Formation of mechanical properties of powdered materials

The assumption that the boundaries of bonds are the sources of load transmission from particle to particle describes the reasons of anisotropy of the properties of semifinished products with nonequiaxial particles and points out the directions of improvement of their properties by enhancing the quality of the boundary.Translated from Problemy Prochnosti. No. 8, pp. 64–67, August, 1990.     

Numerical simulation of entangled materials mechanical properties

A general approach to simulate the mechanical behaviour of entangled materials submitted to large deformations is described in this paper. The main part of this approach is the automatic creation of contact elements, with appropriate constitutive laws, to take into account the interactions between fibres. The construction of these elements at each increment, is based on the determination of intermediate geometries in each region where two parts of beams are sufficiently close to be likely to enter into contact. Numerical tests simulating a 90% compression of nine randomly generated samples of entangled materials are given. They allow the identification of power laws to represent the evolutions of the compressive load and of the number of contacts.     

Biological materials: Structure and mechanical properties

Most natural (or biological) materials are complex composites whose mechanical properties are often outstanding, considering the weak constituents from which they are assembled. These complex structures, which have risen from hundreds of million years of evolution, are inspiring Materials Scientists in the design of novel materials.Their defining characteristics, hierarchy, multifunctionality, and self-healing capability, are illustrated. Self-organization is also a fundamental feature of many biological materials and the manner by which the structures are assembled from the molecular level up. The basic building blocks are described, starting with the 20 amino acids and proceeding to polypeptides, polysaccharides, and polypeptides-saccharides. These, on their turn, compose the basic proteins, which are the primary constituents of ‘soft tissues’ and are also present in most biominerals. There are over 1000 proteins, and we describe only the principal ones, with emphasis on collagen, chitin, keratin, and elastin. The ‘hard’ phases are primarily strengthened by minerals, which nucleate and grow in a biomediated environment that determines the size, shape and distribution of individual crystals. The most important mineral phases are discussed: hydroxyapatite, silica, and aragonite.Using the classification of Wegst and Ashby, the principal mechanical characteristics and structures of biological ceramics, polymer composites, elastomers, and cellular materials are presented. Selected systems in each class are described with emphasis on the relationship between their structure and mechanical response. A fifth class is added to this: functional biological materials, which have a structure developed for a specific function: adhesion, optical properties, etc.An outgrowth of this effort is the search for bioinspired materials and structures. Traditional approaches focus on design methodologies of biological materials using conventional synthetic materials. The new frontiers reside in the synthesis of bioinspired materials through processes that are characteristic of biological systems; these involve nanoscale self-assembly of the components and the development of hierarchical structures. Although this approach is still in its infancy, it will eventually lead to a plethora of new materials systems as we elucidate the fundamental mechanisms of growth and the structure of biological systems.     

Relationships between mechanical properties of cement stabilized materials

A laboratory testing programme of granular materials stabilized with cement has been implemented in order to determine the influence of changes in mix composition on their mechanical characteristics. The elements of the composition of stabilized mixes such as gradation, cement content, cement type, density and curing time were varied. The results obtained were analysed statistically and three types of correlation were determined. It is concluded that the relationship between the dynamic modulus of elasticity and the pulse velocity is almost unaffected by individual composition parameters, while the relationship between compressive and tensile strength, and especially that of compressive strength and dynamic modulus of elasticity, is dependent on these parameters.     

Experimental and theoretical studies of the influence of surface conditions on radiative properties of opaque materials

Radiative properties of opaque materials strongly depend on their surface condition. The fabrication of superficial cavities of various forms and dimensions modifies the directional spectral emissivities or absorptivities. They are usually increased compared to those of optically smooth material; the gain depends on the material, the type of cavities, as well as the wavelength and the direction of the emitted or incident radiation. When grooves of dimensions larger than are fabricated in a sample, the models, taking into account the successive reflections on their inner sides, give a good agreement with experimental data. But a similar theory does not explain the substantial increase of the infrared emissivity of ballblasted samples.     

