129 research outputs found
Evolution and Technological Process (A Review of Methodological Approaches and the Creative Evolution in Historiographic Thought of Vere Gordon Childe)
No full textVere Gordon Childe, an Australian archaeologist and historian, was one of most spectacular figures in archaeology and history in the first half of the 20th century. He took a methodologically synthetic approach to offer particular issues which are still given attention by pre-historians and historians. In this respect, his influential book History, targeting a survey of current theories on the history and historiography, was so given attention by academics in Iran that has been translated and published for several times. In this paper, it is attempted to review the Persian version of the book and that the methodological synthetic approach taken by its author about the historical order and the variables of such interpretation on the history
A Critique on the Book Cultural History of the Islamic World (Introduction to the History of Culture and Civilization in the Islamic Period)
No full textThe compilation of the cultural history of the Islamic world on the one hand requires mastery of cultural theories and definitions, texts and sources and critique and study of the study history of cultural and civilizational history of the Islamic world in its various cultural and civilizational fields and on the other hand requires a clear definition and provide a clear model for understanding culture. The cultural history of the Islamic world, due to its vastness and diversity, has not yet been seriously considered and all the writings that have allegedly arisen due to the lack of definite definitions and frameworks have addressed only some aspects of this issue and have failed to understand its generality. This article examines the form and content of the book of the cultural history of the Islamic world written by Boroumand Azamipour with a descriptive and analytical method. As a result, the author of the work relies on the common approach in writing such works and adopting a common view of the history of Islamic sciences and civilization due to the lack of a clear definition of the concept of cultural history. Therefore, the cultural history of the Islamic world has been reduced to the history of science and education
Ultralight Membrane Structures Toward a Sustainable Environment
No full textThe building construction industry is the largest anthropogenic source of pollution, with massive energy consumption and substantial CO2 emissions. Lightweight tension structures allow the simultaneous implementation of several sustainable strategies by using recyclable low-carbon structural membranes offering a greener alternative to glass and other cladding materials. Their efficient structural load-bearing mechanisms result in significant weight savings in buildings and a drastic reduction of the environmental impact associated with material production, transportation, use, and disposal. A subgroup of lightweight materials, structural fabrics, and foils has been gaining popularity among designers and architects in recent years because of their desirable features such as high stiffness, strength, ductility, durability, and functional properties. While these structural membranes open new crucial perspectives for the clean energy transition and have been recently employed worldwide, their full potential is still limited by the lack of construction codes, advanced optimization tools, and comprehensive viscous-thermo-mechanical constitutive models. This chapter aims to foster the design of membrane structures by presenting their basic principles and recent advancements in the field. It covers the design approaches, employed materials and efforts in their characterization and modeling, implications on the sustainability of the built environment, current challenges, and future pathways from both academic research and engineering design viewpoints.Team Marcel Sluite
Community detection in complex networks, 2015
No full textThis research study has produced advances in the understanding of communities within a complex network. A community in this context is defined as a subgraph with a higher internal density and a lower crossing density with respect to other subgraphs. In this study, a novel and efficient distance-based ranking algorithm called the Correlation Density Rank (CDR) has been proposed and is utilized for a broad range of applications, such as deriving the community structure and the evolution graph of the organizational structure from a dynamic social network, extracting common members between overlapped communities, performance-based comparison between different service providers in a wireless network, and finding optimal reliability-oriented assignment tasks to processors in heterogeneous distributed computing systems. The experiments, conducted on both synthetic and real datasets, demonstrate the feasibility and applicability of the framework
Modeling of Precipitation Sequence and Ageing Kinetics in Al-Mg-Si Alloys
