1,721,028 research outputs found
On Optimal Maintenance of Degrading Multistate Systems With State-Dependent Cost of Repair
No full textFinkelstein, Maxim/0000-0002-3018-8353This article considers an optimal maintenance policy for the multistate systems with the finite number of states. Each state is described by its level of performance ranging from the perfect one to the zero level for the state of failure. Moreover, we assume that the cost of preventive maintenance (PM; i.e., repair/rejuvenation in our case) also depends on the state of a system. Based on the proposed policy, the expected cost per unit of time is defined and the conditions for the existence of the unique and finite PM time are obtained in terms of dynamic reliability characteristics of the system. The results are applied to the three-state Markovian system and a parallel system with n components. The latter is also discussed for the case of the positively dependent components
On some methods of reliability improvement of engineering systems
No full textThe purpose of this thesis was to study some methods of reliability improvement of engineering systems. The reason for selecting the theme “reliability improvement of engineering systems” was first to explore traditional methods of reliability improvement (that is, based on the notion that reliability could be assured by simply introducing a sufficiently high “safety factor” into the design of a component or a system) and then propose new and original concepts of reliability improvement. The latter consists of approaches, methods and best practices that are used at the design phase of a component (system) in order to minimize the likelihood (risk) that the component (system) might not meet the reliability requirements, objectives and expectations.
Therefore, chapter 1 of the thesis, “Introduction to the main methods and concepts of reliability for technical systems” encompasses the introduction section and the main traditional methods available for improvement of technical / engineering systems.
In chapter 2, “Reliability Component Importance Measures” two new and original concepts on reliability improvement of engineering systems are introduced. These are: 1) the study of availability importance of components in coherent systems and 2) the optimal assignment of interchangeable components in coherent multi-state systems.
In chapter 3, “Cannibalization Revisited” two new and original concepts on reliability improvement of engineering systems are introduced. These are: 1) theoretical model to show the effects of cannibalization on mission time availability of systems and 2) new model for cannibalization and the corresponding example. In chapter4, “On the Improvement of Steam Power Plant System Reliability” a new and original model is developed that helps in determining the optimal maintenance strategies which will ensure maximum reliability of the coal-fired generating station.
Conclusions are given, concerning the study conducted and the results thereof, at the end of each chapter. The conclusions for this thesis are annotated in chapter 5.
A set of selected references that were consulted during the study performed for this doctor of philosophy thesis is provided at the end.Afrikaans: Die doel van hierdie tesis was om sekere metodes te bestudeer om ingenieurswese stelsels se betroubaarheid te verbeter. Die rede hoekom die tema “inginieurswese stelsels verbetering” gekies was, was om eerstens die tradisionele metodes van betroubaarheid verbetering te ondersoek, (wat gebaseer is op die ideë dat betroubaarheid kan verseker word deur om net n hoë voldoende veiligheids faktor in die ontwerp van komponente of stelsels voor the stel) asook nuwe, oorspronklike konsepte van betroubaarheid verbetering. Die laasgenoemde bestaan uit benaderings, metodes en beste praktyke wat gebruik kan word by die ontwerps fase van n komponent (stelsel) om die waarskynlikheid (risiko) te minimaliseer wanneer die komponent (stelsel) nie voldoen aan die betroubaarheid vereistes, objektiewe en verwagtinge nie.
Daarom, hoofstuk 1 van die tesis, “Introduction to the main methods and concepts of reliability for technical systems” sluit in die inleiding seksie en die hoof tradisionele metodes beskikbaar om tegniese/ ingenieurswese stelsels te verbeter.
In hoofstuk 2, “Reliability Component Importance Measures” twee nuwe en oorspronklike konsepte oor betroubaarheid verbetering van ingenieurswese stelsels word voorgestel. Hulle is: 1) die studie van hoe belangrik die beskikbaarheid van komponente in samehangende stelsels is en 2) die optimale toewysing van verwisselende komponente in samehangende multi-stadium stelsels.
