Underpinning of a simple blunt flaw fracture initiation relation

With the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture as a basis, 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. In earlier work, a Mode I fracture initiation relation has been derived, subject to the restriction that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius . The relation gives the critical elastic flaw-tip peak stress _pcr, and has been derived using a two-extremes procedure, whereby the separate, and indeed exact, solutions for small and large s/ values are blended together to give an all-embracing relation that is valid for all s/ values. _pcr is expressed in terms of the process zone material parameters and geometrical parameters but, for a wide range of flaw geometry parameters, _pcr essentially depends on only one geometrical parameter . This paper provides underpinning for the general thrust of the two-extremes procedure by appealing to exact results for the complete spectrum of s/ values from analyses of appropriate Mode III models. Results obtained by applying the two-extremes procedure are shown to be in very good agreement with the exact results.     

The stacked lamellar texture on the fracture surfaces of fibre composites

A fracture surface texture, which has been variously termed as lacerations, hackles or serrations, is often observed on the matrix surface of fibre composites, most often in resin-rich regions. This texture, referred to here as a stacked lamellar texture to emphasize its plate-like nature, was studied in an E-glass/epoxy composite. Scanning electron fractographs of these materials suggest that the stacked lamellar texture arises from crack fingers due to a meniscus instability mechanism interacting with a reorienting stress field.     

Some simple flows of a Johnson-Segalman fluid

Summary Unlike most other fluid models, the Johnson-Segalman fluid allows for a non-monotonic relationship between the shear stress and rate of shear in a simple shear flow for certain values of the material parameter. This has been used for explaining a phenomenon such as spurt. Here, we study three simple flows of a Johnson-Segalman fluid with a view towards understanding its response characteristics. We find that boundary conditions can have a very interesting effect on the regularity of the solution; changing them continuously leads to solutions that change their regularity. First, we consider the flow through a circular pipe and find solutions that have discontinuous velocity profiles which have been used to explain the phenomenon of spurt (cf. [10], [11]). Second, we consider the flow past an infinite porous plate and show that it will not admit solutions which have discontinuous velocity gradients, the solutions being necessarity smooth. Lastly, we study Poiseuille flow in a concentric annulus with porous boundaries. While spurt could be explained alternatively by allowing for stick-slip at the wall, the Johnson-Segalman model seems particularly suited in describing the appearance of shear-layers (cf. [13]).     

Where Are We in HTCS?

Address given at the closing of the conference on Major Trends in Superconductivity in the New Millennium, MTSC 2000 and the opening of the Symposium on Itinerant and Localized States in HTCS SILS in HTCS.     

Study of the Temperature Fields in Oil and Gas Strata with Water Injection Based on the Perturbation Method

The problem of the thermal action on oil and gas strata by injection of a heating medium using expansion in a small parameter is reduced to an infinite sequence of boundaryvalue problems that are solved by the method of integral transforms. It is shown that, with an appropriate selection of the small parameter, the zerothorder approximation corresponds to a spaceaveraged (across the stratum thickness) solution of the main problem and leads to a concentratedcapacity scheme that is constructed assuming that the stratum temperature is independent of the vertical coordinate. The first approximation permitted marked refinement of calculations according to the concentratedcapacity scheme and an evaluation of its error. Spacetime temperature distributions are presented that have been calculated using the analytical solutions obtained.     

The influence of stacking fault energy on ductile fracture micromorphology

The influence of stacking fault energy on microvoid coalescence in pure materials has been studied. It was shown that as a material's stacking fault energy (SFE) decreased, the extent of microvoid coalescence that occurred during ductile fracture also decreased. The decrease of microvoid coalescence in low SFE materials was attributed to a hindrance in the development of dislocation cells associated with the restricted motion of dislocations. In pure materials, microvoids are believed to initiate and grow along dislocation cell walls formed during deformation. As such, the absence or scarcity of cells in lower SFE materials limits the formation of these voids during ductile fracture.     

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (⁶³Cu-⁶⁵Cu), C_u =-nT_c/nm_Cu, for two isotopic pairs of oxygen-deficient YBa₂Cu₃O_7–, where varies between 0.06 and 0.52. _Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of _Cu are observed away from the plateau. The dependence of _Cu is similar to that of the pressure effect dnT_c/dP.     

