Christophe L. Martin

Study of particle rearrangement during powder compaction by the Discrete Element Method

This paper presents simulations of cold isostatic and closed die compaction of powders based on the Discrete Element Method. Due to the particulate nature of powders, densification of the compact proceeds both through the plastic deformation at the particle contact and the mutual rearrangement of particles. The relative weight of each mechanism on the macroscopic deformation process depends on the contact law, the relative density, and the type of stress exerted on the particles (shear or pressure). 3D computer simulations have been carried out to investigate the role of these parameters on the deformation mechanisms of powder compacts. The effect of rearrangement is studied by comparing simulations that use a homogeneous strain field solution for which local rearrangement is omitted and simulations that include local rearrangement. It is shown that local rearrangement has some effect on average quantities such as the average coordination number, the average contact area and the macroscopic stress. The effect on averaged quantities is much stronger for closed die compaction than for isostatic compaction. However the main effect of local rearrangement is to widen the distribution of the parameters that define the contact (contact area in particular). The results of these simulations are compared to available experimental data and to statistical models that use a homogeneous strain field assumption.

Study of the cold compaction of composite powders by the discrete element method

We present numerical simulations of the cold isostatic and close die compaction of mixtures of soft and hard powders. The simulations use the discrete element method with periodic boundary conditions on packing of 4000 spherical monosize particles. The two mechanisms that are generally put forward to explain the retarding effect of hard particles on the compaction have been analysed. First, we have studied in which condition the hard particles form a cluster that hinders the homogeneous distribution of the load. Friction between particles is shown to affect significantly the formation of such a cluster. Second, the additional deformation that soft particles must undergo in the presence of hard non-deformable particles has been evaluated. Related to this last issue, the importance of including hardening effects in the constitutive equation of the soft phase is demonstrated.

Pore size distributions calculated from 3-D images of DEM-simulated powder compacts

Abstract Numerical methods for determining pore size distributions from 3-D digitized microstructures of plastically compacted powders are presented. The microstructures are generated by simulations using the discrete element method (DEM). Two types of pore size distribution are considered: first, a “chamber” pore size distribution similar to the pore size distribution measured by quantitative metallography, second, a “throat” pore size distribution that takes into account percolation effects, and is similar to the pore size distribution measured by mercury intrusion porosimetry. The effect of sample size upon the numerically-determined mean pore size is examined. Some evidence that the “throat” pore size distributions accurately reproduce real mercury intrusion porosimetry results is given based upon porosimetry data from the literature. Finally, the evolution of the two types of pore size distribution with relative density is examined. Both homogeneous compacts of plastic spheres and mixtures of plastic and rigid-elastic spheres are studied.

The salt and lipid composition of model cheeses modifies in-mouth flavour release and perception related to the free sodium ion content

Reducing salt and lipid levels in foodstuffs without any effect on acceptability is a major challenge, particularly because of their interactions with other ingredients. This study used a multimodal approach to understand the effects of changes to the composition of model cheeses (20/28, 24/24, 28/20 lipid/protein ratios, 0% and 1% added NaCl) on sodium ion mobility ((23)Na NMR), in-mouth sodium release and flavour perception. An increase in the salt content decreased cheese firmness and perceived hardness, and increased sodium ion mobility, in vivo sodium release and both saltiness and aroma perception. With the same amount of salt, a lower lipid/protein ratio increased the firmness of the cheeses, perceived hardness, and decreased sodium ion mobility, in vivo sodium release, saltiness and aroma perception. These findings suggest on one hand that it could be possible to increase saltiness perception by varying cheese composition, thus inducing differences in sodium ion mobility and in free sodium ion concentration, leading to differences in in-mouth sodium release and saltiness perception, and on the other hand that the reformulation of foods in line with health guidelines needs to take account of both salt content and the lipid/protein ratio.

Fracture behaviour in tension of viscoplastic porous metallic materials saturated with liquid

Abstract The present work extends the investigation which has been initiated in Parts I and II of this study (Martin, C.L., Favier, D., Suery, M., 1997a. Viscoplastic behaviour of porous metallic materials saturated with liquid, part I: constitutive equations. Int. J. Plasticity 13, 215–235; Martin, C.L., Favier, D. Suery, M., 1997b. Viscoplastic behaviour of porous metallic materials saturated with liquid, Part II: experimental identification on a Sn–Pb model alloy. Int. J. Plasticity 13, 237–259) to the tensile behaviour of viscoplastic porous metallic materials saturated with liquid. Simple tensile experiments together with ring extension tests are carried out to study the fracture behaviour of this class of material. Ring tests consist in applying an internal pressure on a specimen with a ring shape. A Sn–Pb model alloy with a dendritic microstructure is used to characterise the behaviour of the material up to fracture. The liquid presence is accounted for to derive the intrinsic behaviour of the solid skeleton. The collected data are then incorporated in the model framework presented in Part I. A simple modification of the model allows the treatment of the strong asymmetry between tension and compression which is exhibited by these materials.

