Showing papers by "Bauhaus University, Weimar published in 2016"

Dual‐horizon peridynamics

Abstract: Summary In this paper, we develop a dual-horizon peridynamics (DH-PD) formulation that naturally includes varying horizon sizes and completely solves the ‘ghost force’ issue Therefore, the concept of dual horizon is introduced to consider the unbalanced interactions between the particles with different horizon sizes The present formulation fulfills both the balances of linear momentum and angular momentum exactly Neither the ‘partial stress tensor’ nor the ‘slice’ technique is needed to ameliorate the ghost force issue We will show that the traditional peridynamics can be derived as a special case of the present DH-PD All three peridynamic formulations, namely, bond-based, ordinary state-based, and non-ordinary state-based peridynamics, can be implemented within the DH-PD framework Our DH-PD formulation allows for h-adaptivity and can be implemented in any existing peridynamics code with minimal changes A simple adaptive refinement procedure is proposed, reducing the computational cost Both two-dimensional and three-dimensional examples including the Kalthoff–Winkler experiment and plate with branching cracks are tested to demonstrate the capability of the method Copyright © 2016 John Wiley & Sons, Ltd

Detection of material interfaces using a regularized level set method in piezoelectric structures

Abstract: An algorithm to solve the inverse problem of detecting inclusion interfaces in a piezoelectric structure is proposed. The material interfaces are implicitly represented by level sets which are identified by applying regularization using total variation penalty terms. The inverse problem is solved iteratively and the extended finite element method is used for the analysis of the structure in each iteration. The formulation is presented for three-dimensional structures and inclusions made of different materials are detected by using multiple level sets. The results obtained prove that the iterative procedure proposed can determine the location and approximate shape of material sub-domains in the presence of higher noise levels.

LCA in architectural design—a parametric approach

Abstract: Life cycle assessment (LCA) has not been widely applied in the building design process because it is perceived to be complex and time-consuming. There is a high demand for simplified approaches that architects can use without detailed knowledge of LCA. This paper presents a parametric LCA approach, which allows architects to efficiently reduce the environmental impact of building designs. First, the requirements for design-integrated LCA are analyzed. Then, assumptions to simplify the required data input are made and a parametric model is established. The model parametrizes all input, including building geometry, materials, and boundary conditions, and calculates the LCA in real time. The parametric approach possesses the advantage that input parameters can be adjusted easily and quickly. The architect has two options to improve the design: either through manually changing geometry, building materials, and building services, or through the use of an optimization solver. The parametric model was implemented in a parametric design software and applied using two cases: (a) the design of a new multi-residential building, and (b) retrofitting of a single-family house. We have successfully demonstrated the capability of the approach to find a solution with minimum environmental impact for both examples. In the first example, the parametric method is used to manually compare geometric design variants. The LCA is calculated based on assumptions for materials and building services. In the second example, evolutionary algorithms are employed to find the optimum combination of insulation material, heating system, and windows for retrofitting. We find that there is not one optimum insulation thickness, but many optima, depending on the individual boundary conditions and the chosen environmental indicator. By incorporating a simplified LCA into the design process, the additional effort of performing LCA is minimized. The parametric approach allows the architect to focus on his main task of designing the building and finally makes LCA practically useful for design optimization. In the future, further performance analysis capabilities such as life cycle costing can also be integrated.

Mechanical responses of borophene sheets: a first-principles study.

Abstract: Recent experimental advances for the fabrication of various borophene sheets introduced new structures with a wide range of applications. Borophene is the boron atom analogue of graphene. Borophene exhibits various structural polymorphs all of which are metallic. In this work, we employed first-principles density functional theory calculations to investigate the mechanical properties of five different single-layer borophene sheets. In particular, we analyzed the effect of the loading direction and point vacancy on the mechanical response of borophene. Moreover, we compared the thermal stabilities of the considered borophene systems. Based on the results of our modelling, borophene films depending on the atomic configurations and the loading direction can yield a remarkable elastic modulus in the range of 163-382 GPa nm and a high ultimate tensile strength from 13.5 GPa nm to around 22.8 GPa nm at the corresponding strain from 0.1 to 0.21. Our study reveals the remarkable mechanical characteristics of borophene films.