Evaluation of the economic effects of Brite-Euram programmes on the European industry

This article deals with an evaluation performed by BETA group about the economic effects of EU R & D programmes (Brite, Euram and Brite-Euram I) on the European industry. The approach used is based on an original methodology designed by BETA, which aims at evaluating those effects at a micro level (i.e. the participants to the programmes) by means of direct interviews of 176 partners involved in 50 projects. The definition of these economic effects is firstly described, as well as the different steps of the evaluation work. Then the overall results of the study are presented, showing the importance of both direct and indirect observed effects in monetary terms. Finally, some more detailed results highlight the positive impact of some aspects of the organization structure set up for the analyzed R & D projects on the amount of observed effects: i) the participation of a university lab; ii) the participation of at least one partner involved in a fundamental research work; iii) the diversity of research tasks over a scale ranging from fundamental research to industrialization work; iv) the combination of user-type and producer-type of activity in one given organisation (integration effect) or in one given project (consortia effect), etc...     

Checking contact angles of radial-thrust bearings

Summary On the basis of the above method of measurement, using expression (4), highly productive semiautomatic devices were made for sorting out bearings by their contact angles into three groups: small, good and large.The adoption by our industry of these measuring devices for checking the contact angles will raise considerably the quality and life of radial-thrust bearings.     

Eine Charakterisierung der Unterhalbstetigkeit von Restriktionsmengen

Zusammenfassung Die vorliegende Arbeit will charakterisieren, wann sich Restriktionsmengen bei konvexen bzw. quasikonvexen Optimierungsproblemen in Abhängigkeit von den Restriktionsfunktionen unterhalbstetig ändern. Zu diesem Zweck wird die Unterhalbstetigkeit des mengentheoretischen Durchschnitts für konvexe Mengen untersucht, und sie erweist sich als gleichwertig mit der Nicht-Trennbarkeit der gegebenen konvexen Mengen. Dieses abstrakte Ergebnis liefert sofort die Lösung für die Unterhalbstetigkeit der Restriktionsmengen.

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Summary This paper tries to characterize lower semicontinuous dependence of constraint sets on the constraint functions in convex and quasiconvex optimization. For this purpose the lower semicontinuity of the set theoretic intersection for convex sets is investigated, and we will see, that it is equivalent with the Non-Separability of the given convex sets. This abstract result contains obviously the solution for the lower semicontinuity of the constraint sets.

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Diese Arbeit stellt eine Weiterentwicklung einiger Ergebnisse in [5] dar. Für die Anregung und hilfreiche Unterstützung möchte der Autor Herrn Prof. Dr. G. Heindl danken.     

The tensile properties of carburized and uncarburized low carbon mild steel

The properties of a low carbon mild steel in monotonic tension loading were compared in the plain normalized and the carbo-nitrided slowly cooled conditions. The application of a carbo-nitriding process raised the yield strength of the steel to that of the nominal tensile strength in the uncarburized samples, and increased the nominal tensile strength to a value 45% above that of the plain normalized steel, whilst still retaining a good measure of ductility. The fractures for the plain samples were cup and cone type whilst those for the reinforced samples revealed slant mode fractures.     

Spin hydrodynamic equations with external disturbances and suitable Green's functions for superfluid3He-B. New onsager relations

The spin hydrodynamic equations for superfluid³He-B have been obtained for the case of external, time-dependent fields. On the basis of a microscopic approach, expressions are found for additional terms in equations containing these fields. Considering the linear response of the system to the switching on of external fields, formulas are found for suitable Green's functions (magnetization-magnetization, rotation-rotation, magnetization-rotation, rotation-magnetization). The rotation-rotation Green's function has the 1/q ² singularity characteristic of superfluid systems. Connections between Green's functions lead to relations among kinetic coefficients v, ₁, and ₂. It is also shown that there is a conserved quantityQ ^(B) = div v _s ^(B) that describes sources or magnetic type charges (monopoles) of the superfluid velocity v _s ^(B) . Comparison with the phenomenological approach suggests thatQ ^(B) is proportional to a pseudoscalar giving the projection of the spin density onto the vector describing the axis of rotation.     

Finite state approximation algorithms for average cost denumerable state Markov decision processes

Summary In this paper we study three finite state, value and policy iteration algorithms for denumerable space Markov decision processes with respect to the average cost criterion. The convergence of these algorithms is guaranteed under a scrambling-type recurrency condition and various tail conditions on the transition probabilities. With the value iteration schemes we construct nearly optimal policies by concentrating on a finite set of important states and controlling them as well as we can. The policy space algorithm consists of a value determination scheme associated with a policy and a policy improvement step where a better policy is determined. Thus a sequence of improved policies is constructed which is shown to converge to the optimal average cost policy.

