Three-dimensional interactions of a crack front with arrays of penny-shaped microcracks

Three-dimensional interactions of a crack front with arrays of penny-shaped microcracks are considered. The work extends the earlier analysis of 2-D crack-microcrack interactions to the 3-D configurations.After analysing simple elementary interaction events (involving only one microcrack) we solve the interaction problem for a number of sample arrays (containing up to 50 microcracks)-realizations of certain microcrack statistics.Statistical aspects of the problem are examined. The interaction effects are found to fluctuate, even qualitatively (from shielding to amplification) along the crack front: the intervals of reduced stress intensity factors (SIFs) alternate with local peaks of SIFs that enhance local front advances. Thus, no statistically stable effect of stress shielding is found (at least, for the microcrack statistics considered): the toughening by microcracking, if it exists, may be due to a statistics of the microcrack centers which is biased towards shielding configurations or to expenditure of energy on nucleation of new microcracks, rather than elastic interactions with them. Similarly to the 2-D case, stochastic asymmetries in the microcrack field produce noticeable secondary modes on the main crack (i.e., modes II and III under mode I loading); this may be partially responsible for crack kinking and an irregular crack path.The short range interactions (several microcracks closest to the main crack tip) play a dominant role. Their impact on the main crack is quite sensitive to the individual microcrack locations and cannot be adequately reproduced by modelling the short range microcracking zone by an effective elastic material of reduced stiffness.The interaction effects in 3-D are found to be weaker than in 2-D.     

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.     

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.     

Relative motion of drops under the action of variable forces

The boundary between moderate and large drops, which is fixed for each drop-medium system in gravitational conditions, shifts toward smaller drops with increase in the forces applied to the drop.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 5, pp. 750–756, November, 1989.     

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.     

Fatigue crack growth behaviour of tungsten carbide-cobalt hardmetals

Fatigue crack propagation studies have been carried out on a range of WC-Co hardmetals of varying cobalt content and grain size using a constant-stress intensity factor double torsion test specimen geometry. Results have confirmed the marked influence of mean stress (throughK _max), which is interpreted in terms of static modes of fracture occurring in conjunction with a true fatigue process, the existence of which can be rationalized through the absence of any frequency effect. Dramatic increases in fatigue crack growth rate are found asK _max approaches that value of stress intensity factor ( 0.9K_IC) for which static crack growth under monotonic load (or static fatigue) occurs in these materials. Lower crack growth rates, however, produce fractographic features indistinguishable from those resulting from fast fracture. These observations, and the important effect of increasing mean free path of the cobalt binder in reducing fatigue crack growth rate, can reasonably be explained through a consideration of the mechanism of fatigue crack advance through ligament rupture of the cobalt binder at the tip of a propagating crack.     

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.     

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.     

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.     

A realistic microscopic theory for the high-Tc cuprates

The electronic structure of the high-T_c cuprates is worked out by decomposing the orbitals around the Fermi level into large-U and small-U components. The large-U orbitals are treated by the slave-fermion method, the small-U orbitals by a mean-field approach, and the hybridization between them is then worked out. It turns out that hopping energy binds the spin and the charge of the large-U orbitals. The state so obtained is either antiferromagnetic, or paramagnetic with antiferromagnetic fluctuations and with violation of parity. Two types of charge carriers are predicted: (i) spinless polarons with a very small bandwidth, explaining the anomalous thermoelectric power, the mid-infrared peak, lattice anomalies, etc.; (ii) anomalous carriers of both charge and spin, explaining marginal-Fermi-liquid behavior and deviations from it, anomalous relaxation time, the systematic behavior of the resistivity, the Hall constant, the Hall angle, etc.     

Description of the Shape Memory Effect of Radiation‐Modified Polymers under Thermomechanical Action

The shape memory effect of crystallizing polymer materials is simulated. The polymer is considered to be an inhomogeneous medium with a moving boundary (temperaturedependent phase composition). Using a model based on the frozen strain hypothesis, the temperature dependences of stresses under isometric heating and cooling have been obtained. On the basis of the known data on the influence of irradiation on the thermomechanical characteristics the dependences of thermorelaxation and thermoshrinkage stresses on the absorbed dose for highdensity polyethylene have been found.     

Cyclic crack resistance of ferritic-pearlitic steels and their weld joints

The cyclic crack resistance of ferritic-pearlitic steels and their weld joints is determined. On the basis of an analysis of our own and literature data a method of evaluation of the endurance of welded structures in the area of multicycle fatigue is proposed. The endurance limit _e, the parameter K_th ^*, and other parameters have been introduced into this method. The concepts of repairless structures and safely damaged structures are proposed.Translated from Problemy Prochnosti, No. 7, pp. 25–30, July, 1991.     

A new acoustic spectroscopy: Resonance spectroscopy by the MIIR

Recent and remarkable advances in the experimental study of acoustic scattering from targets immersed in water are leading to a new spectroscopy: resonance acoustic spectroscopy. The discovery and improvement of an intriguing method, the Method of Isolation and Identification of Resonances (MIIR), has made possible experimental determination of the eigenfrequency spectra of aluminum-elastic cylinders and cylindrical shells. This method gives a quasilinear resonance spectra. In addition, it shows the importance of circumferential waves which generate standing waves. They allow us to explain the reradiation of targets after the end of insonification. The MIIR has numerous applications, especially in underwater acoustics and nondestructive testing.     

