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  • Articles  (440)
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  • 1995-1999  (411)
  • 1980-1984  (29)
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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of the American Ceramic Society 80 (1997), S. 0 
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: X-ray diffraction (XRD) patterns from nominally β-SiC specimens often differ from those expected for the cubic crystal structure. These differences include the presence of additional peaks, enhanced background intensities, peak broadening, changes in relative peak heights, and shifts in peak positions. It has long been recognized that they are due to the presence of stacking faults, and models relating the experimental observations to stacking fault population have continued to evolve. The presence and relative magnitude of these features vary among different β-SiC specimens. In this work, computer simulations were used to show that the variations are closely related to differences in the type and spatial distribution of stacking faults in each specimen. In these simulations, stacking sequences were generated using a selectively activated 1-D Ising model with a Boltzmann-type probability function for specifying errors, which allows a wide variety of fault configurations to be generated. Direct correlations between different features in the XRD data to the underlying fault population are demonstrated, which are discussed in this paper. It is also shown that this computer model is general, in the sense that many of the models presented in prior work can be interpreted as limiting cases of it.
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  • 2
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
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  • 3
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: The Allen and Koomen planner is intractable in two ways: the Allen interval algebra is an intractable temporal reasoner, and the collapsing problem introduces a large branching factor in the search space for a solution plan. We define independence and dependence for networks to address both problems. Independence is used to find a decomposition of an interval network, and dependence is used to focus search when faced with the collapsing problem.
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  • 4
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: We analyze the problem of computing the minimal labels for a network of temporal relations in point algebra. Van Beek proposes an algorithm for accomplishing this task, which takes O(max(n3, n2 m)) time (for n points and m ≠-relations). We show that the proof of the correctness of this algorithm given by van Beek and Cohen is faulty, and we provide a new proof showing that the algorithm is indeed correct.
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  • 5
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: As influence diagrams become a popular representational tool for decision analysis, influence diagram evaluation attracts more and more research interests. In this article, we present a new, two-phase method for influence diagram evaluation. In our method, an influence diagram is first mapped onto a decision graph and then the analysis is carried out by evaluating the decision graph. Our method is more efficient than Howard and Matheson's because, among other reasons, our method generates a much smaller decision graph for the same influence diagram. Like those most recent algorithms reported in the literature, our method also provides a clean interface between influence diagram evaluation and Bayesian net evaluation. Consequently, various well-established algorithms for Bayesian net evaluation can be used in influence diagram evaluation. Furthermore, our method has a few unique merits. First, it takes advantage of asymmetry in influence diagrams to avoid unnecessary computation. Second, by using heuristic search techniques, it provides an explicit mechanism for using heuristic information that may be available in a domain-specific form. These additional merits make our method more efficient than the current algorithms in general. Finally, by using decision graphs as an intermediate representation, the value of perfect information can be computed in a more efficient way.
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  • 6
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Vagueness and uncertainty have attracted the attention of philosophers and logicians for many years. Recently, AI researchers contributed essentially to this area of research. Fuzzy set theory and the theory of evidence are seemingly the most appealing topics. On this note we present a new approach, based on the rough set theory, for looking to these problems. The theory of rough sets seems a suitable mathematical tool for dealing with problems of vagueness and uncertainty. This paper is a modified version of the author's lecture titled “An inquiry into vagueness and uncertainty,” which was delivered at the AI Conference in Wigry (Poland), 1994.
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  • 7
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: This paper presents results of experiments showing how machine learning methods arc useful for rule induction in the process of knowledge acquisition for expert systems. Four machine learning methods were used: ID3, ID3 with dropping conditions, and two options of the system LERS (Learning from Examples based on Rough Sets): LEM1 and LEM2. Two knowledge acquisition options of LERS were used as well. All six methods were used for rule induction from six real-life data sets. The main objective was to lest how an expert system, supplied with these rule sets, performs without information on a few attributes. Thus an expert system attempts to classify examples with all missing values of some attributes. As a result of experiments, it is clear that all machine learning methods performed much worse than knowledge acquisition options of LERS. Thus, machine learning methods used for knowledge acquisition should be replaced by other methods of rule induction that will generate complete sets of rules. Knowledge acquisition options of LERS are examples of such appropriate ways of inducing rules for building knowledge bases.
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  • 8
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Knowledge discovery in databases, or dala mining, is an important direction in the development of data and knowledge-based systems. Because of the huge amount of data stored in large numbers of existing databases, and because the amount of data generated in electronic forms is growing rapidly, it is necessary to develop efficient methods to extract knowledge from databases. An attribute-oriented rough set approach has been developed for knowledge discovery in databases. The method integrates machine-learning paradigm, especially learning-from-examples techniques, with rough set techniques. An attribute-oriented concept tree ascension technique is first applied in generalization, which substantially reduces the computational complexity of database learning processes. Then the cause-effect relationship among the attributes in the database is analyzed using rough set techniques, and the unimportant or irrelevant attributes are eliminated. Thus concise and strong rules with little or no redundant information can be learned efficiently. Our study shows that attribute-oriented induction combined with rough set theory provide an efficient and effective mechanism for knowledge discovery in database systems.
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  • 9
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: This paper presents an empirical study of the use of the rough set approach to reduction of data for a neural network classifying objects described by quantitative and qualitative attributes. Two kinds of reduction are considered: reduction of the set of attributes and reduction of the domains of attributes. Computational tests were performed with five data sets having different character, for original and two reduced representations of data. The learning time acceleration due to data reduction is up to 4.72 times. The resulting increase of misclassification error does not exceed 11.06%. These promising results let us claim that the rough set approach is a useful tool for preprocessing of data for neural networks.
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  • 10
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: In this paper, we introduce the notion of interval structures in an attempt to establish a unified framework for representing uncertain information. Two views are suggested for the interpretation of an interval structure. A typical example using the compatibility view is the rough set model in which the lower and upper approximations form an interval structure. Incidence calculus adopts the allocation view in which an interval structure is defined by the tightest lower and upper incidence bounds. The relationship between interval structures and interval-based numeric belief and plausibility functions is also examined. As an application of the proposed model, an algorithm is developed for computing the tightest incidence bounds.
