Showing papers by "Bauhaus University, Weimar published in 2013"

Molecular dynamics simulations of single-layer molybdenum disulphide (MoS2): Stillinger-Weber parametrization, mechanical properties, and thermal conductivity

Abstract: We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer MoS2 (SLMoS2). The potential parameters are fitted to an experimentally obtained phonon spectrum, and the resulting empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal conductivity of infinite, periodic SLMoS2.

Finite strain fracture of plates and shells with configurational forces and edge rotations

Abstract: We propose a simple and efficient algorithm for FEM-based computational fracture of plates and shells (cf. [1]) with both brittle and ductile materials based on edge rotation and load control. Rotation axes are the crack front nodes and each crack front edge in surface discretizations affects the position of only one or two nodes. Modified positions of the entities maximize the mesh quality complying with the predicted crack path (which depends on the specific propagation theory in use). Compared with XFEM or with classical tip remeshing, the proposed solution has algorithmic and generality advantages. The propagation algorithm is simpler than the aforementioned alternatives and the approach is independent of the underlying element used for discretization. For history-dependent materials, there are still some transfer of relevant quantities between elements. However, diffusion of results is more limited than with tip or full remeshing. To illustrate the advantages of our approach, three prototype models are used: tip energy dissipation (LEFM), cohesive-zone approaches and ductile fracture. Both the Sutton crack path criterion and the path estimated by the Eshelby tensor are employed. Traditional fracture benchmarks, including one with plastic hinges, and newly proposed verification tests are solved. These were found to be very good in terms of crack path and load/deflection accuracy.

Immersive Group-to-Group Telepresence

Abstract: We present a novel immersive telepresence system that allows distributed groups of users to meet in a shared virtual 3D world. Our approach is based on two coupled projection-based multi-user setups, each providing multiple users with perspectively correct stereoscopic images. At each site the users and their local interaction space are continuously captured using a cluster of registered depth and color cameras. The captured 3D information is transferred to the respective other location, where the remote participants are virtually reconstructed. We explore the use of these virtual user representations in various interaction scenarios in which local and remote users are face-to-face, side-by-side or decoupled. Initial experiments with distributed user groups indicate the mutual understanding of pointing and tracing gestures independent of whether they were performed by local or remote participants. Our users were excited about the new possibilities of jointly exploring a virtual city, where they relied on a world-in-miniature metaphor for mutual awareness of their respective locations.

Computational Methods for Fracture in Brittle and Quasi-Brittle Solids: State-of-the-Art Review and Future Perspectives

Abstract: An overview of computational methods to model fracture in brittle and quasi-brittle materials is given. The overview focuses on continuum models for fracture. First, numerical difficulties related to modelling fracture for quasi-brittle materials will be discussed. Different techniques to eliminate or circumvent those difficulties will be described subsequently. In that context, regularization techniques such as nonlocal models, gradient enhanced models, viscous models, cohesive zone models, and smeared crack models will be discussed. The main focus of this paper will be on computational methods for discrete fracture (discrete cracks). Element erosion technques, inter-element separation methods, the embedded finite element method (EFEM), the extended finite element method (XFEM), meshfree methods (MMs), boundary elements (BEMs), isogeometric analysis, and the variational approach to fracture will be reviewed elucidating advantages and drawbacks of each approach. As tracking the crack path is of major concern in computational methods that preserve crack path continuity, one section will discuss different crack tracking techniques. Finally, cracking criteria will be reviewed before the paper ends with future research perspectives.

Elastic bending modulus of single-layer molybdenum disulfide (MoS2): finite thickness effect

Abstract: We derive, from an empirical interaction potential, an analytic formula for the elastic bending modulus of single-layer MoS2 (SLMoS2). By using this approach, we do not need to define or estimate a thickness value for SLMoS2, which is important due to the substantial controversy in defining this value for two-dimensional or ultrathin nanostructures such as graphene and nanotubes. The obtained elastic bending modulus of 9.61 eV in SLMoS2 is significantly higher than the bending modulus of 1.4 eV in graphene, and is found to be within the range of values that are obtained using thin shell theory with experimentally obtained values for the elastic constants of SLMoS2. This increase in bending modulus as compared to monolayer graphene is attributed, through our analytic expression, to the finite thickness of SLMoS2. Specifically, while each monolayer of S atoms contributes 1.75 eV to the bending modulus, which is similar to the 1.4 eV bending modulus of monolayer graphene, the additional pairwise and angular interactions between out of plane Mo and S atoms contribute 5.84 eV to the bending modulus of SLMoS2.