Universal behavior of an isolated phase-slip center: Extension to high pair-breaking parameters

For the first phase-slip center in superconducting whiskers of zinc and zinc-silver alloys the normalized normal-like length and zero-voltage intercept are plotted as a function of the pair-breaking parameter, which ranges between 4 × 10³ and 2 × 10⁴. A comparison is made with results from aluminum microbridges and the prediction of the time-dependent Ginzburg-Landau theory.     

Necklace structure obtained by forging Astroloy supersolidus-sintered preforms

A new and original densification process of nickel-based superalloys consists in forging supersolidus-sintered preforms. From the coarse-grained structure of the material sintered in the presence of a liquid phase, forging at a high deformation ratio can lead to a duplex structure (usually called necklace structure) as can be obtained by conventional HIP + forge or cast/wrought routes. Optimal forging conditions to form a necklace structure have been determined for Astroloy powders. The restoration and recrystallization mechanisms are described in relation to deformation conditions and heat-treatments. Structures and related mechanical behaviour have been characterized. The necklace material compares well with conventional direct HIPed or forged materials. Good creep-rupture properties are due to large warm-worked grains, and a high resistance to crack initiation and propagation results from recrystallized grains of the duplex structure.     

Effects of dimensionality and crystal structure on bound states of point defects in quantum solids

The effects of dimensionality and crystal structure on the bound states of point defects in quantum crystals are investigated. It is shown that tightly bound states tunnel slowly in some lattices (e.g., bcc) while they tunnel quickly through other lattices (e.g., hcp). Explicit band calculations are made for ( ³ He) ₂ impurity molecules in solid host ⁴ He crystals. These illustrate the qualitative differences present in various molecular tunneling modes.     

Stiffness and strength behaviour of woven fabric composites

This paper presents three analytical models for the investigation of the stiffness and strength of woven fabric composites. The mosaic model is effective in predicting the elastic properties of fabric composites. The fibre undulation model takes into account fibre continuity and undulation and has been adopted for modelling the knee behaviour of plain weave fabric composites. The bridging model is developed to simulate the load transfer among the interlaced regions in satin composites. The theoretical predictions coincide extremely well with experimental measurements. The elastic stiffness and knee stress in satin composites are higher than those in plain weave composites due to the presence of the bridging regions in the weaving pattern.On leave from National Aerospace Laboratory 1880 Jindaiji, Chofu, Tokyo, 182, Japan.     

Numerical studies in dynamic fracture mechanics

This paper provides a summary of recent studies concerning numerical modeling of dynamic crack-propagation, Both stationary mesh as well as moving mesh finite-element procedures are examined. Simple procedures, using a moving mesh of conventional isoparametric elements in conjunction with certain path-independent integrals for the evaluation of stress-intensity factors for a dynamically propagating crack are presented.

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Résumé Le mémoire fournit des synthèses des études récentes relatives à la modélisation numérique de la propagation de fissures dynamiques. On examine, à la fois, le maillage stationaire et le maillage mobile utilisés dans les procédures d'éléments finis. On présente des procédures simples utilisant un maillage mobile d'éléments conventionnels isoparamétriques utilisé avec certaines intégrales indépendantes du parcours, en vue d'évaluer les facteurs d'intensité de contrainte dans le cas d'une fissure en cours de propagation dynamique.

     

An electron microscopic study of nickel sulfide inclusions in toughened glass

It has been known since the early sixties that nickel sulfide inclusions cause spontaneous fracture of toughened (thermally tempered) glass, but despite the considerable amount of work done on this problem in the last four decades, failures still occur in the field with regularity. In this study we have classified (by viewing through a 60× optical microscope) inclusions into two groups, which are classic and atypical nickel sulfides. The classics look like the nickel sulfide inclusions found at the initiation-of-fracture of windows that have broken spontaneously. We have compared the structure and composition of the atypical inclusions with the structure and composition of the classics. All of the classic and atypical nickel sulfide inclusions studied in this work were found to have a composition in the range of Ni₅₂S₄₈to Ni₄₈S₅₂. Inclusions on the nickel rich side of stoichiometric NiS were found to be two-phase assemblies, and inclusions on the sulphur rich side of NiS were single phase. It had been proposed that the atypicals were passive, and of a different composition to the classics. However, we found that the difference between passive and dangerous nickel sulfide inclusions was not a difference in composition but rather a difference in the type of material in the internal pore space. The passive's had carbon char in their internal pore space, whereas the pore space of dangerous inclusions contained Na₂O. The presence of Na₂O and carbon char with the inclusions indicates that the formation of the inclusions results from a reaction of a nickel-rich phase with sodium sulphate and carbon.     

Modeling the heat transfer process in an infiltrated granular bed with regard for the temperature difference of the phases

Based on the solutions of the internal and external problems of a gas flow in a granular bed, expressions are obtained for calculating the interphase heat transfer coefficient and the degree of gas flow nonuniformity.Academic Scientific Complex A. V. Luikov Heat and Mass Transfer Institute of the Academy of Sciences of Belarus, Minsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 67, Nos. 3–4, pp. 243–249, September–October, 1994.