An analytical model of the mechanical properties of bulk coal under confined stress

This paper presents the development of an analytical model which can be used to relate the structural parameters of coal to its mechanical properties such as elastic modulus and Poisson’s ratio under a confined stress condition. This model is developed primarily to support process modeling of coalbed methane (CBM) or CO₂-enhanced CBM (ECBM) recovery from coal seam. It applied an innovative approach by which stresses acting on and strains occurring in coal are successively combined in rectangular coordinates, leading to the aggregated mechanical constants. These mechanical properties represent important information for improving CBM/ECBM simulations and incorporating within these considerations of directional permeability. The model, consisting of constitutive equations which implement a mechanically consistent stress–strains correlation, can be used as a generalized tool to study the mechanical and fluid behaviors of coal composites. An example using the model to predict the stress–strain correlation of coal under triaxial confined stress by accounting for the elastic and brittle (non-elastic) deformations is discussed. The result shows a good agreement between the prediction and the experimental measurement.     

Designing nanomaterials with desired mechanical properties by constraining the evolution of their grain shapes

We report a direct observation of Ge hut nucleation on Si(001) during UHV molecular beam epitaxy at 360°C. Nuclei of pyramids and wedges were observed on the wetting layer (WL) (M × N) patches starting from the coverage of 5.1 Å and found to have different structures. Atomic models of nuclei of both hut species have been built as well as models of the growing clusters. The growth of huts of each species has been demonstrated to follow generic scenarios. The formation of the second atomic layer of a wedge results in rearrangement of its first layer. Its ridge structure does not repeat the nucleus. A pyramid grows without phase transitions. A structure of its vertex copies the nucleus. Transitions between hut species turned out to be impossible. The wedges contain point defects in the upper corners of the triangular faces and have preferential growth directions along the ridges. The derived structure of the {105} facet follows the paired dimer model. Further growth of hut arrays results in domination of wedges, and the density of pyramids exponentially drops. The second generation of huts arises at coverages >10 Å; new huts occupy the whole WL at coverages ~14 Å. Nanocrystalline Ge 2D layer begins forming at coverages >14 Å.     

Designing nanomaterials with desired mechanical properties by constraining the evolution of their grain shapes

Grain shapes are acknowledged to impact nanomaterials' overall properties. Research works on this issue include grain-elongation and grain-strain measurements and their impacts on nanomaterials' mechanical properties. This paper proposes a stochastic model for grain strain undergoing severe plastic deformation. Most models deal with equivalent radii assuming that nanomaterials' grains are spherical. These models neglect true grain shapes. This paper also proposes a theoretical approach of extending existing models by considering grain shape distribution during stochastic design and modelling of nanomaterials' constituent structures and mechanical properties. This is achieved by introducing grain 'form'. Example 'forms' for 2-D and 3-D grains are proposed. From the definitions of form, strain and Hall-Petch-Relationship to Reversed-Hall-Petch-Relationship, data obtained for nanomaterials' grain size and conventional materials' properties are sufficient for analysis. Proposed extended models are solved simultaneously and tested with grain growth data. It is shown that the nature of form evolution depends on form choice and dimensional space. Long-run results reveal that grain boundary migration process causes grains to become spherical, grain rotation coalescence makes them deviate away from becoming spherical and they initially deviate away from becoming spherical before converging into spherical ones due to the TOTAL process. Percentage deviations from spherical grains depend on dimensional space and form: 0% minimum and 100% maximum deviations were observed. It is shown that the plots for grain shape functions lie above the spherical (control) value of 1 in 2-D grains for all considered grain growth mechanisms. Some plots lie above the spherical value, and others approach the spherical value before deviating below it when dealing with 3-D grains. The physical interpretations of these variations are explained from elementary principles about the different grain growth mechanisms. It is observed that materials whose grains deviate further away from the spherical ones have more enhanced properties, while materials with spherical grains have lesser properties. It is observed that there exist critical states beyond which Hall-Petch Relationship changes to Reversed Hall-Petch Relationship. It can be concluded that if grain shapes in nanomaterials are constrained in the way they evolve, then nanomaterials with desired properties can be designed.     

受均布载荷内外固支环板应力分析

从基本的环板模型出发,得出承受均布载荷内外固支环板应力分布的解析解,并运用有限元方法和实验电测法对解析解进行了验证,解析解和数值解分析结果一致,并能和实验结果吻合,为内外固支环板结构的强度设计提供了理论基础,具有工程实用价值。     

