A generalized model to predict the effect of voids on modulus in ceramics

An equation recently developed by the present author to describe the modulus of particulate composites as a function of the volume fraction of particles was modified in this study to describe modulus as a function of porosity. This new equation was applied to available modulus literature for ceramics where voids were the particulate phase. By varying the porosity interaction coefficient, σ, this new generalized void/modulus equation was shown to be able to yield equations previously used to predict modulus as a function of voids for ceramics. Wang theoretically described the mode of porosity interaction during compaction with a constant, α, to calculate the void/modulus relationship for three different compaction conditions. The generalized void/modulus equation developed in this study fit Wang's theoretical data exceptionally well, even though the porosity interaction coefficients, σ, obtained did not agree closely with Wang's values of α. Wang also experimentally measured the porosity and Young's modulus of manufactured alumina rods prepared with spherical and “egg-shaped” powders. The optimum fit for spherical particles occurred at σ=0.9 and an initial porosity of P _i=0.405 and for “egg-shaped” particles at σ=1.05 and P _i=0.475. The generalized void/modulus the equation for σ=−1 yields an equation that has the same form as Wang's proposed empirical equation that utilized two empirical constants, b and c. Wang's experimental data fitted with his proposed empirical equation gave a positive value for the constant c of 0.982 which corresponded to a negative value of P _i of -0.0743 which was not defined in the theoretical considerations developed in this study. While this value of the initial porosity, P _i, does give a better fit of the data for the interaction constant σ=-1, it still did not fit all the data as well as the results calculated for interaction coefficients nearer 1.0. The results of this study have shown that an excellent fit of most void/modulus data can be obtained using the generalized void/modulus equation developed in this study without making assumptions inconsistent with the theory presented.     

On laws governing the splitting failure of flat metallic specimens subjected to thermal shock

Computed and experimental data on the splitting failure of copper, nickel, titanium, molybdenum, brass, and bronze metallic foils from 0.005 to 1 mm thick under thermal shock initiated by the x-radiation of a nuclear explosion are presented. It is proposed that the concepts “average energy liberated over the thickness (mass) of the specimen” ε, “specific absorbed energy” W, and “splitting strength of the material” σ be used as criterial characteristics of failure thresholds of optically thin flat metallic specimens (foils). It is demonstrated that the critical average energy liberation ε_*, which results in splitting, decreases logarithmically (ε_* =A _*-B _*log Δ) with increasing thickness Δ of the irradiated specimens in the interval Δ≈0.001–1 mm, and the critical specific potential energy W_* reguired to effect splitting increases with increasing optical mass m of the specimen under the law W_*=−αmlog (βm), where A_*, B_*, α, and β are certain parameters. It is shown that the longevity of the copper, nickel, titanium, molybdenum, brass, and bronze under radiation-induced thermal shock decreases exponentially with increasing amplitude of the failing stress (splitting strength) and can be described on the basis of the kinematic concept of strength. Deceased. Translated from Problemy Prochnosti, No. 1, pp. 37–47, January–February, 1997.     

Influence ff the initial state on the strength of pressed lithium hydride

We present the results of mechanical testing of pressed lithium hydride under the conditions of uniaxial compression, radial compression, and bending. The specimens were made of three different powder fractions. Their density varied depending on the level of compacting pressure. It is shown that, in the investigated range of parameters characterizing the initial state of the material, the dependences of strength on the characteristic size of the original powder d and the density of the material of the specimen ρ can be described by the same formula of the form σ _f =σ₀-klog(d/d ₀)+l(ρ-ρ₀), where d₀ and ρ₀ are arbitrarily chosen reference points, and σ₀, k, and l are coefficients determined by the method of regression analysis. The use of data on the strength of pressed lithium hydride under the conditions of uniaxial and radial tension processed as indicated above enabled us to obtain a similar dependence for the conditions of uniaxial tension. Russian Federal Nuclear Center, All-Russia, Scientific Research Institute for Experimental Physics, Sarov, Russia. Translated from Problemy Prochnosti, No. 6, pp. 134–137, November–December, 1999.     

