Universität Siegen
  • Siegen, NRW, Germany
Recent publications
A suitable combination of different methods and measurement techniques is often required for Structural Health Monitoring (SHM) of objects in situ. In the field of aerospace applications, the current research project “Combined acoustic and modal structure monitoring” is dealing with the development of a robust SHM system for damage identification in carbon-fiber-reinforced polymer (CFRP) structures under realistic and varying loads. The methods combined within the project are based on guided waves, acoustic emission, and different vibration monitoring techniques. The present paper deals with the application of the nullspace-based fault detection algorithm (NSFD) as a part of a holistic concept for the monitoring of an aircraft door surround structure. Due to the great sensitivity to changes in the statistical properties of the measured data, the algorithm reacts very sensitively to structural changes, but also to changes in the environmental conditions, such as changes in the external loads. In this paper, the impact of changing excitation conditions on the NSFD damage indicator is analyzed. Therefore, two different formulations for the NSFD algorithm are studied and compared concerning their sensitivity and robustness against changes of the external loads. Since the sensitivity and the results depend also on the algorithm set-up parameters, these are analyzed and taken into account. The evaluation of the results considers both, the robustness to the influence of the varying excitation and the sensitivity to damages in the analyzed structure. The theory and the algorithms are successfully tested with measured data sets from a CFRP-airplane structure.
The functionality of bolted connections depends significantly on the conservation of the bolt’s preload, which entails the necessity to monitor these components. Traditional monitoring techniques using electro-mechanical impedance spectra are based on the observation of the structural resonances and antiresonances. The change of structural resonances is clearly mirrored for example in the resistance spectrum. This paper introduces a novel approach for such a monitoring process utilizing the susceptance. The susceptance is the imaginary part of the impedance’s reciprocal. In an experimental setup, where piezoelectric wafer active sensors have been attached to the bolt’s head, different preload stages of these components were set. It has been observed that the elastic mechanical deformation of the bolt’s head, which is caused by the preload, affects the piezoelectric wafer active sensor. As the properties of the sensor are well pronounced especially in the susceptance spectrum, a condition index based on the susceptance is proposed. Usually varying ambient temperature can occur in practical applications of bolted connections. It needs to be differentiated from the changes of the preload conditions in order to avoid false alarm of the monitoring system. Therefore, the proposed condition index has been additionally investigated in dependence on varying ambient temperature. The experiments show very promising results.
We apply methods from algebraic geometry to study uniform matrix product states. Our main results concern the topology of the locus of tensors expressed as uMPS, their defining equations and identifiability. By an interplay of theorems from algebra, geometry and quantum physics we answer several questions and conjectures posed by Critch, Morton and Hackbusch.
When it comes to investigating how customers use POS technologies or which factors might promote their adoption, it is important to know which characteristics are particularly relevant from the customers' perspective. Based on three recent approaches to POS technology systematization, this research empirically categorizes POS technologies from the customer's point of view. We investigated POS technologies that are information-oriented and have a certain form of interaction capability. First, we selected seven information-oriented POS technologies for the research. An online study (N = 830) was conducted on POS technologies that were used in a multidimensional scaling (MDS) analysis. As a result, three clusters of POS technologies were identified: (A) comprehendible POS technologies, (B) information integration POS technologies, and (C) entertainment-related POS technologies. Our findings yield important insights for retailers with regard to the customers' perception and qualitative evaluation of the POS technologies in each of the identified clusters.
The accurate simulation of additional interactions at the ATLAS experiment for the analysis of proton–proton collisions delivered by the Large Hadron Collider presents a significant challenge to the computing resources. During the LHC Run 2 (2015–2018), there were up to 70 inelastic interactions per bunch crossing, which need to be accounted for in Monte Carlo (MC) production. In this document, a new method to account for these additional interactions in the simulation chain is described. Instead of sampling the inelastic interactions and adding their energy deposits to a hard-scatter interaction one-by-one, the inelastic interactions are presampled, independent of the hard scatter, and stored as combined events. Consequently, for each hard-scatter interaction, only one such presampled event needs to be added as part of the simulation chain. For the Run 2 simulation chain, with an average of 35 interactions per bunch crossing, this new method provides a substantial reduction in MC production CPU needs of around 20%, while reproducing the properties of the reconstructed quantities relevant for physics analyses with good accuracy.