动力载荷作用下橡胶混凝土的力学性能

采用体积分数分别为15%、30%和45%的橡胶粉或橡胶块颗粒对混凝土中粗骨料进行替换,并用自由振动法对橡胶混凝土简支梁的振动频率和阻尼性能进行测试,考察了小变形时阻尼比与橡胶颗粒大小和数量之间的关系。采用弹性波法和梁单元法对橡胶混凝土的动力模量进行了测量,并对其静弹性模量与动弹性模量进行了对比。结果表明,橡胶混凝土的动力弹性模量低于普通混凝土。橡胶块对橡胶混凝土动、静弹性模量的影响均大于橡胶粉。普通混凝土的动弹性模量比静弹性模量高37.4%,而橡胶混凝土的动弹性模量比静弹性模量高约50%。橡胶混凝土的阻尼比随最大响应幅值的增大而增大。相比于普通混凝土,橡胶混凝土的阻尼对振动的响应更为敏感。橡胶混凝土的阻尼比随橡胶含量的增加而显著增大。当橡胶的体积分数为30%时,橡胶混凝土综合性能较佳。     

Chemorheological study of vulcanized rubbers under mechanical stress

At lower than the glass transition temperature, the cleavage of the molecular chains of extended vulcanized rubbers will occur. The crosslinking density of vulcanized rubbers, which were placed at room temperature for about 4 hr in nitrogen atmosphere, increased about 17% after extension of 200% at ?76°C. This may be due to the recombination of cleft chains, and this can be shown by the Maxwellian type of curves between crosslinking density and forced strain. Considering the decreasing proportion of crosslinking density by the radical acceptors included in the rubbers and the fact that crosslinking density increases because of recombination of cleft chains, this was attributed to the recombination of unstable chains produced by the added mechanical stimulus.     

Analysis of the mechanical properties of TiN/Ti multilayer coatings using indentation under a broad load range

In this study, physical vapor deposition was used to prepare TiN/Ti multilayer coatings as well as the corresponding monolithic coatings for comparison. Nanoindentation using a large load range (5–4800 mN) and finite element method (FEM) simulations were conducted to investigate the influence of various multilayer structures on the mechanical behavior of multilayer coatings. The nanoindentation results show that the TiN/Ti multilayer coating has the maximum hardness and Young's modulus while retaining good crack resistance and fracture toughness. The FEM results show that increasing the number of layers in the multilayer coatings reduced the hardness and Young's modulus as well as the maximum stress, while it increased the equivalent plastic strain. As the layer thickness ratio increased, both the hardness and Young's modulus gradually increased, and the stress in the coating reached its maximum at the highest thickness ratio. In addition, to consider the effect of the indentation depth on the coating, the influence of the number of layers and the layer thickness ratio on the multilayer coating is combined into the indentation response of the multilayer coating. Therefore, we establish an expression describing the relationship between the number of layers and the ratio of the layer thickness to the mechanical properties of TiN/Ti multilayer coatings.     

Synthesis of high-performance composites by in situ polycondensation and their mechanical properties

High-performance composites of vinylpyridine-styrene copolymers and polyamic acid (PAA) were prepared by the so-called ‘in situ polymerization method’. Poly(4-vinylpyridine-co-styrene) (P4VPy-St) and poly(2-vinyl pyridine-co-styrene) (P2VPy-St) were used as flexible matrix polymers. A molecular composite could be obtained from a polymer pair having an attractive interaction such as a coulombic interaction. Their morphologies were observed by scanning electron microscopy (SEM); mechanical properties of these composites were studied by tensile tests. The PAA content dependence of tensile strength for the composite films obtained by the in situ polymerization method was investigated. The tensile strength of the resulting composite was about 1.5 times higher than that of PAA film. The coulombic interaction between the pyridine moiety in the matrix copolymer and resulting PAA enhanced both the miscibility and mechanical properties of the composites. Furthermore, a polyimide (PI) structure was formed by stepwise heat-treatment and greatly enhanced the tensile strength of the composite films.     

Residual stress and mechanical properties of polyimide thin films

Four different structure polyimide thin films based on 1,4‐phenylene diamine (PDA) and 4,4′‐oxydianiline (ODA) were synthesized by using two different dianhydrides, pyromellitic dianhydride (PMDA) and 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA), and their residual stress behavior and mechanical properties were investigated by using a thin film stress analyzer and nanoindentation method. The residual stress behavior and mechanical properties were correlated to the morphological structure in polyimide films. The morphological structure of polyimide thin films was characterized by X‐ray diffraction patterns and refractive indices. The residual stress was in the range of ?5 to 38 MPa and increased in the following order: PMDA‐PDA < BPDA‐PDA < PMDA‐ODA < BPDA‐ODA. The hardness of the polyimide films increased in the following order: PMDA‐ODA < BPDA‐ODA < PMDA‐PDA < BPDA‐PDA. The PDA‐based polyimide films showed relatively lower residual stress and higher hardness than the corresponding ODA‐based polyimide films. The in‐plane orientation and molecularly ordered phase were enhanced with the increasing order as follows: PMDA‐ODA < BPDA‐ODA < BPDA‐PDA ～ PMDA‐PDA. The PDA‐based polyimides, having a rigid structure, showed relatively better‐developed morphological structure than the corresponding ODA‐based polyimides. The residual stress behavior and mechanical properties were correlated to the morphological structure in polyimide films. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dynamic mechanical properties of crosslinked polyisoprene under deformation

Summary Molecular motions of elastomers under deformations were observed through dynamic mechanical measurements. Composite master curves of dynamic moduli E and E and loss tangent tan over a wide range of frequency and in a state of elongation were obtained by the time-temperature superposition procedure. It is found that both moduli increase with strain, . The slope of the dispersion curve of E become more gradual with the increase in , while that of E is almost unchanged. The increment of E is generally larger than that of E, which does not agree with the N. W. Tschoegl prediction, E ^* ()=f() E _o ^* (), where E ^* () and E _o ^* () are complex moduli at the strain of and O, respectively, and f() is the function of only . The difference in the strain dependence of E from E was found to correspond to the strain dependence of the equilibrium modulus.     