Fabrication of indium arsenic-antimony-bismuthide multilayer heterostructures by “capillary” liquid-phase epitaxy

Low-temperature “capillary” liquid-phase epitaxy is used to grow InAs_{1−x− y} Sb_xBi_y/InSb_1−y Bi_y multilayer epitaxial heterostructures on InSb(111)A substrates. The heterostructures contained up to sixty epitaxial layers of thickness between 0.05 and 0.15 μm, which was controlled by the epitaxial growth conditions. The heterostructures were investigated by scanning electron microscopy and secondary-ion mass spectrometry, and results are presented. Pis’ma Zh. Tekh. Fiz. 23, 51–55 (April 12, 1997)     

Specimen Size Requirements for Two-parameter Fracture Toughness Testing

Using a single parameter fracture mechanics theory, a minimum specimen size requirement of min(a, b, B) >200J/σ₀ in tension and min(a, b, B) >25J/σ₀ in bending, where B is the thickness, b the remaining ligament and a is the crack length of the specimen, were derived [Shih and German (1981), International Journal of fracture 17, 27–43] which have provided the basis for modern fracture toughness testing procedures. Two-parameter fracture toughness testing including the constraint, on the other hand, is desirable since it offers a solution to the transferability issue. A size requirement for a valid two-parameter fracture toughness testing based on the J-A₂ three-term solution was determined as min(a, b, B) > 11J/σ₀ [Chao and Zhu (1998), International Journal of fracture 89, 285–307] in which the limiting case is bend specimens under large scale yielding (LSY). Recent work by Chao et al. (2004, International Journal of fracture, 27, 283–302) has shown that the J-A₂ dominance at a crack tip can be significantly enhanced for bending specimens under LSY if a modified J-A₂ solution is adopted. This current paper further studies the size of the J-A₂ dominant zone using the modified J-A₂ solution for deep bend specimens with hardening from low to high and loading from SSY to LSY using finite element analysis. Based on the results, a rather relaxed specimen size requirement min(a, b, B) >6J/σ₀ is developed and recommended for a valid two-parameter fracture toughness testing using the J-A₂ fracture criterion. Validity of the size requirement is demonstrated by using the experimental J-R curves from non-standard bending specimens for A285 steel.     

Electronic Transport Behaviors of Insulating Icosahedral Al–Pd–Re Bulk Samples

Electronic transport measurements are summarised on two insulating icosahedral quasicrystalline (IQC) Al_69.0 Pd_22.8 Re_8.2 samples. Data were taken between 0.020 K ≤ T ≤ 292 K and in magnetic fields Bs up to 18 T. For a weakly insulating IQC, the electronic conductivity followed the expression σ(T) = σ₀T^zexp(T/ T₀) over four decades of temperature. For a strongly insulating sample, the conductivity above 0.3 K followed simple temperature power laws, σ (T) = σ₀ T^z. Below 0.2 K, the conductivity displayed an activated variable-range hopping (VRH) law. These fits included a conductivity contribution arising from the presence of a second metallic phase, which caused “weak saturation” of the measured resistances below 1 K.     

Magnetic crystallographic anisotropy of Fe/GaAs(100) epitaxial films

An investigation was made of the anisotropic properties of Fe films obtained by molecular-beam epitaxy on GaAs(100) substrates. It is shown that at thicknesses t=40–50 Å the Fe films begin to exhibit cubic magnetic anisotropy. At thicknesses t>100 Å the first constant of cubic anisotropy K₁ has values similar to those for “bulk” Fe single crystals. Films of intermediate thickness 50<t<100 Å possess both isotropic and anisotropic phases. Pis’ma Zh. Tekh. Fiz. 25, 88–95 (January 26, 1999)     

An investigation of the effect of high-temperature cyclic straining and creep loading on tensile mechanical properties of GH4145/SQ alloy