The goal of this study was to examine whether a subject’s emotional competence correlates to attachment styles and parenting styles in children and their parents. The study was conducted with fifty children (9–11 years old) and their parents, both of whose emotional competence (EKF) and parenting style (PAQ) were measured. The attachment styles of parents and children were measured with the Adult Attachment Scale (AAS) and the Bochumer Bindungstest (BoBiTe), respectively. The findings provide initial support to the assumption that attachment is related to emotional competence in parents. This relationship, however, was not significantly correlated in children. In addition, authoritative parenting and permissive parenting were significantly associated with emotional competence in parents. Emotional competence in children showed to be associated with an authoritative parenting style.
Surface atomic structures of transition metal phosphides determine the electrocatalytic activities toward hydrogen evolution reaction (HER). In this work, we developed a new facet-guided phosphorization strategy to synthesize Rh2P electrocatalysts with tunable facet exposure and further unveil their intrinsic facet-dependent HER activity in pH-universal media. The Rh2P nanoparticles feature {1 1 1} and {2 0 0} facets when the precursors of Rh nanoctahedron and nanocube are used, respectively. The Rh2P{2 0 0} catalyst showed a remarkable HER performance with the overpotential of 23, 22 and 54 mV at a current density of 10 mA cm⁻² in 1.0 M KOH, 0.5 M H2SO4 and 1.0 M phosphate buffer, respectively. The mass activity and specific activity of Rh2P{2 0 0} were 3~5-times higher than those of Rh2P{1 1 1} and Pt/C catalysts. As elucidated using theoretical calculations, the overlapping of Rh-4d and P-3p of Rh2P{2 0 0} and Rh2P{1 1 1} results in distinct electronic states of surface Rh atoms, leading to varied free energies for water dissociation and hydrogen adsorption/desorption.
Metacognition is a well-researched construct important to successful learning. Recent studies show that state-level metacognition regarding self-control conflicts is also important for successfully resolving these conflicts. Because there exists no scale to assess trait-level metacognition in self-control and because of limitations of commonly used measures in self-control research, we adapted a scale that is widely used to assess trait-level metacognition in self-regulated learning, the Metacognitive Awareness Inventory (MAI). In two studies (N = 315 and N = 503), we constructed the 12-item Metacognition in Self-Control Scale (MISCS), which loaded on the two factors metacognitive knowledge and metacognitive regulation. The MISCS showed a good fit with good internal consistencies. In the 10-day experience sampling part of study 2, which included 9639 reports of self-control conflicts, higher trait-levels of metacognition as measured with the MISCS predicted higher state-levels of success in resolving these conflicts, as well as higher state-levels of the subcomponents of metacognition, namely metacognitive knowledge, planning, monitoring, and evaluation. Most of these associations persisted when controlling for trait self-control, supporting the usefulness of the scale beyond the most commonly used scale in self-control research. The MISCS showed adequate test-retest reliability. Correlations with other scales, limitations, and future directions are discussed.
Thermodynamic modelling is an effective approach to accelerate the development of novel materials such as V-based alloys. In the present work, the results of thermodynamic modelling of the ternary system V–Ti–B using the “CALculation of PHAse Diagrams” (CALPHAD) method are presented. The thermodynamic descriptions of the binary systems V–Ti, V–B, and Ti–B are based on available publications whereas thermodynamic parameters for the ternary system stem from the iterative approximation to the experimental investigations, which further serve for verifying the calculated phase diagram. Thus, eight different alloys in the compositional range of 10–40 at.% Ti and B were analyzed aiming at identifying the evolving phases. A set of thermodynamic parameters is proposed. The calculated isothermal phase diagram at 1673 K exhibits a high degree of agreement with the literature and experimental observations. In our further work, the thermodynamic data generated in this study will be useful to develop the quaternary V–Ti–Si–B system.
The present study reports on the existence of a new ternary phase, V8SiB4, in the V–Si–B system. The new phase was found in alloys heat-treated at 1400 °C for 100 h and 200 h within the Vss–V3Si–V5SiB2 phase field at 1600 °C. The crystal structure of V8SiB4 was determined by combining energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and density functional theory (DFT) calculations. To further examine the stability of V8SiB4, electronic density-of-states (EDOS), phononic density-of-states (PDOS), the chemical bonding and the elastic properties of V8SiB4 were calculated using DFT and compared with the properties of V5SiB2 (T2).
