Formalization of Workflows and Correctness Issues in the Presence of Concurrency

In this paper, main components of a workflow system that are relevant to the correctness in the presence of concurrency are formalized based on set theory and graph theory. The formalization which constitutes the theoretical basis of the correctness criterion provided can be summarized as follows:-Activities of a workflow are represented through a notation based on set theory to make it possible to formalize the conceptual grouping of activities.-Control-flow is represented as a special graph based on this set definition, and it includes serial composition, parallel composition, conditional branching, and nesting of individual activities and conceptual activities themselves.-Data-flow is represented as a directed acyclic graph in conformance with the control-flow graph.The formalization of correctness of concurrently executing workflow instances is based on this framework by defining two categories of constraints on the workflow environment with which the workflow instances and their activities interact. These categories are:-Basic constraints that specify the correct states of a workflow environment.-Inter-activity constraints that define the semantic dependencies among activities such as an activity requiring the validity of a constraint that is set or verified by a preceding activity.Basic constraints graph and inter-activity constraints graph which are in conformance with the control-flow and data-flow graphs are then defined to represent these constraints. These graphs are used in formalizing the intervals among activities where an inter-activity constraint should be maintained and the intervals where a basic constraint remains invalid.A correctness criterion is defined for an interleaved execution of workflow instances using the constraints graphs. A concurrency control mechanism, namely Constraint Based Concurrency Control technique is developed based on the correctness criterion. The performance analysis shows the superiority of the proposed technique. Other possible approaches to the problem are also presented.     

Einfluss und Bedeutung chemischer Entmischungen auf mechanische Eigenschaften thixogeformter Bauteile

Chemical Segregation Effect on Mechanical Properties of Thioxo Formed Components Thixoforming is a near net shape forming process of semi solid billets with a stiffness, which is comparable to “butter”. Advantages of conventional forming and casting processes can be combined due to this modern forming process technology. The objective of this investigation is to identify the effect of local chemical segregation regarding the microstructure and mechanical properties in various parts of a component. Different components and component regions – with diverse cross‐sections and flow lengths – have been tested. Tensile, charpy and fatigue tests are completed by metallography and local chemical analysis. The investigation was focused on AlSi7Mg0.3 and AlSi6Cu3 aluminium casting alloys. It can be confirmed, that chemical composition of the raw material and segregation are important effects on mechanical properties of components. Chemical segregation can be caused by abrupt changes in cross‐section. The inhomogeneity verified by chemical analysis and metallography are important effects on mechanical properties of Thixoforming components. In future the counteract between metal structure and processing steps regarding mechanical properties of components must be examined more intensively. The aim is to create a fundamental understanding of the relationship between the structure, chemical composition, homogeneity and characteristics of aluminium materials.     

Evaluation of hybrid solar-wind-hydrogen energy system based on methanol electrolyzer

In this study, it is aimed to meet the annual electricity and heating needs of a house without interruption with the photovoltaic panel, wind turbine, methanol electrolyzer, and high temperature proton exchange membrane fuel cell system. The system results show that the use of the 2 WT with 18 PV was enough to provide the need of the methanol electrolyzer, which provides requirements of the high temperature proton exchange membrane fuel cell. The produced heat by the fuel cell was used to meet the heat requirement of the house with combined heat and power system. Electrical, thermal and total efficiencies of fuel cell system with combined heat and power were obtained as 38.54%, 51.77% and 90%, respectively. Additionally, the levelized cost of energy of the system was calculated as 0.295 $/kWh with combined heat and power application. The results of this study show that H₂ is useful for long-term energy storage in off-grid energy systems and that the proposed hybrid system may be the basis for future H₂-based alternative energy applications.     

Analytical models for high performance milling. Part II: Process dynamics and stability

Chatter is one of the most important limitations on the productivity of milling process. In order to avoid the poor surface quality and potential machine damage due to chatter, the material removal rate is usually reduced. The analysis and modeling of chatter is complicated due to the time varying dynamics of milling chatter which can be avoided without sacrificing the productivity by using analytical methods presented in this paper.     

Identification and modeling of process damping in turning and milling using a new approach

Process damping can be a significant source of increased stability in machining particularly at low cutting speeds. However, it is usually ignored in chatter analysis as there is no model available to estimate process damping coefficients. In this study, a practical identification and modeling method is presented where process damping coefficients are obtained from chatter tests. The method is generalized by determining the indentation force coefficient responsible for the process damping through energy analysis. This coefficient is then used for process damping and the stability limit prediction in different cases, and predictions are verified by time domain simulations and experimental results.     

Prediction of workpiece dynamics and its effects on chatter stability in milling

The workpiece dynamics affect stability in machining of flexible parts. However, it is not a straightforward task to include it in the analysis since the workpiece dynamics continuously change due to mass removal and variation of the cutter contact. In this paper, a methodology for prediction of in-process workpiece dynamics is presented, which is based on a structural dynamic modification using the FE model of the workpiece. The cutter location (CL) file is used to determine the removed elements at each tool location along a cycle. The proposed approach is demonstrated on example cases, and simulations are verified through experiments.     

Peripheral milling conditions for improved dimensional accuracy

Peripheral milling with flexible helical cutters is analyzed and modelled. The end mill is modelled as a cantilevered beam clamped to the collet with linear springs. Cutting forces and resulting tool deflection marks on the surface are analytically expressed. It is shown that by proper selection of cutting conditions, the material removal rate can be increased significantly without sacrificing the dimensional accuracy of the finished product. A method of identifying optimal feedrate and width of cut for given cutter dimensions and cutting constants is presented with experimental verification.