The effect of the reverse cyclic straining and the creep loading on the resultant tensile mechanical properties, such as the strength parameter (σ_0.2 and σ_b), the ductile parameter (δ and ϕ_f) and the composite parameter of the strength and ductility, the static toughness (U_t), for the precipitation-strengthened nickel-based superalloy GH4145/SQ used for high-temperature turbine and valve studs/bolts in power plant was investigated systematically at a temperature of 538°C. The experimental results show that in the case of the reverse cyclic straining both σ_0.2 and σ_b increases at early stages of cyclic straining and, after reaching their saturated values, σ_0.2 remains relatively constant until about 90% of fatigue life, while σ_b exhibits continuous reduction up to a level equal to the maximum applied stress amplitude. With the increasing number of straining cycles, both δ and ϕ_f as well as U_t decrease significantly until final fracture. In the case of creep loading the strength parameters (σ_0.2 and σ_b) tend to increase, as a whole, while the ductile parameters (δ and ϕ_f) and the static toughness (U_t) exhibit continuous decrease characterization as the amount of the creep deformation increases. The variation of the aforementioned various tensile mechanical properties during cyclic straining and creep loading of the alloy was further discussed by means of the observations of the deformation microstructures as well as the examinations of the fracture features of the specimens.     

On the “local approach” to the brittle fracture of structural materials

We consider the principles and sources of an important scientific direction in fracture mechanics-the so-called “local approach” to the problem of fracture of materials. This approach provides a possibility to describe their crack resistance based on conventional mechanical properties and opens up a direct way to determining the limiting state of structural elements. As the first example of the application of this approach, we present the principal propositions of the two-parameter K _μ-model of fracture, which relates the “global” K _Ic and “local” ρ_c and σ_c parameters via the yield condition on the contour of a small plastic zone near the tip of a crack. For the last decades, this direction was developed substantially and extended to the regions of quasi-brittle and ductile fracture. We also present the basic statements of this approach that were corroborated for the last decades. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 2, pp. 39–44, March–April, 2006.     

Some electrical properties of the defectiveγ phases of spinel structure during their transformation to the rhombohedral α phases

During the transformation of “cubic” iron sesquioxides, γ-Fe₂O₃, substituted by divalent or trivalent ions, to rhombohedral α phases, the electrical conductivity yields discontinuities and a change in plots of logσ againstf(1/T). The average temperature of these discontinuities is influenced by the particle size, and extent of oxidation and substitution. For divalent substituted defective spinels, the activation energy for the haematite precipitation appears to depend on the extent of substitution and the size of the particle.     

Estimating a transformation and its effect on Box-CoxT-ratio

This article concernsi) the stochastic behavior of the Box-Cox transformation estimator andii) the effect of estimating a transformation on the Box-CoxT-ratio used for the post-transformation analysis. It is shown that the transformation estimator depends on three factors: the model structure, the mean-spread and the error standard deviation σ₀. In general, a structured model is able to estimate the transformation very well; an unstructured model can do well also unless the mean-spread and σ₀ are both small; and a one-mean mode can give a poor-estimate if σ₀ is small. When the sample is not large, it is shown that the unconditional effect of estimating a transformation on the Box-CoxT-ratio is generally small, and the “conditional” effect is also negligible in most of the situations except the case of one-way ANOVA with small σ₀. Extensive Monte Carlo simulations are performed to support the theoretical findings.     