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  • 11
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: “Simpson's paradox,” first described nearly a century ago, is an anomaly that sometimes arises from pooling data. Dramatic instances of the paradox have occurred in real life in the domains of epidemiology and admissions policies. Many writers have recently described hypothetical examples of the paradox arising in other areas of life and it seems possible that the paradox may occur frequently in mundane domains but with less serious implications. Thus, it is not surprising that the paradox should arise in commonsense reasoning, that subarea of artificial intelligence that seeks to axiomatize reasoning in such mundane domains. It arises as the problem “approximate proof by cases” and the question of whether to accept it may well depend on whether we wish to construct performance or competence models of reasoning. This article gives a brief history of the paradox and discusses its occurrence in our own discipline. It argues that if the paradox occurs frequently but undramatically in real life, every uncertain reasoning system will have to deal with the problem in some way.
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  • 12
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: We present the syntax and proof theory of a logic of argumentation, LA. We also outline the development of a category theoretic semantics for LA. LA is the core of a proof theoretic model for reasoning under uncertainty. In this logic, propositions are labeled with a representation of the arguments which support their validity. Arguments may then be aggregated to collect more information about the potential validity of the propositions of interest. We make the notion of aggregation primitive to the logic, and then define strength mappings from sets of arguments to one of a number of possible dictionaries. This provides a uniform framework which incorporates a number of numerical and symbolic techniques for assigning subjective confidences to propositions on the basis of their supporting arguments. These aggregation techniques are also described with examples.
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  • 13
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: In this paper we focus on the temporal constraints between causes and effects of causal relations, and, to deal correctly with such relations, we stress the importance of analyzing the action-types (aspectual category) of causes and effects. In particular, we propose a domain-independent ontology in which the distinctions between action-types (e.g., the distinction between durative and punctual situations) are dealt with, and different types of causal relations are distinguished, on the basis of (i) the temporal constraints they impose between causes and effects (these constraints are expressed in a temporal formalism that extends Vilain's point interval algebra) and (ii) the action-types of their causes and effects. Our ontology allows one to capture precisely the temporal constraints imposed by causation and the action-types of the related situations. Moreover, in case the user has no accurate knowledge about the action-types of some situations and/or the types of some causal connections to be dealt with, our formalism allows the user to leave the descriptions underspecified, and more specific pieces of information may be inferred. Inferences provide a flow of information about action-types to information about temporal constraints in causation and vice versa, and demonstrate that a deep integration of time and causation is provided also at the inferential level.
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  • 14
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: The Galois (or concept) lattice produced from a binary relation has proved useful for many applications. Building the Galois lattice can be considered a conceptual clustering method because it results in a concept hierarchy. This article presents incremental algorithms for updating the Galois lattice and corresponding graph, resulting in an incremental concept formation method. Different strategies are considered based on a characterization of the modifications implied by such an update. Results of empirical tests are given in order to compare the performance of the incremental algorithms to three other batch algorithms. Surprisingly, when the total time for incremental generation is used, the simplest and less efficient variant of the incremental algorithms outperforms the batch algorithms in most cases. When only the incremental update time is used, the incremental algorithm outperforms all the batch algorithms. Empirical evidence shows that, on the average, the incremental update is done in time proportional to the number of instances previously treated. Although the worst case is exponential, when there is a fixed upper bound on the number of features related to an instance, which is usually the case in practical applications, the worst-case analysis of the algorithm also shows linear growth with respect to the number of instances.
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  • 15
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: We propose a procedure for estimating DBLEARN's potential for knowledge discovery, given a relational database and concept hierarchies. This procedure is most useful for evaluating alternative concept hierarchies for the same database. The DBLEARN knowledge discovery program uses an attribute-oriented inductive-inference method to discover potentially significant high-level relationships in a database. A concept forest, with at most one concept hierarchy for each attribute, defines the possible generalizations that DBLEARN can make for a database. The potential for discovery in a database is estimated by examining the complexity of the corresponding concept forest. Two heuristic measures are defined based on the number, depth, and height of the interior nodes. Higher values for these measures indicate more complex concept forests and arguably more potential for discovery. Experimental results using a variety of concept forests and four commercial databases show that in practice both measures permit quite reliable decisions to be made; thus, the simplest may be most appropriate.
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  • 16
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: The inverted pendulum poses serious problems for qualitative modeling methods, so it is a good benchmark to lest their performance. This paper shows how a new data analysis method known as rough set theory can be utilized to swing up and stabilize the pendulum. Our approach to this task consists of deriving control rules from the actions of a human operator stabilizing the pendulum and subsequently using them for automatic control. Rule derivation is based on the “learning from examples” principle and does not require knowledge of a quantitative model of the system.
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  • 17
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    Oxford, UK : Blackwell Publishing Ltd
    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: We present some methods, based on the rough set and Boolean reasoning approaches, for extracting laws from decision tables. First we discuss several procedures for decision rules synthesis from decision tables. Next we show how to apply some near-to-functional relations between data to data filtration. Two methods of searching for new classifiers (features) are described: searching for new classifiers in a given set of logical formulas, and searching for some functions approximating near-to-functional relations.
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  • 18
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    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: We apply rough set constructs to inductive learning from a database. A design guideline is suggested, which provides users the option to choose appropriate attributes, for the construction of data classification rules. Error probabilities for the resultant rule are derived. A classification rule can be further generalized using concept hierarchies. The condition for preventing overgeneralization is derived. Moreover, given a constraint, an algorithm for generating a rule with minimal error probability is proposed.
