Multiscale modeling of heat conduction in graphene laminates

We developed a combined atomistic-continuum hierarchical multiscale approach to explore the effective thermal conductivity of graphene laminates. To this aim, we first performed molecular dynamics simulations in order to study the heat conduction at atomistic level. Using the non-equilibrium molecular dynamics method, we evaluated the length dependent thermal conductivity of graphene as well as the thermal contact conductance between two individual graphene sheets. In the next step, based on the results provided by the molecular dynamics simulations, we constructed finite element models of graphene laminates to probe the effective thermal conductivity at macroscopic level. A similar methodology was also developed to study the thermal conductivity of laminates made from hexagonal boron-nitride (h-BN) films. In agreement with recent experimental observations, our multiscale modeling confirms that the flake size is the main factor that affects the thermal conductivity of graphene and h-BN laminates. Provided information by the proposed multiscale approach could be used to guide experimental studies to fabricate laminates with tunable thermal conduction properties

Mechanical properties and thermal conductivity of graphitic carbon nitride: A molecular dynamics study

Graphitic carbon nitride nanosheets are among 2D attractive materials due to presenting unusual physicochemical properties.Nevertheless, no adequate information exists about their mechanical and thermal properties. Therefore, we used classical molecular dynamics simulations to explore the thermal conductivity and mechanical response of two main structures of single-layer triazine-basedg-C3N4 films.By performing uniaxial tensile modeling, we found remarkable elastic modulus of 320 and 210 GPa, and tensile strength of 47 GPa and 30 GPa for two different structures of g-C3N4sheets. Using equilibrium molecular dynamics simulations, the thermal conductivity of free-standing g-C3N4 structures were also predicted to be around 7.6 W/mK and 3.5 W/mK. Our study suggests the g-C3N4films as exciting candidate for reinforcement of polymeric materials mechanical properties

An adaptive continuum/discrete crack approach for meshfree particle methods

A coupled continuum/discrete crack model for strain softening materials is implemented in a meshfree particle code. A coupled damage plasticity constitutive law is applied until a certain strain based threshold value - this is at the maximum tensile stress of the equivalent uniaxial stress strain curve - is reached. At this point a discrete crack is introduced and described as an internal boundary with a traction crack opening relation. Within the frame-work of meshfree particle methods it is possible to model the transition from the continuum to the discrete crack since boundaries and particles can easily be added and removed. The EFG method and an explicit time integration scheme is used. The integrals are evaluated by nodal integration, an integration with stress points and also a full Gauss quadrature. Some results are compared to experimental data and show good agreement. Additional comparisons are made to a pure continuum constitutive law

Fluid-structure interaction in lower airways of CT-based lung geometries

In this study, the deformability of airway walls is taken into account to study airflow patterns and airway wall stresses in the first generations of lower airways in a real lung geometry. The lung geometry is based on CT-scans that are obtained from in-vivo experiments on humans. A partitioned fluid-structure interaction (FSI) approach, realized within a parallel in-house finite element code, is employed. It is designed for the robust and eficient simulation of the interaction of transient incompressible Newtonian flows and (geometrically) nonlinear airway wall behavior. Arbitrary Lagrangian Eulerian (ALE)-based stabilized tetrahedral finite elements are used for the fluid and Lagrangian-based 7-parametric mixed/hybrid shell elements are used for the airway walls using unstructured meshes due to the complexity of the geometry. Air flow patterns as well as airway wall stresses in the bronchial tree are studied for a number of different scenarios. Thereby, both models for healthy and diseased lungs are taken into account and both normal breathing and mechanical ventilation scenarios are studied

Steiner-point free edge cutting of tetrahedral meshes with applications in fracture

Realistic 3D finite strain analysis and crack propagation with tetrahedral meshes require mesh refinement/ division. In this work, we use edges to drive the division process. Mesh refinement and mesh cutting are edge- based. This approach circumvents the variable mapping procedure adopted with classical mesh adaptation algorithms. The present algorithm makes use of specific problem data (either level sets, damage variables or edge deformation) to perform the division. It is shown that global node numbers can be used to avoid the Schönhardt prisms. We therefore introduce a nodal numbering that maximizes the trapezoid quality created by each mid-edge node. As a by-product, the requirement of determination of the crack path using a crack path criterion is not required. To assess the robustness and accuracy of this algorithm, we propose 4 benchmarks. In the knee-lever example, crack slanting occurs as part of the solution. The corresponding Fortran 2003 source code is provided