On the theory of the indentation test for the measurement of tensile strength of brittle materials

A theoretical solution has been obtained for the state of stress in a rectangular plate under a pair of symmetrically placed rigid indenters. The stress distributions along the two central axes have been calculated for a square plate assuming the pressure distribution under the indenters as uniform, parabolic and one resulting from 'constant displacement' on a semiinfinite boundary, for different ratios of indenter-width to side of square. The results are compared with those of photoelastic analysis of Berenbaum and Brodie and the validity of the solution is discussed. The solution has been extended to orthotropic materials and numerical results for one type of coal are given.

Generalizations and Models in Ecology : Lawlikeness, Invariance, Stability, and Robustness

The question at issue in this dissertation is the epistemic role played by ecological generalizations and models. I investigate and analyze such properties of generalizations as lawlikeness, invariance, and stability, and I ask which of these properties are relevant in the context of scientific explanations. I will claim that there are generalizable and reliable causal explanations in ecology by generalizations, which are invariant and stable. An invariant generalization continues to hold or be valid under a special change called an intervention that changes the value of its variables. Whether a generalization remains invariant during its interventions is the criterion that determines whether it is explanatory. A generalization can be invariant and explanatory regardless of its lawlike status. Stability deals with a generality that has to do with holding of a generalization in possible background conditions. The more stable a generalization, the less dependent it is on background conditions to remain true. Although it is invariance rather than stability of generalizations that furnishes us with explanatory generalizations, there is an important function that stability has in this context of explanations, namely, stability furnishes us with extrapolability and reliability of scientific explanations. I also discuss non-empirical investigations of models that I call robustness and sensitivity analyses. I call sensitivity analyses investigations in which one model is studied with regard to its stability conditions by making changes and variations to the values of the model s parameters. As a general definition of robustness analyses I propose investigations of variations in modeling assumptions of different models of the same phenomenon in which the focus is on whether they produce similar or convergent results or not. Robustness and sensitivity analyses are powerful tools for studying the conditions and assumptions where models break down and they are especially powerful in pointing out reasons as to why they do this. They show which conditions or assumptions the results of models depend on. Key words: ecology, generalizations, invariance, lawlikeness, philosophy of science, robustness, explanation, models, stability

Improved Conditions For L2-Stability Of Nonstationary Feedback-Systems

Concerning the L2-stability of feedback systems containing a linear time-varying operator, some of the stringent restrictions imposed on the multiplier as well as the linear part of the system, in the criteria presented earlier, are relaxed.

