564 research outputs found
Experimental analysis and numerical simulation of sintered micro-fluidic
This paper investigates the use of numerical simulations to describe solid state diffusion of a sintering stage during a Powder Hot Embossing (PHE) process for micro-fluidic components. Finite element analysis based on a thermo-elasto-viscoplastic model was established to describe the densification process of a PHE stainless steel porous component during sintering. The corresponding parameters such as the bulk viscosity, shearing viscosity and sintering stress are identified from dilatometer experimental data. The numerical analyses, which were performed on a 3D micro-structured component, allowed comparison between the numerical predictions and experimental results of during a sintering stage. This comparison demonstrates that the FE simulation results are in better agreement with the experimental results at high temperatures
Physical modelling of amorphous thermoplastic polymer and numerical simulation of micro hot embossing process
Micro hot embossing process is considered as one of the most promising micro replication processes for manufacturing of polymeric components, especially for the high aspect ratio components and large surface structural components. A large number of hot embossing experimental results have been published, the material modelling and processes simulation to improve the quality of micro replication by hot embossing process are still lacking. This paper consists to 3D modelling of micro hot embossing process with amorphous thermoplastic polymers, including the mechanical characterisation of polymers properties, identification of the viscoelastic behaviour law of the polymers, numerical simulation and experimental investigation of micro hot embossing process. Static compression creep tests have been carried out to investigate the selected polymersâ viscoelastic properties. The Generalized Maxwell model has been proposed to describe the relaxation modulus of the polymers and good agreement has been observed. The numerical simulation of the hot embossing process in 3D has been achieved by taking into account the viscoelastic behaviour of the polymers. The microfluidic devices with the thickness of 2 mm have been elaborated by hot embossing process. The hot embossing process has been carried out using horizontal injection/compression moulding equipment, especially developed for this study. A complete compression mould tool, equipped with the heating system, the cooling system, the ejection system and the vacuum system, has been designed and elaborated in our research. Polymer-based microfluidic devices have been successfully replicated by the hot embossing process using the compression system developed. Proper agreement between the numerical simulation and the experimental elaboration has been observed. It shows strong possibility for the development of the 3D numerical model to optimize the micro hot embossing process in the future
Water soluble Invar 36 feedstock development for ”PIM
A water soluble binder system based on cellulose acetate butyrate (CAB) and polyethylene glycol (PEG) is proposed and investigated to carry out a micro powder injection moulding (muPIM) process with an Invar 36 alloy powders. The overall process was optimised with an emphasis on the determination of the optimal solid loading. Several methodologies were evaluated and compared to determine this parameter. A full muPIM was performed with different powder content feedstocks. Dog bone-type micro test parts were fabricated thereof and their mechanical properties were evaluated. Solid loadings up to 65 vol.% resulted to have the most equilibrated properties to successfully fabricate Invar 36 micro parts with the selected powder and binder system.The authors wish to thank GUZMĂN GLOBAL S.L. and MIMTECH ALFA for their collaboration on the ECOPIM project (Ref. IPT-2011-0931-20000) that was funded by the Spanish Ministry of the Economy and Competitiveness. Furthermore, the authors would like to acknowledge the strong support from the ESTRUMAT projects (Ref. S2009/MAT-1585), which were funded by the CAM ConsejerĂa EducaciĂłn Dir. Gral. Universidades e InvestigaciĂłn, and from the COMETAS project (Ref. MAT2009/14448-C02-02), which was funded by the Spanish Ministry of the Economy and Competitiveness.Publicad
Dynamics of a one-dimensional Holstein polaron: The multiconfigurational Ehrenfest method
