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    15226 research outputs found

    Comparison of high-temperature compression and compression-compression fatigue behavior of magnesium alloys DieMag422 and AE42

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    Due to their high strength-to-weight-ratio, magnesium alloys are very attractive for use in automotive engineering. For application at elevated temperatures, the alloys must be creep-resistant. Therefore, the influence of the operating temperature on the material properties under quasistatic and cyclic load has to be understood. A previous study investigated tensile-tensile fatigue behavior of the magnesium alloys DieMag422 and AE42 at room temperature (RT). The aim of this study was the comparison of both alloys regarding compression, tensile, and compression-compression fatigue behavior. The quasistatic behavior was determined by means of tensile and compression tests, and the tensile-compression asymmetry was analyzed. In temperature increase fatigue tests (TIFT) and constant amplitude tests (CAT), the temperature influence on the cyclic creeping (ratcheting) behavior was investigated, and mechanisms-relevant test temperatures were determined. Furthermore, characteristic fracture mechanisms were evaluated with investigations of the microstructure and the fracture surfaces. The initial material was analyzed in computed tomographic scans and energy dispersive X-ray (EDX) analyses

    An asymptotic test for constancy of the variance under short-range dependence

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    We present a novel approach to test for heteroscedasticity of a non-stationary time series that is based on Gini's mean difference of logarithmic local sample variances. In order to analyse the large sample behaviour of our test statistic, we establish new limit theorems for U-statistics of dependent triangular arrays.We derive the asymptotic distribution of the test statistic under the null hypothesis of a constant variance and show that the test is consistent against a large class of alternatives, including multiple structural breaks in the variance. Our test is applicable even in the case of non-stationary processes, assuming a locally stationary mean function. The performance of the test and its comparatively low computation time are illustrated in an extensive simulation study. As an application, we analyse data from civil engineering, monitoring crack widths in concrete bridge surfaces

    Optimal control of perfect plasticity Part I: Stress tracking

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    The paper is concerned with an optimal control problem governed by the rate-independent system of quasi-static perfect elasto-plasticity. The objective is to optimize the stress field by controlling the displacement at prescribed parts of the boundary. The control thus enters the system in the Dirichlet boundary conditions. Therefore, the safe load condition is automatically fulfilled so that the system admits a solution, whose stress field is unique. This gives rise to a well defined control-to-state operator, which is continuous but not Gˆateaux-differentiable. The control-to-state map is therefore regularized, first by means of the Yosida regularization and then by a second smoothing in order to obtain a smooth problem. The approximation of global minimizers of the original non-smooth optimal control problem is shown and optimality conditions for the regularized problem are established. A numerical example illustrates the feasibility of the smoothing approach

    Benchmarking and Validation of a Combined CFD-Optics Solver for Micro-Scale Problems

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    In this work we present a new approach for coupled CFD-Optics problems that consists of a combination of a Finite Element Method (FEM) based flow solver with a ray tracing based tool for optic forces that are induced by a laser. This is a setup that is mainly encountered in the field of optical traps. We combined the open-source computational fluid dynamics (CFD) package FEATFLOW with the raytracing software of the LAT-RUB with this task in mind. We benchmark and analyze the solver first based on a configuration with a single spherical particle that is subjected to the laser forces of an optical trap. The setup is based on an experiment which is then compared to the results of our combined CFD-Optics solver. As an extension of the standard procedure to simulate such problems we present a method with a time-stepping scheme that contains a macro step approach. The results showthat this macro time-stepping scheme provides a significant acceleration of the standard procedure while still maintaining good accuracy. A second configuration is analyzed that involves non-spherical geometries such as micro rotors. We describe a procedure that is able to efficiently and accurately calculate optical forces with surface triangulations as input geometries. Then we proceed to compare simulation results of the final angular velocity of the micro rotor with experimental measurements

    Phosphoinositide 3-kinase inhibitor AS605240 ameliorates streptozotocin-induced Alzheimer’s disease like sporadic dementia in experimental rats