Tablettien soveltuvuus annosjakeluun

Koneellinen annosjakelu on kasvava lääkehuollon osa-alue, jossa lääkkeet pakataan koneellisesti pieniin annoskertakohtaisiin pusseihin kahden viikon erissä. Aikaisemmin lääkevalmisteiden soveltuvuutta koneelliseen annosjakeluun ei ole systemaattisesti tutkittu. Tutkimus tehtiin yhteistyössä Espoonlahden apteekin annosjakeluyksikön kanssa ja sen tavoitteena oli määrittää annosjakeluprosessin kannalta optimaaliset ominaisuudet annosjaeltavalle tabletille rikkoutumisten ja siirtymien vähentämiseksi. Rikkoutuminen on lääkevalmisteen murentumista, puolittumista tai muuta rikkoutumista annosjakelun aikana. Siirtymä on lääkevalmisteen jakelu väärään annospussiin. Prosentuaalisesti rikkoutumisia ja siirtymiä on jakelumäärästä hyvin vähän, mutta määrällisesti paljon ja koko ajan enemmän koneellisen annosjakelun yleistyessä. Rikkoutumiset ja siirtymät aiheuttavat paljon lisätyötä pussien korjaamisen takia, joten niiden määrää on pyrittävä vähentämään. Lisäksi tavoitteena oli selvittää lääkkeiden valmistajilta kysyttävissä olevat asiat lääkevalmisteiden ominaisuuksista ja säilyvyydestä, jotta voitaisiin päätellä valmisteen soveltuvuus koneelliseen annosjakeluun kirjallisen tiedon perusteella. Tutkimuksen tulosten perusteella rikkoutumisten ja siirtymien vähentämiseksi optimaalinen tablettivalmiste annosjakeluun on pienehkö tai keskisuuri, päällystetty, luja ja jakouurteeton ja optimaalinen ilman suhteellinen kosteustaso annosjakeluyksikön tuotantotiloissa olisi noin 30 – 40 %. Lääkkeiden valmistajilta kysyttäviä seikkoja ovat koon, päällysteen, murtolujuuden ja jakouurteen lisäksi valmisteen säilyvyys alkuperäispakkauksen ulkopuolella sekä valmisteen valo-, lämpö- ja kosteusherkkyys. Rikkoutumisten ja siirtymien lisäksi tutkittiin myös kosteusherkän asetyylisalisyylihappovalmisteen (Disperin 100 mg) säilyvyyttä 25 °C ja 60 % RH olosuhteissa, koska tuotantotilojen ilman kosteustasoa ei ole säädelty. Säilyvyystutkimuksen kesto oli neljä viikkoa. Se on riittävä, koska se on enimmäisaika, jonka tabletit ovat annosjakeluprosessin yhteydessä pois alkuperäispakkauksestaan ennen käyttöä. Tabletteja säilytettiin avoimessa alkuperäispakkauksessa (purkki), suljetussa alkuperäispakkauksessa, annosjakelukoneen kasetissa ja kahdessa erilaisessa annospussissa (uusi ja käytössä oleva materiaali). Tulosten mukaan annosjakelukoneen kasetti suojaa kosteudelta yhtä huonosti kuin avoin purkki. Uusi pussimateriaali sen sijaan suojaa kosteudelta paremmin kuin tällä hetkellä käytössä oleva materiaali. Raman -spektroskopiamittausten perusteella asetyylisalisyylihappotableteissa ei ehdi neljän viikon seurannan aikana tapahtua asetyylisalisyylihapon hajoamista salisyylihapoksi. Kosteus heikentää tablettien murtolujuutta, mikä saattaa aiheuttaa enemmän rikkoutumisia. Kosteustaso olisi hyvä olla säädettävissä vakioksi tuotantotiloissa tai purkaa tabletit kasetteihin mahdollisimman lähellä jakelua rikkoutumisten ehkäisemiseksi, etenkin ilman kosteustason ollessa korkea. Lisäksi tutkittiin lääkevalmisteen lämpöherkkyyttä koska annosjakelukoneen saumauslaite altistaa annospussit noin 75 °C lämmölle, jos annosjakelukone pysäytetään kesken työn. Tutkimus tehtiin XRPD:llä, jolla voidaan säätää näytteen lämpötilaa. Lämpöherkkyystutkimusten perusteella 75 °C lämpö ei ehdi tunnin aikana aiheuttaa muutoksia karbamatsepiinitabletissa (Neurotol 200 mg). Tuloksista selvisi, että tutkitun valmisteen sisältämä karbamatsepiini ei kuitenkaan ole lämpöherkin muoto, joten muita lämpöherkkiä lääkevalmisteita tulisi tutkia lisätiedon saamiseksi lämmön vaikutuksista.

L2-Stability Of A Class Of Nonlinear-Systems

Sufficient conditions are given for the L2-stability of a class of feedback systems consisting of a linear operator G and a nonlinear gain function, either odd monotone or restricted by a power-law, in cascade, in a negative feedback loop. The criterion takes the form of a frequency-domain inequality, Re[1 + Z(jω)] G(jω) δ > 0 ω ε (−∞, +∞), where Z(jω) is given by, Z(jω) = β[Y1(jω) + Y2(jω)] + (1 − β)[Y3(jω) − Y3(−jω)], with 0 β 1 and the functions y1(·), y2(·) and y3(·) satisfying the time-domain inequalities, ∝−∞+∞¦y1(t) + y2(t)¦ dt 1 − ε, y1(·) = 0, t < 0, y2(·) = 0, t > 0 and ε > 0, and , c2 being a constant depending on the order of the power-law restricting the nonlinear function. The criterion is derived using Zames' passive operator theory and is shown to be more general than the existing criteria

Time-Domain Criteria For L2-Stability Of Nonstationary Feedback-Systems

Criteria for the L2-stability of linear and nonlinear time-varying feedback systems are given. These are conditions in the time domain involving the solution of certain associated matrix Riccati equations and permitting the use of a very general class of L2-operators as multipliers.

A Durable PEFC with Carbon-Supported Pt-TiO2 Cathode: A Cause and Effect Study

Durability is central to the commercialization of polymer electrolyte fuel cells (PEFCs). The incorporation of TiO2 with platinum (Pt) ameliorates both the stability and catalytic activity of cathodes in relation to pristine Pt cathodes currently being used in PEFCs. PEFC cathodes comprising carbon-supported Pt-TiO2 (Pt-TiO2/C) exhibit higher durability in relation to Pt/C cathodes as evidenced by cell polarization, impedance, and cyclic voltammetry data. The degradation in performance of the Pt-TiO2/C cathodes is 10% after 5000 test cycles as against 28% for Pt/C cathodes. These data are in conformity with the electrochemical surface area and impedance values. Pt-TiO2/C cathodes can withstand even 10,000 test cycles with nominal effect on their performance. X-ray diffraction, transmission electron microscope, and cross-sectional field-emission-scanning electron microscope studies on the catalytic electrodes reflect that incorporating TiO2 with Pt helps in mitigating the aggregation of Pt particles and protects the Nafion membrane against peroxide radicals formed during the cathodic reduction of oxygen. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3421970] All rights reserved.

