Said Ahzi

Strain rate effects on the mechanical response of polypropylene-based composites deformed at small strains

The mechanical properties and response of two polypropylene (PP)-based composites have been determined for small strains and for a range of strain rates in the quasi-static domain. These two materials are talc-filled and unfilled high-impact PP. Uniaxial tensile tests were performed at different strain rates in order to characterize the mechanical response and the strain rate effect. The experimental results showed that both unfilled and talc-filled high-impact PP were sensitive to strain rate and exhibited nonlinear behavior even at relatively low strains. SEM analysis was conducted to obtain a better comprehension of deformation mechanisms involved during loading by observations of the microstructure evolution. For each of these two materials, two existing modeling approaches are proposed. The first one is a three-parameter nonlinear constitutive model based on the experimental results. The second is a micromechanically based approach for the elastic-viscoplastic behavior of the composite materials. The stress-strain curves predicted by these models are in fairly good agreement with our experimental results.

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.

Evolution of the three-dimensional collagen structure in vascular walls during deformation: An in situ mechanical testing under multiphoton microscopy observation

The collagen fibers’ three-dimensional architecture has a strong influence on the mechanical behavior of biological tissues. To accurately model this behavior, it is necessary to get some knowledge about the structure of the collagen network. In the present paper, we focus on the in situ characterization of the collagenous structure, which is present in porcine jugular vein walls. An observation of the vessel wall is first proposed in an unloaded configuration. The vein is then put into a mechanical tensile testing device. As the vein is stretched, three-dimensional images of its collagenous structure are acquired using multiphoton microscopy. Orientation analyses are provided for the multiple images recorded during the mechanical test. From these analyses, the reorientation of the two families of collagen fibers existing in the vein wall is quantified. We noticed that the reorientation of the fibers stops as the tissue stiffness starts decreasing, corresponding to the onset of damage. Besides, no relevant evolutions of the out of plane collagen orientations were observed. Due to the applied loading, our analysis also allowed for linking the stress relaxation within the tissue to its internal collagenous structure. Finally, this analysis constitutes the first mechanical test performed under a multiphoton microscope with a continuous three-dimensional observation of the tissue structure all along the test. It allows for a quantitative evaluation of microstructural parameters combined with a measure of the global mechanical behavior. Such data are useful for the development of structural mechanical models for living tissues.

A PSO algorithm for the calculation of the series and shunt resistances of the PV panel one-diode model

Different equivalent circuit models can be found in the literature to describe the electrical performance of PV panels. Among all the models, the one diode models are widely used. However the determination of the electrical parameters of these models is not an easy task. In this paper, the particle swarm optimization (PSO) method was applied to extract the equivalent circuit parameters of PV panels. The PSO method was implemented in Matlab to calculate the series and shunt resistances of two modified one-diode models used to determine the electrical performance of a photovoltaic panel. The PSO method allowed us to plot I-V curves close to those provided by the PV manufacturer and proved to be a good approach capable of working with any electrical model.

Thermal Analysis of Solar Panels

In this work, we propose to analyze the thermal behavior of PV panels using finite element simulations (FEM). We applied this analysis to compute the temperature distribution in a PV panel BP 350 subjected to different atmospheric conditions. This analysis takes into account existing formulations in the literature and, based on NOCT conditions, meteorological data was used to validate our approach for different wind speed and solar irradiance. The electrical performance of the PV panel was also studied. The proposed 2D FEM analysis is applied to different region’s climates and was also used to consider the role of thermal inertia on the optimization of the PV device efficiency.

Quasistatic to Dynamic Behavior of Particulate Composites for Different Temperatures

In automotive industry, polypropylene based composites are generally used in the manufacturing of car bumpers. In such a case, a detailed study of high strain rate and temperature sensitivities of polypropylene based composites is essential since the bumpers undergo compressive impact loading in a wide range of temperatures. The considered fillers consisted of ethylene octane copolymer and talc. The investigated filler content was 0, 10. and 20 wt. The dynamic behavior was investigated by using split Hopkinson pressure bars, at different strain rates and temperatures. We found that neat polypropylene and polypropylene talc composites were brittle at low temperatures. The addition of ethylene octane copolymer inclusions improved the impact resistance of materials. The dynamic properties are correlated with the morphological investigations of the studied composites by means of the optical microscopy. Finally, we also identify the effect of soft and rigid fillers in the quasi static to dynamic transition.

Simulation of Solidification, Relaxation and Long‐Term Behavior of a Borosilicate Glass

High-level radioactive waste (HLW) vitrification is a manufacturing process designed to dispose of nuclear energy fission products over long-term timescales. We studied and modeled the thermomechanical phenomena occurring during the processing of the glass blocks, e.g. during their solidification and their cooling down. The thermomechanical modeling takes place in 3D FEM simulations. The relaxations of the borosilicate glass are to be taken into account through scripted algorithms. They allow us to describe accurately the evolution of the glass properties over its phase transition (the glass transition temperatures are non-uniform in the HLW package). A damage behavior within the frame of Continuum Damage Mechanics is also used to predict the glass cracking surface area.

Bridging microstructure, properties, and processing of polymer based advanced materials

This is a guest editorial for a special issue in Journal of Engineering Materials and Technology. The papers collected in this special issue emphasize significant challenges, current approaches and future strategies necessary to advance the development of polymer-based materials. They were partly presented at the symposium of Bridging microstructure, properties and processing of polymer based advanced materials in the TMS 2011 annual conference meeting, which was held in San Diego, US, on Feb 28 to March 3, 2011. This symposium was organized by the Pacific Northwest National Laboratory (USA) and the Institute of Mechanics of Fluids and Solids of the University of Strasbourg (France). The organizers were D.S. Li, S. Ahzi, and M. Khaleel.