No full textAl-Mg-Si alloys are heat treatable alloys in which strength is obtained by precipitation hardening. Precipitates, formed from a supersaturated solid solution during ageing heat treatment, are GP-zones, B", B´ and B-Mg2Si. Precipitation kinetics and strength vary with alloy composition and process parameters. There is still a need for property and process optimization and therefore to investigate precipitation in this system. Therefore, the general objective is to develop process models to study precipitation kinetics and precipitation sequence in Al-Mg-Si alloys. Precipitates in Al-Mg-Si alloys usually nucleate with a spherical morphology but grow with an elongated shape. An age-hardening model is used to study the effects of morphology of precipitate on the strengthening of Al-Mg-Si alloys (Chapter 2). The results show that age-hardening models, assuming elongated precipitates of constant aspect ratio, do not give an overall better prediction of precipitation and strength evolution during ageing and the assumption of spherical precipitates remains, as a first approximation, an acceptable assumption. The Kampmann-Wagner numerical (KWN) framework, as a base ageing model, is introduced in chapter 3. The KWN model is a method for modelling coupled nucleation, growth, and coarsening. In this method, the precipitate size distribution is simulated using a finite difference method. Current precipitation models, applied to aluminium alloys, usually assume that thermodynamic equilibrium is always fulfilled at the precipitate-matrix interface. This implies the assumption of an infinite interface mobility, which means very fast transformation of matrix to precipitate as soon as the local equilibrium is disturbed by diffusion. The validity of this assumption has been investigated in chapter 4. A modified version of KWN model is introduced in which a mixed-mode growth model has been implemented instead of the only diffusion-controlled growth equation. Using this model a comprehensive systematic study has been done on the effects of diffusivity, and type of precipitate, i.e. interface energy and mobility, on the kinetics and character of precipitation in Al-Mg-Si alloys. The results show that changes in the interfacial energy have almost no effect on the precipitation character. However, changes in diffusivity and interface mobility have significant influence on the character of precipitation. For example, it is shown that there is a certain radius below which precipitation character is always interface controlled. In chapter 5, the complexity of the precipitation sequence and its effect on the precipitation kinetics during ageing has been investigated with a multi-component multi-precipitate model, based on the assumption of maximum Gibbs free energy dissipation. In this modelling framework it is possible to consider simultaneous formation of GP-zones, B'', B', B, and free-Si. The model predicts that a large fraction of nuclei of different precipitate species form during quenching from solutionizing temperature and during heating to ageing temperature. Nucleation is first followed by the growth of the less stable species, which dissolve at some point in favor of more stable precipitates. In the end, only thermodynamically stable precipitates like b and free-Si remain in the alloy. The model also confirms that maximum strength is reached when B'' is the dominant precipitate. The effects of secondary precipitates, induced by interrupted ageing, on the age hardening of Al-Mg-Si alloys have been presented in chapter 6. In the interrupted ageing the alloy is first aged at an elevated temperature (e.g. 170 °C), quenched and then exposed to a lower temperature (e.g. 25-100 °C), and aged again at elevated temperatures. From the results it appears that the influence of secondary precipitates is highly dependent on the interruption temperature. Secondary precipitation stimulated by interruption at temperatures below 50 °C has almost no influence on the alloy strength, while when the interruption temperature is above 50 °C, it increases the hardness significantly. The proposed scenario to explain this behavior is based on the temperature-dependent competitive growth of GP-I and GP-II precipitates. According to this model, interruption temperature below 50 °C stimulates the formation of GP-I zones, which have a very slow kinetics of transformation and therefore they have almost no influence on the mechanical properties. On the other hand, when the alloy is interrupted at temperatures above 50 °C, the formation of GP-II zones is more likely to take place, consequently resulting in the higher density of ?" precipitates during re-ageing and better mechanical properties. Previous precipitation models were linked to a strength model to predict the yield strength evolution during ageing. However, the whole work-hardening behavior is also important for many applications of Al-Mg-Si alloys. In the last chapter, the influence of ageing on work-hardening is investigated from tensile tests. A modified version of Kocks-Mecking-Estrin (KME) model is then employed to simulate the work-hardening response as a function of the precipitation state. Results reveal that underaged material shows a linear decrease of the work-hardening rate with flow stress, while overaged material shows an initial constant work-hardening rate before decreasing linearly. This distinct behavior has been related to the stability of the Orowan loops. In conclusion, the thesis addresses several important issues concerning precipitation and work-hardening behavior of Al-Mg-Si alloys, including precipitates morphology, precipitation sequence, precipitation character, and interrupted ageing. This allows for a better understanding of precipitation sequence, precipitation and hardening kinetics in these alloys. The results of this study can be used for optimization of both chemical composition and ageing parameters in order to achieve desirable microstructure and mechanical properties.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
