Theoretical estimation of the effect of interfacial energy on the mechanical strength of spinodally decomposed alloys

New interfaces are produced on the slip plane when a crystal with continuous composition fluctuation arising from spinodal decomposition is deformed by slip. In this work, the energy of such interfaces is evaluated for both modulated and mottled structures, and their effects on slip behaviour are discussed. It is concluded that the contribution of this interfacial energy is large enough to account for the age-hardening concomitant with spinodal decomposition.     

Cyclic deformation behavior of a low carbon steel in the temperature range between room temperature and 850 K

Stress-controlled uniaxial push-pull fatigue tests were carried out with cylindrical specimens of low carbon steel with a carbon content of 0.1 percent. Resistent heated grips provided testing temperatures up to 550°C. During all fatigue tests the time-dependent strain was measured. The strain at zero load, the plastic strain amplitude plotted against the number of cycles is known as the cyclic deformation curve. At room temperature, this curve depends on the applied stress amplitude. The shape of the cyclic deformation curve at the same stress amplitude is strongly temperature-dependent. In the temperature region of blue brittleness the endurance shows a maximum connected with a very low plastic strain amplitude. Also the cyclic stress strain curves have a different appearance within varied temperature ranges.The observation of the dislocation structure of the specimen's interior after fatigue damage by means of the transmission electron microscopy showed different pictures within different temperature regions. For instance, besides other structures at other temperatures, dislocation channeling was found, connected with a cyclic softening process which is evident from the cyclic deformation curve. The dislocation structures of specimens fatigued at various conditions differ from clouds to cells, channels, walls, and blocks.

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Résumé Des essais de fatigue traction compression uniaxiale en contrainte contrôlée ont été éffectués sur des éprouvettes cylindriques d'acier à bas C, le carbone étant de 0,1%. Grâce à des mordaches résistant à la température, on a pû réaliser des essais jusque 550°C. Durant tous les essais de fatigue, on a mesuré la dilatation en fonction du temps.La déformation à charge nulle ou amplitude de déformation plastique portée en diagramme en fonction du nombre de cycles est connu comme la courbe de déformation cyclique. A la température ambiante, cette courbe dépend de l'amplitude des contraintes appliquées. La forme de la courbe de déformation cyclique à même amplitude de contrainte dépend directement de la température. Dans la région de température correspondant à la fragilité au bléu, l'endurance fait état d'un maximum en relation avec une très faible amplitude de déformation plastique. On constate également que les courbes de contrainte et de déformation cyclique ont une forme différente suivant la gamme de températures.L'observation de la structure de dislocation à l'intérieur d'un échantillon soumis à dommage de fatigue montre, en utilisant une microscopie électronique par transmission différents aspects suivant les zones de températures différentes atteintes. Ainsi, parmi diverses structures correspondant à d'autres températures, on a constaté des alignements de dislocations en connexion avec un processus d'adoucissement cyclique qui apparait clairement à partir des courbes de déformation cyclique. Les structures des dislocations des éprouvettes soumisses à fatigue sous diverses conditions prennent la forme de nuages, de cellules dalignements de parois et de blocs.

     

Nonlinear behavior of some hydrostatically stressed isotropic elastomeric foams

Summary We calculate the stress-strain relation for elastomeric foam from anab initio theory, which shows that the plateau and densification regions should be described by a hyperbola. The theory seems to agree reasonably well with experiment.     

Interfacial and mechanical properties of environment-friendly “green” composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin

Physical and tensile properties of pineapple fibers were characterized. Tensile properties of pineapple fibers, like most natural fibers, showed a large variation. The average interfacial shear strength between the pineapple fiber and poly(hydroxybutyrate-co-valerate) (PHBV) was 8.23 MPa as measured by the microbond technique. Scanning electron microscopy (SEM) photomicrographs of the microbond specimens revealed an adhesive failure of the interface. Fully degradable and environment-friendly green composites were prepared by combining pineapple fibers and PHBV with 20 and 30% weight content of fibers placed in a 0°/90°/0° fiber arrangement. Tensile and flexural properties of these green composites were compared with different types of wood specimens. Even though tensile and flexural strength and moduli of these green composites were lower than those of some wood specimens tested in grain direction, they were significantly higher than those of wood specimens tested in perpendicular to grain direction. Compared to PHBV virgin resin, both tensile and flexural strength and moduli of these green composites were significantly higher. SEM photomicrographs of the fracture surface of the green composites, in tensile mode, showed partial fiber pull-out indicating weak bonding between the fiber and the matrix.     

