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Time Increment (time + increment)
Selected AbstractsA minimization principle for finite strain plasticity: incremental objectivity and immediate implementationINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 12 2002Eric Lorentz Abstract A finite strain plasticity formulation is proposed which meets several requirements that often appear contradictory. On a physical ground, it is based on a multiplicative split of the deformation, hyperelasticity for the reversible part of the behaviour and the maximal dissipation principle to define the evolution laws. On a numerical ground, it is incrementally objective and the integration over a time increment can be expressed as a minimization problem, a proper framework to examine the questions of existence and uniqueness of the solutions. Last but not least, the implementation is immediate since it relies on the same equations for finite and infinitesimal transformations. Finally, the formulationis applied to von Mises plasticity with isotropic linear hardening and introduced in the finite element software Code_Aster®. The numerical computation of a cantilever beam shows that it leads to results in good agreement with those obtain with common approaches. Copyright © 2002 John Wiley & Sons, Ltd. [source] Geometry update driven by material forces for simulation of brittle crack growth in functionally graded materialsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 13 2009Rolf Mahnken Abstract Functionally graded materials (FGMs) are advanced materials that possess continuously graded properties, such that the growth of cracks is strongly dependent on the gradation of the material. In this work a thermodynamic consistent framework for crack propagation in FGMs is presented, by applying a dissipation inequality to a time-dependent migrating control volume. The direction of crack growth is obtained in terms of material forces as a result of the principle of maximum dissipation. In the numerical implementation a staggered algorithm,deformation update for fixed geometry followed by geometry update for fixed deformation,is employed within each time increment. The geometry update is a result of the incremental crack propagation, which is driven by material forces. The corresponding mesh is generated by combining Delaunay triangulation with local mesh refinement. Furthermore a Newton algorithm is proposed, taking into account mesh transfer of displacements for crack propagation in incremental elasticity. In two numerical examples brittle crack propagation in FGMs is investigated for various directions of strength gradation within the structures. Copyright © 2008 John Wiley & Sons, Ltd. [source] Numerical finite element formulation of the Schapery non-linear viscoelastic material modelINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 1 2004Rami M. Haj-Ali Abstract This study presents a numerical integration method for the non-linear viscoelastic behaviour of isotropic materials and structures. The Schapery's three-dimensional (3D) non-linear viscoelastic material model is integrated within a displacement-based finite element (FE) environment. The deviatoric and volumetric responses are decoupled and the strain vector is decomposed into instantaneous and hereditary parts. The hereditary strains are updated at the end of each time increment using a recursive formulation. The constitutive equations are expressed in an incremental form for each time step, assuming a constant incremental strain rate. A new iterative procedure with predictor,corrector type steps is combined with the recursive integration method. A general polynomial form for the parameters of the non-linear Schapery model is proposed. The consistent algorithmic tangent stiffness matrix is realized and used to enhance convergence and help achieve a correct convergent state. Verifications of the proposed numerical formulation are performed and compared with a previous work using experimental data for a glassy amorphous polymer PMMA. Copyright © 2003 John Wiley & Sons, Ltd. [source] Application of the high-order symplectic FDTD scheme to the curved three-dimensional perfectly conducting objectsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 4 2007Wei Sha Abstract A high-order symplectic finite-difference time-domain (SFDTD) scheme using the diagonal split-cell model is presented to analyze electromagnetic scattering of the curved three-dimensional perfectly conducting objects. On the one hand, for the undistorted cells, the fourth-order accurate spatial difference is employed. On the other hand, for the completely distorted cells, the treatment of the curved surfaces is based on the diagonal split-cell model. Finally, for the partially distorted cells, the interpolation strategy is proposed to keep the field components continuous. The numerical experiments suggest that the diagonal SFDTD scheme can obtain more accurate results than both the staircased SFDTD scheme and the traditional diagonal FDTD method. Furthermore, in view of the high numerical stability, the improved symplectic scheme does not need to decrease time increment to comply with the stability criterion. