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1.
Chemical Vapor Deposition (CVD) of carbon nanotubes from a gas mixture consisting of methane (carbon precursor) and hydrogen (a carrier gas) in the presence of cobalt, nickel or iron catalytic particles in a cylindrical reactor is modeled at the reactor length-scale by solving a continuum-based coupled boundary-layer laminar-flow hydrodynamics, heat-transfer, gas-phase chemistry and surface chemistry problem. The model allows determination of the gas-phase fields for temperature, velocity, and various species as well as the surface-species coverages and the carbon deposition rate. Various available experimental and theoretical assessments are used to construct the necessary database for gas-phase and surface chemistry and gas-phase transport parameters. A reasonably good agreement is found between the model predicted and the experimentally measured carbon nanotubes deposition rates over a relatively large range of processing conditions. 相似文献
2.
The commercial finite element package ABAQUS has been used to analyse the crack bridging process by Ti-15 at%V -phase particles dispersed in -TiAl matrix in the presence of particle–matrix decohesion. Both the particle–matrix decohesion potential and the -phase materials constitutive relations are found to have a major effect on the ductility, fracture toughness and failure mode of the – two-phase material. The interface potential is found to primarily affect the distribution of the normal interface strength ahead of the advancing interfacial crack and the mode (gradual versus sudden) of decohesion. The -phase materials constitutive relations are found to influence the location of nucleation of the interfacial cracks and, in turn, the mode of decohesion. A metastable -phase that can plastically deform at low stress levels by undergoing a stress-assisted martensitic transformation, but experience a high rate of strain hardening is found to give rise to the largest levels of ductility and fracture toughness is the – two-phase material. © 1998 Kluwer Academic Publishers 相似文献
3.
Grégory Chatelain Micaël Ripert Carole Farre Salomé Ansanay-Alex Carole Chaix 《Electrochimica acta》2012
We report the use of a four-ferrocene modified oligonucleotide as a probe for DNA detection with a gold electrode microsystem. This oligonucleotide is synthesized by automated solid-phase synthesis with four successive ferrocene moieties at the 5′-end and a C6-thiol modifier group at the 3′-end. The grafting of this 4Fc-DNA probe on a gold electrode microsystem results in the appearance of the ferrocene redox couple in cyclic voltammetry. The probe sequence is a stem-loop structure that folds efficiently on the electrode, thus optimizing electron transfer. Such architecture serves as sensor for DNA detection which is based on hybridization. The resulting disposable voltammetric sensor allowed direct, reagentless DNA detection in a small volume (20 μL). Electrochemical response upon hybridization with complementary short sequence (30-base length) and long sequence (50-base length) strands was observed by differential pulse voltammetry. Current variations were compared. The longer the sequence, the greater the decrease in current. The system's detection limit was estimated at 3.5 pM (0.07 fmol in 20 μL) with the 50-base length target and provided a dynamic detection range between 3.5 pM and 5 nM. Single mismatch detection showed a good level of sensitivity. The system was regenerated twice with no significant loss of Fc signal. Finally, 1 pM sensitivity was reached with a long chain analog of DNA PCR products of Influenza virus. 相似文献
4.
M. Grujicic B. Pandurangan C.-F. Yen B. A. Cheeseman 《Journal of Materials Engineering and Performance》2012,21(11):2207-2217
Johnson-Cook strength material model is frequently used in finite-element analyses of various manufacturing processes involving plastic deformation of metallic materials. The main attraction to this model arises from its mathematical simplicity and its ability to capture the first-order metal-working effects (e.g., those associated with the influence of plastic deformation, rate of deformation, and the attendant temperature). However, this model displays serious shortcomings when used in the engineering analyses of various hot-working processes (i.e., those utilizing temperatures higher than the material recrystallization temperature). These shortcomings are related to the fact that microstructural changes involving: (i) irreversible decrease in the dislocation density due to the operation of annealing/recrystallization processes; (ii) increase in grain-size due to high-temperature exposure; and (iii) dynamic-recrystallization-induced grain refinement are not accounted for by the model. In this study, an attempt is made to combine the basic physical-metallurgy principles with the associated kinetics relations to properly modify the Johnson-Cook material model, so that the model can be used in the analyses of metal hot-working and joining processes. The model is next used to help establish relationships between process parameters, material microstructure and properties in friction stir welding welds of AA5083 (a non-age-hardenable, solid-solution strengthened, strain-hardened/stabilized Al-Mg-Mn alloy). 相似文献
5.
