Computational modeling of the cold compaction of ceramic powders

A finite-element model of the cold compaction of ceramic powders by uniaxial pressing is developed and validated by comparison with experimental data. The mechanical behavior of processing powders is assumed according to the modified Drucker-Prager cap model. The frictional effects and the mechanical behavior of tools involved in the process are taken into account. The proposed model allows evaluation of the density distribution into the processed part, as well as stress and strain fields. Variations of the density distribution due to the unloading and the ejection of the part are evaluated Published in Prikladnaya Mekhanika, Vol. 42, No. 10, pp. 135–143, October 2006.     

Numerical investigation into the influence of the punch shape on the mechanical behavior of pharmaceutical powders during compaction

During the production of pharmaceutical tablets using powder compaction, certain common problems can occur, such as sticking, tearing, cutting, and lamination. In the past, the compressibility of the powder was calculated only along the axis of the device; consequently, critical areas of the material throughout the volume could not be identified. Therefore, finite element method （FEM） can be used to predict these defects in conjunction with the use of an appropriate constitutive model. This article summarizes the current research in the field of powder compaction, describes the Drucker-Prager Cap model calibration procedure and its implementation in FEM, and also examines the mechanical behavior of powder during compaction. In addition, the mechanical behavior of pharmaceutical powders in relation to changes in friction at the wall of the system is examined, and the dependence of lubrication effect on the geometry of the compaction space is also investigated. The influence of friction on the compaction process for the flat-face, fiat-face radius edge, and standard convex tablets is examined while highlighting how the effects of friction change depending on the shape of these tablets.     

Dynamic compaction model for a granular medium

A model for dynamic compaction of granular medium is proposed for the case in which the external action far exceeds the yield strength of the material. A radial axisymmetric compaction problem is solved for a granular medium with nanosize structure in the presence of a rigid rod at the symmetry axis. Simulated data are compared with experimental data on magnetic pulsed compaction of oxide nanopowders. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 2, pp. 211–215, March–April, 2008.     

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.     

Measurement of compacted soil density in a compaction of thick finishing layer

The compaction of a soil is one of the important construction operations that influences the durability of soil structure. Therefore, the measurement of soil density, used to judge the degree of compaction, has to be performed exactly. Since a compaction of a thick finishing layer could be executed with the enlargement of compaction machinery and the improvement of productivity, new equipment which can measure the soil density in a deep stratum has to be developed. In this paper, we propose a method of accurately estimating compacted soil density based on the three dimensional stresses measured in the ground during compaction by a stress state transducer (SST). A tracked vehicle mounted with a vertical oscillator was used to compact a decomposed granite soil of 45 cm depth. A model experiment was executed at a frequency that was varied from 16 to 25 Hz, setting the load ratio of maximum oscillating force to the vehicle weight (4.9 kN) to be 1.2, 1.6 and 2.0. The three dimensional stresses in the ground were measured by use of the SST. Comparing the dry density converted from cone penetrometer test results and the dry density estimated from Baily’s formula, the compacted soil density at the lowest soil stratum could be estimated by measuring earth pressure using SST.     

A modified Drucker-Prager Cap model for die compaction simulation of pharmaceutical powders

In this paper, we present a modified density-dependent Drucker-Prager Cap (DPC) model to simulate the compaction behaviour of pharmaceutical powders. In particular, a nonlinear elasticity law is proposed to describe the observed nonlinear unloading behaviour following compaction. To extract the material parameters for the modified DPC model, a novel experimental calibration procedure is used, based on uniaxial single-ended compaction tests using an instrumented cylindrical die. The model is implemented in ABAQUS by writing a user subroutine, and a calibration process on microcrystalline cellulose (MCC) Avicel PH101 powders is detailed. The calibrated parameters are used for the manufacturing process simulation of two kinds of typical pharmaceutical tablets: the flat-face tablet and the concave tablet with single or double radius curvatures. The model developed can describe not only the compression and decompression phases, but also the ejection phase. The model is validated by comparing finite element simulations with experimental loading–unloading curves during the manufacture of 8 and 11 mm round tablets with flat-face (FF), single radius concave (SRC) and double radius concave (DRC) profiles. Moreover, the density and stress distributions during tabletting are used to analyse and explain the failure mechanism of tablets. The results show that the proposed model can quantitatively reproduce the compaction behaviour of pharmaceutical powders and can be used to obtain the stress and density distributions during compression, decompression and ejection.     