Dynamic mechanical properties of some nylons and their blends

The dynamic mechanical properties of nylons 6, 11, 12, 66, 610, 612, and 666 are compared. The spectra are very similar with peaks in the loss curves at about ?120, ?40, and 85°C. The similarity suggests that attempts to determine whether the nylons could in fact be incompatible when blended might not be successful. Calculations based in turn on calculated cohesive energy densities and interaction parameters also suggest that only nylon, 6 and 66 would be compatible. By using the nature of the major loss peak at the glassy transition which is high and narrow for nylons 11 and 12 and broader and shorter for nylons 6, 66, and 666, it is possible to deduce that nylons 6 and 12 are somewhat incompatible but that the other combinations are most likely dynamically compatible.     

Dynamic mechanical properties of polypentenamers with pendant ionic groups and their hydrogenated derivatives

Dynamic mechanical properties of substituted polypentenamers having thioglycolate side groups and their hydrogenated derivatives have been studied. Methyl esters, acids and salts were investigated at two concentrations, 5.5 mole (%) and 9.2 mole (%). In the un-hydrogenated derivatives one principal relaxation, labelled β, exists in the temperature range from-160 to 100°C. This relaxation arises from microbrownian segmental motion accompanying the glass transition. Its temperature is affected by the concentration of substituent but not the chemical nature of the substituent. A ‘rubbery plateau’ region exists in the acid and salt derivatives and in the latter the salt groups act as reinforcing fillers in this plateau region, consistent with the ionic domain model for their structures. In the hydrogenated derivatives, three relaxations occur in the temperature range from ?160 to 120°C and these are labelled α, β, and γ in order of decreasing temperature. The α relaxation arises from motions associated with the crystal phase except in the case of the 9.2 mole (%) sodium salt derivatives where it arises from motions occurring within ionic domains. The β relaxation arises from microbrownian segmental motions accompanying the glass transition, and the γ relaxation arises from localised motions which may involve both amorphous and crystalline phase components.     

Behaviour of cordierite materials under mechanical and thermal biaxial stress

Abstract

A commercial cordierite powder (< 0.17 wt-% impurities) was selected for a study of material behaviour under mechanical and thermal stresses. Disks were slip cast, sintered for 2 h at 1450°C, and indented (Vickers, 44.1 N) at the centre of the surface to be subjected to mechanical and thermal shock tests. The sintered bodies (84 wt-% cordierite, 10 wt-% mullite, 6 wt-% glass) reached 95% of theoretical density. The microstructure consisted of homogeneous, mainly equiaxed grains (mean size ≈0.5 μm) and a few elongated grains (aspect ratio ≈1.9). A glass phase was identified at triple points, and intergranular pores (< 10 μm) and a few isolated larger pores (up to 40 μm) were observed. The fracture strength σ_F was measured by biaxial flexure, employing a ball on discontinuous ring configuration with displacement con1 trol (0.05 mm min ^-1). In each thermal shock test, the indented specimen was heated to a selected temperature and the disk centre was then suddenly cooled using a high velocity air jet at room temperature. The initial temperature was increased by increments of 10°C until crack propagation was detected and the value of the thermal shock resistance Δ T_C was evaluated. The values obtained were compared with cordierite disks without indents and with alumina materials. The fracture features of the specimens broken in both mechanical and thermal shock tests (crack patterns and fracture surfaces) were characterised, taking into account the developed microstructures (grains, phases, pores) and the fracture origin at the controlled size defect introduced by indentation.     

Reaction sintering of β-tricalcium phosphates and their mechanical properties

Various kinds of calcium oxides, carbonates and phosphates were used as the raw materials, and β-TCP ceramics was fabricated by reaction sintering at 1100 °C, and the sinterability, the reaction sintering behavior and mechanical properties of reaction-sintered β-TCP were investigated. Reaction-sintered bodies using CaHPO₄ + HAp consisted of single β-TCP phase, and bulk density and bending strength increased with extending sintering time. On the contrary, normal-sintered β-TCP synthesized using CaHPO₄ + HAp did not change in bulk density and bending strength with extending sintering time. Reaction-sintered body using CaHPO₄ + HAp as the raw materials showed higher bulk density and bending strength than normal sintered β-TCP.