M. Grujicic B. Pandurangan B. A. Cheeseman C.-F. Yen 《Journal of Materials Engineering and Performance》2012,21(9):1813-1823
Spallation is a fracture mode commonly observed in ballistically/blast-wave-loaded structures. The interaction between decompression waves generated within the target structure produces tensile stresses which, if of a sufficient magnitude, may cause material damage and ultimate fracture (spallation). In this study, the phenomenon of spall-fracture is analyzed within a one-dimensional Lagrangian framework. Two distinct analyses are carried out. Within the first analysis, decompression waves are treated as decompression shocks, which simplified the analysis and enabled the formation of spallation-strength-based material index. In the second analysis, decompression waves are treated as smooth (centered simple) waves. This increased the fidelity of the computational analysis, but the material-selection procedure could be done only numerically and an explicit formulation of the spallation-strength-based material-selection index could not be carried out. Overall, the two analyses yielded similar results for the spallation-strength-based material-selection criterion suggesting that the simpler (decompression shock based) one is still adequate for use in the material-selection process. 相似文献
6.
Mica Grujicic B. Pandurangan W. C. Bell S. Bagheri 《Journal of Materials Engineering and Performance》2012,21(2):167-179
The propagation of uniaxial-stress planar shocks in granular materials is analyzed using a conventional shock-physics approach.
Within this approach, both compression shocks and decompression waves are treated as (stress, specific volume, particle velocity,
mass-based internal energy density, temperature, and mass-based entropy density) propagating discontinuities. In addition,
the granular material is considered as being a continuum (i.e., no mesoscale features like grains, voids, and their agglomerates
are considered). However, while the granular material is treated as a (smeared-out) continuum, it is recognized that it contains
a solid constituent (parent matter), and that the structurodynamic properties (i.e., Equations of State (EOS) and Hugoniot
relations) of the granular material are related to its parent matter. Three characteristic shock loading regimes of granular
material are considered and, in each case, an analysis is carried out to elucidate shock attenuation and energy dissipation
processes. In addition, an attempt is made to identify a metric (a combination of the material parameters) which quantifies
the intrinsic ability of a granular material to attenuate a shock and dissipate the energy carried by the shock. Toward that
end, the response of a typical granular material to a flat-topped compressive stress pulse is analyzed in each of the three
shock loading regimes. 相似文献
7.
M. Grujicic G. Arakere W. C. Bell H. Marvi H. V. Yalavarthy B. Pandurangan I. Haque G. M. Fadel 《Journal of Materials Engineering and Performance》2010,19(3):301-313
The effect of materials processing- and component manufacturing-induced uncertainties in material properties and component
shape and size on the reliability of component performance is investigated. Specifically, reliability of a suspension system
component from a high-mobility multipurpose wheeled vehicle which typically can fail under low-cycle strain-based fatigue
conditions is analyzed. Toward that end, the most advanced reliability-based design optimization methods available in the
literature were combined with the present understanding of low-cycle fatigue durability and applied to the component in question.
This entailed intricate integration of several computational tools such as multibody vehicle dynamics, finite-element simulations,
and fatigue strain-life assessment/prediction techniques. The results obtained clearly revealed the importance of consideration
of material property uncertainties in attaining vehicle performance of critical structural components in complex systems (e.g.,
a vehicle). 相似文献
8.