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    The quest for chemical entities able to curb the action of the phosphoinositide 3-kinase, (PI3K)/protein kinase B (AKT) signaling pathways is evolving as a potential therapeutic strategy for the treatment and/or prevention of neurodegenerative disorders including Alzheimer’s disease (AD). In this study, the effects of a PI3K inhibitor, AS605240 on cognitive dysfunction and antioxidative defense parameters against intra-cerebroventricular-streptozotocin (ICV-STZ)-induced rat model of sporadic AD was evaluated. ICV administration of a single dose of STZ (3 mg/kg) was performed to induce behavioral and biochemical changes in rats using the stereotaxic technique. Animals were administered with varying doses of AS605240 (5, 10 and 15 mg/kg) orally, 1 h before ICV-STZ on day 1 and continued once daily for four weeks. The behavioral parameters (passive avoidance and Morris water maze), antioxidative defense parameters, amyloid-beta (Aβ) protein expression by Western blotting and immunostaining technique were estimated in brain tissue. AS605240 dose-dependently and significantly (p < 0.05 and p < 0.01 and p < 0.001) improved ICV-STZ-induced cognitive impairment and attenuated the altered antioxidative related parameters including superoxide dismutase, lipid peroxidation, glutathione and nitrite levels. Further, the increased Aβ protein expression levels in brain tissue were markedly restored with AS605240 treatment. In conclusion, our study demonstrated that AS605240 exhibited immense potential in attenuating STZ-induced sporadic AD features in rats and may be developed as a therapeutic agent in the treatment and management of sporadic AD

    Tätigkeitsbericht 2019

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    Amtliche Mitteilungen der Technischen Universität Dortmund Nr. 7/2020

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    Amtliche Mitteilungen der Technischen Universität Dortmund Nr. 6/2020

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    An algebraic flux correction scheme facilitating the use of Newton-like solution strategies

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    Building on recent advances in the analysis and design of algebraic flux correction (AFC) schemes, new differentiable limiter functions are constructed for efficient nonlinear solution strategies. The proposed scaling parameters are used to limit artificial diffusion operators incorporated into the residual of a high order target scheme to produce accurate and bound-preserving finite element approximations to hyperbolic problems. Due to this stabilization procedure, the occurring system becomes highly nonlinear and the efficient computation of corresponding solutions is a challenging task. The presented regularization approach makes the AFC residual twice continuously differentiable so that Newton’s method converges quadratically for suÿciently good initial guesses. Furthermore, the performance of each nonlinear iteration is improved by expressing the Jacobian as the sum and product of matrices having the same sparsity pattern as the Galerkin system matrix. Eventually, the AFC methodology constructed for stationary problems is extended to transient test cases and validated numerically by applying it to several numerical benchmarks

    Tests based on sign depth for multiple regression

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    The extension of simplicial depth to robust regression, the so-called simplicial regression depth, provides an outlier robust test for the parameter vector of regression models. Since simplicial regression depth often reduces to counting the subsets with alternating signs of the residuals, this led recently to the notion of sign depth and sign depth test. Thereby sign depth tests generalize the classical sign tests. Since sign depth depends on the order of the residuals, one generally assumes that the D-dimensional regressors (explanatory variables) can be ordered with respect to an inherent order. While the one-dimensional real space possesses such a natural order, one cannot order these regressors that easily for D > 1 because there exists no canonical order of the data in most cases. For this scenario, we present orderings according to the Shortest Hamiltonian Path and an approximation of it. We compare them with more naive approaches like taking the order in the data set or ordering on the basis of a single quantity of the regressor. The comparison bases on the computational runtime, stability of the order when transforming the data, as well as on the power of the resulting sign depth tests for testing the parameter vector of different multiple regression models. Moreover, we compare the power of our new tests with the power of the classical sign test and the F-test. Thereby, the sign depth tests based on our distance based approaches show similar power as the F-test for normally distributed residuals with the additional benefit of being much more robust against outliers

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