Finite-element modeling of rate dependent mechanical properties in nanocrystalline materials

A generalized self-consistent polycrystal model is used to study the mechanical properties of polycrystalline metals as the grain size decreases from the ultra-fine size to the nanometer scale. The model takes each oriented grain and its immediate grain boundary to form a pair. Then by making use of a composite model, the nonlinear behavior of the nanocrystalline polycrystal is determined. The finite-element method is employed in conjunction with the unit cell of the composite to investigate the rate-dependent tensile behavior of the system. A dislocation density based constitutive equation is used to describe the plastic flow behavior of the grain interior. The boundary phase is assumed to have the mechanical properties of quasi-amorphous material. The constitutive equations for both grain interior and boundary phase are implemented into a finite-element program and the results of the calculations are compared with previously published experimental data. For some cases, an optimization procedure was used to tune some parameters of the model in order to decrease the distance between the calculated and experimental stress–strain curves. The agreement between results indicates the suitability of the updated model for nanocrystalline materials.     

The nonequilibrium thermodynamic conditions and properties of materials

The basic principles for classifying the properties of materials, measured under nonequilibrium thermodynamic conditions, are formulated. It is proposed to supplement the nonlinear nonequilibrium thermodynamic mode of operation by new submodes, the area of action of which is determined by the dimensionless criteria introduced. The heat flux relaxation time and the temperature gradient time are considered as new nonlinear properties. Translated from Izmeritel'naya Tekhnika, No. 11, pp. 41–46, November, 2008.     

Nanostructural metallic materials: Structures and mechanical properties

The trade-off of strength and ductility of metals has long plagued materials scientists. To resolve this issue, great efforts have been devoted over the past decades to developing a variety of technological pathways for effectively tailoring the microstructure of metallic materials. Here, we review the recent advanced nanostructure design strategies for purposely fabricating heterogeneous nanostructures in crystalline and non-crystalline metallic materials. Several representative structural approaches are introduced, including (1) hierarchical nanotwinned (HNT) structures, extreme grain refinement and dislocation architectures etc. for crystalline metals; (2) nanoglass structure for non-crystalline alloys, i.e. metallic glasses (MGs); and (3) a series of supra-nano-dual-phase (SNDP) nanostructures for composite alloys. The mechanical properties are further optimized by manipulating these nanostructures, especially coupling multiple advanced nanostructures into one material. Particularly, the newly developed SNDP nanostructures greatly enrich the nanostructure design strategies by utilizing supra-nano sized crystals and MGs, which exhibit unique size and synergistic effects. The origins of these gratifying properties are discussed in this review. Furthermore, based on a comprehensive understanding of microscopic mechanisms, a broad vision of strategies towards high strength and high ductility are proposed to promote future innovations.     

Verification of relationships between mechanical properties of concrete-like materials

It was proposed previously that a proper consideration of the concrete porosity can provide an improved method for the derivation of sound, reliable relationships between mechanical properties, such as strength and modulus of elasticity. The method of derivation is based on the differing effects of porosity in concrete on its mechanical properties. Several new formulas, such as equations 18) through 25) were derived from appropriate members of two groups of underlying equations: from equations 8) through 14), and equations 15) through 17). It is shown in figures 4 through 12 of this paper that the new formulas and, consequently, the outlined method of derivation are supported by experimental results despite the several simplifying assumptions applied. The fundamental advantage of the outlined method of derivation is that it is mathematical as compared to the empirical nature of the presently used methods.     

Statistical analysis of the mechanical properties of composite materials

The Weibull statistic is currently used in designing mechanical components made of composite materials. This work presents useful formulae to describe the behaviour of the Weibull modulus estimator, which in turn may be described by means of a three-parameter Weibull distribution. Expressions for the parameters of this latter distribution, dependent on the sample size, are also given in the paper, so, the percentage points, published until now in tabular form, may be directly calculated. Empirical expressions are derived for determining the A-basis and the B-basis material properties as a function of the sample size.     

