Denis Favier

Theoretical and experimental study of the isothermal mechanical behaviour of alloys in the semi-solid state

Abstract An isothermal constitutive model for semi-solid alloys based on the concepts of mechanics of continuous media and the theory of mixtures is presented. The model is applicable to semi-solid states obtained either by solidification from liquid state or partial remelting from solid state in which each of the solid and the liquid phases is contiguous. During deformation their behaviours are coupled: the densification of the solid matrix considered as a porous viscoplastic medium saturated with a liquid drives the fluid flow behaviour, and the resulting pressure distribution in the liquid affects in turn the stresses and the densification of the solid. The identification procedure of the model uses two types of mechanical tests: uniaxial compression and drained die pressing (filtration) carried out with A356 alloy. The identification results are then validated using drained triaxial compression.

Shear and compression behaviour of sheet moulding compounds

At the present time, the rheology of sheet moulding compounds (SMC) during forming is not well known. In order to provide better experimental data, an experimental program was carried out as a result of the development of a new rheometer especially dedicated to this kind of material. Homogeneous simple compression and simple shear tests are presented. They allow the evaluation of the influence of the main parameters on the SMC behaviour: strain rate, temperature and fibre fraction. It is shown that the SMC can be considered as a strongly anisotropic non-linear viscous medium.

Rheology of highly concentrated planar fiber suspensions

The rheology of highly concentrated fibers suspended in power-law fluids is investigated by upscaling the physics at the fiber scale. A deterministic upscaling technique is used, namely the homogenization method for periodic discrete structures. This micro-macro approach is used to carry out a quantitative study of concentrated fiber suspensions with planar fiber orientation, performing “numerical rheometry experiments” on a set of representative elementary volumes of fiber suspensions. The simulations underline the significant influence of the fiber volume fraction and orientation, as well as of the non-Newtonian properties of the suspending fluid on the resulting macroscopic rheological behavior. The predictions of the model are compared with experimental results obtained on an industrial thermoset short fiber-bundle polymer composite (SMC).

Shear deformation of high solid fraction (>0.60) semi-solid SnPb under various structures

Abstract The behavior of high solid fraction (>0.60) semi-solid SnPb alloys was studied in pure shear deformation using a Couette geometry rheometer. Three structures were investigated: a classical coarse dendritic structure, a globular structure obtained by reheating a fine dendritic structure (R.G.) and a globular structure generated by vigorous stirring during solidification (S.G.). The slurries were subjected to shear rates ranging from 0.01 to 50 s −1 . Experimental data for dendritic structure and for R.G. are compared to the model proposed by Nguyen et al. ( Proc. 2nd Int. Conf. on Processing of Semi-Solid Alloys and Composites , Boston, USA, 1992, p.296; and Int. J. Plast. 10 (1994) 693). The S.G. structure exhibited a very different behavior compared with globular structures obtained from a fine dendritic precursor (R.G.). This behavior is interpreted in terms of an agglomeration/disagglomeration model.

Influence of SiC particle volume fraction on the compressive behaviour of partially remelted AlSi-based composites

Abstract Al-7%Si-0.3%Mg (A356) matrix composites containing 10, 20 and 30 vol.% of SiC particles were fabricated by the compocasting technique followed by rapid reheating up to the liquid state, casting and solidification under pressure. The obtained composite ingots were then extruded at 480 °C with an extrusion ratio of 9:1. Specimens obtained from the extruded bars were subjected, after a globularization treatment, to semisolid state filtration to produce samples with various volume fractions of the liquid phase. These samples were thereafter deformed in uniaxial compression in the semisolid state at different constant strain rates ranging from 2.4 × 10 −3 to 3.0 × 10 −1 s −1 . The influence of the parameters characterizing the volume fractions of the liquid phase and the particles for different strain rates was investigated. A rheological analysis shows that the partially remelted composites behave as non-newtonian fluids, following a power-law model of the typeσ = K έ m with m in the range 0.25–0.4. Increasing the liquid volume fraction decreases strongly the compressive stress, whereas the influence of the particle volume fraction is more complex and depends on the liquid volume fraction.

