Papers by Keyword: Multi-Scale Modelling

Abstract: Commercial graphites are used for a wide range of applications. For example, Gilsocarbon graphite is used within the reactor core of Advanced Gas Cooled Reactors (UK) as a moderator. In service, the mechanical properties of the graphite are changed as a result of neutron irradiation induced defects and porosity arising from radiolytic oxidation. In this paper, we discuss measurements undertaken of mechanical properties at the micro-length-scale for virgin and irradiated material. These data provide the necessary inputs to an experimentally-informed model that predicts the deformation and fracture properties of Gilsocarbon graphite at the centimetrelength-scale. The results provide an improved understanding of how the mechanical properties and fracture characteristics of this type of graphite change as a result of exposure to the service environment.

Abstract: The use of sandwich structures is well established in industrial sectors where high stiffness and strength combined with lightweight are required, like in marine, wind turbine and railway applications. However, the vulnerability of sandwich structures to low-velocity impacts limits its use in primary aircraft structures. Pin reinforcement of the foam core enhances the out-of-plane properties and the damage tolerance of the foam core. In this paper, a finite element model is proposed to predict the impact behaviour of pin-reinforced sandwich structure. An approach based on the building block approach was used to develop the model. Multi-scale modelling in the impact region that considers the delamination of the face sheet using cohesive zone elements was employed to increase the accuracy of the simulation. Impact tests were performed to validate the numerical model. A good agreement between numerical and experimental results in terms of contact-force displacement history and failure mode was found.

Abstract: Fracture in heterogeneous materials under dynamic loading is modelled using a multi-scale method. Computational homogenization is considered, in which the overall properties at the global-scale are obtained by solving a boundary value problem for a representative volume element (RVE) assigned to each material point of the global-scale model. In order to overcome the problems with upscaling of localized deformations, a non-standard failure zone averaging scheme is used. Discontinuous cohesive macro-cracking is modelled using the XFEM and a gradient-enhanced damage model is used to model diffuse damage at the local-scale. A continuous-discontinuous computational homogenization method is employed to obtain the traction-separation law for macro-cracks using averaged properties calculated over the damaged zone in the RVE. In the multi-scale model, a dynamic analysis is performed for the global-scale model and the local-scale model is solved as a quasi-static problem. Dispersion effects are then captured by accounting for the inertia forces at the local-scale model via a so-called dispersion tensor which depends on the heterogeneity of the RVE. Numerical examples are presented and the multi-scale model results are compared to direct numerical simulation results. Objectivity of the multi-scale scheme with respect to the RVE size is examined.

Abstract: Precipitate-host lattice interface studies have not traditionally been viewed as requiring hybrid model schemes for accurate determination of the interfacial and strain energies. On the other hand, the interfaces of main hardening precipitates of age hardenable alloys are often characterized by both high levels of coherency and considerable subsystem misfits. Near the interface, linear elasticity theory evidently fails in such cases to fully correctly predict the subsystem strains. Further, density functional theory based studies on isolated supercells may prove inadequate in capturing strain influences on the chemical interactions underlying the interfacial energy. Recent work within the group has focussed on the implementation of a first principles based hierarchical multi-scale model scheme, capable of determining the interfacial and strain energies for the same model system. Choosing the fully coherent Al-Mg-Si alloy main hardening phase β'' as our test system and limiting our studies to 2D, we discuss the variation in these energies with changing precipitate cross-section morphology and size.

Abstract: The impregnation stage of the Resin Transfer Moulding process can be simulated by solving the Darcy equations on a mould model, with a ‘macro-scale’ finite element method. For every element, a local ‘meso-scale’ permeability must be determined, taking into account the local deformation of the textile reinforcement. This paper demonstrates that the meso-scale permeability can be computed efficiently and accurately by using meso-scale simulation tools. We discuss the speed and accuracy requirements dictated by the macro-scale simulations. We show that these requirements can be achieved for two meso-scale simulators, coupled with a geometrical textile reinforcement modeller. The first solver is based on a finite difference discretisation of the Stokes equations, the second uses an approximate model, based on a 2D simulation of the flow.