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  • 19
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    Expert systems 12 (1995), S. 0 
    ISSN: 1468-0394
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Abstract: Increased interest in problem-solving decisions in the area of engineering leads to an increased demand for expert system applications. This paper presents an expert system that detects faults in hydraulic systems by simulating the reasoning of the expert, using as guide the hydraulic circuit of the system on the screen and extensive graphical information. A graphical user interface, appropriate explanations by using hypertext and user modelling have been implemented in order to produce a system that overcomes the usual interface problems of expert systems in engineering. This system has been developed in close collaboration with the company Automation Systems SA (Mannesmann Rexroth — Greece) and is currently used to detect faults in hydraulic presses, a fact that presents evidence of its usefulness.
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  • 20
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    Expert systems 12 (1995), S. 0 
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    Topics: Computer Science
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  • 21
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    Expert systems 12 (1995), S. 0 
    ISSN: 1468-0394
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Abstract: Retrieving ad hoc data from information systems is difficult for non-expert users. Despite the efforts made in improving query tools (e.g. visual query construction, Query By Example, query templates), empirical research shows that constructing a request is still difficult (Reisner 1988). The core of the problem seems to be in the difference between the way the user perceives the application domain and the way the system requires the user to see it (Carroll & Olsen 1988). In this paper we describe the design and implementation of an ad hoc query tool developed by RCC for a personnel information system, and how AI techniques contributed to this module. The main novel idea incorporated in the query tool is to present the user with a means to question the conceptual model of the system instead of the technical model. To support this, a blackboard architecture has been designed and implemented with knowledge sources that translate the user's questions to database queries. The query tool is used in daily practice by over 100 users.
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  • 22
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    Expert systems 12 (1995), S. 0 
    ISSN: 1468-0394
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Abstract: A fuzzy reasoning algorithm is presented in this paper. It is based on the concept of a generalised fuzzy production rule that adds new conjunctions such as ADD and REL to the usual AND and OR conjunctions for the linkage of premises in conventional rules. A definition is given for weights and related factors, to express and measure a relation between the premises. A formal representation for fuzzy premises is presented, together with the introduction of a fuzzy match method. Multiple thresholds are used to convert the uncertainty measures of reasoning results into linguistic variables. We have applied the fuzzy reasoning algorithm to a frame-based hybrid expert system tool. An object-oriented approach is used in implementing the system. Some results of the application are presented and discussed.
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  • 23
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    Expert systems 12 (1995), S. 0 
    ISSN: 1468-0394
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Abstract: Architectural design is a complex task which requires expert knowledge, a great deal of experience and no small amount of creativity and artistic flair. However, while humans are very good at creativity, intuition, artistry and evaluation, they are notoriously bad at routine mundane tasks. Searching through large databases of past designs can be such a task. This can result in useful design knowledge being ignored. In this paper we present a way to aid architects in their work by providing a software tool aimed at recording, retrieving and evaluating architectural designs.
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  • 24
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    Expert systems 12 (1995), S. 0 
    ISSN: 1468-0394
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Abstract: The purpose of this research is to present a comprehensive bibliography of neural network application research in business. One hundred and twenty-seven neural network application papers and reports are identified through exhaustive literature searches. A classification of these articles by application area reveals that neural networks are being used for a diverse range of corporate functional activities, particularly in the areas of production/operations and finance. Information on the neural network development language/tool, the learning paradigm and the computer operating environment as reported in each article is included. Those journals which have published the most neural network business applications are also presented.
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  • 25
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    Expert systems 12 (1995), S. 0 
    ISSN: 1468-0394
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Abstract: A real-time visual servo tracking system for an industrial robot has been developed. Instead of a charge coupled device (CCD), a position sensitive detector (PSD) is used as the real-time vision sensor due to its fast response (the light position is transduced to analogue current). A neural network learns the complex association between the 3D object position and its sensor reading, and uses it to track that object, either moving or stationary. It also turns out that this scheme lends itself to a user-friendly way to teach workpaths for industrial robots. Furthermore, for real-time use of the neural net, an efficient neural network architecture has been developed based on the concept of input space partitioning and local learning. Real experiments indicate the system's characteristics of fast processing and learning as well as optimal usage of network resources.
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    Expert systems 12 (1995), S. 0 
    ISSN: 1468-0394
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Abstract: Computational support for concurrent engineering design presents a number of issues in the design of knowledge-based systems. A fundamental concern is the need to provide the life-cycle perspective in recommending design alternatives. It is argued that such a complex knowledge-based system should be distributed, due to the inherent problems with large knowledge bases and the maintenance of consistency. Inconsistent local views of a problem and different priorities on design issues are expected between different knowledge-based design perspectives. These inconsistencies and different priorities will lead inherently to conflict which has to be resolved. This paper addresses the issues and key variables in conflict resolution and presents a strategy that can be applied for managing the conflict resolution process. We propose negotiation as the method of analysing a conflict situation, applying an appropriate conflict resolution strategy and monitoring its performance. We present an overview of the nature of conflict and some conflict resolution strategies, and see in what situations certain strategies are appropriate.
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  • 27
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    Expert systems 12 (1995), S. 0 
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    Topics: Computer Science
    Notes: Abstract: Research into the development and impacts of expert systems has accumulated to a point where it may be said to constitute a distinct field of study. As such, the evolution, organisation and dissemination of expert system research are themselves topics for investigation. Here we focus on the dissemination aspect, providing objective rankings of journals that have had the greatest impacts on recent expert system research. Our study examines the citation patterns of all research published in five expert systems journals during the past five years. Over 14 000 citations to more than 800 journals are analysed under two kinds of ranking schemes. These are based on cumulative citation counts and normalised citation counts. The latter adjusts for the number of years a journal article has been available to be cited. The reported rankings will be of interest to all who want to know where scientific developments that influence expert system research are reported: those who conduct, support, evaluate and study expert system research.
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    Expert systems 12 (1995), S. 0 
    ISSN: 1468-0394
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Abstract: In this paper we describe TheMD (The Methods Designer) shell and its use in the development of an expert system, known as Dust-Expert, for the relief of dust explosions. We begin by providing the motivation for the development of TheMD Shell by evaluating a prototype implementation of Dust-Expert using a conventional shell. This evaluation is carried out in terms of the transparency, reliability and extensibility of the system.The lessons learned from this evaluation have led to the development of TheMD Shell. We describe the main features of the shell which include more appropriate schemas, more complete explanation capabilities, a ranking scheme and equation-solving capabilities. We illustrate the use of the schemas and the importance of the equation-solving capabilities by examples from the domain of dust explosions. We also provide a formal specification of the ranking scheme that we adopt. We conclude by highlighting the benefits of our approach in comparison to the usual process of developing knowledge-based systems.
