Ahmed Makradi

Micromechanical modeling and characterization of the effective properties in starch-based nano-biocomposites

The aim of this work was to predict the effective elastic properties of starch-based nano-biocomposites. Experiments (materials elaboration, morphological characterization and determination of mechanical properties) were conducted on both the pristine matrix (plasticized starch) and the matrix filled with montmorillonite nanoclay. Aggregated/intercalated and exfoliated nano-biocomposites were produced and mechanically tested under uniaxial tension to understand the effect of montmorillonite morphology/dispersion on the stiffness properties of starch-based nano-biocomposites. Micromechanical models, based on the classical bounds and the Mori-Tanaka approaches, were developed taking into consideration the influence of the clay concentration, the exfoliation ratio, the relative humidity and the storage time (ageing). Predicted results are in a good agreement with our experiments and show that the micromechanical model can be used as an indirect characterization technique to quantify the exfoliation/aggregation degree in the plasticized starch/clay nano-biocomposites.

Diffusion and viscosity of liquid tin: Green-Kubo relationship-based calculations from molecular dynamics simulations

Molecular dynamics (MD) simulations of liquid tin between its melting point and 1600 °C have been performed in order to interpret and discuss the ionic structure. The interactions between ions are described by a new accurate pair potential built within the pseudopotential formalism and the linear response theory. The calculated structure factor that reflects the main information on the local atomic order in liquids is compared to diffraction measurements. Having some confidence in the ability of this pair potential to give a good representation of the atomic structure, we then focused our attention on the investigation of the atomic transport properties through the MD computations of the velocity autocorrelation function and stress autocorrelation function. Using the Green-Kubo formula (for the first time to our knowledge for liquid tin) we determine the macroscopic transport properties from the corresponding microscopic time autocorrelation functions. The selfdiffusion coefficient and the shear viscosity as functions of temperature are found to be in good agreement with the experimental data.

On high-cycle fatigue of 316L stents

This paper deals with fatigue life prediction of 316L stainless steel cardiac stents. Stents are biomedical devices used to reopen narrowed vessels. Fatigue life is dominated by the cyclic loading due to the systolic and diastolic pressure and the design against premature mechanical failure is of extreme importance. Here, a life assessment approach based on the Dang Van high cycle fatigue criterion and on finite element analysis is applied to explore the fatigue reliability of 316L stents subjected to multiaxial fatigue loading. A finite element analysis of the stent vessel subjected to cyclic pressure is performed to carry out fluctuating stresses and strain at some critical elements of the stent where cracks or complete fracture may occur. The obtained results show that the loading path of the analysed stent subjected to a pulsatile load pressure is located in the safe region concerning infinite lifetime.

A Reference Benchmark Solution for Free Convection in A Square Cavity Filled with A Heterogeneous Porous Medium

The Fourier-Galerkin (FG) method is used to produce a highly accurate solution for free convection in a square cavity filled with heterogeneous porous medium. To this end, the governing equations are reformulated in terms of the temperature and the stream function. These unknowns are then expanded in infinite Fourier series truncated at given orders. The accuracy of the FG solution is investigated for different truncation orders and compared to the results of an advanced finite-element numerical model using fine-mesh discretization. The obtained results represent a set of high-quality data that can be used for benchmarking numerical models.

An XFEM model for cracked porous media: effects of fluid flow and heat transfer

In this work, a numerical model is developed to investigate the influence of fluid flow and heat transfer on the thermo-mechanical response of a cracked porous media. The fluid flow, governed by the Darcy’s law, is discretized with the nonconforming finite element method. Time splitting is used with the energy conservation equation to solve the fluid and the solid phases separately. A combination of Discontinuous Galerkin (DG) and multi-point flux approximation methods is used to solve the advection-diffusion heat transfer equation in the fluid phase. While the conductive heat transfers equation in the solid phase is solved using the eXtended finite element method (XFEM) to better handle the temperature discontinuities and singularities caused by the cracks. Further, the resulted temperature is used as body force to solve the thermo-mechanical problem using the XFEM. In the post processing stage, the thermal stress intensity factor is computed using the interaction integral technique at each time step and used to validate the obtained results. A good agreement was found when the results were compared with the existing ones in the literature.