A Phantom-Node Method with Edge-Based Strain Smoothing for Linear Elastic Fracture Mechanics

Abstract: This paper presents a novel numerical procedure based on the combination of an edge-based smoothed finite element (ES-FEM) with a phantom-node method for 2D linear elastic fracture mechanics. In the standard phantom-node method, the cracks are formulated by adding phantom nodes, and the cracked element is replaced by two new superimposed elements. This approach is quite simple to implement into existing explicit finite element programs. The shape functions associated with discontinuous elements are similar to those of the standard finite elements, which leads to certain simplification with implementing in the existing codes. The phantom-node method allows modeling discontinuities at an arbitrary location in the mesh. The ES-FEM model owns a close-to-exact stiffness that is much softer than lower-order finite element methods (FEM). Taking advantage of both the ES-FEM and the phantom-node method, we introduce an edge-based strain smoothing technique for the phantom-node method. Numerical results show that the proposed method achieves high accuracy compared with the extended finite element method (XFEM) and other reference solutions.

A case study on automated safety compliance checking to assist fall protection design and planning in building information models

Abstract: Worldwide occupational safety statistics show that the construction industry in many countries experiences one of the highest accident rates of all industry sectors. Falls remain a major concern as they contribute to very serious injuries or even fatalities on construction projects around the world. Since the standards and rules for protective safety equipment vary by country, the growing numbers of internationally operating companies are in need of tools that allow ubiquitous understanding and planning of safety regardless of the country where they operate. The problem is examined using a customizable automatic safety rule-checking platform for building information models. The applied rule-based checking algorithms are designed to be add-ons to existing building information modelling (BIM) software and can check models for safety hazards early in the design and planning process. Once hazards have been identified preventative safety equipment can be designed, estimated, and included in the construction sc...

Paraphrase acquisition via crowdsourcing and machine learning

Abstract: To paraphrase means to rewrite content while preserving the original meaning. Paraphrasing is important in fields such as text reuse in journalism, anonymizing work, and improving the quality of customer-written reviews. This article contributes to paraphrase acquisition and focuses on two aspects that are not addressed by current research: (1) acquisition via crowdsourcing, and (2) acquisition of passage-level samples. The challenge of the first aspect is automatic quality assurance; without such a means the crowdsourcing paradigm is not effective, and without crowdsourcing the creation of test corpora is unacceptably expensive for realistic order of magnitudes. The second aspect addresses the deficit that most of the previous work in generating and evaluating paraphrases has been conducted using sentence-level paraphrases or shorter; these short-sample analyses are limited in terms of application to plagiarism detection, for example. We present the Webis Crowd Paraphrase Corpus 2011 (Webis-CPC-11), which recently formed part of the PAN 2010 international plagiarism detection competition. This corpus comprises passage-level paraphrases with 4067 positive samples and 3792 negative samples that failed our criteria, using Amazon's Mechanical Turk for crowdsourcing. In this article, we review the lessons learned at PAN 2010, and explain in detail the method used to construct the corpus. The empirical contributions include machine learning experiments to explore if passage-level paraphrases can be identified in a two-class classification problem using paraphrase similarity features, and we find that a k-nearest-neighbor classifier can correctly distinguish between paraphrased and nonparaphrased samples with 0.980 precision at 0.523 recall. This result implies that just under half of our samples must be discarded (remaining 0.477 fraction), but our cost analysis shows that the automation we introduce results in a 18p financial saving and over 100 hours of time returned to the researchers when repeating a similar corpus design. On the other hand, when building an unrelated corpus requiring, say, 25p training data for the automated component, we show that the financial outcome is cost neutral, while still returning over 70 hours of time to the researchers. The work presented here is the first to join the paraphrasing and plagiarism communities.