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    Notes: Subjective Probability George Wright and Peter Ayton (Eds.)Knowledge-based Information Systems D. Partridge and K. M. HussainIntelligent Agents: the New Revolution in Software Christine Guilfoyle and Ellie WarnerHigh Performance Computing Demystified D. Loshin, AP ProfessionalKnowledge Processing and Applied Artificial Intelligence Soumitra Dutta
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    Notes: Abstract: A knowledge-based system (KBS) could be an ideal decision tool to reduce the complexity of planning non-destructive evaluation (NDE) or scheduling inspections. Such a KBS would have to evaluate the advantages and limitations of each available technique, given the specificity of a problem, and incorporate the fundamental knowledge derived from failure analysis. The KBS should include topics such as materials vs. defect size and type, probability of failure, and basic reliability information. In order to organise knowledge of materials degradation efficiently, the parameters that control various forms of failure first have to be rationalised in a general framework. This framework and their factors would then constitute a quantitative and easily programmable description of the independent variables controlling the intensity of a failure. This paper describes such a framework which could guide the general selection of NDE for materials failure, with a particular emphasis on corrosion related failures. The framework architecture itself was constructed using an object-oriented methodology for maximum flexibility, since it was anticipated that the materials parameters could easily be described as multidimensional objects.
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    Notes: Abstract: They all stink: food and drink, perfumery, household products, soaps, shampoos, paints, manufacturing processes, printing processes, waste products, contaminated air, and automotive emissions and environmental testing. In every case, small is a criterion of quality.Automated techniques to ‘smell’ or ‘taste’ liquids using mass spectrometry and gas chromatography are time consuming, require skilled personnel and often do not give the information required for qualitative ‘testing’. A new technique is now available due to the advances in neural computing technology and multi-sensor array technology. The combination of these two approaches tries to simulate the human olfactory system in a simplified form. This paper shows that the recognition ability of an odour sensor array will be significantly improved using a neural computing approach in order to discriminate between similar odours.
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    Notes: Abstract: This paper presents a knowledge-based system for task allocation in a network of processors. Since the problem is NP-complete, optimisation techniques are computationally prohibitive and impractical. Our approach reduces computational burden by using (1) a two-level hierarchical scheduling model to alleviate the problem complexity, and (2) a search strategy that uses knowledge from the knowledge base to limit the search space to a manageable size. The major goal of our approach is to generate good but not necessarily optimal solutions quickly. The system is implemented on a Sun SPARCstation 1 + using Common Lisp. Our computational experience in using this system for task allocation indicates that the two-level hierarchical approach generates better schedules in shorter computational time than a non-hierarchical approach.
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    Notes: Abstract: This paper proposes a fuzzy post adjustment (FPA) mechanism so that human knowledge and machine knowledge can be integrated more synergistically to improve the performance of expert systems. Machine knowledge means knowledge algorithmically derived from past instances. Human knowledge implies (1) expert knowledge judging the trends of external factors and (2) user knowledge representing users’personal views about information given by both expert knowledge and machine knowledge. We consider an expert system that uses the FPA mechanism to incorporate the effect of external factors effectively into its inference process. The goal of this expert system is stock market timing prediction, which is divided into four kinds: bull, edged-up, edged-down and bear. Empirical tests showed that the proposed FPA mechanism can improve the performance of an expert system significantly, even in a turbulent decision-making environment.
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    Notes: Abstract: There is still a need for new knowledge elicitation techniques and tools. Laddering is a technique which has a long history in a wide range of disciplines and which has proved extremely useful in knowledge elicitation. There have, however, been few attempts to describe and survey the technique per se. This paper describes the technique, its background, its use, analysis, and automation, with particular reference to knowledge elicitation. The advantages and disadvantages of the technique are described. It is concluded that laddering is a valuable technique which could be used in a wide range of settings.
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    Notes: The Art and Science of Smalltalk: An Introduction to Object Oriented Programming Using VisualWorks Simon LewisThe Book of GENESIS: Exploring Realistic Neural Models with the General Neural Simulation System James Bower and D. BeemanThe Book of GENESIS: Exploring Real istic Neural Models with the General Neural Simulation System James Bower and D. BeemanThe ISDN Literacy Book Gerald L. HopkinsThe Virtual Reality Casebook Carl Eugene LoeHer and Tim
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    Notes: Abstract: The design of telecommunication networks is a complex activity involving a vast amount of input data and numerous design rules. Much of the data is estimated and many of the rules are based upon experiential knowledge. The use of existing uncertainty techniques in expert systems is not really suited to the complexity of telecommunication network design. Having investigated the uncertainty problem, it is believed that a more rational method has been found for use in this application area. The paper presents the ‘Uncertainty Window’technique and its testing on one aspect of network design, namely that of network synchronisation.The essence of this uncertainty process is one of associating individual uncertainty windows with the relevant rules, data and facts of a design problem, which results in providing the network designer with an insight into the possible uncertainties within a design at each stage of the process. It indicates which information sources provide the dominant uncertainty and which assumptions are critical, needing further investigation, in order to improve the level of confidence in the overall design. The windowing technique works by virtue of its ability to retain the composition of the uncertainties within the problem and their associated values, assumptions, and so forth and allows for better solutions to be obtained.
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    Notes: Abstract: Prolog/Rex supports the construction of knowledge-based systems, allowing frames (Prolog/Rex concepts) and rules to be combined, and providing language level support for hypothetical reasoning and assumption-based truth maintenance (Prolog/Rex viewpoint mechanism). To increase run-time performance during rule execution, Prolog/Rex provides two complementary rule compilation techniques: the simple indexing scheme and the heavily modified Rete expansion method. To improve the efficiency of the inference engine, we built an agenda-based rule manager that uses priority mechanism and/or heuristics embedded in meta-rules to control the rule firing order. Forward- and backward-chaining can be combined within the same program.
