength by almost 300 MPa compared to the unreinforced alloy. A subsequent heat treatment leads to a further increase in hardness and strength that can be related to changes in the defect structure. Our study provides detailed information on how processing steps, microstructures, and mechanical behavior are interrelated in this technologically relevant class of materials.     

Deformation and Fracture of Micron‐Sized Metal‐Coated Polymer Spheres: An In Situ Study

In situ imaging and analysis of the mechanical behavior of micron‐sized metal‐coated polymer particles under compression is reported. A nanoindentation set‐up mounted in a scanning electron microscope is used to observe the deformation and fracture of 10 μm polymer spheres with Ni, Ni/Au, Au, and Ag coatings. The spheres fracture in one of two metallization‐dependent modes, brittle, and ductile, depending only on the presence of a nickel layer. The metal coating always fractures parallel to the direction of compression. The mechanical properties up to the point of coating fracture are rate‐dependent due to the viscoelastic polymer core. Metal‐coated polymer spheres are an important composite material in electronics packaging, and this study demonstrates a novel method of evaluating the mechanical properties of particles to tailor them for electronic materials.

     

Nature‐Inspired Lightweight Cellular Co‐Continuous Composites with Architected Periodic Gyroidal Structures

Shell‐core cellular composites are a unique class of cellular materials, where the base constituent is made of a composite material such that the best distinctive physical and/or mechanical properties of each phase of the composite are employed. In this work, the authors demonstrate the additive manufacturing of a nature inspired cellular three‐dimensional (3D), periodic, co‐continuous, and complex composite materials made of a hard‐shell and soft‐core system. The architecture of these composites is based on the Schoen's single Gyroidal triply periodic minimal surface. Results of mechanical testing show the possibility of having a wide range of mechanical properties by tuning the composition, volume fraction of core, shell thickness, and internal architecture of the cellular composites. Moreover, a change in deformation and failure mechanism is observed when employing a shell‐core composite system, as compared to the pure stiff polymeric standalone cellular material. This shell‐core configuration and Gyroidal topology allowed for accessing toughness values that are not realized by the constituent materials independently, showing the suitability of this cellular composite for mechanical energy absorption applications.

     

Verhalten laserschockverfestigter und festgewalzter Randschichten der Ti‐Legierung Ti‐6Al‐4V bei schwingender Beanspruchung unter erhöhten Temperaturen

Residual stress stability and near‐surface microstructures in high temperature fatigued mechanically surface treated Ti‐6Al‐4V It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly‐stressed metallic components. Deep rolling is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of deep rolling on the low‐cycle and high‐cycle fatigue behavior of a Ti‐6Al‐4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near‐surface microstructures. Preliminary results on laser shock peened Ti‐6Al‐4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ～450 °C, i.e., at an homologous temperature of ～0.4 T/T_m (where T_m is the melting temperature). Based on cyclic deformation and stress/life (S/N) fatigue behavior, together with the X‐ray diffraction and in situ transmission electron microscopy observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti‐6Al‐4V at such higher temperatures, despite the almost complete relaxation of the near‐surface residual stresses. In the absence of such stresses, it is shown that the near‐surface microstructures, which in Ti‐6Al‐4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.     

Increasing the Dimensionality of Soft Microstructures through Injection‐Induced Self‐Folding

Devices fabricated using soft materials have been a major research focus of late, capturing the attention of scientists and laypersons alike in a wide range of fields, from microfluidics to robotics. The functionality of such devices relies on their structural and material properties; thus, the fabrication method is of utmost importance. Here, multilayer soft lithography, precision laser micromachining, and folding to establish a new paradigm are combined for creating 3D soft microstructures and devices. Phase‐changing materials are exploited to transform actuators into structural elements, allowing 2D laminates to evolve into a third spatial dimension. To illustrate the capabilities of this new fabrication paradigm, the first “microfluidic origami for reconfigurable pneumatic/hydraulic” device is designed and manufactured: a 12‐layer soft robotic peacock spider with embedded microfluidic circuitry and actuatable features.     

Microstructure and mechanical properties of an Al‐Si alloy reinforced by Al64Cu24Fe12 quasi‐crystalline particles

In this work, aluminum‐silicon alloy reinforced with Al₆₄Cu₂₄Fe₁₂ quasi‐crystalline particles have been prepared by a traditional casting method with proper heat treatment process. The microstructures of the composites were examined using optical microscopy, scanning, X‐ray and energy dispersive spectrometer. The results indicate that there was no quasi‐crystalline phase remained and a new phase‐β (Al_65‐75Si_13‐26Fe_3‐10) phase formed, which may act as a new reinforcing phase. There are three primary phases in the final composite: eutectic silicon, β‐phase and the α‐aluminum phase. Also the changes of mechanical properties were studied by tensile test and Vickers hardness test. The test results demonstrate that the mechanical properties of the composite is remarkably improved by adding proper amount of quasi‐crystalline particles. After adding 7.70 vol.% quasi‐crystalline particles, the tensile strength and the hardness increased by 70 % and 62 % respectively. However, adding a big amount (10.09 vol.%) of particles to molten aluminum‐silicon alloy will cause the particles aggregation and adhesion, which resulted in the decrease of the mechanical properties.