A Multiscale Framework for Predicting the Performance of Fiber/Matrix Composites

To enable higher fidelity studies of laminated and 3D textile composites, a scalable finite element framework was developed for predicting the performance of fiber/matrix composites across scales. Effective design paradigms and lessons learned are presented. Using the developed framework, new insights into the behavior of laminated and woven composites were discovered. For a [0/90]s and [±45/0/90]s laminated composite, the classical free-edge problem was revisited with the heterogeneous microstructure directly modeled, which showed that the local heterogeneity greatly affects the predicted stresses along the ply interface. Accounting for the microscale heterogeneity removed the singularity at the ply interface and dramatically reduced the predicted interlaminar stresses near a free-edge. However, the heterogeneous microstructure was also shown to induce a complex stress distribution away from the free-edge due to the interaction of fibers near the ply interface, since close fibers were shown to induce compressive stress concentrations. The fiber arrangement had a significant effect on the local stresses, with a more uniform fiber arrangement resulting in lower peak stresses. Finally, the region needed to accurately predict the microscale stresses near the ply interface was shown to be much smaller then entire ply. For two types of orthogonally woven textile composites, nonidealized textile models were created and subjected to a variety of loads, providing insight into how load is distributed throughout the complex tow architecture and the locations of critical stresses. By comparing the stresses of a textile model with and without binders, the binders were shown to greatly affect the distributions of stress a tensile load but not in-plane shear. Variations in the local fiber volume fraction within the tows were shown to significantly affect the magnitude of critical stress concentrations but did not change where the critical stresses occurred. Finally, accounting for plasticity in the neat matrix pocket of the textile was shown to only affect the localized region near where binders traverse the thickness of the textile

QuantumATK: An integrated platform of electronic and atomic-scale modelling tools

QuantumATK is an integrated set of atomic-scale modelling tools developed since 2003 by professional software engineers in collaboration with academic researchers. While different aspects and individual modules of the platform have been previously presented, the purpose of this paper is to give a general overview of the platform. The QuantumATK simulation engines enable electronic-structure calculations using density functional theory or tight-binding model Hamiltonians, and also offers bonded or reactive empirical force fields in many different parametrizations. Density functional theory is implemented using either a plane-wave basis or expansion of electronic states in a linear combination of atomic orbitals. The platform includes a long list of advanced modules, including Green's-function methods for electron transport simulations and surface calculations, first-principles electron-phonon and electron-photon couplings, simulation of atomic-scale heat transport, ion dynamics, spintronics, optical properties of materials, static polarization, and more. Seamless integration of the different simulation engines into a common platform allows for easy combination of different simulation methods into complex workflows. Besides giving a general overview and presenting a number of implementation details not previously published, we also present four different application examples. These are calculations of the phonon-limited mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model simulation of lithium ion drift through a battery cathode in an external electric field, and electronic-structure calculations of the composition-dependent band gap of SiGe alloys.Comment: Submitted to Journal of Physics: Condensed Matte

Proceedings of the FEniCS Conference 2017

Proceedings of the FEniCS Conference 2017 that took place 12-14 June 2017 at the University of Luxembourg, Luxembourg

Generation and Analysis of Stop-Hole Geometries for Crack-Like Structures in Auxetic Materials

In this investigation, the main interest was studying low porosity auxetic metamaterials generated out of linearly elastic materials, meaning bodies made out of linearly elastic materials (e.g., metals) that, due to alternating patterns of elongated voids perforated on them, exhibit a negative effective Poisson ratio (property commonly called auxeticity). This kind of metamaterials, often obtained by a pattern of elongated ellipsis, generally face the issue of presenting high stress concentration when loaded. The objective of this study is to solve this problem by adding rounded shapes (stop-holes) at the end of elongated grooves, as a replacement for the previously mentioned elongated elliptical voids. Particularly, the “superformula”, a generalized ellipse equation in polar coordinates, was utilized in this investigation as a way of parametrization to determine the shapes to be added in a flexible through way by the alteration of 6 parameters. For the process of choosing adequate parameters to ensure optimum stress concentration, firstly, a careful selection from a catalog of shapes took place. Then, static FEA simulations of a totally parametric Representative Volume Element model that included the selected shapes were executed. To do this computer scripts were developed for interconnecting the operation of multiple engineering software tools. Finally, the effect that the size of the stop-holes, thus the porosity, had over the stress induced in the material and its auxetic deformation response due to the new geometry of the pattern of voids was evaluated. The investigation successfully found shapes that produced a significant stress reduction, by reducing the stress concentration, and in the process found several corollaries and observations of the behavior of the metamaterial depending on the shape and size of the stop-holes