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 931,934, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22306 [source] A space-time mixed-hybrid finite element method for the damped wave equationNUMERICAL METHODS FOR PARTIAL DIFFERENTIAL EQUATIONS, Issue 2 2008A. Serghini Mounim Abstract A space-time finite element method is introduced to solve the linear damped wave equation. The scheme is constructed in the framework of the mixed-hybrid finite element methods, and where an original conforming approximation of H(div;,) is used, the latter permits us to obtain an upwind scheme in time. We establish the link between the nonstandard finite difference scheme recently introduced by Mickens and Jordan and the scheme proposed. In this regard, two approaches are considered and in particular we employ a formulation allowing the solution to be marched in time, i.e., one only needs to consider one time increment at a time. Numerical results are presented and compared with the analytical solution illustrating good performance of the present method. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2008 [source] Dynamic Modeling and Simulation of Steam Cracking FurnacesCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 7 2007M. Ghashghaee Abstract The transient modeling of thermal cracking furnaces is developed. This representation is capable of describing and predicting the unsteady-state behavior of cracking furnaces during start-up. To accurately predict the heat transfer to the reactor tube, the fireside conditions are coupled with the process side. The mutual interaction of these two sections is found to be very stiff in terms of convergence of the computations. The two-dimensional transient zone model is developed for the radiative heat exchange calculation. A simplified model for the convection section is also used to predict the crossover temperature at each time increment. The main simulation outputs are the flue gas properties as well as the distributions of heat flux, refractory wall and coil skin temperatures versus time. The dynamic simulation is implemented for a conventional procedure used in the start-up run of the olefin furnaces. [source] NMR Characterization of Complex p- Oligophenyl Scaffolds by Means of Aliasing Techniques to Obtain Resolution-Enhanced Two-Dimensional SpectraHELVETICA CHIMICA ACTA, Issue 9 2004Damien Jeannerat The usefulness of computer-assisted aliasing to secure maximal resolution of signal clusters in 1H- and 13C-NMR spectra (which is essential for structure determination by HMBC 2D NMR spectroscopy) in minimal acquisition time is exemplified by the complete characterization of the two complementary p -octiphenyls 1 and 2 with complex substitution patterns. The need for digital resolution near 1,Hz/pt to dissect the extensive signal clusters in the NMR spectra of these refined oligomers excluded structure determination under routine conditions. High resolution was secured by exploiting the low signal density in the 13C dimension of HMBC spectra by using computer-assisted aliasing to maximize signal density. Based on the observed shifts in DEPT and 1H-decoupled 13C-NMR spectra of 1 and 2, computer-assisted aliasing allowed to reduce the number of required time increments by a factor of 20 to 30 compared to full-width spectra with identical resolution. Without signal-to-noise constraints, this computer-assisted aliasing reduced the acquisition time for high-resolution NMR spectra needed for complete characterization of refined oligomers 1 and 2 by the same factor (e.g., from over a day to about an hour). With resolved signal clusters in fully aliased HSQC and HMBC spectra, unproblematic structure determination of 1 and 2 is demonstrated by unambiguous assignment of all C- and H-atoms. These findings demonstrate that computer-assisted aliasing of the underexploited 13C dimension makes extensive molecular complexity accessible by conventional multidimensional heteronuclear NMR experiments without extraordinary efforts. [source] Numerical characteristics of a simple finite element formulation for consolidation analysisINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 10 2004Guofu Zhu Abstract The spatial oscillation of values in the consolidation analysis when using small time increments has been a common problem for most existing methods. In this paper, the numerical characteristics of a simple finite element formulation for 1-D consolidation analysis recently proposed by the authors have been examined in detail. This paper proves that the commonly encountered phenomenon of spatial oscillation due to small time increments does not occur in the simple finite element formulation. The criterion of minimum time step used in most existing methods can be eliminated at least for linear situations by using the simple formulation proposed by the authors. Thus, the consolidation analysis can be carried easily for many situations, such as the one involving a relatively impermeable clay layer sandwiched between sandy layers. Copyright © 2004 John Wiley & Sons, Ltd. [source] Time-domain BEM solution of convection,diffusion-type MHD equationsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 11 2008N. Bozkaya Abstract The two-dimensional convection,diffusion-type equations are solved by using the boundary element method (BEM) based on the time-dependent fundamental solution. The emphasis is given on the solution of magnetohydrodynamic (MHD) duct flow problems with arbitrary wall conductivity. The boundary and time integrals in the BEM formulation are computed numerically assuming constant variations of the unknowns on both the boundary elements and the time intervals. Then, the solution is advanced to the steady-state iteratively. Thus, it is