Abstract: This paper describes an overview of a new meshless solution procedure for calculation of one-domain coupled macroscopic heat, mass, momentum and species transfer problems as well as phase-field concepts of grain evolution. The solution procedure is defined on the macro [1] as well as on the micro levels [2] by a set of nodes which can be non-uniformly distributed. The domain and boundary of interest are divided into overlapping influence areas. On each of them, the fields are represented by the multiquadrics radial basis functions (RBF) collocation on a related sub-set of nodes. The time-stepping is performed in an explicit way. All governing equations are solved in their strong form, i.e. no integrations are performed. The polygonisation is not present and the formulation of the method is practically independent of the problem dimension. The solution can be easily and efficiently adapted in node redistribution and/or refinement sense, which is of utmost importance when coping with fields exhibiting sharp gradients. The concept and the results of the multiscale solidification modeling with the new approach are compared with the classical mesh-based [3] approach. The method turns out to be extremely simple to code and accurate, inclusion of the complicated physics can easily be looked over. The coding in 2D or 3D is almost identical.

Abstract: This paper introduces a multiscale modeling approach for carbon nanotubes (CNTs), in which a fine continuum model that has been developed by the present authors is employed to implement the continuum modleing. The entire domain is decomposed into a continuum modeling region and an atomic region with an overlapping region. For the atomic region, the atomic finite element method (AFEM) is used to trace the individual atomic motion. Whereas, the continuum region is viewed as the higher-order continuum media, and the mesh-free method is adopted to implement the continuum numerical discrization. For the overlapping region, the bridging domain method is used to efficiently couple two scales. Numerical computation is carried out and several examples are discussed.

Abstract: Modelling the constitutive behaviour of metallic materials based on their microstructural features and the micromechanical mechanisms in the framework of continuum mechanics is addressed. Deformation at the lengthscale of grains is described by crystal plasticity. The macroscopic behaviour is obtained either by a homogenisation process yielding phenomenological equations or by a submodel technique. The modelling processes for two light-weight materials, namely magnesium and titanium aluminides are presented.

Abstract: This paper aims at providing a preliminary understanding in biomechanics with respect to the effect of the particle size of Fully Porous-Coated (FPC) dental implant on osseointegration. 2D multiscale finite element models are created for a typical dental implantation setting. Under a certain mastication force (<200N), a global response is first obtained from a macro-scale model (without considering morphological details on the coated surface), and then it is transferred to micro-scale models (with coated surface morphology details in three different particle sizes). An equivalent strain is analyzed to investigate the effect of particle size of the FPC materials on osseointegration and initiation of bone remodelling. The result reveals that increasing particle sizes has a significant effect on biomechanical and bone remodelling responses.

Abstract: We focus this work on multi-scale modeling of the ion-beam-induced amorphization and recrystallization in Si, although our scheme can be applied to other materials. We use molecular dynamics to study the formation mechanisms of amorphous regions. We have observed that along with energetic ballistic collisions that generate Frenkel pairs, low energy interactions can produce damage through the melting and quenching of target regions. By quantifying these results, we have developed an improved binary collision approximation model which gives a damage description similar to molecular dynamics. We have successfully applied our model to ion and cluster implantations. In order to define the energetic of defects in a more computationally efficient Kinetic Monter Carlo code, we have used molecular dynamics results related to the recrystallization behavior of local amorphous regions. The combination of all these simulation tools, molecular dynamics (fundamental studies of damage formation and recrystallization), improved binary collisions (including ballistic and melting-related damage) and Kinetic Monte Carlo (for efficient defect kinetics modeling during the implantation and the subsequent annealing), allows us to model the effect of ion mass, beam current and implant temperature on the amount and morphology of residual defects in Si.