Eldorado - Repositorium der TU Dortmund

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Bei diesem Service handelt es sich um das Institutionelle Repositorium der Technischen Universität Dortmund. Hier werden Ressourcen aus und für Lehre, Studium und Forschung gespeichert, erschlossen und der Öffentlichkeit zugänglich gemacht.

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Aktuellste Veröffentlichungen

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    Joining-by-hydroforming of aluminum and poly(ether ether ketone)
    (Elsevier BV, 2025-12-30) Weber, Florian; Handge, Ulrich A.; Rakshit, Tanmoy; Dardaei Joghan, Hamed; Hahn, Marlon; Korkolis, Yannis P.; Tekkaya, A. Erman
    Joining-by-hydroforming is a process in which components are joined through expansion under internal pressure. Depending on the required fluid pressure and application rate, this process can be technically demanding and challenging to implement on industrial equipment. To address this, a simplified experimental setup was developed to investigate the fundamental joining mechanisms. In this setup, aluminum 6061-T6 (AA6061-T6) and poly(ether ether ketone) (PEEK) rings are force-fitted using a conical punch and segmented conical expansion elements, enabling controlled radial expansion. The resulting assemblies are subsequently separated in a dedicated push-out test. Experimental results show that the required separation force increases with rising elastic strain in the polymer, attributable to an increase in contact pressure according to Coulomb's friction law. This effect diminishes once plastic deformation of the thermoplastic initiates. Furthermore, stress relaxation in PEEK causes a time-dependent decrease in joint strength, reaching a quasi-equilibrium after approximately 10^4 s, as confirmed by relaxation experiments on PEEK coupons. To analyze thermal effects, the entire ring assembly is preheated to defined temperatures in a laboratory furnace. An inverse correlation between joining temperature and joint strength is observed, consistent with the trend identified in the dynamic-mechanical-thermal analysis (DMTA) of PEEK. The proposed experimental method enables rapid identification of the most influential parameters for joining-by-hydroforming, without requiring dedicated hydroforming equipment or production machine time.
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    Understanding the impact of different nucleation strategies on bis(2-hydroxyethyl) terephthalate crystallization from a glycolysis reaction mixture
    (MDPI AG, 2026-05-22) Seppelfricke, Lukas; Loos, Henning; Sander, Leonard; Möller, Louisa-Marie; Wohlgemuth, Kerstin
    The recycling of polyethylene terephthalate (PET) is gaining increasing importance, as it enables the conversion of plastic waste into valuable raw materials and contributes to a circular economy. Recent research has primarily focused on optimizing the depolymerization step of PET glycolysis, while downstream processes often overlook what are at least equally critical downstream steps in recovering the monomer bis(2-hydroxyethyl) terephthalate (BHET). The implementation of a water-free PET glycolysis process eliminates challenges related to internal solvent and homogeneous catalyst recycling that commonly occur in conventional processes. This study, therefore, focuses on BHET crystallization and filtration as key downstream unit operations. Two nucleation strategies, gassing and seeding, were investigated and compared with experiments without a nucleation strategy. The aim was to achieve reproducible process control during crystallization and to obtain crystals with good filterability, which can be critical for subsequent steps in the product purification process. Experiments without a nucleation strategy showed poor reproducibility. In contrast, gassing and seeding improved crystallization control, particularly regarding nucleation temperature and relative crystallization yield. However, these strategies also resulted in significantly prolonged filtration times due to differences in filter cake properties. The anisotropic crystals exhibited a broad particle size distribution with a high fraction of fine particles, leading to small and heterogeneous pores in the filter cake. Limited crystal growth was identified as the main cause of the unfavorable filtration behavior.
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    A robust test for equal predictive accuracy
    (2026) Demetrescu, Matei; Hanck, Christoph; Hoga, Yannick
    This paper studies what we call a "robust" Diebold–Mariano type test. The unique feature of our test is that - even in the absence of any knowledge of the forecasting method - it is robust to estimation noise in the forecasts, i.e., size is kept irrespective of estimation effects induced by model fitting. We obtain this feature by a test statistic that is based on rolling-window means whose length is a vanishing fraction of the total evaluation sample. This leads to non-standard Gumbel limit laws. Other desirable features of our test are that it is easily robustified against time-varying volatility, and that it naturally uncovers time-varying differences in predictive ability under the alternative. Simulations demonstrate the benefits of our multiply robust implementation vis-à-vis several competitors. An empirical application to forecasts for several variables, horizons, vintages and methods from the Survey of Professional Forecasters illustrates the relevance of the new approach, allowing us to identify forecasters with superior models. Such conclusions are in fact impossible to infer by extant tests, since information on the models and estimation procedures behind the forecasts are typically proprietary and, hence, estimation effects cannot be factored out.
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    Analyzing Paroli and Let-It-Ride Betting Systems: Expected Gain, Variance, and Risk Measures in Roulette
    (2026-06-24) Pflaumer, Peter
    This study presents a comprehensive analysis of the Paroli (reverse Martingale) and Let-It-Ride betting systems in roulette, combining exact analytical formulas, recursive sequence methods, and Monte Carlo simulations to quantify expected gain, variance, and normalized risk measures. We derive closed-form expressions for net gain, total wager, and associated risk metrics, enabling systematic comparisons across sequences and bet sizes. A key theoretical insight is that Paroli constitutes a mathematical mirror of the classical Martingale system: their performance formulas are related by swapping win and loss probabilities. In the fair-game limit, Paroli and Martingale are statistically equivalent, with identical moments, differing only in whether bets increase after wins or losses. Extending this framework to Let-It-Ride variants, we analyze covariances and correlations between gain and total wager to characterize the risk structure of each system. Sequence-based tables illustrate outcome distributions, while simulations validate analytical results and highlight comparative volatility. Our findings confirm that, although Paroli and Let-It-Ride strategies can generate large individual gains, the underlying house edge remains unchanged.
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