Effect of directionality of unidirectional grinding marks on friction and transfer layer formation of Mg on steel using inclined scratch test

Surface topography has been known to play an important role in the friction and transfer layer formation during sliding. In the present investigation, EN8 steel flats were ground to attain different surface roughness with unidirectional grinding marks. Pure Mg pins were scratched on these surfaces using an Inclined Scratch Tester to study the influence of directionality of surface grinding marks on coefficient of friction and transfer layer formation. Grinding angle (i.e., the angle between direction of scratch and grinding marks) was varied between 0 degrees and 90 degrees during the tests. Experiments were conducted under both dry and lubricated conditions. Scanning electron micrographs of the contact surfaces of pins and flats were used to reveal the surface features that included the morphology of the transfer layer. It was observed that the average coefficient of friction and transfer layer formation depend primarily on the directionality of the grinding marks but were independent of surface roughness on the harder mating surface. In addition, a stick-slip phenomenon was observed, the amplitude of which depended both on the directionality of grinding marks and the surface roughness of the harder mating surface. The grinding angle effect on the coefficient of friction, which consists of adhesion and plowing components, was attributed to the variation of plowing component of friction. (c) 2006 Elsevier B.V. All rights reserved.

Optimum Design for External Seismic Stability of Geosynthetic Reinforced Soil Walls: Reliability Based Approach

In this paper, an analytical study considering the effect of uncertainties in the seismic analysis of geosynthetic-reinforced soil (GRS) walls is presented. Using limit equilibrium method and assuming sliding wedge failure mechanism, analysis is conducted to evaluate the external stability of GRS walls when subjected to earthquake loads. Target reliability based approach is used to estimate the probability of failure in three modes of failure, viz., sliding, bearing, and eccentricity failure. The properties of reinforced backfill, retained backfill, foundation soil, and geosynthetic reinforcement are treated as random variables. In addition, the uncertainties associated with horizontal seismic acceleration and surcharge load acting on the wall are considered. The optimum length of reinforcement needed to maintain the stability against three modes of failure by targeting various component and system reliability indices is obtained. Studies have also been made to study the influence of various parameters on the seismic stability in three failure modes. The results are compared with those given by first-order second moment method and Monte Carlo simulation methods. In the illustrative example, external stability of the two walls, Gould and Valencia walls, subjected to Northridge earthquake is reexamined.

Structural stability of slender aerospace vehicles: Part I Mathematical modeling

A detailed mechanics based model is developed to analyze the problem of structural instability in slender aerospace vehicles. Coupling among the rigid-body modes, the longitudinal vibrational modes and the transverse vibrational modes due to asymmetric lifting-body cross-section are considered. The model also incorporates the effects of aerodynamic pressure and the propulsive thrust of the vehicle. The model is one-dimensional, and it can be employed to idealized slender vehicles with complex shapes. Condition under which a flexible body with internal stress waves behaves like a perfect rigid body is derived. Two methods are developed for finite element discretization of the system: (1) A time-frequency Fourier spectral finite element method and (2) h-p finite element method. Numerical results using the above methods are presented in Part II of this paper. (C) 2010 Elsevier Ltd. All rights reserved.

Optimum Design for External Seismic Stability of Geosynthetic Reinforced Soil Walls: Reliability Based Approach

In this paper, an analytical study considering the effect of uncertainties in the seismic analysis of geosynthetic-reinforced soil (GRS) walls is presented. Using limit equilibrium method and assuming sliding wedge failure mechanism, analysis is conducted to evaluate the external stability of GRS walls when subjected to earthquake loads. Target reliability based approach is used to estimate the probability of failure in three modes of failure, viz., sliding, bearing, and eccentricity failure. The properties of reinforced backfill, retained backfill, foundation soil, and geosynthetic reinforcement are treated as random variables. In addition, the uncertainties associated with horizontal seismic acceleration and surcharge load acting on the wall are considered. The optimum length of reinforcement needed to maintain the stability against three modes of failure by targeting various component and system reliability indices is obtained. Studies have also been made to study the influence of various parameters on the seismic stability in three failure modes. The results are compared with those given by first-order second moment method and Monte Carlo simulation methods. In the illustrative example, external stability of the two walls, Gould and Valencia walls, subjected to Northridge earthquake is reexamined.