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Zusammenfassung Für Markovsche Entscheidungsprozesse mit abzählbarem Zustandsraum untersuchen wir für den Fall des Durchschnittskostenkriteriums drei endliche Wertiterations- und Politikiterations-Algorithmen. Die Konvergenz der Algorithmen wird durch scramblingtype Rekurrenzbedingungen und verschiene tail Bedingungen an die Übergangswahrscheinlichkeiten gesichert. Mit den Wertiterationsverfahren konstruieren wir fast optimale Politiken, indem wir uns auf eine endliche Menge von wichtigen Zuständen konzentrieren und diese bestmöglich kontrollieren. Der Politikiterations-Algorithmus besteht aus einem Schritt zur Wertbestimmung für eine Politik und einem Schritt zur Verbesserung der Politik. Auf diese Weise wird eine Folge verbesserter Politiken konstruiert, die Konvergenz zur optimalen Politik wird gezeigt.

     

Thermal Conductivity of the Four-Leg Spin-Ladder System La2Cu2O5 Single Crystal

We have investigated the magnetic susceptibility, , and the thermal conductivity, , in magnetic fields for the four-leg spin-ladder system La₂Cu₂O₅ single crystal. The in a magnetic field parallel to the ladder exhibits a kink at 130 K in correspondence to the magnetic ordering. The along the ladder exhibits a peak at 25 K and a shoulder at 14 K, which are probably related to the thermal conductivity due to magnons, _magnon, and that due to phonons, _phonon, respectively. The perpendicular to the ladder, on the other hand, exhibits only one broad peak related to _phonon. The observed large anisotropy of has been explained based upon the anisotropy of _magnon.     

A fractographic study of hydrogen-assisted cracking and liquid-metal embrittlement in nickel

Metallographic and fractographic studies of crack growth in nickel polycrystals and single crystals in a number of environments are described. Brittle intercrystalline and transcrystalline cleavage-like fractures were observed for specimens tested in liquid mercury, liquid lithium, liquid sodium, gaseous hydrogen, and for hydrogen-charged specimens tested in air. Brittle fractures were associated with considerable slip, and dimples/tear ridges were observed on fracture surfaces, suggesting that crack growth occurred by localized plastic flow. There were remarkable similarities between adsorption-induced liquid-metal embrittlement and hydrogen-assisted cracking which, along with other observations, suggested that adsorbed hydrogen at crack tips was responsible for hydrogen-assisted cracking. It is concluded that adsorbed atoms weaken interatomic bonds at crack tips thereby facilitating the nucleation of dislocations and promoting crack growth by localized plastic flow.     

Ultrasonic crack characterization via a constrained inversion algorithm

The Kirchhoff approximation is used to show that the time domain impulse response of an isolated flat crack can be given a simple geometrical interpretation in terms of the derivative of a projected length function. For an elliptical crack, this derivative can be obtained explicitly to yield the two edge-diffracted waves which originate from the flashpoints of the crack. An explicit coordinate invariant expression is obtained from this elliptical crack solution which relates the time difference, t, between the arrival of these edge-diffracted waves and the crack size and orientation. Previously, we have proposed that this expression, together with t measurements in different scattering directions, could be used in a regression analysis as the basis for performing a constrained inversion of crack scattering data (i.e., where we attempt to obtain the best equivalent flat elliptical crack that fits the scattering measurements). Here we will demonstrate some results of applying the proposed algorithm using noisy synthetic data. The sensitivity of the results to both, number of measurements and transducer orientation, will be discussed.     

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.     

Yield and transient effects during the plastic deformation of solid polymers

Tensile tests were performed on seven commercial polymers at 22° C and at constant true strain rates of 10^–4 to 10^–1 sec^–1. The constant strain rates were imposed on the minimum section of each sample with the aid of a diametral transducer, an exponential function generator and a closed-loop hydraulic testing machine. The polymers investigated were: high and low density polyethylene, polytetrafluoroethylene, polypropylene. polyvinylchloride and polyamide 6 and 66. True yield drops were observed in the rigid glassy polymers, whereas yielding was more gradual in the semi-crystalline or plasticized polymers. Strain rate change tests were also performed, during which one order of magnitude increases and decreases were imposed on the specimens. Normal transients were observed at small strains in the samples containing a rubbery phase, while the transients were of an inverse nature in the samples containing a glassy phase. With an increase in the strain at which the change was initiated, the normal transients changed in character to inverse. Transient tests were also performed in which straining was interrupted to permit a period of stress relaxation or of holding in the unloaded condition prior to the resumption of straining. A quantitative model is proposed, based on the dynamics of plastic waves which accounts for the transition from normal to inverse transient behaviour with increasing strain, and also explains the opposite effects of stress relaxation and of specimen unloading on the restraining transients.     

Problems of quantitative analysis of scientific activities: The non-additivity of data. Part I

A viewpoint is given, according to which, addivitity may be defined only at the intuition level and quantitative latent variables are origin additive. The proposed solution to the non-additivity problem consists in restricting quantitative indicator scales by the so-called natural, in particular, open scales.