Cold compaction of ceramic powder: Computational analysis of the effect of pressing method and die shape

This paper deals with a computational analysis of the influence of the pressing method and part geometry on the final density distribution in the cold compaction process of ceramic alumina powders. The analysis is based on the explicit finite-element model proposed and validated in a previous study. The mechanical behavior of the processing material is described using a multisurface elastoplastic model, the modified Drucker-Prager/Cap model Published in Prikladnaya Mekhanika, Vol. 43, No. 10, pp. 129–134, October 2007.     

Numerical simulation of incompressible flow driven by density variations during phase change1

A change in density during the solidification of alloys can be an important driving force for convection, especially at reduced levels of gravity. A model is presented that accounts for shrinkage during the directional solidification of dendritic binary alloys under the assumption that the densities of the liquid and solid phases are different but constant. This leads to a non‐homogeneous mass conservation equation, which is numerically treated in a finite element formulation with a variable penalty coefficient that can resolve the velocity field correctly in the all‐liquid region and in the mushy zone. The stability of the flow when shrinkage interacts with buoyancy flows at low gravity is examined. Copyright © 1999 John Wiley & Sons, Ltd.     

Elasticity, fracture and yielding of cold compacted metal powders

The behaviour of powder compacts is modelled by explicitly introducing the possibility of plastic loading, elastic unloading and decohesion at contacts. The study is limited to cold compaction and to perfectly plastic materials for which the analysis of Mesarovic and Johnson (J. Mech. Phys. 48 (2000) 2009) is used. We model the compact behaviour both with an analytical approach based upon a mean field assumption and with the discrete element method (DEM) that allows force equilibrium to be treated in a realistic manner. Using these two approaches, we are able to predict the effective elastic properties of a powder compact at the onset of unloading. The knowledge of the conditions that lead to decohesion at the contact scale is used to model the fracture of the powder compact (green strength). It is shown that, in first approximation, green strength is inversely proportional to the size of the powder particles. The two methods are used to generate failure and yield surfaces for axisymmetric conditions. Both isostatic and close die conditions are studied.     

Pressure and temperature history in diamond silicon mixtures during dynamic compaction

Diamond powders with silicon additives were shock compressed by using a flyer impact technique. Pressure and temperature histories in the powder mixtures were numerically simulated in order to determine the optimum experimental condition which resulted in the highest Vicker's hardness. This was found to be: an initial diamond particle size of 2–4m at 7.2 % silicon by volume. The results of the simulations were consistent with the distribution of the microstructure and the microhardness in the compact.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1990.     

Numerical simulation for deformation of nano-grained metals

Electro-deposition technique is capable of producing nano-grained bulk copper specimens that exhibit superplastic extensibility at room temperature. Metals of such small grain sizes deform by grains sliding, with little distortion occurring in the grain cores. Accommodation mechanisms such as grain boundary diffusion, sliding and grain rotation control the kinetics of the process. Actual deformation minimizes the plastic dissipation and stored strain energy for representative steps of grain neighbor switching. Numerical simulations based on these principles are discussed in this paper. The project supported by the National Natural Science Foundation of China (19972031)     

Computation of compaction in compressible granular material

Equations modeling compaction in a mixture of granular high explosive and interstitial gas are solved numerically. Both phases are modeled as compressible, viscous fluids. This overcomes well known difficulties associated with computing shock jumps in the inviscid version of the equations, which cannot be posed in a fully conservative form. One-dimensional shock tube and piston-driven compaction solutions compare favorably with experiment and known analytic solutions. A simple two-dimensional extension is presented.     

Analysis of the dynamic radial compaction of granular media

The previously developed continual approximation is used to analyze the radial axisymmetric compaction of a granular medium in the presence of a rigid undeformable rod on the symmetry axis. It is shown that, during pulsed loading, high densities close to those corresponding to the nonporous state can be attained due to inertia effects. The influence of the initial radial dimensions of the rod-powder-medium system on the compaction process is analyzed. The problem is found to be scale invariant under various constraints imposed on the ratio of the characteristic dimensions. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 6, pp. 181–189, November–December, 2008     

Effect of Wave Formation during Shock-Wave Compaction of Powders

The problem of shock-wave compaction of a metal powder enclosed in a metal container with a transverse partition is solved. A model of wave formation on the partition and in the compact adjacent to the partition is proposed; the model is based on the loss of strength in the powder due to collapsing of pores and to development of instability of the partition being compressed in the shock wave. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 1, pp. 119–130, January–February, 2006.     

