Uniaxial strength asymmetry in cellular materials: An analytical model

The ability to predict failure of cellular materials depends on the knowledge of microstructural mechanisms that contribute to macroscopic behavior. In this paper, we develop microstructural models to examine the mechanisms responsible for differences in tensile and compressive strength observed in cellular materials. We limit our analyses to those materials that fail by the same mechanism (yielding or microcracking) in tension and compression. Using both a honeycomb and an open-cell foam model, we demonstrate that density-dependent, compression-strong strength asymmetry arises when two conditions are met: the cell wall material has a higher yield strength in compression than in tension, and the cell walls are loaded simultaneously by axial forces and bending moments. Our models predict that strength asymmetry (defined as the ratio of compressive to tensile yield strength of the cellular material) increases with relative density (as observed in real materials such as rigid polyurethane foams and trabecular bone), and that strength asymmetry is more pronounced in anisotropic materials (where oblique struts are more closely aligned with the direction of loading).     

石墨铝基自润滑材料的制备及性能表征

以石墨为固体润滑剂,铝为基体材料,添加硅、铜、铁等元素作为强化成分,添加Cr3C2、粉煤灰来提高自润滑材料的耐磨性,通过液态铸造法制备铝基自润滑材料。借助MM-200磨损试验机研究不同石墨含量以及粉煤灰的添加对铝基复合材料摩擦磨损性能的影响;在Olympus金相显微镜下观察材料显微组织;用S-3000N扫描电子显微镜对材料拉伸断口和磨损表面进行观察。结果表明:随着石墨含量的增加,摩擦因数逐渐降低,磨损率逐渐升高,抗拉强度、硬度都逐渐降低;粉煤灰的加入使得初生硅得到细化,自润滑材料的强度上升,摩擦因数得到进一步的改善。     

新型硬质合金刀具材料的组织与性能分析

以Ti C和VC为抑制剂,采用先球磨再烧结的两步法制备了新型硬质合金刀具材料,并进行了显微组织、力学性能和抗氧化性能的测试与分析。结果表明,该新型硬质合金刀具材料组织均匀、晶粒细小,具有优异的力学性能和抗氧化性能。该合金材料的平均晶粒尺寸为1±0.5μm,显微硬度达1582HV,横向断裂强度达2600MPa,断裂韧度达21.8MPa·m1/2,700℃氧化5h后的氧化增重量低于6mg/cm2,氧化层有WO3、Co3O4、Co WO4、Ni O等氧化物。     

Organic optoelectronics ： materials, devices and applications

1Introduction Beforethediscoveryofelectricallyconduc tivepropertiesthroughdoping,polymerswere widelyusedaselectricalinsulatorsduetothesu periorinsulatingpropertiestheyexhibited.In1977,thediscoveryofthefirsthighlyconductive polymer,chemicallyandelectrochemicallydoped polyacetylene,wasreported[12].Thediscovery ofdopedpolyacetylenehasopenedanentirenew fieldforpolymersandorganicmaterialsintheir applicationstobothconductorsandsemiconduc tors.Althoughtheinitialemphasiswasonthe conductivepropertiesob…     

The importance of static friction characteristics of brake friction couple,and methods of testing

Detailed knowledge about static friction materials is required for the accurate calculation of the braking torque needed to hold a load at rest. This is particularly important for brakes in cranes, elevators, hoists and mining winding machines, which must meet specifications such as the definite value of the static safe braking factor. The study of static friction is also a useful supplement to the dynamic testing of brake friction materials. In such a study, precise control of the temperature on the surfaces is possible, as well as surface roughness, and existence of the third-body can be accurately identified. It is an important fact that the coefficient of static friction, μ_s, is not an invariant, and it cannot be adequately represented in many engineering applications as a single number. The study of static friction dependence upon factors such as stationary contact time, rate of tangential loading, and surface temperature, contributes to a better understanding of friction phenomena. In this paper, a test apparatus is presented, and a series of experiments is described. The experiments reveal the static friction characteristics of some brake friction materials.     

Histochemical and structural characterization of egg extra‐cellular matrix in bufonid toads,Bufo bufo and Bufotes balearicus: Molecular diversity versus morphological uniformity

The extra‐cellular matrix of fertilized eggs in the bufonid toads Bufo bufo and Bufotes balearicus was studied to clear the relationships between structural and molecular diversity. Histochemical (PAS, AB pH 2.5 and pH 1.0, Beta‐elimination PAS) and lectin‐histochemical (Con A, WGA, Succinyl‐WGA, PNA, RCA‐1, DBA, SBA, AAA, UEA‐I, LTA) techniques were used and the observations were made under light and electron microscopy. Both species present a fertilization envelope (FE) and two jelly layers (J₁ and J₂). The fibers of J₂ are shared among the eggs of a clutch in a jelly ribbon. The FE of both species presents neutral glycoproteins, mostly N‐linked. In B. bufo there are also residuals of mannose and/or glucose and N‐acetylglucosamine. In the FE fibers run parallel to egg's surface or are in bundles or looser hanks with no clear orientation. The J₁ layer of both species presents sialosulfoglycoproteins, mostly O‐linked, with lactosaminylated, galactosaminylated, glycosaminylated, and fucosylated residuals. A lower amount of galactosaminylated residuals is observed in B. balearicus in respect to B. bufo, whereas the opposite is seen in the amount of fucosylated residuals. The J₂ layer is similar in composition to J₁ but in B. balearicus there are no glucosaminylated residuals. J layers present fibers and granules that reduce towards J₂. Several microorganisms, in particular blue algae, are observed in the J₂ layer of both species. In respect to other species, B. bufo and B. balearicus have a lower number of jelly layers, but a comparable number of glycan types. Microsc. Res. Tech. 77:910–917, 2014. © 2014 Wiley Periodicals, Inc.     

Computational micro- and macroscopic models of contact and friction: formulation, approach and applications

This paper is dedicated to new asperity-based constitutive models of contact interfaces. These models have been obtained through a combination of finite element analysis of surface asperities and statistical homogenization techniques, to predict macroscopic, phenomenological behavior of the interface. This new approach has generalized the existing asperity-based models of contact and friction by considering realistic, complex shapes and mechanical properties of surface asperities, as opposed to previous simplified analytical solutions. This has been achieved by application, at the stage of asperity modeling, of the finite element method, which takes into account arbitrary shapes of asperities, non-linear material properties, molecular-range adhesion forces, and sliding resistance on the contact surface. The h–p adaptive mesh refinement techniques, adaptive timestepping and other adaptive methods are used to assure high accuracy of the solution. The result of this development is a new family of constitutive interface models, consistent with surface micromechanics and applicable to studies of static and dynamic friction phenomena. They are also extendible to calculation of thermal or electrical resistances, wear modeling, and other applications. This paper presents the theoretical formulation, numerical methodology and sample models of contact, adhesion and friction obtained through these homogenization techniques.