A novel three-dimensional (3-D) dynamic model considering nonlinearity of tension is proposed to explore in-line (IL), cross-flow (CF) and axial (AX) vortex-induced vibration (VIV) responses of riser under platform motion. The interaction between riser and external flow is simulated by the van der Pol equation, and the nonlinear equations of motion are established by Hamilton's principle and the Galerkin method, and solved by Runge-Kutta method. The influences of heave and swing motions of platform on VIV responses of the riser are investigated. It is found that for large velocity of external flow, the axial displacement of riser results in an increase in IL excitation mode and also an increase in IL oscillating displacement. When the riser is excited by swing motion of platform, with the increase of excitation amplitude, the maximum response amplitudes will also increase in all the three directions, and the amplitudes in high frequency domain increase more obviously. Combined effect of heave and swing motions of the platform results in the increase of VIV amplitudes in IL and AX directions, and combined action of forced and parametric excitations has little effect on response amplitude of riser in CF direction.
By the stiffness matrix method based on the nonlocal elastic continuum theory, the transmission spectra of the transverse elastic waves propagating normally in nanoscaled periodic piezoelectric laminates with defects are investigated in detail in this paper. Defect is introduced by replacing a constituent layer or inserting another layer in a unit-cell. The thicknesses of all sub-layers and defect layer are at the nanoscale. Simultaneously, complex band structures and localization factors are calculated and employed to verify the band structures described by the transmission spectra. The influences of the defect type, the defect material, the volume fraction of the defect layer in the unit-cell, the piezoelectric constants of the defect layer, and the nanoscale size-effect on the transmission spectra are analyzed and discussed in detail. The present study provides a theoretical guide for the design and applications of the nanoscaled acoustic filters or transducers with defects.
In this paper, the photo-thermoelastic wave propagation analysis in a semiconductor nanorod resonator under laser excitation employing the strain-gradient Moore–Gibson–Thompson (MGT) and Love–Bishop theories is presented, which is for the first time to the authors’ knowledge. The governing equations of the photo-thermoelastic wave propagation are derived using the proposed novel size-dependent MGT heat conduction model, strain-gradient and Love–Bishop theories. The size-dependent MGT coupled photo-thermoelasticity theory is developed by taking into account of five nano-scale parameters for the first time, which is employed to derive the governing equations. The governing equations are converted into the Laplace-transformed domain and then analytical solution is obtained for a semiconductor Love–Bishop nanorod resonator subjected to plasma and thermal shock loadings. To obtain the transient field variables in the time-domain, the Talbot Laplace-inversion technique is employed. The size-effects on the propagation of the photo-thermoelastic waves are studied in detail. The transient behaviors of the displacement, temperature and carrier density (plasma density) fields are also investigated with considering the nano-scale effects by performing the parametric studies. The effects of the carrier density, the relaxation time in the size-dependent MGT theory and the nano-scale parameters on the plasma-affected photo-thermoelastic wave propagation are revealed. Numerical examples demonstrate that the derived governing equations and also the proposed analytical solutions can be employed to determine the realistic behaviors of the field variables in a semiconductor Love–Bishop nanorod resonator under laser shock loading with considering the nano-scale effects.
This paper investigates the efficiency-restoring (= Pigovian) taxation of emissions in economies with pollution damage and consumers who are morally motivated to reduce their emissions and who are heterogeneous with respect to their degree of morality or other preference parameters. If the conventional concept of efficiency is applied that relates to the maximum utilitarian welfare disregarding the consumers’ morality, we show that the Pigovian tax is consumer-type specific and smaller than the uniform Pigovian tax in economies with consumers without moral preferences. Since ‘personalized’ Pigovian taxes cannot be implemented in practice due to informational infeasibility, we also characterize the second-best uniform emissions tax. Finally, we investigate the Pigovian taxation when the emissions tax crowds out the moral consumers’ voluntary incentives to reduce emissions.