Use of key performance indicators for PPP transport projects to meet stakeholders’ performance objectives

Abstract: Purpose – The need for more efficient and effective public-private partnership (PPP) projects makes performance management of increasing importance, especially with respect to stakeholder expectations. This paper seeks to examine the use of key performances indicators (KPI) based on the analysis of critical success factors (CSF) for monitoring of PPP transport projects from the perspective of different stakeholders. Design/methodology/approach – The list of stakeholders’ performance objectives including CSF was defined using the brainstorming technique applied to group of experts in PPP transport sectors and refined through literature review. Input of the expert group was validated against a feedback from a respective survey. Three main groups of stakeholders were identified: public sector, private sector, and users. Finally, the review and comparative analysis of existing KPIs in PPP transport projects was performed based on stakeholders’ objectives and project attributes using the “frequency of mention”...

The mechanical properties of three types of carbon allotropes

Abstract: The mechanical properties of supergraphene, cyclicgraphene and graphyne are studied using molecular dynamics simulations based on the AIREBO potential. In particular, we present the chirality-dependence of their mechanical properties, including Young's moduli, shear moduli, Poisson's ratios, ultimate strength and ultimate strains. The relationship of their Young moduli, shear moduli and Poisson ratios is in the order of Y(su) (super) v(cy) > v(gy) > v(ge) in corresponding zigzag and armchair sheets, respectively. Their intersheet adhesion energy is obtained as γ(su) = 30, γ(cy) = 99 and γ(gy) = 149 mJ m(-2), which are much lower than that of γ(ge) = 291 mJ m(-2) (the value is in good agreement with the latest experimental result γ(ge) = 310 ± 30 mJ m(-2)). The obtained adhesion energy is accurately characterized by continuum modeling of the van der Waals interactions. Our study is very useful for the future applications of graphene-like materials in nanoelectromechanical systems.

Detection of flaws in piezoelectric structures using extended FEM

Abstract: SUMMARY An iterative method to treat the inverse problem of detecting cracks and voids in two-dimensional piezoelectric structures is proposed. The method involves solving the forward problem for various flaw configurations, and at each iteration, the response of piezoelectric material is minimized at known specific points along the boundary to match measured data. Extended finite element method (XFEM) is employed for solving the forward problem as it allows the use of a single regular mesh for a large number of iterations with different flaw geometries. The minimization of cost function is performed by multilevel coordinate search (MCS) method. The algorithm is an intermediate between purely heuristic methods and methods that allow an assessment of the quality of the minimum obtained and is in spirit similar to the direct method for global optimization. In this paper, the XFEM-MCS methodology is applied to two-dimensional electromechanical problems where flaws considered are straight cracks and elliptical voids. The results show that this methodology can be effectively employed for damage detection in piezoelectric materials. Copyright © 2013 John Wiley & Sons, Ltd.

Cultural Techniques: Or the End of the Intellectual Postwar Era in German Media Theory

Abstract: This paper seeks to introduce cultural techniques to an Anglophone readership. Specifically geared towards an Anglophone readership, the paper relates the re-emergence of cultural techniques (a con...

Initially rigid cohesive laws and fracture based on edge rotations

Abstract: We propose alternative methods for performing FE-based computational fracture: a mixed mode extrinsic cohesive law and crack evolution by edge rotations and nodal reposition. Extrinsic plastic cohesive laws combined with the discrete version of equilibrium form a nonlinear complementarity problem. The complementarity conditions are smoothed with the Chen-Mangasarian replacement functions which naturally turn the cohesive forces into Lagrange multipliers. Results can be made as close as desired to the pristine strict complementarity case, at the cost of convergence radius. The smoothed problem is equivalent to a mixed formulation (with displacements and cohesive forces as unknowns). In terms of geometry, our recently proposed edge-based crack algorithm is adopted. Linear control is adopted to determine the displacement/load parameter. Classical benchmarks in computational fracture as well as newly proposed tests are used in assessment with accurate results. In this sense, the proposed solution has algorithmic and accuracy advantages, at a slight penalty in the computational cost. The Sutton crack path criterion is employed in a preliminary path determination stage.