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.

Unloading of powder compacts and their resulting tensile strength

Abstract We examine the unloading of an agglomerate of perfectly plastic spherical particles that have been compacted beyond their elastic limit and that exhibit some adhesion. The behaviour at the contact scale is derived from the model of Mesarovic and Johnson for the decohesion of two spherical particles [1] . The resulting springback that the powder compact experiences is calculated using the Discrete Element Method. An analytical equation for the extent of springback is proposed and coincides well with simulation results. The alteration of the compact during unloading is investigated through contact loss and the decrease of the contact area. The tensile strength of the resulting compact is calculated in isostatic conditions. While isostatic conditions represent the basis of this work, the effect of closed die conditions on the anisotropy of springback is also investigated. Finally, the consequences of adding hard elastic particles to form a composite are analysed in terms of the springback extent and the resulting loss of tensile strength of the agglomerate.

Salivary Composition Is Associated with Liking and Usual Nutrient Intake.

Salivary flow and composition have an impact on flavor perception. However, very few studies have explored the relationship between saliva, individual liking and usual dietary intake. The aim of our study was to evaluate the association of salivary flow and composition with both a liking for fat, saltiness and sweetness and the usual nutrient intake in an adult French population. Liking for fat, saltiness, and sweetness were inferred from liking scores obtained during hedonic tests on 32 food products among 282 French adults participating in the Nutrinet-Santé Study. Before assessing liking, resting saliva was collected. Standard biochemical analyses were performed to assess specific component concentrations and enzymatic activities. Dietary data were collected using three web-based 24h records. Relationships between salivary flow and composition, sensory liking and nutrient intake were assessed using linear regression. Total antioxidant capacity was positively associated with simple carbohydrate intake (β = 31.3, 95% CI = 1.58; 60.99) and inversely related to complex carbohydrate consumption (β = -52.4, 95% CI = -87.51; -19.71). Amylolysis was positively associated with both total (β = 0.20, 95% CI = 0.01; 0.38) and simple carbohydrate intake (β = 0.21, 95% CI = 0.01; 0.39). Salivary flow was positively associated with liking for fat (β = 0.14, 95% CI = 0.03; 0.25). Proteolysis was positively associated with liking for saltiness and for fat (β = 0.31, 95% CI = 0.02; 0.59; β = 0.28, 95% CI = 0.01; 0.56, respectively). Amylolysis was inversely associated with liking for sweetness (β = -10.13, 95% CI = -19.51; -0.75). Carbonic anhydrase 6 was inversely associated with liking for saltiness (β = -46.77, 95% CI = -86.24; -7.30). Saliva does not substantially vary according to a usual diet, except for carbohydrate intake, whereas the specific association between salivary flow/composition and sensory liking suggests the influence of saliva characteristics in food acceptance.

Rheological behavior of the mushy zone at small strains

The modeling of the hot-tearing phenomena at the end of solidification requires the development of specific constitutive equations for the mushy zone. The presence of liquid films in the solid-liquid mixture has important consequences on the rheological behavior of the mushy zone. This aspect is particularly treated in order to propose a constitutive equation for the mushy zone in the low strain region. Starting from the behavior of the fully solid material, the mush is modeled as a porous medium saturated with liquid with partial cohesion. The cohesion of the mush is treated as an internal variable for which an evolution equation is proposed. Experimental data on a Sn-Pb model alloy both in the fully solid state and in the mushy state are used in order to study the advantages and the limitations of the proposed constitutive equation.

Micromodeling of Functionally Graded SOFC Cathodes

One approach to improve the performances of a solid oxide fuel cell (SOFC) cathodes is to use a composite electrode that typically consists of a two-phase porous mixture of a solid electrolyte (yttria-stabilized zirconia) and an electrocatalytic material. The best electrochemical performances with such a type of cathode are obtained when the composition varies within the composite cathode called graded electrode. The present modeling has been performed in order to give a complete description of the electrode microstructure, as well as the process occurring therein. A one-dimensional, homogeneous model and a discrete element method was used. The present study proposes a one-dimensional dc and ac model that takes into account mass and charge conservation, transport of species, and reaction kinetics. The influence of transport parameters and microstructure on the shape of both polarization curves and impedance diagrams is discussed.