Conventional two-dimensional (2D) re-entrant auxetic honeycomb is one of the earliest examples of auxetic metamaterials. Recently, plenty of 2D re-entrant honeycomb variants are developed to enhance their stiffness. However, the stiffness enhancement for most of the existing designs is generally obtained with compromising the conjugated auxeticity. Moreover, most of the existing designs are hard to be fabricated by low-cost traditional manufacturing technology due to their complex architectures. The present work aims to enhance the stiffness without compromising the conjugated auxeticity, and yet retains its convenience and low-cost for fabrication. By introducing zigzag inclined ligaments, a novel 2D re-entrant auxetic honeycomb metamaterial is firstly proposed. Based on Castigliano’s second theorem, a theoretical model of the proposed design is established to facilitate the understanding of the underlying microstructural mechanisms. The obtained analytical solutions, as validated by systematic finite element (FE) analyses, elucidate different roles of the microstructural geometry on the effective mechanical properties of the proposed re-entrant auxetic honeycomb metamaterial. Compared with the conventional re-entrant auxetic honeycomb, which is a particular case of the present design, both the stiffness and the auxeticity along specific principal directions of the present design are improved remarkably.
Purpose Small photon beams used in radiotherapy techniques have inherent characteristics of charge particle disequilibrium and high-dose gradient making accurate dosimetry for such fields very challenging. By means of a 3D manufacturing technique, it is possible to create arrays of pixels with a very small sensitive volume for radiotherapy dosimetry. We investigate the impact of 3D pixels size on absorbed dose sensitivity, linearity of response with dose rate, reproducibility and beam profile measurements. Methods Diamond detectors with different pixel sizes have been produced in the 3DOSE experiment framework. To investigate the pixels size impact, they were tested using an Elekta Synergy LINAC. Dose rate dependence, absorbed dose sensitivity, reproducibility and beam profile measurement accuracy have been investigated and compared with PTW 60019 and IBA SFD reference dosimeters. Results All of the 3D pixels had a linear and reproducible response to the dose rate. The sensitivity of a pixel decreases with its size, although even the smallest pixel has a high absorbed dose sensitivity (15 nC/Gy). The penumbra width measured with the smallest pixel size was consistent with the PTW microDiamond and differed by 0.2 mm from the IBA SFD diode. Conclusions The study demonstrates that variation in pixel size do not affect the linearity of response with dose rate and the reproducibility of response. Due to the 3D geometry, the absorbed dose sensitivity of the detector remains high even for the smallest pixel, furthermore the pixel size was demonstrated to be of fundamental importance in the measurement of beam profiles.
The trends in the evolution of the production structure of the automotive industry and the development of alternative drive concepts are accompanied by the fact that comparatively smaller batch sizes are produced for a large variety of models. These trends require production techniques to meet the demand of flexibility. The requirements for manufacturing processes, especially for forming processes, are low-tool production, fast set-up, and quick product changes. 3D-swivel-bending is a developed manufacturing technique to produce cross-section and load-adapted sheet metal parts and profile components. Such components are used in lightweight structures, often found in developments in automotive and aerospace industries.
Planning a manufacturing factory involves a high number of planners who deliver their domain-specific planning information. In Building Information Modeling, a central entity needs to check the information from different planning domains for coherence to each other. Especially for non-geometrical information, this is a manual, time-consuming and error-prone approach. The paper at hand therefore aimed at developing an expert system that automates the decision making in the checking process of factory planning information. On the basis of a case study, we have applied knowledge engineering methods from the Semantic Web and calculated an F1 score for validating our approach.
Profiles with variable cross-sections allow for a load-adapted design of components. Using a newly developed bending process, new lightweight construction potentials can be exploited that were not feasible with the current design restrictions. This helps to save material costs as well as other resources. To bend profiles with a cross-section that varies along the longitudinal axis, it was necessary to develop a new type of bending process. The already established three-roll-push-bending process was further developed so that the individual tool positions in the process can be adapted to the current profile cross-section.
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4,853 members
Volker Michel
  • Department of Mathematics
Goutam Das
  • Workgroup of Theoretical Physics 1 (Particle Physics)
Heiner Stahl
  • Department of History
Tobias Michael Scholz
  • Chair for Human Resource Management and Organizational Behavior
Kristof Van Laerhoven
  • Department of electrical engineering and computer science
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