Thermal comfort in naturally ventilated primary school classrooms

Abstract: An investigation is presented of children-s thermal sensation trends and their perception of overall comfort and tiredness in school classrooms. Findings are reported from a field survey in a naturally ventilated primary school in Southampton, UK, which included thermal comfort surveys and simultaneous measurements of indoor environmental variables. Approximately 230 pupils aged 7-11 years were surveyed from April to July 2011. During this period of the summer term, there was no heating in the school. The results are compared with findings from other studies in schools and from studies with adults in offices. The results reveal a tendency of pupils towards warm thermal sensations which paradoxically is not complemented by an equally strong preference for cooler environments. There was no difference in the thermal sensation responses in terms of gender, but boys generally preferred cooler environments at high temperatures than girls. Overall, the results indicate that children in a classroom have a different thermal perception compared with adults, suggesting that current adult-based comfort standards may not apply to school children. The study shows that adjustments may be required to current comfort criteria in order to address the thermal perception of children. More child-centred research is needed to address pupils- thermal requirements under current and future climates. © 2013 Taylor and Francis Group, LLC.

Finite element model calibration of a railway vehicle based on experimental modal parameters

Abstract: This article describes the experimental calibration of a three-dimensional numerical model of an Alfa Pendular train vehicle based on modal parameters. The dynamic tests of the carbody and bogie of the vehicle allowed the determination of the frequencies and modal configurations of 13 vibration modes, by applying the data-driven stochastic subspace identification method. In addition, a dynamic characterisation test of the passenger-seat system was also conducted. The calibration of the model was performed using a submodelling/multistep approach involving two phases, the first one focused on the calibration of the model of the bogie under test conditions and the second one focused on the calibration of the complete model of the vehicle. The calibration was performed through an iterative method based on a genetic algorithm and allowed to obtain optimal values of 17 parameters of the numerical model. For the pairing of the vibration modes, real and complex, a recent technique was used based on the calculation of the modal strain energy. The stability of a significant number of parameters considering different initial populations demonstrated the robustness of the algorithm. The comparison of experimental and numerical responses before and after calibration revealed significant improvements in the numerical model and a very good correlation between the responses obtained with the calibrated model and the experimental responses.

Recent Trends in Digital Text Forensics and Its Evaluation

Abstract: This paper outlines the concepts and achievements of our evaluation lab on digital text forensics, PANi¾?13, which called for original research and development on plagiarism detection, author identification, and author profiling. We present a standardized evaluation framework for each of the three tasks and discuss the evaluation results of the altogether 58i¾?submitted contributions. For the first time, instead of accepting the output of software runs, we collected the softwares themselves and run them on a computer cluster at our site. As evaluation and experimentation platform we use TIRA, which is being developed at the Webis Group in Weimar. TIRA can handle large-scale software submissions by means of virtualization, sandboxed execution, tailored unit testing, and staged submission. In addition to the achieved evaluation results, a major achievement of our lab is that we now have the largest collection of state-of-the-art approaches with regard to the mentioned tasks for further analysis at our disposal.

Orientation Dependent Thermal Conductance in Single-Layer MoS 2

Abstract: We investigate the thermal conductivity in the armchair and zigzag MoS2 nanoribbons, by combining the non-equilibrium Green's function approach and the first-principles method. A strong orientation dependence is observed in the thermal conductivity. Particularly, the thermal conductivity for the armchair MoS2 nanoribbon is about 673.6 Wm−1 K−1 in the armchair nanoribbon and 841.1 Wm−1 K−1 in the zigzag nanoribbon at room temperature. By calculating the Caroli transmission, we disclose the underlying mechanism for this strong orientation dependence to be the fewer phonon transport channels in the armchair MoS2 nanoribbon in the frequency range of [150, 200] cm−1. Through the scaling of the phonon dispersion, we further illustrate that the thermal conductivity calculated for the MoS2 nanoribbon is esentially in consistent with the superior thermal conductivity found for graphene.