Vandalism Detection in Wikidata

Abstract: Wikidata is the new, large-scale knowledge base of the Wikimedia Foundation. Its knowledge is increasingly used within Wikipedia itself and various other kinds of information systems, imposing high demands on its integrity. Wikidata can be edited by anyone and, unfortunately, it frequently gets vandalized, exposing all information systems using it to the risk of spreading vandalized and falsified information. In this paper, we present a new machine learning-based approach to detect vandalism in Wikidata. We propose a set of 47 features that exploit both content and context information, and we report on 4 classifiers of increasing effectiveness tailored to this learning task. Our approach is evaluated on the recently published Wikidata Vandalism Corpus WDVC-2015 and it achieves an area under curve value of the receiver operating characteristic, ROC-AUC, of 0.991. It significantly outperforms the state of the art represented by the rule-based Wikidata Abuse Filter (0.865 ROC-AUC) and a prototypical vandalism detector recently introduced by Wikimedia within the Objective Revision Evaluation Service (0.859 ROC-AUC).

Topology optimization of piezoelectric nanostructures

Abstract: We present an extended finite element formulation for piezoelectric nanobeams and nanoplates that is coupled with topology optimization to study the energy harvesting potential of piezoelectric nanostructures. The finite element model for the nanoplates is based on the Kirchoff plate model, with a linear through the thickness distribution of electric potential. Based on the topology optimization, the largest enhancements in energy harvesting are found for closed circuit boundary conditions, though significant gains are also found for open circuit boundary conditions. Most interestingly, our results demonstrate the competition between surface elasticity, which reduces the energy conversion efficiency, and surface piezoelectricity, which enhances the energy conversion efficiency, in governing the energy harvesting potential of piezoelectric nanostructures.

Predictions of J integral and tensile strength of clay/epoxy nanocomposites material using phase field model

Abstract: We predict macroscopic fracture related material parameters of fully exfoliated clay/epoxy nanocomposites based on their fine scale features. Fracture is modeled by a phase field approach which is implemented as user subroutines UEL and UMAT in the commercial finite element software Abaqus. The phase field model replaces the sharp discontinuities with a scalar damage field representing the diffuse crack topology through controlling the amount of diffusion by a regularization parameter. Two different constitutive models for the matrix and the clay platelets are used; the nonlinear coupled system consisting of the equilibrium equation and a diffusion-type equation governing the phase field evolution are solved via a Newton–Raphson approach. In order to predict the tensile strength and fracture toughness of the clay/epoxy composites we evaluated the J integral for different specimens with varying cracks. The effect of different geometry and material parameters, such as the clay weight ratio (wt.%) and the aspect ratio of clay platelets are studied.

Using Argument Mining to Assess the Argumentation Quality of Essays

Abstract: Argument mining aims to determine the argumentative structure of texts. Although it is said to be crucial for future applications such as writing support systems, the benefit of its output has rarely been evaluated. This paper puts the analysis of the output into the focus. In particular, we investigate to what extent the mined structure can be leveraged to assess the argumentation quality of persuasive essays. We find insightful statistical patterns in the structure of essays. From these, we derive novel features that we evaluate in four argumentation-related essay scoring tasks. Our results reveal the benefit of argument mining for assessing argumentation quality. Among others, we improve the state of the art in scoring an essay’s organization and its argument strength.

A News Editorial Corpus for Mining Argumentation Strategies

Abstract: Many argumentative texts, and news editorials in particular, follow a specific strategy to persuade their readers of some opinion or attitude. This includes decisions such as when to tell an anecdote or where to support an assumption with statistics, which is reflected by the composition of different types of argumentative discourse units in a text. While several argument mining corpora have recently been published, they do not allow the study of argumentation strategies due to incomplete or coarse-grained unit annotations. This paper presents a novel corpus with 300 editorials from three diverse news portals that provides the basis for mining argumentation strategies. Each unit in all editorials has been assigned one of six types by three annotators with a high Fleiss’ Kappa agreement of 0.56. We investigate various challenges of the annotation process and we conduct a first corpus analysis. Our results reveal different strategies across the news portals, exemplifying the benefit of studying editorials—a so far underresourced text genre in argument mining.