Deterministic Model of Homophase and Heterophase Fluctuations in a Liquid–Vapor System

Within the framework of the assembly-type catastrophe model, a nonlinear dynamic equation (DE) homogeneous in the parameter t with an aftereffect is constructed, in which t characterizes the deviation of the reduced density of a thin surface layer on the liquid-vapor interface from the mean density of the vapor-liquid system. This equation is used to treat a second-order nonlinear DE with a variable damping coefficient for a vapor-liquid system excited by periodic impacts (acts of evaporation and condensation of molecules). This DE is integrated over a finite time interval to find a two-dimensional mapping whose numerical solution describes the chaotic dynamics of the density in time, including homophase and heterophase fluctuations. For this system, the bifurcation diagrams are constructed and the Lyapunov exponents are found.     

Output measures of cooperative research: The case of the Pulp and Paper Research Institute of Canada

Applying various quantitative techniques, this paper attempts to describe and analyze the scientific output of a cooperative industrial research institute (Pulp and Paper Research Institute of Canada, PAPRICAN) by comparing its impact on the employment patterns of McGill graduate students who have done their thesis research under the auspices of the industrial laboratory with graduate students from the same departments who have not worked at PAPRICAN; and a comparison of the publication practices of three groups: PAPRICAN staff not associated with the university (McGill), the PAPRICAN staff who also hold academic appointments at McGill, and the faculty of the Chemistry Department at McGill who do not hold staff positions at PAPRICAN.It is found that the academic association with PAPRICAN during graduate research has a significant impact on the number of students who go on to careers in industry. However, close examination of those who remain in Canada indicates that the impact is increasingly felt in only the Pulp and Paper industry. Different macro standards are applied to this micro example, and policy implications are discussed.The publication record is again compared to various macro standards so as to judge various qualities of the scientific output of the different groups. The PAPRICAN staff performs as would be expected of industrial researchers and the McGill faculty show normal characteristics for an academic group. However, those who hold positions in both the industrial institute and the academic sector, reveal the special role they play in linking the science of the second with the technology of the first.     

Local Dissipation Phenomena in Crystalline Forms of Boron

A classification is proposed for dissipation (in particular, relaxation) processes in crystalline systems and inorganic materials. The concepts of modifying and aggregation subsystems are defined. Relations are derived for determining the temperature dependences of the local quasi-viscous dissipation parameter r _iand the local quasi-elastic parameter K _ifor each relaxation transition in the tan (T) spectrum.     

Quantitative estimation of “big” and “little” science interrelation

Qantitative analysis of the interrelation of big and little science on the example of Research and Development of higher education in the USA has been made. The difference in the growth rates of big and little science is explained with the help of scientometric index of capital expenditures per researcher. An attempt has been made to compare the dynamics of efficiency of big and little science on the base of mean duration of a research project. Possibilities of an alternative index of a relative amount of preliminary researches (preprojects) are pointed out.     

Generalizing the finite element method: Diffuse approximation and diffuse elements

This paper describes the new diffuse approximation method, which may be presented as a generalization of the widely used finite element approximation method. It removes some of the limitations of the finite element approximation related to the regularity of approximated functions, and to mesh generation requirements. The diffuse approximation method may be used for generating smooth approximations of functions known at given sets of points and for accurately estimating their derivatives. It is useful as well for solving partial differential equations, leading to the so called diffuse element method (DEM), which presents several advantages compared to the finite element method (FEM), specially for evaluating the derivatives of the unknown functions.     

The effect of Si,Zr, Al and Mo on the structure and strength of Ti martensite

The structure and strength of martensite in near titanium alloys have been studied in the composition range (wt %) up to 10% Zr, 6%, Al, 1/2% Mo, 2.4% Si. [0001], 1/3 11¯20 dislocations, 1/3 10¯10 stacking faults and approximately {10¯11} twin related martensite plates are found to be common features of the martensite. Martensite midribs consist either of finely transformed material between martensite plates, or regions of low dislocation density within martensite plates.The martensite morphology is related to the alloy composition, changing from massive to plate-like with increasing solute content. The strength of the martensite is controlled largely by solid solution strengthening.     