We have extended the multiconfigurational Ehrenfest (MCE) approach to investigate the dynamics of a one-dimensional Holstein molecular crystal model. It has been shown that the extended MCE approach yields results in perfect agreement with benchmark calculations by the hierarchy equations of motion method. The accuracies of the MCE approach in describing the dynamical properties of the Holstein polaron over a wide range of exciton transfer integrals and exciton-phonon couplings are carefully examined by a detailed comparison with the fully variational multiple Davydov D2 ansatz. It is found that while the MCE approach and the multi-D2 ansatz produce almost exactly the same results for a small transfer integral, the results obtained by the multi-D2 ansatz start to deviate from those by the MCE approach at longer times for a large transfer integral. A large number of coherent state basis functions are required to characterize the delocalized features of the phonon wavefunction in the case of large transfer integral, which becomes computationally too demanding for the multi-D2 ansatz. The MCE approach, on the other hand, uses hundreds to thousands of trajectory guided basis functions and converges very well, thus providing an effective tool for accurate and efficient simulations of polaron dynamics
Effect of the particle size and solids volume fraction on the thermal degradation behaviour of Invar 36 feedstocks
Degradation kinetics and the thermal stability of Invar 36 powder injection moulding feedstocks (PIM) based on cellulose acetate butyrate (CAB) and polyethylene glycol (PEG) binders were investigated using simultaneous thermogravimetric analysis (STA) and differential scanning calorimetry (DSC). The initial decomposition temperature (IDT) and the integral procedure decomposition temperature (IPDT) were used to analyse the thermal stability of the binder system as a function of the solid loading content and powder particle size. The degradation kinetics was studied, and the process apparent activation energies were assessed using isoconversional methods. All the methodologies revealed changes in the thermal degradation behaviours of the feedstocks for solid loadings that were previously determined to correspond to optimal solid loadings using other experimental procedures. The studies also contrast previous similar findings with a ceramic powder. Therefore these results strengthen the proposal of thermodynamic degradation studies of feedstocks as an alternative or complementary technique to determine optimal solid loading contents in metal injection moulding (MIM).The authors a wish to thank GUZMĂN GLOBAL S.L. and MIM
TECH ALFA for their collaboration on the ECOPIM project (ref. IPT
2011 0931 20000) that was funded by the Spanish Ministry of
the Economy and Competitiveness. Furthermore, the authors
would like to acknowledge the strong support from the ESTRUMAT
projects (ref. S2009/MAT 1585), which were funded by the CAM
ConsejerĂa EducaciĂłn Dir. Gral. Universidades e InvestigaciĂłn, and
from the COMETAS project (ref. MAT2009/14448 C02 02), which
was funded by the Spanish Ministry of the Economy and Compet
itiveness. J.P.F. B. acknowledges support from âMarie Curieâ
Amarout Europe Program
Role of the Subunits Interactions in the Conformational Transitions in Adult Human Hemoglobin: an Explicit Solvent Molecular Dynamics Study
Hemoglobin exhibits allosteric structural changes upon ligand binding due to
the dynamic interactions between the ligand binding sites, the amino acids
residues and some other solutes present under physiological conditions. In the
present study, the dynamical and quaternary structural changes occurring in two
unligated (deoxy-) T structures, and two fully ligated (oxy-) R, R2 structures
of adult human hemoglobin were investigated with molecular dynamics. It is
shown that, in the sub-microsecond time scale, there is no marked difference in
the global dynamics of the amino acids residues in both the oxy- and the deoxy-
forms of the individual structures. In addition, the R, R2 are relatively
stable and do not present quaternary conformational changes within the time
scale of our simulations while the T structure is dynamically more flexible and
exhibited the T\rightarrow R quaternary conformational transition, which is
propagated by the relative rotation of the residues at the {\alpha}1{\beta}2
and {\alpha}2{\beta}1 interface.Comment: Reprinted (adapted) with permission from J. Phys. Chem. B
DOI:10.1021/jp3022908. Copyright (2012) American Chemical Societ
A unified approach to the derivation of work theorems for equilibrium and steady-state, classical and quantum Hamiltonian systems
We present a unified and simple method for deriving work theorems for
classical and quantum Hamiltonian systems, both under equilibrium conditions
and in a steady state. Throughout the paper, we adopt the partitioning of the
total Hamiltonian into the system part, the bath part, and their coupling. We
rederive many equalities which are available in the literature and obtain a
number of new equalities for nonequilibrium classical and quantum systems. Our
results can be useful for determining partition functions and (generalized)
