Browsing by Subject "Distinct Element Method"
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Item Mesoscopic Distinct Element Method for Carbon Nanotubes: From Workstation to Massively Parallel Simulations(2022-02) Drozdov, GrigoriiCarbon nanotubes (CNT), artificially synthesized hollow cylinders with graphitic walls, have attracted significant attention as components for developing ultrastrong materials. However, scaling up the superb mechanical properties of individual CNTs to the material level poses significant challenges related mainly to the poor inter-tube load transfer between CNTs. The design of better CNT materials could benefit from guidance through numerical modeling, but neither molecular dynamics techniques nor finite element modeling could provide necessary tools for modeling the mechanics of CNT assemblies. New mesoscopic methods are needed and among coarse-grained mesoscopic models available, our mesoscopic distinct element method (mDEM) model provides premium accuracy and efficiency of bonded and non-bonded interactions as well as unprecedented length scales. This thesis is devoted to the development, enhancement, and application of the mDEM model to the modeling of CNT materials. In this work, mDEM, previously implemented in the DEM code PFC3D, is cast into an enhanced vector format and scalable parallelized with the message passing interface (MPI) technology, as enabled by waLBerla DEM multiphysics framework. The new capability allows for the modeling of large assemblies of CNTs, while distributing the computation over thousands of computational cores. With the parallelized implementation of the mDEM model in waLBerla we are able to perform unprecedented simulations, including single-walled CNT films densification and nanoindentation processing, and double-walled CNT yarns formation by stretching CNT "sock" materials. Furthermore, with a tabulation technique, mDEM is expanded to non-cylindrical collapsed CNTs. Finally, in another enhancement of mDEM model, we lay a foundation for simulation of CNT assemblies interaction with fluids.Item Mesoscopic Distinct Element Method for Computational Design of Carbon Nanotube Materials(2017-07) Wang, YuezhouCarbon Nanotubes (CNTs) are hollow molecular cylinders conceptually formed by rolling single or multiple layers of graphene into tubes. CNT materials have become an attractive research subject during the last decades owning to the superior mechanical and electronic properties of individual CNTs. Developing applications, such as structural materials, supercapacitors, batteries or nanomechanical devices, depend on our ability to understand, model, and design the structure and properties of realistic CNT assemblies. Toward this goal, here we have applied a recently developed mesoscale computational method, titled the mesoscopic distinct element method (MDEM) that makes it possible to simulate the formation, stability, and mechanics of CNT aggregates and ultrathin CNT films. We first combine experiments and distinct element method simulations to understand the stability of rings and rackets formed by single-walled carbon nanotubes assembled into ropes. The obtained agreement validates MDEM and indicates that the stability of the experimental aggregates can be largely explained by the competition between bending and van der Waals adhesion energies. Next, we have considered the geometry and internal packing in twisted CNT ropes. Compared to the state of the art, MDEM accounts in a computationally tractable manner for both the deformation of the fiber and the distributed van der Waals cohesive energy between fibers. These features enable us to investigate the torsional response in a new regime where the twisted rope develops packing rearrangements and aspect-ratio-dependent geometric nonlinearities, in agreement with phenomenological models. Finally, we have performed MDEM simulations and developed an atomic-scale picture of the CNT network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. Silicon nanoparticles are integrated into the model and the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications. As a distinct direction, MDEM was explored for modeling the mechanics of nanocrystalline particles. Simulations that rely on the fitting of the peak stress, strain, and failure mode on the experimental testing of Au and CdS hollow nanocrystalline particles illustrate the promising potential of MDEM for bridging the atomistic-scale simulations with experimental testing data.Item Mesoscopic Distinct Element Method for Multiscale Modeling of Carbon Nanotubes(2019-07) xu, haoSuper strong lightweight material systems comprising carbon nanotubes (CNTs) are especially suitable for aerospace applications. Assembles of CNTs obtained by mechanically stretching the CNT sheets, represent a promising material platform for developing composite materials with mechanical attributes approaching those of individual CNTs. In this quest, the guidance power of computational materials modelling is critical. Ideally one would like to investigate CNT assembles with all atom simulation methods, but this approach is computationally prohibitive. Due to the inherent spatial and temporal limitations of atomistic modeling and the lack of mesoscale models, mesoscopic simulation methods for CNT systems are missing. My work focuses on deriving ultra-coarse-grained models based on mesoscopic dintinct element method (mDEM). Our mDEM model is informed by atomistic data obtained with molecular dynamics (MD) and density functional theory-based tight-binding (DFTB) objective molecular modeling. Our mDEM model is capable of reproducing the atomistic elastic and frictional properties of CNTs. With the mDEM model, tensile tests of mesoscale CNT network were carried out, showing results in good agreement with experiments. The tensile tests revealed nanofriction was a key factor deciding the load transfer of CNT network. Our mDEM model serves as a powerful tool to expand the understanding and guide the development of CNT materials.