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    Notes: Abstract: Process supervision consists of following the temporal evolution (change) of process behaviours. This task has usually been performed based on the knowledge and experience of domain experts and operators. Actually, these experts and operators almost always express their experience and knowledge about process evolution in an imprecise, fuzzy and vague way. A good supervision system should be capable of dealing at once with two different kinds of knowledge: time and uncertainty.For many years, time and uncertainty have been two of the most important topics in Artificial Intelligence research and applications. Many approaches have been proposed to deal with either one or the other. Among the various approaches for time, reified logic has been considered as the most influent one. Possibilistic logic, on the other hand, has shown its ability to handle uncertain knowledge and information. This paper describes an approach for managing temporal uncertainty based on fuzzy logic and possibility theory. A fuzzy temporal expert system shell has been developed to perform process supervision tasks.
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    Notes: Abstract: During the last decade, cumulative trauma disorders have become a major concern both to occupational health and safety professionals and manufacturing industries. This paper will present the development of a knowledge-based system for predicting the potential of developing strain injuries among employees exposed to repetitive and forceful activities. This potential is identified through the use of an expert system incorporating a fuzzy set algorithm. Recommendations for the prevention of the resulting CTDs are then provided by the system. The system executes on a microcomputer platform, and was constructed utilising a commercial object-oriented expert system shell.
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    Notes: Abstract: The recent surge of interest in connectionist models arose through the availability of high speed parallel supercomputers and the advent of new learning algorithms. The computations performed on concurrent architectures are less costly than similar ones performed on sequential machines. In this paper, the design and implementation of a parallel version of fuzzy ARTMAP (Carpenter et al. 1992), which encompasses both neural and fuzzy logic, is discussed. Fuzzy ARTMAP is a supervised learning algorithm utilising two fuzzy ART modules and an associated mapping network. A simplified version of fuzzy ARTMAP (SFAM) was designed by incorporating a simplification of the match tracking concept on unsupervised fuzzy ART paradigms. The proposed simplified version consists of only one fuzzy ART module and an associated mapping network. A parallel fuzzy ARTMAP (PFAM) algorithm is then designed and implemented on a hypercube simulator (iPSC). The algorithm is parallelised for any architecture and, with the exception of issues related to communications, the implementation remains the same on any type of parallel machine. PFAM enjoys the advantage of reduced training time that makes the algorithm a successful candidate for applications that require both online testing and training. Such applications can range from underwater sonar detection and chemical plant processing control to nuclear reactor process control, flexible manufacturing and systems analysis.
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    Notes: Abstract: Expert systems have traditionally been designed with one particular purpose in mind, whether problem-solving, critiquing etc. When the system is intended for users with varying background knowledge it becomes necessary to provide different facilities for different groups of users, which in turn puts demands on how the knowledge must be represented. This paper describes an industrial prototype—Protein Purification Advisor (PPA)—in the area of protein purification, and how a suitable choice of knowledge representation has allowed us to use the same knowledge for a number of purposes including those mentioned above. Particular interest is laid on the topic of how to represent the heuristic knowledge and how to use it in actual decision-making.
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    Notes: Abstract: The KADS methodology (Schreiber et al. 1993; Tansley & Hayball 1993) and its successor, CommonKADS (Wielinga et al. 1992) have proved to be very useful approaches for modelling the various transformations involved between eliciting knowledge from an expert and encoding this knowledge in a computer program. These transformations are represented in a series of models. While it is widely agreed that these methods are excellent approaches from a theoretical viewpoint, the documentation provided concentrates on defining what models should be produced, with only general guidance on how the models should be produced. This has the advantage of making KADS and CommonKADS widely applicable, but it also means that considerable training and experience is required to become proficient in them. This paper reviews three projects that investigated the feasibility of producing specific guidance for certain decisions which are required when using KADS or CommonKADS to develop a knowledge-based system. Guidance was produced for the identification of the generic task addressed by a knowledge-based system; for the selection of appropriate AI techniques for implementing the analysed knowledge; and for selecting a suitable tool for implementing the system. Each set of guidance was encoded in its own knowledge-based system, which was itself developed with the assistance of KADS or CommonKADS. These projects therefore both studied and applied KADS and CommonKADS in order to produce knowledge-based guidance for knowledge engineers. The projects showed that it was feasible to produce heuristic guidance which could be understood, applied and occasionally overridden by knowledge engineers. The guidance provides reasonably experienced knowledge engineers with a framework for making the key decisions required by CommonKADS, in the same way that CommonKADS provides knowledge engineers with a framework for representing knowledge. The projects also produced some new insights about CommonKADS domain modelling and about the process of task identification.
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    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— This paper examines the application of the Jk, L and M integrals, in complex-variable form, to the Boussinesq wedge. The wedge is symmetrical and subjected to a point couple and point forces at the apex of the wedge. In the case of a point couple acting at the wedge apex the Jy, L and M integrals are found to vanish for all wedge angles whereas Jx displays a 1/r3 path-dependence; where r is a radial dimension measured from the wedge apex. When the wedge is subjected to point forces at the wedge apex then Jx and Jy are 1/r path-dependent whereas L and M are path-independent.The property that the L and M integrals are path-independent for the Boussinesq wedge is applied to the problem of determining the modes I and II stress intensity factors for a corner-loaded edge crack in a half-plane subjected to both normal and parallel point forces to the free surface of the half-plane.
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    Notes: Abstract— Fatigue tests were performed on thin-walled tubular specimens of S45C steel under tension-compression, pure torsion, in-phase and out-of-phase axial-torsional loadings. The relationship between cracking behaviour and stress components on the crack plane was investigated. Measurement of microcrack density showed that microcracking was governed predominantly by the shear stress amplitude acting on the crack plane for all loading conditions. The failure crack was formed by coalescence of many cracks initiated near the maximum shear planes. The cracks grew turning their orientation to the direction perpendicular to the maximum normal stress. The transition of crack orientation occurred at relatively longer crack lengths at a higher stress ratio. The crack growth behaviour for all loading modes can be correlated using an equivalent strain intensity parameter based on shear and normal strains on the crack plane.
