37,383 research outputs found
Numerical Methods for the Modelling of Debonding in Composites
No full textThis monograph starts with a discussion of various phenomena in laminated composite structures that can lead to failure: matrix cracking, delamination between plies, and debonding and subsequent pull-out between fibres and the matrix material. Next, the different scales are discussed at which the effect of these nonlinearities can be analysed and the ways to couple analyses at these different length scales. From these scales—the macro, meso and micro-levels — the meso-level is normally used for the analysis of delamination, which is the focus of this monograph. At this level, the plies are modelled as continua and interface elements between them conventionally serve as the framework to model delami-nation and debonding. After a brief discussion of the cohesive—zone concept and its importance for the analysis of delamination, a particular finite element model for the plies is elaborated: the solid—like shell. This is followed by a derivation of interface elements. In the second part of this monograph more recent methods to numerically model delamination are discussed: meshfree methods, methods that exploits the partition—of—unity property of finite element shape functions, and discontinuous Galerkin methods. These approaches offer advantages over the more traditional approach that uses interface elements, as will be discussed in detail. From these more modern discretisation concepts the partition-of-unity approach seems the most promising for modelling debonding in composite structures, one advantage being that it can rather straightforwardly be incorporated in solid-like shell elements, thus enabling large-scale analyses of layered composite structures that take into account the possibility of debonding
Discontinuous modelling of strain localisation and failure
No full textThe computational simulation of failure in solids poses many challenges. A proper understanding of how structures respond under loading, both before and past the peak load, is important for safe and economical constructions. This requires numerical models for failure which are both faithful to the physical reality and mathematically well founded. A serious computational issue is that of objectivity with respect to the spatial discretisation of a problem. This requires that upon refinement of the spatial discretisation of a problem, a unique, physically meaningful result is approached. One approach to ensure objectivity with respect to spatial discretisation when simulating failure in solids is to allow displacement discontinuities in the solution. In this work, different techniques, of varying complexity, are developed to simulate displacement discontinuities which are independent of the spatial discretisation using finite elements. The different techniques are then critically evaluated. The first model examined involves adding only the effect of a displacement discontinuity to a finite element as an incompatible strain mode. This allows a traction–separation relationship to be applied at an interface and can be implemented simply in a standard finite element code. It is however shown that this type of model can be cast in an equivalent continuum format, a form which is known to be sensitive to the spatial discretisation. The second approach developed involves the addition of the Heaviside function to the underlying finite element interpolation basis. This method is based on the partition of unity concept, and allows the Heaviside function to be added locally to a finite element mesh to simulate a propagating displacement discontinuity. The approach is formulated for geometrically linear, geometrically nonlinear, quasi-static and dynamic problems. It is shown to be completely independent of the spatial discretisation. The partition of unity-based model is used also to simulate failure using a regularised strain softening model. When a critical level of inelastic deformation is reached, a displacement discontinuity is inserted. This model is better suited to modelling the entire failure process than a continuum or discontinuous model alone. Through numerical examples, it is shown that the inclusion of a displacement discontinuity during the failure process can lead to a different failure mode than for a continuum-only model
Synapse distribution on VCH, an inhibitory, motion-sensitive interneuron in the fly visual system
No full textGauck V, Egelhaaf M, Borst A. Synapse distribution on VCH, an inhibitory, motion-sensitive interneuron in the fly visual system. JOURNAL OF COMPARATIVE NEUROLOGY. 1997;381(4):489-499
Implicit integration of a generalized plasticity constitutive model for partially saturated soil
No full textExtends the stress update algorithm and the tangent operator recently proposed for generalized plasticity by De Borst and Heeres to the case of partially saturated soils, where on top of the hydrostatic and deviatoric components of the (effective) stress tensor suction has to be considered as a third independent variable. The soil model used for the applications is the Bolzon-Schrefler-Zienkiewicz model, which is an extension of the Pastor-Zienkiewicz model to partial saturation. The algorithm is incorporated in a code for partially saturated soil dynamics. Back calculation of a saturation test and simulation of surface subsidence above an exploited gas reservoir demonstrate the advantage of the proposed algorithm in terms of iteration convergence of the solution
Some observations on the regularizing field for gradient damage models
No full textGradient enhanced material models can potentially preserve well-posedness of incremental boundary value problems also after the onset of strain softening. Gradient dependent constitutive relations are rooted in the assumption that some scalar or tensor field, which appears in the yield function, has to be enriched by adding a term involving its second-order gradient field. For gradient-dependent plasticity this term is universally accepted to be the equivalent plastic strain. For gradient-dependent damage models different choices have been presented in the literature. They all possess the desired regularization of the solution, but they are not identical as regards the structural response. In this paper the implications of the choice of the regularization field are discussed. As an example a plasticity-like damage material model is formulated. Finally, a well-known one-dimensional problem is solved analytically, and the results are compared with results obtained via other gradient approaches
Despenalización de la eutanasia en los Países Bajos. El proyecto de ley Korthals/Borst
No full textSUMARIO:1. Introducción.-2. ¿Por qué Holanda?- 3. La Comisión Estatal sobre la Eutanasia- 4. Evidencias empíricas.-5. El Informe Remmelink.-6. Legislación vigente.-7. Jurisprudencia- 8. Análisis político de las reformas legislativas: de la proposición Kohnstamm a la reforma de
la Ley ele Enterramientos.-9. La Ley Korthals/Borst.-lO. Conclusiones.Publicad
Conservation under Incompatibility for Fluid-Solid-Interaction Problems: the NPCL Method
No full textFinite-element discretizations of fluid-solid-interaction problems only trivially preserve the conservation properties of the underlying problem under restrictive compatibility conditions on the approximation spaces for the fluid and the solid. The present work introduces a new general method for enforcing interface conditions that maintains the conservation properties under incompatibility. The method is based on a nonlinear variational projection of the velocity field to impose the kinematic condition, and a consistent evaluation of the load functional that accounts for the dynamic condition. Numerical results for a projection problem are presented to illustrate the properties of the method
An evaluation of the accuracy of discontinuous finite elements in explicit dynamic calculations
No full textThe use of partition of unity based discontinuous finite element formulations for the simulation of crack propagation with implicit simulations is now well established. However, in explicit simulations the accuracy is still a point of concern. Some outstanding issues will be addressed in this paper
On axially symmetric shell problems with reinforced junctions
No full textWithin the framework of the six-parameter nonlinear resultant shell theory we consider the axially symmetric deformations of a cylindrical shell linked to a circular plate. The reinforcement in the junction of the shell and the plate is taken into account. Within the theory the full kinematics is considered. Here we analyzed the compatibility conditions along the junction and their influence on the deformations and stressed state
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