Numerical simulation of axisymmetric turbulent jets

The flow in axisymmetric turbulent jets is numerically simulated with the use of a semi-empirical second-order turbulence model including differential transport equations for the normal Reynolds stresses. Calculated results are demonstrated to agree with experimental data. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 5, pp. 55–60, September–October, 2008.     

Numerical simulation of drop migration in channell flow under zero-gravity

The migration of deformable drops in the channel flow neglecting the gravity influence is investigated numerically by solving the incompressible Navier-Stokes equations using the finitedifference method coupled with the front-tracking technique. The objectives of this study are to examine the effectiveness of the present approach for predicting the migration of drops in a shear flow and to investigate the behavior of the drop migration in the channel flow under zero-gravity. To validate the present calculation, some typical results are compared with available computational and theoretical data, which confirms that the present approach is reliable in predicting the drop migration. With respect to the drop migration in the channel flow at finite Reynolds numbers, the drops either move to an equilibrium lateral position or undergo an oscillatory motion under different conditions. The effects of some typical parameters, e.g., the Reynolds number, the Weber number, the viscosity ratio and the density ratio of the drop fluid to the suspending medium, and the drop size, on the migration of drops are discussed and analyzed. The project supported by the National Natural Science Foundation of China (10125210) and the Hundred-Talent Programme of the Chinese Academy of Sciences     

Numerical methods for the simulation of a coalescence-driven droplet size distribution

The droplet size distribution in a turbulent flow field is considered and modeled by means of a population balance system. This paper studies different numerical methods for the 4D population balance equation and their impact on an output of interest, the time-space-averaged droplet size distribution at the outlet, which is known from experiments. These methods include different interpolations of the experimental data at the inlet, various discretizations in time and space, and different schemes for computing the coalescence integrals. It will be shown that noticeable changes in the output of interest might occur. In addition, the computational efficiency of the studied methods is discussed.     

Particle morphology and packing effects on the shock loading of powders

A physically based model for the shock Hugoniot of a powdered material is described which allows separate identification of the cold and thermal contributions to pressure and specific internal energy. Special features of this model are provision for the effects of porosity on the stress state and an empirically determined cold loading contribution to pressure. The model was tested against published Hugoniot data for iron and gave excellent agreement for shock pressures ranging from low to high values.This shock Hugoniot was used to explore the shocked state of 4 samples of iron powder derived from commercially available material. The purpose of this study was to investigate the effect of powder particle characteristics and initial starting densities on the shocked state.The powder samples investigated had a range of morphologies and sizes. Powders with either a large shape factor or high internal friction, as determined in shear cell experiments, exhibited a higher stiffness in the cold loading curve. In the shocked state, this translated into a higher cold component of pressure and energy than found in the other powders.The effect of initial powder density was studied by applying the Hugoniot model to two impact initiated shock loadings, one for a stainless steel flyer impacting at 0.5 km/s and one at the higher velocity of 2.0 km/s. Both were applied to iron powder targets preloaded to a range of initial densities. For a given impact event, the proportion of shock energy in the thermal mode was found to decrease with increasing initial density. This decrease was more pronounced at higher shock strengths. As a result of the decreasing component of thermal energy with higher initial density, there was a reduction in the continuum temperature behind the shock. However, the corresponding increase in the component of cold energy with the falling relative contribution from the thermal energy lead to increasing density behind the shock suggesting that there is a trade off in terms of temperature and density achievable with a given impact event.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Numerical simulation on compressible turbulence by spectral method

The numerical and physical issues of simulations on compressible turbulence are reviewed in the present paper. An outline of the global spectral methods and the progress of recent local spectral methods are illustrated. Several typical subjects in this field are studied, including homogeneous isotropic turbulence, autoignition in premixed turbulence, interaction between flames and turbulence, and shock wave in turbulence. The results of the numerical simulations are discussed, enabling us to discover and to understand the physical phenomena which have not been solved by experiments.     

Numerical simulation of VOCs distribution in a room with a new carpet

This paper investigates the effect of volatile organic compounds (VOCs) emission from a new carpet in a room. Two cases with and without flow are modeled numerically. The commercial software Fluent 6 has been employed to solve the continuity, momentum, turbulence and concentration equations. The equivalent air phase concentration is solved as the dependent variable in the concentration equation. VOCs concentration in the breathing plane of an adult is carefully investigated. Numerical results show that the concentration near the center of recirculating flow is the highest in the case with flow while the concentration near the center of the carpet is the highest in the case without flow. The concentration in the right annex is lower than that in the left annex when the ventilation exists. Computation shows that there still exists much VOCs in the carpet at 240 h. Some additive ventilation is still necessary to maintain indoor air quality.