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    Notes: Abstract— A ductile medium strength steel has been modelled by means of the Gurson model, and been used to investigate the effect of crack tip constraint in several fracture mechanics specimens. Both numerical and experimental results have been obtained, in the course of the crack extension process, for single edge notch bending specimens with different crack length-to-width ratios. The geometries with the shorter cracks always exhibited higher J values at initiation and steeper J crack growth resistance curves, and these results have been explained in terms of the stress and strain fields and damage development in the region ahead of the crack tip.
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    Notes: Abstract— It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick-walled tubes against pulsating internal pressure. The physical reason is based on the well-known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick-walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between-110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low-temperature autofrettage in enhancing both the fatigue life in the finite-life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low-temperature autofrettage performance are discussed.
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    Notes: Abstract— A new single-specimen testing method, the normalization method with the so-called LMN calibration function, based on the load separation principle and function calibrations from an individual test record, was used to construct J-R curves directly from load versus load-line displacement records without any additional on-line crack-length monitoring equipment. The research was done on CT-specimens of a glassy polymer PVC at different crosshead speeds ranging from 0.01 to 50 mm/min. The J-R curves evaluated from the normalization method are in good agreement with those from the conventional multiple-specimen testing method in the whole range of the tested crosshead speeds. The results demonstrated the applicability of the normalization method for developing J-R curves at different crosshead speeds in PVC. The crack initiation J-integral values, J0.2, showed a two-regime dependence on the crosshead speeds in the tested crosshead speed range.
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    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The development of fatigue damage in Co45Ni specimens during push—pull and reversed torsion tests, performed inside a scanning electron microscope, was observed and the different stress states compared. It appeared that transgranular crack initiation and development is delayed and intergranular crack initiation promoted under torsional loading. This was explained in terms of reduced surface distortion at the emergence of persistent slip bands (PSBs) and smaller compatibility stresses at the PSB-matrix interfaces. The influence of the mechanical strength of grain boundaries on the difference between tensile and torsional fatigue lives is discussed.
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  • 81
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A Fourier series approach is proposed to calculate stress intensity factors using weight functions for semi-elliptical surface cracks in flat plates subjected to two-dimensional stress distributions. The weight functions were derived from reference stress intensity factors obtained by three-dimensional finite element analyses. The close form weight functions derived are suitable for the calculation of stress intensity factors for semi-elliptical surface cracks in flat plates under two-dimensional stress distributions with the crack aspect ratio in the range of 0.1 ≤a/c≤ 1 and relative depth in the range of 0 ≤a/t≤ 0.8. Solutions were verified using several two-dimensional non-linear stress distributions; the maximum difference being 6%.
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A conventional finite element method may show a weakness when determining the hot spot stress distributions in the brace/chord intersection region of offshore tubular joints. This is because the chosen element displacement functions do not implicitly satisfy the conditions which prevail on the free surfaces. A procedure has been proposed to modify the conventional finite element method so as to allow the hot spot stresses, which occur at the free boundary of the weld toe of tubular joints, to be determined with improved accuracy. The results obtained by this modified method are compared with both an experimental and a traditional finite element solution. The comparison shows that the modified solution is in better agreement with the experimental data as compared with the traditional solution.
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 84
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Simple extensions to the standard deep notch bend test procedure are suggested to allow the collection of data relevant to the energy dissipation rate, D, crack opening angle, COA, and J, all for arbitrarily large amounts of growth in extensive plasticity. The methods of analysis are detailed for real elastic-plastic behaviour of a high strength low-hardening type metal with a view to encouraging use on a wider range of materials. A proposal is made, and equations given, that the particular version of J used for an R-curve derived from the area under the loading diagram, should correspond to the value of the far-field integral, Jff.The relationship between the global measure of COA that emerges from D and the local crack tip opening angle, CTOA, as used in computational studies, is established. Transferability of CTOA data is examined in the light of effects of size and configuration. An explicit rule of the form CTOA √G =f (material and configuration) is proposed for the modelling of ductile growth in finite element studies. It is applied to a set of data in the literature, for the variation of CTOA with size in the deep notch bend test and for the configurations, bending, double edge and centre cracked tension.
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  • 85
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— In this investigation the Electron Channelling Contrast (ECC) technique in scanning electron microscopy (SEM) was applied to reveal the dislocation structures in the vicinity of surface fatigue cracks in comparison to those of cyclically-deformed recrystallized polycrystalline copper. The plastic zone around a fatigue crack was found to consist of an innermost region containing cells, followed by a region containing dense veins and PSBs, surrounded by a structure of loose veins, bundles and loop patches typical of the cyclically deformed matrix. A relation between plastic strain amplitude values deduced from cyclic stress-strain investigations and the dislocation structures near fatigue cracks are given. Typical regions of damage accumulation were identified and plastic strain contours for surface fatigue cracks established. The essentially non-destructive ECC technique is particularly suited to identify the changes in mesoscopic dislocation structures from surface layers to the interior of specimens over large specimen areas.
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  • 86
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 87
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The propagation behaviour of fatigue cracks emanating from pre-cracks was numerically simulated to evaluate the development of crack closure with crack growth. The crack opening stress intensity factor at the threshold was approximated as a function of the applied stress and the amount of crack extension. Pre-cracked specimens of a medium-carbon steel with a small surface crack and a single-edge crack were fatigued to investigate experimentally the initiation and propagation of cracks from pre-cracks. Crack closure was dynamically measured by using an interferometric strain/displacement gauge. The threshold condition of crack initiation from pre-cracks was given by a constant value of the effective stress intensity range which was equal to the threshold value for long cracks. The cyclic R-curve was constructed in terms of the threshold value of the maximum stress intensity factor as a function of crack extension approximated on the basis of the experimental and numerical results. The cyclic R-curve method was used to predict the fatigue thresholds of pre-cracked specimens. The predicted values of the fatigue limits for crack initiation and fracture, and the length of non-propagating cracks agreed very well with the experimental results.