In hoofstuk 3, “Cannibalization Revisited” twee nuwe en oorspronklike konsepte oor betroubaarheid verbetering van ingenieurswese stelsels word voorgestel. Hulle is: 1) n teoretiese model om die effekte van kannibalisering op missie tyd beskikbaarheid van stelsels te toon en 2) n nuwe model vir “cannibalization” en die oorstemende voorbeeld. In hoofstuk 4, “On the Improvement of Steam Power Plant System Reliability” n nuwe en oorspronklike model is ontwikkel en dit sal help om die optimale instandhouding strategieë wat die maksimum betroubaarheid van die steenkool-aangedrewe kragsentraal verseker.
Die gevolgtrekkings word gegee, met betrekking tot die studie en die resultate daarvan is aan die einde van elke hoofstuk. Die gevolgtrekking vir hierdie tesis is geannoteer in hoofstuk 5. n Stel geselekteerde verwysings wat gekonsulteer was gedurende die studie wat uitgevoer was vir hierdie doctors filosofie tesis word aan die einde voorsien
Mixture failure rate modeling with applications
No full textThis thesis is mostly on mixture failure rate modeling with some applications. The topic is very important in the modern statistical analysis of real world populations, as mixtures is the tool for modeling heterogeneous populations. Neglecting heterogeneity can result in serious errors in analyzing the corresponding statistical data. Many populations are heterogeneous in nature and the homogeneous modeling can be considered as some approximation. It is well known that the failure (mortality) rate in heterogeneous populations tends (as time increases) to that of the strongest subpopulation. However, this basic result had to be considered in a much more generality dealing with the shape of the failure rate and the corresponding properties for other reliability indices as well. This is done in the dissertation, which is (we believe), its main theoretical contribution which can have practical implications as well. We focus on describing aging characteristics for heterogeneous populations. A meaningful case of a population which consists of two subpopulations, which we believe was not sufficiently studied in the literature, is considered. It is shown that the mixture failure rate can decrease or be a bathtub (BT) shaped: initially decreasing to some minimum point and eventually increasing as t or show the reversed pattern (UBT). Otherwise, the IFR property is preserved. The mean residual life’s (MRL) ‘shape properties’ are analyzed and some relations with the failure rate are highlighted. We show that this function for some specific cases with, e.g., IFR or UBT shaped failure rates is decreasing for certain values of parameters, whereas it is UBT for other values. Some findings on the bending properties of the mixture failure rates are presented. It follows from conditioning on survival in the past interval of time that the mixture failure rate is majorized by the unconditional one. These results are extended to other main reliability indices. The mixture failure rate before and after a shock for ordered heterogeneous populations are compared. It turns out that the failure rate after the shock is smaller than the one without a shock, which means that shocks under some assumptions can improve the probabilities of survival for items in a heterogeneous population. We show that the population failure/mortality rate decreases with age and, even tend to reach a plateau for some specific cases of mortality (hazard) rate process induced by the non-homogeneous Poisson process of shocks. Our model can be used to model and analyze the damage accumulated by organisms experiencing external shocks. In this case, the cumulated damage is reflected by jumps in the failure rate. The focus in the literature has been mostly on the study of expectations for mixtures, however, the obtained results show that the variability characteristics in heterogeneous populations may change dynamically
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Actuarial risk management of investment guarantees in life insurance
No full textInvestment guarantees in life insurance business have generated a lot of research in recent years
due to the earlier mispricing of such products. These guarantees generally take the form of exotic
options and are therefore difficult to price analytically, even in a simplified setting. A possible
solution to the risk management problem of investment guarantees contingent on death and
survival is proposed through the use of a conditional lower bound approximation of the corresponding
embedded option value. The derivation of the conditional lower bound approximation
is outlined in the case of regular premiums with asset-based charges and the implementation is
illustrated in a Black-Scheles-Merton setting. The derived conditional lower bound approximation
also facilitates verifying economic scenario generator based pricing and valuation, as well as
sensitivity measures for hedging solutions
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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