Finite element implementation of a two-phase model for compression molding of composites

A two-phase approach is proposed to model the rheology of polymer glass-fiber compounds such as SMC or GMT during processing. The anisotropic behavior of the composite, which is related to the microstructure of the fiber network, is reduced to the simple case of transverse isotropy. The rheology of the two media, e.g. the matrix and the fiber network, as well as their interaction follow non-linear viscous behaviors. The equations of this model are simplified to the case of the compression of SMC, giving the formulation of a shell model whose equations are written into a finite element code. Simple simulation examples thus show the strong influence of material and process parameters on the phenomenon of phase separation.

Radiopaque poly(ε-caprolactone) as additive for X-ray imaging of temporary implantable medical devices

Implantable polymeric medical devices suffer from a lack of visibility under current clinical imaging techniques. To circumvent this problem, poly(e-caprolactone-co-α-triiodobenzoate-e-caprolactone)s (PCL-TIB) containing from 3.5% to 24% of triiodobenzoate-e-caprolactone units were synthesized as new X-ray macromolecular contrast agents. Physico-chemical and thermal properties of PCL-TIBs were evaluated by 1H NMR, SEC, DSC and TGA. Their potential as radio-opacifying additive for medical devices was evaluated by preparing polymeric blends of PCL-TIB with various (co)polyesters. At first, in vitro X-ray visibility of PCL/PCL-TIB blends was evaluated. A more in depth characterization was then carried out based on PCL/PLA50-PEG-PLA50 blends. The impact of PCL-TIB content on the mechanical properties of blends was evaluated by tensile tests. Stability of X-ray visibility was evaluated by ex vivo implantation of non-degraded blends and of blends degraded for 6 weeks in vitro. Finally, cytocompatibility was assessed by evaluating the proliferation of L929 fibroblasts on the blends.

Influence of processing parameters on the macroscopic mechanical behavior of PVA hydrogels

This paper investigates the influence of three different processing parameters on the global mechanical behavior of PVA (Polyvinyl alcohol)/DMSO (Dimethylsulfoxide) hydrogels: the initial concentration of PVA, the DMSO:H2O ratio and the number of freeze/thaw cycle applied to the material. A specific thermo-regulated testing apparatus for hydrophilic materials is presented, along with the performed cyclic and rupture tests. The observed mechanical responses are explained by an in-depth analysis of the cross-linking phenomenon. Using the Neo-Hookean hyperelastic model, the experimental data is fitted and a link between the density of macro-molecular chains in the material and its mechanical behavior is established. Strong differences are observed and discussed.

Mechanical behaviour׳s evolution of a PLA-b-PEG-b-PLA triblock copolymer during hydrolytic degradation.

PLA-b-PEG-b-PLA is a biodegradable triblock copolymer that presents both the mechanical properties of PLA and the hydrophilicity of PEG. In this paper, physical and mechanical properties of PLA-b-PEG-b-PLA are studied during in vitro degradation. The degradation process leads to a mass loss, a decrease of number average molecular weight and an increase of dispersity index. Mechanical experiments are made in a specific experimental set-up designed to create an environment close to in vivo conditions. The viscoelastic behaviour of the material is studied during the degradation. Finally, the mechanical behaviour is modelled with a linear viscoelastic model. A degradation variable is defined and included in the model to describe the hydrolytic degradation. This variable is linked to physical parameters of the macromolecular polymer network. The model allows us to describe weak deformations but become less accurate for larger deformations. The abilities and limits of the model are discussed.

Tube Drawing Process Modelling By A Finite Element Analysis

Drawing process is used in manufacturing thin‐walled tubes, while reducing progressively their wall thickness and their inner and outer diameters. In this paper a stainless steel 316LVM and a cobalt alloy L605 are studied with two drawing processes, hollow sinking and plug drawing. This study gets into different issues including elastoplastic behaviour, contacts, friction and numerical convergence. Experimental drawings are realized on a testing bench where forces and dimensional data are recorded. In a first approach, tensile tests lead up to apply an elastoplastic constitutive equation with an isotropic hardening law. In simulations, an axisymetric steady‐state model, with numeric stabilization if needed, is used. Numerical results are compared with experimental results. Finally, in spite of some defaults, this study shows that finite element modelling is able to foresee accurately the behaviour of a tube during a drawing process. A better understanding and modelling of the mechanical behaviour of mater...