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  • 88
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A basic study was performed on the evolution of three-dimensional shapes of small surface fatigue cracks during fatigue, and the effect of this evolution on small-crack growth behavior of a titanium-base alloy. Specifically, the nature and the magnitude of variations in crack aspect ratio, a/c (a is the crack depth and c is the half-surface crack length), during cyclic crack growth and its impact on growth rates have been studied. Experiments were performed on naturally initiated micro-cracks in a microstructure consisting of equiaxed primary-α2 phase in a Widmanstätten (transformed β) matrix. Several cracks under stress ratio (R) levels of 0.1 and −1, were studied. A specialized experimental system, consisting of a laser interferometer (to measure precisely the small-crack surface displacements), and a photo microscope (to automatically and continuously photograph the fatigue micro-cracks) was employed in the study. Apparent aspect ratios of surface cracks were calculated from the compliance response and the surface crack length data as a function of fatigue cycles. These data enabled accurate calculations of growth rates at the surface crack tip as well as the tip at depth in the bulk over the entire crack growth period, thus giving an insight into the crack growth process. Measurements of closure levels of small cracks were also performed and were used to partly account for the differences in growth rates. In the comparisons of small-crack growth data with the large-crack data, surface growth rates correlated relatively well with the large-crack data. Growth rates at depth exhibited large variations due to the irregularity of crack fronts at this location, and these rates deviated significantly from the large-crack behavior. Additionally, these growth rates varied between different cracks. An attempt was made to rationalize these observations in terms of the effects of inhomogeneities present in the microstructure.
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  • 89
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— A new technique, known as crack modelling, is used here to predict fatigue failure in a crankshaft component. The technique uses a linear elastic finite element (FE) analysis to derive a stress intensity factor (K) for the component under load. The novel feature of the technique is that K is calculated without introducing a crack into a component; the stress field around the maximum stress point is examined and compared to that for a standard centre-cracked plate. The fatigue limit for a crankshaft was successfully predicted, when compared to experimental data. The only material parameter required for this prediction was the threshold stress intensity range, ΔKth.
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The effects of bluing, associated with drawing strain, on the fatigue strength of eutectoid steel wires have been investigated. The fatigue limit increases by bluing and the increase is more significant with higher drawing strain. The peak in the fatigue limit with regard to the drawing strain in the wires, at a strain of 2.5, disappears after bluing. On the other hand, in the ferritic steel wires investigated for comparison, the fatigue limit gradually increases with the drawing strain up to 7.7. Furthermore, no appreciable change in the fatigue limit due to bluing is found. Based on the results of hardness tests on fatigue specimens with- and without-bluing, it is deduced that the decrease of the fatigue limit beyond the peak drawing strain in the eutectoid steel wire can partly be attributed to insufficient locking of the high-density dislocations by solute atoms. The effect of relaxation of residual stress during bluing is also briefly discussed.
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  • 91
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— It is well known that for very short cracks the stress intensity factor K is not a suitable parameter to estimate the stress level over the small but finite Stage II process zone activation region of size rs near the crack tip, within which crack growth events take place. A critical appreciation of the reasons for the limitations on the applicability of ΔK as a fatigue crack propagation (FCP) parameter, when the crack length a is of the same order of magnitude or smaller than the size of the ‘fatigue-fracture activation region’, rs is presented. As an alternative to ΔK the range Δσs of the cyclic normal stress at a point situated at the fixed distance s=rs/2, ahead of the crack tip, inside the fatigue-fracture activation region, is proposed. It is observed that the limitation on the use of ΔK when the crack is short, is mathematical (and not physical) but this inconvenience is easily circumvented if the stress Δσs at the prescribed distance is used instead of ΔK since nowadays Δσs can be obtained numerically by using finite element methods (FEM). It follows that the parameter Δσs is not restricted by the mathematical limitations on ΔK and so it would seem that there is, a priori, no reason why the validity of the parameter Δσs cannot be extended to short cracks. It is shown that if the Paris law is expressed in terms of Δσs (πrrs)½ instead of ΔK the validity of the modified Paris law can be extended to short cracks.A coherent estimate of the value of the fatigue-fracture activation region rs is derived in terms of the fatigue limit ΔσFL obtained from S-N tests and of the threshold value ΔKth obtained from tests on long cracks where both relate to Stage II crack growth that ends in failure, namely, rs= (ΔKth/ΔσFL)2/π. An overall, threshold diagram is presented based on the simple criterion that, for sustained Stage II FCP, Δσs must be greater than ΔσFL. The study is based on a simple continuum mechanics approach and its purpose is the investigation of the suitability of both ΔK and Δσs to characterise the crack driving force that activates complex fracture processes at the microstructure's scale. The investigation pertains to conditions that lead to the ultimate failure of the component at values of Δσs 〉 ΔσFL.
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The boundary value problem for an arbitrarily shaped plane crack embedded in a 3D linear elastic solid can be reduced to a governing hyper-singular integral equation. A discretizing procedure based on a triangulation of the crack area has been offered in Part I of this work. The main goal of Part I is to introduce the analytical results for the 18 resulting finite-part integrals defined over a triangular mesh area. The finite-part integrals occur in those triangles where the source point coincides with one of the element nodes. Mostly the source point lies outside of the considered triangle. In these cases the occurring area integrals are regular.The aim of Part II is, therefore, the derivation of the closed form expressions for the relevant 18 regular area integrals. The resulting relations are of algebraic form which can easily be coded in compact form. Their numerical proof by two different methods shows the highest accuracy and, therefore, the correctness of the final solutions. The relevant numerical results are offered in Appendix I.With the formulae provided in Part I and Part II of the paper the determination of the coefficient matrix, necessary for the calculation of COD values from a linear equation system, is precise and needs only minimum computer time.