free energies through simulations and/or measurements performed on
nonequilibrium systems
A False Start in the Race Against Doping in Sport: Concerns With Cyclingâs Biological Passport
Professional cycling has suffered from a number of doping scandals. The sportâs governing bodies have responded by implementing an aggressive new antidoping program known as the biological passport. Cyclingâs biological passport marks a departure from traditional antidoping efforts, which have focused on directly detecting prohibited substances in a cyclistâs system. Instead, the biological passport tracks biological variables in a cyclistâs blood and urine over time, monitoring for fluctuations that are thought to indirectly reveal the effects of doping. Although this method of indirect detection is promising, it also raises serious legal and scientific concerns. Since its introduction, the cycling community has debated the reliability of indirect biological-passport evidence and the clarity, consistency, and transparency of its use in proving doping violations. Such uncertainty undermines the legitimacy of finding cyclists guilty of doping based on this indirect evidence alone. Antidoping authorities should address these important concerns before continuing to pursue doping sanctions against cyclists solely on the basis of their biological passports
Effects of Three Months of Low Molecular Weight Heparin (dalteparin) Treatment After Bypass Surgery for Lower Limb IschemiaâA Randomised Placebo-controlled Double Blind Multicentre Trial
AbstractObjectivesTo test the hypothesis that long-term postoperative dalteparin (FragminÂź, Pharmacia Corp) treatment improves primary patency of peripheral arterial bypass grafts (PABG) in lower limb ischemia patients on acetylsalicylic acid (ASA) treatment.DesignProspective randomised double blind multicenter study.Materials and methodsUsing a computer algorithm 284 patients with lower limb ischemia, most with pre-operative ischemic ulceration or partial gangrene, from 12 hospitals were randomised, after PABG, to 5000IU dalteparin or placebo injections once daily for 3 months. All patients received 75mg of ASA daily for 12 months. Graft patency was assessed at 1, 3 and 12 months.ResultsAt 1 year, 42 patients had died or were lost to follow-up. Compliance with the injection schedule was 80%. Primary patency rate, in the dalteparin versus the control group, respectively, was 83 versus 80% (n.s.) at 3 months and 59% for both groups at 12 months. Major complication rates and cardiovascular morbidity were not different between the two groups.ConclusionsIn patients on ASA treatment, long-term postoperative dalteparin treatment did not improve patency after peripheral artery bypass grafting. Therefore, low molecular weight heparin treatment cannot be recommended for routine use after bypass surgery for critical lower limb ischemia
Comparing faceted and smoothed tool surface descriptions in sheet metal forming simulation
This study deals with different tool surface description
methods used in the finite element analysis of sheet metal
forming processes. The description of arbitrarily-shaped tool
surfaces using the traditional linear finite elements is compared
with two distinct smooth surface description approaches:
(i) BĂ©zier patches obtained from the ComputerAided
Design model and (ii) smoothing the finite element
mesh using Nagata patches. The contact search algorithm is
presented for each approach, exploiting its special features in
order to ensure an accurate and efficient contact detection. The
influence of the tool modelling accuracy on the numerical
results is analysed using two sheet forming examples, the
unconstrained cylindrical bending and the reverse deep drawing
of a cylindrical cup. Smoothing the contact surfaces with
Nagata patches allows creating more accurate tool models,
both in terms of shape and normal vectors, when compared
with the conventional linear finite element mesh. The computational
efficiency is evaluated in this study through the total
number of increments and the required CPU time. The mesh
refinement in the faceted description approach is not effective
in terms of computational efficiency due to large discontinuities
in the normal vector field across facets, even when
adopting fine meshes.The authors gratefully acknowledge the financial
support of the Portuguese Foundation for Science and Technology (FCT)
via the projects PTDC/EME-TME/118420/2010 and PEst-C/EME/
UI0285/2013 and by FEDER funds through the program COMPETE â
Programa Operacional Factores de Competitividade, under the project
CENTRO-07-0224-FEDER-002001 (MT4MOBI). The first author is
also grateful to the FCT for the PhD grant SFRH/BD/69140/2010.info:eu-repo/semantics/publishedVersio
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