A high-accurate solution for Darcy-Brinkman double-diffusive convection in saturated porous media

ABSTRACT The main purpose of this article is to present a high-accurate reference solution for double-diffusive convection in a confined saturated porous medium. The solution is developed using the Fourier-Galerkin spectral method for the coupled flow, heat, and mass transfer equations. The accuracy of the obtained solution is investigated in terms of the truncation orders of the Fourier series and by comparison against a newly developed finite-element model. Results of simulations highlight the accuracy of the proposed reference solution and show its worthiness for benchmarking numerical models dealing with Darcy-Brinkman double-diffusive convection.

A combination of Crouzeix-Raviart, Discontinuous Galerkin and MPFA methods for buoyancy-driven flows

Purpose – The purpose of this paper is to develop an efficient non-iterative model combining advanced numerical methods for solving buoyancy-driven flow problems. Design/methodology/approach – The solution strategy is based on two independent numerical procedures. The Navier-Stokes equation is solved using the non-conforming Crouzeix-Raviart (CR) finite element method with an upstream approach for the non-linear convective term. The advection-diffusion heat equation is solved using a combination of Discontinuous Galerkin (DG) and Multi-Point Flux Approximation (MPFA) methods. To reduce the computational time due to the coupling, the authors use a non-iterative time stepping scheme where the time step length is controlled by the temporal truncation error. Findings – Advanced numerical methods have been successfully combined to solve buoyancy-driven flow problems on unstructured triangular meshes. The accuracy of the results has been verified using three test problems: first, a synthetic problem for which t...

Simulation of the densification of semicrystalline polymer powders during the selective laser sintering process: Application to Nylon 12

The processes of heating and densification of semicrystalline polymer powders during the selective laser sintering process are simulated using the finite element method. Based on a previously developed three-dimensional approach for the sintering of amorphous polymer powders, the modeling methodology is extended to semicrystalline polymers by taking into account the effects of latent heat during melting. In these simulations, the temperature-dependent thermal conductivity, the specific heat, the density, and the effect of latent heat are computed and then used as material constants for the integration of the heat equation. Results for the temperature and density distribution using Nylon-12 powder are presented and discussed. The effects of processing parameters on the density distribution are also presented.

Comparison of micromechanical models for the prediction of the effective elastic properties of semicrystalline polymers: Application to polyethylene

In this paper, we discuss the application of different micromechanical composite models to compute the effective elastic properties of semicrystalline polymers. The morphology of these two-phase materials consists of crystalline lamellae and amorphous domains which may form a spherulitic microstructure. The selected models are the Mori-Tanaka type models, the Double-Inclusion models, and the Self-Consistent models. We applied these composite estimates to both fully isotropic and transverse isotropic transcrystalline polyethylene. The results from these different models are compared to the experimental results for different crystallinities. The Generalized Mori-Tanaka (GMT) model and the Self-Consistent Composite-Inclusion (SCCI) model give the best predictions of the effective elastic constants compared to the other models.

Fatigue life prediction of cardiovascular stent using finite element method

Failure of a stent due to fatigue may result in loss of radial support of the stented blood vessel or in perforation of the vessel by the stent struts. Fatigue analysis, combined with stress analysis and accelerated durability testing, provides an indication of device durability. In this study, we show a methodology that is based on a finite element method (FEM) for the fatigue life prediction (FLP) of a cardiovascular stent made of stainless steel AISI316L. FLP of a cardiovascular stent was conducted by two different approaches. The first approach is related to the well-known Goodman diagram (GD) and the second one is an application of the theory of critical distance (TCD). Numerical simulation and finite element analysis (FEA) of the stent deployment inside the blood vessel were carried out using Abaqus finite element code.