Study of the growth of YBa2Cu3O7−x films on a Al2O3 single crystal with a CeO2 buffer sublayer

Superconducting YBa₂Cu₃O_7−x films were prepared by magnetron sputtering on Al₂O₃ single crystals with a CeO₂ sublayer. Scattering of moderate-energy ions and x-ray diffraction were used to show that the films exhibit good single-crystal properties over the entire thickness up to 2.6 μm. The hypothesis is advanced that the indentations formed by the growth of films above “extraneous” phase grains may act as defect sinks. Pis’ma Zh. Tekh. Fiz. 24, 91–95 (January 12, 1998)     

The effect of flaw shape on fracture initiation at a blunt flaw

The author is involved in a wide-ranging research programme, the objective being to extend the fracture mechanics methodology for sharp cracks to blunt flaws, so as to take credit for the blunt flaw geometry. The approach is based on the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture. An earlier paper has derived a blunt flaw fracture initiation relation which gives the critical elastic flaw-tip peak stress σ_pcr (a “signifier” of a critical condition in the process zone) in terms of the process zone material parameters, subject to the proviso that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius ρ. The relation has been derived using a “two-extremes” procedure, whereby the separate σ_pcr solutions for small and large s/ρ are blended together to give an all-embracing relation that is valid for all s/ρ. A key feature of the relation is that σ_pcr essentially depends on only one geometrical parameter: the flaw root radius ρ. Though the relation has evolved from a consideration of the characteristics of one model, i.e. that of an elliptical flaw in an infinite solid that is subjected to an applied tensile stress, it is anticipated that the relation can be applied equally well for a wide range of geometrical configurations involving different flaw shapes. It is against this background that the present paper demonstrates that the relation also applies to the behaviour of an intrusion type flaw in the surface of a semi-infinite solid subjected to an applied tensile stress.     

Effects of loading rate, notch geometry and loading mode on the local cleavage fracture stress of a C–Mn steel

In this work, notched specimens with two notch geometries were tested in two loading modes (four-point bending (4PB) and three-point bending (3PB)) at various loading rates at a temperature of − 110°C for a C–Mn steel. An elastic–plastic finite-element method (FEM) is used to determine the stress distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ _f is measured. The results obtained and combining with previous studies by the authors show that the local cleavage fracture stress σ _f is closely related to the cleavage fracture mechanism (critical events) in steels. The σ _f values do not change with loading rate, notch geometry and loading mode, as long as the critical event of cleavage fracture does not change at various testing conditions. The σ _f is mainly determined by the steel microstructure, and its scatter is mainly caused by the size distribution of the weakest constituent in steels (ferrite grain or pearlite colony with large sizes and large second phase particles) and the change of the critical events in cleavage process. The σ _f can characterize the intrinsic toughness of steels and may be used in a “local approach” model for assessing integrity of flawed structures. The σ _f values could be measured by both 4PB and 3PB tests.     

On the conditions for contraction of a multielectrode corona discharge in He/Ar, Kr, Xe mixtures

The results of an investigation of the contraction of a corona discharge in “needle-grid” and “needle-plane” electrode systems maintained by a constant negative voltage are presented. The discharge was ignited in working media of infrared lasers on p-d transitions of atoms of heavy inert gases (He/Ar, Kr, Xe mixtures). Investigation of the current-voltage characteristics showed that in the mixtures He/Ar/Kr there is no hysteresis on the rising section of the current-voltage characteristic, while hysteresis does appear for a corona discharge in the mixture He/Xe. For voltages ≥5.5 kV on the needles and ballast resistance R_b≥0.5 MΩ dynamical contraction of a corona discharge together with current pulses with f=3–5 kHz and hysteresis loops were found on the descending section of the curve I=f(U), where I and U are the average current and voltage on the discharge gap. The maximum content of atoms of heavy inert gases for [He]=200–300 kPa is [Ar]≤12, [Kr]≤8, and [Xe]≤4 kPa. Pis’ma Zh. Tekh. Fiz. 25, 90–94 (September 12, 1999)     

Transition to chaotization and loss of self-averaging in two-dimensional two-phase media at the flow threshold

Giant fluctuations of the electric field recently identified experimentally in two-dimensional two-phase media at the percolation threshold are discussed. An example of a hierarchical realization of these media is used to show that for Re σ _i=0 (where σ₁ and σ₂ are the phase conductivities) and Im σ ₂/Im σ₁>0, the hierarchy construction procedure yields the Dykhne expression , whereas for Im σ ₂/Im σ₁<0, the procedure becomes randomized and the medium loses its property of self-averaging. Pis’ma Zh. Tekh. Fiz. 23, 89–95 (July 12, 1997)     