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Circumferentially notched cylindrical specimens are tested in torsion to obtain critical J values from crack resistance curves. The specimens are explosion cladded, half ferrite, half austenite, with the interface perpendicular to the cylinder axis and the circumferential notch at, or parallel to, the interface. Critical J values for crack extension in mode III were found to be a factor 1.1 to 2.1 higher than under comparable mode I loading.
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  • 94
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
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    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Strength measurements are becoming increasingly important for electroceramics. Bending of specimens small enough to be cut out of small electroceramic components may be one possibility. Therefore the miniaturisation of the 4-point bend-test for ceramic specimens is now being attempted. In this paper the errors in determining the flexural strength arising from the test principle itself, plus the geometry and measuring inaccuracies are calculated and expressed as a function of the outer span length. Contact pressure and a tolerable total measuring inaccuracy determines the dimensions of miniature specimens and fixtures. The possibilities of appropriate specimen preparation are also investigated.Ceramic materials show a volume (i.e. a specimen size) dependence of strength which is described by Weibull's statistical theory. The applicability of the miniature bend-fixtures is demonstrated by measuring this volume effect.
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  • 95
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— This paper describes a versatile technique for simulating the fatigue growth of a wide range of planar cracks of practical significance. Crack growth is predicted on a step-by-step basis from the Paris law using stress intensity factors calculated by the finite element method. The crack front is defined by a cubic spline curve from a set of nodes. Both the 1/4-node crack opening displacement and the three-dimensional J-integral (energy release rate) methods are used to calculate the stress intensity factors. Automatic remeshing of the finite element model to a new position which defines the new crack front enables the crack propagation to be followed. The accuracy and capability of this finite element simulation technique are demonstrated in this paper by the investigation of various problems of both theoretical and practical interest. These include the shape growth trend of an embedded initially penny-shaped defect and an embedded initially elliptical defect in an infinite body, the growth of a semi-elliptical surface crack in a finite thickness plate under tension and bending, the propagation of an internal crack in a round bar and the shape change of an external surface crack in a pressure vessel.
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Cold-expansion of fastener holes is now commonly used within the aerospace industry to increase the fatigue endurance of airframes. Although a number of methods of cold expansion are possible, the split-sleeve cold-expansion process is the most widely accepted and is frequently used in the repair and manufacture stages of both military and civil aircraft. In the present work, the redistribution of residual hoop stresses due to the application of constant amplitude fatigue loading at 4% cold-expanded holes has been studied. A modified Sachs method was adopted to evaluate the residual stress profiles and a replication technique was used to quantify crack growth. It was found that the decay of the residual hoop stress profile near the bore of the hole was due to the initiation and growth of small fatigue cracks. Cracks were found to initiate both near and below the fatigue limit, but subsequently arrested so stabilising the overall residual stress profile.
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    Fatigue & fracture of engineering materials & structures 20 (1997), S. 0 
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— The factors affecting the fatigue strength of nitrided titanium were clarified. The fatigue strength depended strongly on the fracture strength of the compound layer formed on the surface by nitriding. We found a Hall-Petch relationship between the fatigue strength of nitrided titanium and the grain size. The findings indicated that the reduction in the fatigue strength by nitriding results from both the formation of the compound layer possessing low fracture strength and grain growth occurring from ordinary nitriding. Furthermore, low-temperature nitriding (620°C, 24 h) was proposed to suppress grain growth. This treatment method improved not only the wear resistance and the corrosion resistance but also the fatigue strength of titanium.
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    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Control strategies that decompose the competence of an agent into independent, task-achieving control structures are emerging as viable alternatives to the notion that a robot is an explicit symbol processing device with input and output extensions in the form of sensors and actuators. This approach draws inspiration from ethology and the cognitive sciences, where the competence of biological creatures is seen to be a result of the successful combination of several distinct behavior patterns that enable them to interact effectively with their environments.The distinction between classical approaches that view intelligent behavior as the outcome of symbolic manipulations and an alternate approach that asserts that it is the careful integration of distributed, goal-competent control structures that lead to intelligent behavior is analyzed in detail. It is found that this dichotomy can be said to arise from the paradigm that is chosen to represent the knowledge at the disposal of the agent. Structurally encoded knowledge requiring an active interpreter for the knowledge to be exhibited in action leads to the symbol processing paradigm for intelligent behavior, whereas the interactive model for knowledge representation assumes that the knowledge at the disposal of an active agent resides within goal-competent, interactive control structures.A formalism is developed for representing the goal-directed nature of these interactive control structures with clear control-module boundaries. An implementation is then presented to demonstrate some of the basic features of this methodology.
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    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: We have previously reported a number of tractable planning problems defined in the SAS+ formalism. This article complements these results by providing a complete map over the complexity of SAS+ planning under all combinations of the previously considered restrictions. We analyze the complexity of both finding a minimal plan and finding any plan. In contrast to other complexity surveys of planning, we study not only the complexity of the decision problems but also the complexity of the generation problems. We prove that the SAS+-PUS problem is the maximal tractable problem under the restrictions we have considered if we want to generate minimal plans. If we are satisfied with any plan, then we can generalize further to the SAS+-US problem, which we prove to be the maximal tractable problem in this case.
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    Computational intelligence 11 (1995), S. 0 
    ISSN: 1467-8640
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Computer Science
    Notes: Sentential theories of belief hold that propositions (the things that agents believe and know) are sentences of a representation language. To analyze quantification into the scope of attitudes, these theories require a naming map a function that maps objects to their names in the representation language. Epistemic logics based on sentential theories usually assume a single naming map, which is built into the logic. I argue that to describe everyday knowledge, the user of the logic must be able to define new naming maps for particular problems. Since the range of a naming map is usually an infinite set of names, defining a map requires quantification over names. This paper describes an epistemic logic with quantification over names, presents a theorem-proving algorithm based on translation to first-order logic, and proves soundness and completeness. The first version of the logic suffers from the problem of logical omniscience; a second version avoids this problem, and soundness and completeness are proved for this version also.
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