M. Grujicic G. Arakere B. Pandurangan V. Sellappan A. Vallejo M. Ozen 《Journal of Materials Engineering and Performance》2010,19(8):1116-1127
A multi-disciplinary design-optimization procedure has been introduced and used for the development of cost-effective glass-fiber
reinforced epoxy-matrix composite 5 MW horizontal-axis wind-turbine (HAWT) blades. The turbine-blade cost-effectiveness has
been defined using the cost of energy (CoE), i.e., a ratio of the three-blade HAWT rotor development/fabrication cost and
the associated annual energy production. To assess the annual energy production as a function of the blade design and operating
conditions, an aerodynamics-based computational analysis had to be employed. As far as the turbine blade cost is concerned,
it is assessed for a given aerodynamic design by separately computing the blade mass and the associated blade-mass/size-dependent
production cost. For each aerodynamic design analyzed, a structural finite element-based and a post-processing life-cycle
assessment analyses were employed in order to determine a minimal blade mass which ensures that the functional requirements
pertaining to the quasi-static strength of the blade, fatigue-controlled blade durability and blade stiffness are satisfied.
To determine the turbine-blade production cost (for the currently prevailing fabrication process, the wet lay-up) available
data regarding the industry manufacturing experience were combined with the attendant blade mass, surface area, and the duration
of the assumed production run. The work clearly revealed the challenges associated with simultaneously satisfying the strength,
durability and stiffness requirements while maintaining a high level of wind-energy capture efficiency and a lower production
cost. 相似文献
9.
M. Grujicic W. C. Bell B. Pandurangan P. S. Glomski 《Journal of Materials Engineering and Performance》2011,20(6):877-893
To combat the problem of traumatic brain injury (TBI), a signature injury of the current military conflicts, there is an urgent
need to design head protection systems with superior blast/ballistic impact mitigation capabilities. Toward that end, the
blast impact mitigation performance of an advanced combat helmet (ACH) head protection system equipped with polyurea suspension
pads and subjected to two different blast peak pressure loadings has been investigated computationally. A fairly detailed
(Lagrangian) finite-element model of a helmet/skull/brain assembly is first constructed and placed into an Eulerian air domain
through which a single planar blast wave propagates. A combined Eulerian/Lagrangian transient nonlinear dynamics computational
fluid/solid interaction analysis is next conducted in order to assess the extent of reduction in intra-cranial shock-wave
ingress (responsible for TBI). This was done by comparing temporal evolutions of intra-cranial normal and shear stresses for
the cases of an unprotected head and the helmet-protected head and by correlating these quantities with the three most common
types of mild traumatic brain injury (mTBI), i.e., axonal damage, contusion, and subdural hemorrhage. The results obtained
show that the ACH provides some level of protection against all investigated types of mTBI and that the level of protection
increases somewhat with an increase in blast peak pressure. In order to rationalize the aforementioned findings, a shockwave
propagation/reflection analysis is carried out for the unprotected head and helmet-protected head cases. The analysis qualitatively
corroborated the results pertaining to the blast-mitigation efficacy of an ACH, but also suggested that there are additional
shockwave energy dissipation phenomena which play an important role in the mechanical response of the unprotected/protected
head to blast impact. 相似文献
10.
Mode-I fracture behavior of fully-lamellar polycrystalline -TiAl + 2-Ti3Al intermetallic alloys and the role of Ti-V base -phase precipitates of different thermodynamic stability have been studied using a finite element method. A rate-dependent, finite-strain, crystal-plasticity based materials constitutive model is used to represent the deformation behavior of both the -TiAl + 2-Ti3Al lamellar matrix and the -phase precipitates. Within the matrix colonies, fracture is assumed to take place throughout the 2-Ti3Al lamellae. In addition, fracture along colony boundaries and matrix/precipitate interfaces is considered. The constitutive behavior of all fracture interfaces is modeled using a cohesive-zone formulation. The analysis is carried out using the commercial finite element program Abaqus/Standard within which the material state is integrated using an Euler-backward implicit formulation. The results obtained show that the main mechanism of crack growth is nucleation of secondary cracks along 2-Ti3Al lamellae ahead of the main crack and their subsequent link-up with the tip of the main crack. The resulting fracture resistance curve acquires the characteristic step-wise shape. Both stable and metastable -phase precipitates are found to have a beneficial effect on the fracture resistance of the material. However, the effect is not very significant and metastable -phase precipitates appear to be a little bit more beneficial. All these findings are consistent with their experimental counterparts. 相似文献