Film thickness effects on craze micromechanics

Air crazes have been grown from indentor “crack” tips in polystyrene films of thicknesses 0.11, 0.57 and 1.2μm. Quantitative transmission electron microscopy is used to measure craze thickness and fibril volume fraction profiles. From these, profiles of craze fibril extension ratio, λ(x), craze surface displacement,w(x), and craze surface stress,S(x), have been computed. For all thicknesses of film, the λ(x) profiles prove that the craze thickens by drawing more material into the fibrils from the craze-matrix interface, rather than by fibril creep. The form ofS(x) is also similar for all thicknesses of film, with a maximum at the craze tip and a minimum approximately half way along the craze. The extension ratio profiles also show a maximum at the craze tip. The midrib, which develops in the high stress region behind the craze tip as the craze propagates, has a value of λ comparable to that found at the stationary craze tip. When an isolated craze grows in an initially homogeneous stress field the midrib is observed to be of constant thickness. In contrast the midribs of crazes grown from crack tips decrease in thickness to a constant value with distance from the crack tip. All these observations are explained by the surface drawing mechanism of craze thickening. The computed values of λ and the fibril stress,σ _t, for the thinnest film, are significantly lower than for the thicker two films. These changes are attributed to the absence of plastic constraint in the thinnest film, which decreases the fibril true stress necessary for surface drawing.     

Pressure stimulated electrical emissions from cement mortar used as failure predictors

The electrical signals emitted during the application of uniaxial compressive mechanical stress upon cement mortar specimens are observed and discussed in this paper. This work discusses the electrical signals that are detected when the specimens are excited by a stepwise uniaxial stress increase from a low level (σ _L ) to a higher level (σ _H ) at a fast or slow rate and consequently remain at a high pressure regime for a long time. When maintaining constant mechanical stress for a long time, creep phenomena are evident in the specimen and the corresponding electrical emissions are recorded and analyzed. The characteristics of the electrical signal give clear information regarding the breaking stress (σ _F ) of the material. The electrical emission recordings are of great interest when the applied σ _H is located in the vicinity of the failure stress; the emitted electrical current increases greatly due to the sequential formation and propagation of cracks that occurs in this stress region. Thus, by correlating the strain rate variations to the electrical emissions this methodology can be used to predict failure due to compressive stress in cement mortars.     

Mathematical Model for Transitional Processes in Josephson Cryotrons Based on Tunnel Junctions

In this work, we consider controlling of logical states of Josephson memory cells (cryotrons) based on superconductor-insulator-superconductor (SIS) Josephson tunnel junctions by external current impulses. A mathematical model for the transitional processes that take place during direct logical transitions “0” → “1” and inverse logical transitions “1” → “0” is proposed. By means of mathematical modeling, we investigate transitional processes in cryotrons during the change of their logical state and obtain their transitional characteristics for operational temperatures T ₁=11.6 K and T ₂=81.2 K, close to the boiling temperatures of helium and nitrogen, respectively. It is shown that such memory cells can efficiently operate under the temperature T ₂=81.2 K. The behavior of the Josephson cryotrons as well as their operational stability is explored.     

Evaporation of NH3 in NH3/H2 atmosphere by 25 bar for neutral gas absorption refrigeration units

The present work considers the evaporation process ofNH ₃ inNH ₃/H₂ atmosphere, that takes place in neutral gas absorption refrigeration units. The data obtained are used to calculate the “psychrometric problem” and the evaporation process within an insulated tube. The “psychrometric problem” deals with the determination of the mass fraction of theNH ₃/H ₂ gas mixture from known “dry” and “wet bulb” temperatures for Lewis numbers not equal to one. The results are in good agreement with some existing experimental data.