Surface effects on the mechanical properties of nanoporous materials

Using the theory of surface elasticity, we investigate the mechanical properties of nanoporous materials. The classical theory of porous materials is modified to account for surface effects, which become increasingly important as the characteristic sizes of microstructures shrink to nanometers. First, a refined Timoshenko beam model is presented to predict the effective elastic modulus of nanoporous materials. Then the surface effects on the elastic microstructural buckling behavior of nanoporous materials are examined. In particular, nanoporous gold is taken as an example to illustrate the application of the proposed model. The results reveal that both the elastic modulus and the critical buckling behavior of nanoporous materials exhibit a distinct dependence on the characteristic sizes of microstructures, e.g. the average ligament width.     

Processing and mechanical properties of autogenous titanium implant materials

Pure titanium and some of its alloys are currently considered as the most attractive metallic materials for biomedical applications due to their excellent mechanical properties, corrosion resistance, and biocompatibility. It has been demonstrated that titanium and titanium alloys are well accepted by human tissues as compared to other metals such as SUS316L stainless steel and Co–Cr–Mo type alloy. In the present study, highly porous titanium foams with porosities 80% are produced by using a novel powder metallurgical process, which includes the adding of the selected spacers into the starting powders. The optimal process parameters are investigated. The porous titanium foams are characterized by using optical microscopy and scanning electron microscopy. The distribution of the pore size is measured by quantitative image analyses. The mechanical properties are investigated by compressive tests. This open-cellular titanium foams, with the pore size of 200–500 m are expected to be a very promising biomaterial candidates for bone implants because its porous structure permits the ingrowths of new-bone tissues and the transport of body fluids.     

Dynamic mechanical properties of polyethylene- and exfoliated-graphite-base composition materials

Results of experimental investigation of the dynamic mechanical and relaxation properties of composition materials of the polyethylene/exfoliated-graphite system, where the exfoliated-graphite content is varied from 0 to 1 volume fraction in the — 180–80°C interval are presented. As an effective filler, exfoliated graphite is demonstrated to improve the elasticity of composition materials. The major influence exerted by spatial graphite-particle structures on the mechanical-relaxation processes that take place in polyethylene and composition materials is established. The data obtained are examined from positions of the percolation theory. Expressions of this theory are used to describe mathematically the variation in the dynamic shear modulus as a function of graphite content in a certain range of component concentrations and temperatures.Translated from Problemy Prochnosti, No. 7, pp. 84–91, July, 1994.     

Structure and mechanical properties of materials after hydroexplosive forming

The relationship between structure and mechanical properties of nickel, nichrome and steel Kh18N10T deformed at strain rates of 10⁴–10⁵ sec^–1 was studied. X-ray diffraction analysis was carried out, and static and high-speed deformation of these materials were compared.     

Prediction of mechanical properties in magnesia based refractory materials using ANN

Refractory materials are heterogeneous materials having complex microstructures with different constituent’s properties. The mechanical properties of these materials change depending on their chemical composition and temperature. Therefore, it is important to select a refractory material, which is suitable for working conditions and is fit to place of use. Artificial neural network (ANN) model is established to investigate the relationship among processing parameters (chemical composition, temperature) and mechanical properties (bending strength, Young’s modulus) in magnesia based refractory materials. The mechanical properties of magnesia based refractory materials having four different chemical compositions were investigated using three point bending test at temperatures of 25, 400, 500, 600, 700, 800, 900, 1000 and 1400 °C.The bending strength (σ) and Young’s modulus (E) were theoretically calculated by ANN method and theoretical results were compared with experimental values for each temperature. There were insignificant differences between experimental values and ANN results meaning that ANN results can be used instead of experimental values. Thus, mechanical properties of refractory materials having different chemical composition can be predicted by using ANN method regardless of the treatment temperature.