The development of strength in reaction sintered silicon nitride

The development of strength in reaction sintered silicon nitride has been investigated by determining the elastic moduli, fracture mechanics parameters, strengths and critical defect sizes of silicon compacts reacted to various degrees of conversion using static or flowing nitrogen. The relationship between each property and the nitrided density is shown to be independent of the green silicon compact density but is influenced by the nitriding conditions employed. Young's moduli, rigidity moduli and strengths vary linearly with the nitrided density. After an initial period when increases may occur, the critical defect sizes in both static and flow materials decrease continuously with increasing nitrided density, although at any particular density they are larger in material produced under flow conditions. A model is suggested for the development of the structure of reaction sintered silicon nitride involving the development of a continuous silicon nitride network within the pore space of the original silicon compact. The experimental data are discussed in terms of the proportion of silicon nitride which contributes effectively to the continuous network.     

Uncertainty of a measured result and confidence error of measurements

An analysis is made of the known information concerning the concepts of error and uncertainty and it is concluded that they are identical.Translated from Izmeritel'naya Tekhnika, No. 8, pp. 14–17, August, 1994.     

Relationship between superconductivity and band structures of electrons and phonons

The relationship between superconductivity and band structures of electrons and phonons is established on the basis of a generalized Bardeen-Cooper-Schrieffer model in which the interaction strengths (V ₁₁,V ₁₂,V ₁₂) among and between electron (1) and hole (2) Cooper pairs are differentiated. Elemental superconductors must have local hyperboloidal Fermi surfaces called necks or inverted double caps.     

Thermal shock resistance of miniaturized multilayer ceramic capacitors

The thermal shock resistance of miniaturized multilayer ceramic capacitors (MLCs), of sizes 0402, 0603, 0805 and 1206, was investigated by comparing the leakage currents before and after thermal shock. It was generally found that smaller capacitors have a higher thermal shock resistance than larger ones. The 0402 MLC possesses a thermal shock resistance in excess of 420 C. The linear interdependence of thermal shock resistance and reciprocal of half thickness, as predicted by conventional thermal shock analysis, was not observed. Instead, the thermal shock resistance of an MLC was found to be inversely proportional to the total area of its ceramic surface. This confirms that pre-existing flaws on the ceramic surface dominate the crack initiation process and are therefore primarily responsible for determining the thermal shock resistance of an MLC.     

Formation of Friction Layers and Wear Resistance of Steels under Conditions of Boundary Friction

We studied the influence of the lubricant compositions Grafitol with 10% graphite (1), Aerol containing 17% MoS₂ (2), Limol containing 60% MoS₂ (3), Limol + 10% chlorine-paraffin (4), and Limol + 10% copper powder (5) on the wear rate and formation of the fine structure of surface friction layers of structural steels. We established a correlation between the tribological characteristics of steels and lubricants. The abrasive wear of 40KhFA steel was minimum if it was lubricated with Limol + 10% copper powder. In this case, its wear was smaller by a factor of 10, 2, 1.25, and 7.25 as compared with lubricant compositions 1–4, respectively. In the course of minimum wear of 40KhFA steel, in the surface friction layers, we observed the minimum values of second-kind distortions (a/a) and of the true size of domains of X-ray coherent scattering (D) as well as the minimum difference between the crystal lattice constants (a) of steel before and after friction.     

Effect of Intense Self-Irradiation on the Phase Composition of Metallic Plutonium

The crystal structure of film samples of "high-level" (based on ²³⁸Pu) and low-level (based on ²³⁹Pu) metallic plutonium during their prolonged (up to 343 days) storage (self-irradiation) at room temperature was studied by X-ray diffraction. In the samples of high-level plutonium, the -Pu and -Pu lattices coexist. In the period of 40-60 days, the other known crystal modifications of plutonium (-Pu, -Pu, -Pu, and -Pu) are also present. Low-level plutonium had only the -Pu lattice. A possible origin of this phenomenon is discussed.     

Three-dimensional problems of strongly interacting arbitrarily located penny-shaped cracks

Three-dimensional elastic interactions of arbitrarily located and oriented penny-shaped cracks are considered, and solutions for the stress intensity factors (SIFs) are obtained for several crack geometries. The work is based on the method of analysis of many cracks problems developed by Kachanov [1]. Unlike most of the results available in literature, the obtained solutions remain accurate for closely spaced, strongly interacting cracks. The effects of stress shielding and amplification and the influence of various geometrical factors are examined. The simplest crack-microcrack interactions in three dimensions are considered. The approximations of weak interactions and of widely spaced cracks are discussed. The interactions that are weak in their impact on the effective elastic constants may be strong in their impact on SIFs; this indicates that no simple relation between the deterioration of elastic stiffness of a microcracking solid and its progression towards failure seems to exist.