possible to use quite large time increments and stability problems are not encountered. The time-domain BEM solution procedure is tested on some convection,diffusion problems and the MHD duct flow problem with insulated walls to establish the validity of the approach. The numerical results for these sample problems compare very well to analytical results. Then, the BEM formulation of the MHD duct flow problem with arbitrary wall conductivity is obtained for the first time in such a way that the equations are solved together with the coupled boundary conditions. The use of time-dependent fundamental solution enables us to obtain numerical solutions for this problem for the Hartmann number values up to 300 and for several values of conductivity parameter. Copyright © 2007 John Wiley & Sons, Ltd. [source] Topical bovine thrombin: a 21-year review of topical bovine thrombin spontaneous case safety reports submitted to FDA's Adverse Event Reporting SystemPHARMACOEPIDEMIOLOGY AND DRUG SAFETY, Issue 2 2010John A. Clark MD Abstract Purpose To review topical bovine thrombin spontaneous adverse event (AE) reports that were forwarded to the US Food and Drug Administration's (FDA) Adverse Event Reporting System (AERS) between January 1986 and December 2006. Methods Forty-one spontaneous AE reports were summarized for reported AE profile and chronological reporting patterns. Each AE report was adjudicated by a hematologist for the topical bovine thrombin product that was given and the AE(s) that were reported. AEs were grouped as allergic, coagulopathy/bleeding, and all other AEs combined. Grouped AE serial analyses were carried out using successive 3-year time increments between 1986 (the year an AE report was first noted for a bovine thrombin product) and 2006 (the first full year that was available at the time of initiation of the data summary). Main outcome measures The primary outcome measures were every 3-year trend lines for all-AE reports, all reporters, and topical bovine thrombin brand mentions for 2 AE groups of interest (allergic events and coagulopathy/bleeding events). Results The all-AE spontaneous reporter trend showed a downward appearance for AE reporting activity that started in 1995,1998 and continued through 2004,2006. The all-AE reports trend showed two potential safety signals that could be identified serially: (1) a prominent 1989,1991 peak that was attributable to allergic events (in particular, anaphylaxis), and (2) a small 1995,2000 broad peak that was attributable in part to coagulopathy/bleeding events. Allergic events were predominantly reported with products approved prior to 1995, were not temporally associated with prior medical literature case reports, and continued to be forwarded to the FDA at low levels up to the end of this study in 2006. Coagulopathy/bleeding events were reported only with products approved prior to 1995, were temporally associated with medical literature case reports, and were not forwarded to the FDA after 2000. Conclusions Overall, spontaneous AE reporting for topical bovine thrombin occurs at very low levels, and appears to have been decreasing since 1995. The serial reporting patterns for topical bovine thrombin are best explained as a strong safety signal for allergic events with ongoing, low level reporting, and a weak safety signal for coagulopathy/bleeding events that ceased on or before 2000. Although this descriptive trend analysis cannot measure associations or causation, the coagulopathy/bleeding signal may have been prompted by multiple, antecedent published case reports. The subsequent diminishment of signal attributed to thrombin likewise may coincide with lack of such reporting in larger follow-up clinical trials or, alternatively, in the introduction and growing market share of thrombin brands of greater purity. Currently marketed topical bovine thrombin formulations are rarely volunteered as possible causes of adverse events. Copyright © 2009 John Wiley & Sons, Ltd. [source] Dynamics of a protected black rhino (Diceros bicornis) population: Pilanesberg National Park, South AfricaANIMAL CONSERVATION, Issue 3 2005Halszka Hrabar Achieving maximum productivity in remnant populations of black rhinoceros is crucial to the persistence of this species. It was, therefore, investigated whether the black rhino population of Pilanesberg National Park had become regulated by resource limitation 22 years after introduction in 1979. Inter-calving intervals (which are not restricted to yearly time increments, due to asynchronous reproduction) decreased with an increase in rainfall, while the percentage of male calves born increased with increasing rainfall. The percentage of reproductive cows achieving maternal success increased with increasing density until 0.085 rhinos/km2, after which it decreased. This positive relationship at low densities is largely due to changes in the female age structure and the adult female/male ratio. The age at first calving tended to increase with increasing density, while mortality was not related to rainfall or density. It is concluded that the Pilanesberg black rhino population is showing the first signs of density dependence. It is proposed that black rhino conservators should monitor the percentage of cows achieving maternal success to detect early indications of density dependent resource limitation and use this as a criteria for decisions regarding metapopulation management. 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