Application of waste ceramic powder as a cement replacement in reinforced concrete beams toward sustainable usage in construction
No full textThis article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/policies/article-withdrawal). The paper is retracted at the request of the Editor-in-Chief. The Editors no longer have confidence in this paper. Following a complaint received by the editors, it was found upon investigation that 5 citations were added to this paper at the author proof stage. In most cases the added citations were not immediately relevant to the paper topic and in some cases, the added citations appear to benefit co-corresponding authors and a few other related individuals. This is an instance of citation farming (or citation dumping) and is a violation of publication ethics standards. Corresponding author Özkılıç acknowledges adding the citations but disagrees with the retractions. Corresponding author Bahrami disagrees with these retractions. The journal apologises for not identifying this breach during production. © 2024 The AuthorsNajran University, NU; Ministry of Education - Kingdom of Saudi Arabia, MoE, (NU/IFC/2/SERC/-/24); Ministry of Education - Kingdom of Saudi Arabia, Mo
Spark plasma sintering of Stellite®-6 superalloy
No full textThis paper aims at studying microstructure and mechanical properties of spark plasma sintered (SPSed) Stellite®-6 cobalt-based superalloy. SPS is a sintering technique, based on a relatively fast resistance heating using a pulsed current. Fast sintering process, associated with minimum grain growth, results in excellent mechanical properties. Samples were sintered at temperatures ranging from 950 to 1100 °C. Microstructure of samples were studied using scanning electron microscope (SEM), energy-dispersive X-ray spectroscope (EDS), X-Ray diffraction (XRD), and optical microscope. Hardness, impact test, as well as room and high temperature compression tests were used to evaluate the effects of sintering temperature and duration on the mechanical properties of SPSed samples. Results show that optimum mechanical properties can be obtained after sintering at 1050 °C for 10 min. The correlation between sintering parameters, microstructure, and mechanical properties are discussed.Accepted Author ManuscriptElectronic Components, Technology and Material
Non-Intrusive Appliance Load Monitoring using the Viterbi Algorithm (NIALM-VA)
No full textThe goal of Non-Intrusive Appliance Load Monitoring (NIALM), or energy disaggregation, is to deduce which devices are active and how much energy they consume from observation of the time evolution of the total voltage or current in the electrical network. In this thesis, energy disaggregation is performed from the time series of power meter readings, by making use of the fact that different types of appliance can be distinguished by the statistical properties of their signatures, i.e., power consumption behaviour over time. Optimal detectors, using the Viterbi algorithm, are derived for three types of signature: 1) only power levels, no time information, 2) power levels with exponentially distributed lifetimes and 3) power levels with Gaussian lifetimes with given means and variances. The detectors have been implemented in MATLAB, and their performance is compared for various inputs.TelecommunicationsElectrical Engineering, Mathematics and Computer Scienc
A study on the failure of AISI 304 stainless steel tubes in a gas heater unit
No full textThis paper investigates a failure in convection section tubes of a gas heater unit in a petrochemical plant. Tubes are made of AISI 304 stainless steel. The failure is reported after 5 years of service at working temperature 500 °C. The failure is in the form of circumferential cracks in the vicinity of the weld. Various characterization techniques, including optical and electron microscopes as well as energy-dispersive X-ray spectroscopy (EDS), were used to study the failure. Results showed that that the damage has initiated in the heat affected zone (HAZ) area parallel to the weld/base metal interface. Cracks have propagated alongside grain boundaries, resulting in an intergranular fracture. The main cause of failure was concluded to be attributable to the grain boundary sensitization and intergranular grain boundary attack due to improper welding and long time exposure of tubes to high temperature. Possible mitigation strategies to minimize similar failures will be discussed
Creep failure of reformer tubes in a petrochemical plant
No full textThis paper investigates a failure in HP-Mod radiant tubes in a petrochemical plant. Tubes fail after 90,000 h of working at 950 °C. Observed failure is in the form of excessive bulging and longitudinal cracking in reformer tubes. Cracks are also largely branched. The microstructure of service-exposed tubes was evaluated using optical and scanning electron microscopes (SEM). Energy-dispersive X-ray spectroscopy (EDS) was used to analyze and characterize different phases in the microstructure. The results of this study showed that carbides are coarsened at both the inner and the outer surface due to the long exposure to a carburizing environment. Metallography examinations also revealed that there are many creep voids that are nucleated on carbide phases and scattered in between dendrites. Cracks appeared to form as a result of creep void coalescence. Failure is therefore attributed to creep due to a long exposure to a high temperature(OLD) MSE-
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