Let's jam the reactable: Peer learning during musical improvisation with a tabletop tangible interface

Abstract: There has been little research on how interactions with tabletop and Tangible User Interfaces (TUIs) by groups of users change over time. In this article, we investigate the challenges and opportunities of a tabletop tangible interface based on constructive building blocks. We describe a long-term lab study of groups of expert musicians improvising with the Reactable, a commercial tabletop TUI for music performance. We examine interaction, focusing on interface, tangible, musical, and social phenomena. Our findings reveal a practice-based learning between peers in situated contexts, and new forms of participation, all of which is facilitated by the Reactable's tangible interface, if compared to traditional musical ensembles. We summarise our findings as a set of design considerations and conclude that construction processes on interactive tabletops support learning by doing and peer learning, which can inform constructivist approaches to learning with technology.

An edge-based smoothed finite element method for analysis of laminated composite plates

Abstract: This paper promotes a novel numerical approach to static, free vibration and buckling analyses of laminated composite plates by an edge-based smoothed finite method (ES-FEM). In the present ES-FEM formulation, the system stiffness matrix is established by using the strain smoothing technique over the smoothing domains associated with the edges of the triangular elements. A discrete shear gap (DSG3) technique without shear locking is combined into the ES-FEM to give a so-called edge-based smoothed discrete shear gap method (ES-DSG3) for analysis of laminated composite plates. The present method uses only linear interpolations and its implementation into finite element programs is quite simple. Numerical results for analysis of laminated composite plates show that the ES-DSG3 performs quite well compared to several other published approaches in the literature.

Free and forced vibration analysis using the n-sided polygonal cell-based smoothed finite element method (nCS-FEM)

Abstract: A n-sided polygonal cell-based smoothed finite element method (nCS-FEM) was recently proposed to analyze the elastic solid mechanics problems, in which the problem domain can be discretized by a set of polygons with an arbitrary number of sides. In this paper, the nCS-FEM is further extended to the free and forced vibration analyses of two-dimensional (2D) dynamic problems. A simple lump mass matrix is proposed and hence the complicated integrations related to computing the consistent mass matrix can be avoided in the nCS-FEM. Several numerical examples are investigated and the results found of the nCS-FEM agree well with exact solutions and with those of others FEM.

Scale-integrated atmospheric simulations to assess thermal comfort in six different urban tissues in the warm humid summer of São Paulo, Brazil

Abstract: The microclimate of six different urban tissues was simulated on a typical summer day in Sao Paulo using a combination of the meso-scale model BRAMS and the micro-scale model ENVI-met. The simulated air temperature and relative humidity were fairly similar in all areas, whereas both wind speed and mean radiant temperature varied greatly between and within the areas. The index Temperature of Equivalent Perception (TEP) was used to assess human comfort during the period 11:00–15:00. Thermal comfort at this period of the day is well above the comfort zone defined for TEP, i.e. above 27.2 °C. Thermal comfort is especially poor in non-shaded areas where TEP reaches above 45 °C. The bare soil, wooded area, with a complete cover of tree canopy, was the most comfortable with TEPs below 35 °C. In the case the streets have no shading trees, high-rise buildings are better than low-rise since they give more shade at street level. For high-rise buildings a street pattern oriented northwest–southeast and southwest–northeast gives more shade than a north–south and east–west pattern. It was shown that street trees greatly improve the thermal comfort in built-up areas. With street trees the difference between low-rise and high-rise areas diminishes.

Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects

Abstract: We perform molecular dynamics simulations to investigate the reduction of the thermal conductivity by kinks in silicon nanowires. The reduction percentage can be as high as 70% at room temperature. The temperature dependence of the reduction is also calculated. By calculating phonon polarization vectors, two mechanisms are found to be responsible for the reduced thermal conductivity: (1) the interchanging effect between the longitudinal and transverse phonon modes and (2) the pinching effect, i.e a new type of localization, for the twisting and transverse phonon modes in the kinked silicon nanowires. Our work demonstrates that the phonon interchanging and pinching effects, induced by kinking, are brand new and effective ways in modulating heat transfer in nanowires, which enables the kinked silicon nanowires to be a promising candidate for thermoelectric materials.