Mechanical response of all-MoS2 single-layer heterostructures: a ReaxFF investigation

Abstract: Molybdenum disulfide (MoS2) is a highly attractive 2D material due to its interesting electronic properties. Recent experimental advances confirm the possibility of further tuning the electronic properties of MoS2 through the fabrication of single-layer heterostructures consisting of semiconducting (2H) and metallic (1T) MoS2 phases. Nonetheless, despite significant technological and scientific interest, there is currently limited information concerning the mechanical properties of these heterostructure systems. This investigation aims at extending our understanding of the mechanical properties of all-MoS2 single-layer structures at room temperature. This goal was achieved by performing extensive classical molecular dynamics simulations using a recently developed ReaxFF force field. We first studied the direction dependent mechanical properties of defect-free 2H and 1T phases. Our modelling results for pristine 2H MoS2 were found to be in good agreement with the experimental tests and first-principles theoretical predictions. We also discuss the mechanical response of 2H/1T single layer heterostructures. Our reactive molecular dynamics results suggest all-MoS2 heterostructures as suitable candidates for providing a strong and flexible material with tuneable electronic properties.

A new peridynamic formulation with shear deformation for elastic solid

Abstract: We propose a new peridynamic formulation with shear deformation for linear elastic solid. The key idea lies in subtracting the rigid body rotation part from the total deformation. Based on the strain energy equivalence between classic local model and non-local model, the bond force vector is derived. A new damage rule of maximal deviatoric bond strain for elastic brittle fracture is proposed in order to account for both the tensile damage and shear damage. 2D and 3D numerical examples are tested to verify the accuracy of the current peridynamics. The new damage rule is applied to simulate the propagation of Mode I, II and III cracks.

Mechanical properties of borophene films: a reactive molecular dynamics investigation

Abstract: The most recent experimental advances could provide ways for the fabrication of several atomic thick and planar forms of boron atoms. For the first time, we explore the mechanical properties of five types of boron films with various vacancy ratios ranging from 0.1–0.15, using molecular dynamics simulations with ReaxFF force field. It is found that the Young's modulus and tensile strength decrease with increasing the temperature. We found that boron sheets exhibit an anisotropic mechanical response due to the different arrangement of atoms along the armchair and zigzag directions. At room temperature, 2D Young's modulus and fracture stress of these five sheets appear in the range 63–136 N m−1 and 12–19 N m−1, respectively. In addition, the strains at tensile strength are in the ranges of 9%–14%, 11%–19%, and 10%–16% at 1, 300, and 600 K, respectively. This investigation not only reveals the remarkable stiffness of 2D boron, but establishes relations between the mechanical properties of the boron sheets to the loading direction, temperature and atomic structures.

Anisotropic thermal conductivity and mechanical properties of phagraphene: a molecular dynamics study

Abstract: Phagraphene is a novel 2D carbon allotrope with interesting electronic properties which has been recently theoretically proposed. Phagraphene is similar to a defective graphene structure with an arrangement of pentagonal, heptagonal and hexagonal rings. In this study we investigate the thermal conductivity and mechanical properties of phagraphene using molecular dynamics simulations. Using the non-equilibrium molecular dynamics method, we found the thermal conductivity of phagraphene to be anisotropic, with room temperature values of 218 ± 20 W m−1 K−1 along the armchair direction and 285 ± 29 W m−1 K−1 along the zigzag direction. Both values are one order of magnitude smaller than for pristine graphene. The analysis of the phonon group velocities also shows a significant reduction in this quantity for phagraphene in comparison to graphene. By performing uniaxial tensile simulations, we studied the deformation process and mechanical response of phagraphene. We found that phagraphene exhibits a remarkable high tensile strength around 85 ± 2 GPa, whereas its elastic modulus is also anisotropic along the in-plane directions, with values of 870 ± 15 GPa and 800 ± 14 GPa for the armchair and zigzag directions, respectively. The lower thermal conductivity of phagraphene along with its predicted electronic properties suggests that it could be a better candidate than graphene in future carbon-based thermoelectric devices.

A new peridynamic formulation with shear deformation for elastic solid

Abstract: We propose a new peridynamic formulation with shear deformation for linear elastic solid. The key idea lies in subtracting the rigid body rotation part from the total deformation. Based on the strain energy equivalence between classic local model and non-local model, the bond force vector is derived. A new damage rule of maximal deviatoric bond strain for elastic brittle fracture is proposed in order to account for both the tensile damage and shear damage. 2D and 3D numerical examples are tested to verify the accuracy of the current peridynamics. The new damage rule is applied to simulate the propagation of Mode I, II and III cracks.