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Résumé On considère les interactions élastiques sur trois dimensions de fissures en forme d'onglets disposées et orientées de façon arbitraire, et on obtient des solutions pour les facteurs d'intensité de contraintes pour diverses géométries de fissures. L'étude est basée sur la méthode d'analyse des problèmes de fissuration multiples, développées par Kachanov. A l'encontre de la plupart des résultats disponibles dans la littérature, les solutions obtenues demeurent exactes pour des fissures groupées et à interactions sévères. On examine les effets de bouclier ou d'amplification pour les contraintes, ainsi que l'influence de divers facteurs géométriques. On considère les cas les plus simples d'interactions en trois dimensions entre fissure et micro-fissure. On discute les approximations que constituent les notions de faibles interactions et de fissures largement espacées. Les interactions qui sont faibles au plan de leur impact sur les constantes élastiques effectives peuvent se révéler sévères par rapport au facteur d'intensité de contraintes. Ceci indique qu'il ne semble pas exister de relations simples entre la détérioration de la rigidité élastique d'un corps du fait d'une micro-fissuration, et son évolution vers la rupture.

     

Application of dynamic shear crack models to the study of the earthquake faulting process

The earthquake faulting process can be modelled as propagating two or three dimensional shear cracks. Since analytical methods cannot be used to study finite cracks, several numerical approaches to this problem have been taken, of which the numerical boundary integral method is here described in detail. The process of crack growth can be determined by the use of a fracture criterion. Since the material properties such as fracture strength and frictional properties vary over the surfaces of real faults, two types of complex faulting models-the barrier model and the asperity model-are discussed in the context of seismology.

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Résumé Un processus de glissement dans une faille souterraine peut être modélisé sous forme de fissure de cisaillement à 2 ou 3 dimensions en cours de propagation. Comme les méthodes analytiques ne peuvent pas être utilisées pour des fissures finies, on a considéré diverses approches numériques à ce problème parmi laquelle la méthode des intégrales numériques aux limites est décrite en détail. Le processus de croissance d'une fissure peut être déterminé par l'utilisation d'un critère de rupture. Comme les propriétés du matériau telles que la résistance à la rupture ou les propriétés de friction varient sur les surfaces des failles réelles, deux types de modèles complexes de glissement, le modèle Barrière et le modèle Aspérité, sont discutés dans le contexte de la seismologie.

     

Electromagnetic radiation emitted during friction process

A series of studies of electromagnetic radiation (EMR) emitted during fracture of materials, enabled us to obtain a relationship between the width of a fracture and the wavelength of the emitted EMR. Applying this relation to friction we could check one of its suggested mechanisms, namely the Bowden-Tabor model, which states that during a friction process, asperities on the two contacting surfaces are welded together (at a microscopic level) and fractured. A uniaxial tension machine was used, whereby two half cylinders of chalk (CaCO3) bound together, were moved one against the other, generating friction. Calculations based on EMR observations showed that the average width of the fractured asperities was 26.3 m, while mechanical profilograph measurements of the average width of the total number of long asperilies before and after the experiment, yielded values of 15.6 and 18.4 m, respectively, implying that 25% of long asperities were fractured during a single friction process.     

Dynamics of a nonlinear periodic structure with cyclic symmetry

Summary The free and forced motions of a nonlinear periodic structure with cyclic symmetry are studied. The structure consists of a number of identical linear flexural members coupled by means of nonlinear stiffnesses of the third degree. It is found that this system can only possessn similar nonlinear modes of free oscillation, and that no other modes are possible. Moreover, there exist pairs of nonlinear modes with mutually orthogonal nodal diameters having, in general, distinct backbone curves. A multiple-scales averaging analysis is used to study the nonlinear interaction between a pair of modes with orthogonal nodal diameters. As a result of this analysis, it is found that all pairs of nonliner modes along with all their linear combinations are orbitally unstable, and the only possible orbitally stable periodic motions are free travelling waves, that propagate through the structure in the clockwise and anti-clockwise directions. Under harmonic forcing, a bifuraction of a stable branch of forced travelling waves from a branch of forced normal mode motions is detected, and jump phenomena between branches of periodic solutions are observed. The analytical results are in agreement with experimental observations of an earlier work, and, in addition, are verified by numerical simulations.