| 1. |
|
|
| 2. |
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
- Karlsson, Tomas, et al.
(författare)
-
Finite element simulation of granular material flow in plane silos with complicated geometry
- 1998
-
Ingår i: Powder Technology. - 0032-5910 .- 1873-328X. ; 99:1, s. 29-39
-
Tidskriftsartikel (refereegranskat)abstract
- A model with few material parameters is proposed for finite element simulation of the transient flow of cohesionless granular materials in silos. The constitutive model used is essentially a fluid model and, consequently, it is best established in the Eulerian format. The model allows the simulation of initial stress transients at the beginning of the discharging process. After long time, when the solution approaches steady state, the flow pattern that is developed represents either a mass or funnel flow situation. Numerical examples of transient stress fields are given, and the flow patterns in advanced silo geometries are studied. The numerical results are in good agreement with analytical solutions and experiments known from the literature
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
- Klisinski, Marek, et al.
(författare)
-
Discussion on application of piece-wise linear weight functions in 2D contact problems
- 2003
-
Ingår i: Computer Assisted Mechanics and Engineering Sciences. - 1232-308X. ; 10:3, s. 321-337
-
Tidskriftsartikel (refereegranskat)abstract
- Standard higher order finite elements often perform unsatisfactory in contact problems. The major difficulties are caused by uneven distribution of nodal forces resulting in oscillating contact pressures. The paper presents a new approach that eliminates this drawback. The weight functions are chosen in such a way that even distributions of nodal forces are obtained. It is achieved by applying piece-wise linear functions. Two new 2D isoparametric quadratic elements are derived: 6-node triangle and 8-node quadrilateral, and tested in many examples. The new elements have unsymmetric stiffness matrices, but the provided examples show their good performance in contact problems
|
|
| 9. |
|
|
| 10. |
|
|
| 11. |
- Klisinski, Marek, et al.
(författare)
-
Implementation of 3D frictional contact condition
- 2001
-
Ingår i: Handbook of conveying and handling of particulate solids. - Amsterdam : Elsevier. - 0444502351 ; , s. 271-279
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
|
|
| 12. |
- Klisinski, Marek
(författare)
-
Inconsistency errors of constant velocity multi-time step integration algorithms
- 2001
-
Ingår i: Computer Assisted Mechanics and Engineering Sciences. - 1232-308X. ; 8:1, s. 121-139
-
Tidskriftsartikel (refereegranskat)abstract
- Previously known multi-time step integration methods for finite element computations in structural dynamics have been shown to be unstable due to interpolation error propagation. New algorithms of multi-time step integration based on constant velocity during subcycling are investigated. The assumption of constant velocity gives linear variation of displacements so the errors connected to interpolation at the interface between different time step partitions are eliminated. As a consequence, the new constant velocity algorithms give bounded solutions and have been shown to be conditionally stable by their authors. However, numerical investigation demonstrates that if time steps close to the stability limit are used, the errors for higher natural modes are so huge that the obtained solutions can only be considered as incorrect. The main reason for this behaviour is that the constant velocity time integration algorithms are inconsistent. Displacements can be calculated either by direct integration or from the equation of motion leading to different solutions. Based on the numerical results it is concluded that use of time steps below stability limit is insufficient to assure proper solutions. Therefore, significant time step reductions are often required to assure acceptable error levels. As a consequence, the new subcycling algorithms can be more expensive than ordinary time integration. Because they also lead to larger errors the constant velocity subcycling algorithms are useless from practical point of view. Since subcycling is available as an option in LS-DYNA a serious warning is issued to potential users.
|
|
| 13. |
|
|
| 14. |
- Klisinski, Marek, et al.
(författare)
-
Modelling of cracking of concrete with mixed mode inner softening band
- 1995
-
Ingår i: Engineering mechanics. - : American Society of Civil Engineers (ASCE). ; , s. 1095-1098
-
Konferensbidrag (refereegranskat)abstract
- The inner softening band approach is used to model the mixed mode (mode I and II) cracking of concrete. The softening behaviour is attributed to a displacement discontinuity within an element. The constitutive relation between tractions and relative displacements is based on the Mohr-Coulomb criterion with tension cut-off. Softening affects both the tensile strength and the cohesion of the material. The cracking pattern is controlled by an energy criterion leading to fairly unique results. Some numerical examples show that the results are objective and independent, to a certain degree, on finite element mesh
|
|
| 15. |
|
|
| 16. |
- Klisinski, Marek
(författare)
-
On constitutive equations for arbitrary stress-strain control in multi-surface plasticity
- 1998
-
Ingår i: International Journal of Solids and Structures. - 0020-7683 .- 1879-2146. ; 35:20, s. 2655-2678
-
Tidskriftsartikel (refereegranskat)abstract
- The structure of incremental constitutive equations in multi-surface plasticity is discussed with respect to different choices of state and control variables. The state and control variables can combine stresses and strains as long as they are decomposed into energy-conjugate parts. A general uniqueness condition is established for non-associated flow rules and any choice of control variables. Furthermore, proper tangent constitutive matrices are given within each loading/unloading region. The theory is demonstrated in a simple example involving Tresca's yield condition
|
|
| 17. |
- Klisinski, Marek, et al.
(författare)
-
On stability of multitime step integration procedures
- 1998
-
Ingår i: Journal of engineering mechanics. - 0733-9399 .- 1943-7889. ; 124:7, s. 783-793
-
Tidskriftsartikel (refereegranskat)abstract
- Stability of multitime step integration methods for finite-element computations in structural dynamics is analyzed. Multitime step procedures based on the Newmark family of methods are described. The basic idea of multitime step methods is to utilize various time step sizes in different domains of an element mesh. Interpolated nodal values from the large time step domain are used in the computation of displacements, velocities, and accelerations in the small time step domain. An analytical study of the errors introduced by the interpolation is given. The analysis shows instability because of the resonance phenomena and because of the propagation of spurious high frequencies caused by unbalanced forces originating from interpolation errors. Numerical results exhibiting instability problems are presented. To complete the study, examples of solutions stabilized with numerical damping of high frequencies are included in the presentation. The conclusion from the present study is that multitime step methods are unstable in their nature. Numerical damping can stabilize the computations but alters the response of free vibrating undamped systems.
|
|
| 18. |
|
|
| 19. |
|
|
| 20. |
- Luo, Chouping, et al.
(författare)
-
Application of piece-wise linear weight functions for 2D 8-node quadrilateral element in contact problems
- 2004
-
Ingår i: International Journal for Numerical Methods in Engineering. - : Wiley. - 0029-5981 .- 1097-0207. ; 61:2, s. 159-188
-
Tidskriftsartikel (refereegranskat)abstract
- The present study is a continuation of our previous work with the aim to reduce problems caused by standard higher order elements in contact problems. The difficulties can be attributed to the inherent property of the Galerkin method which gives uneven distributions of nodal forces resulting in oscillating contact pressures. The proposed remedy is use of piece-wise linear weight functions. The methods to establish stiffness and/or mass matrix for 8-node quadrilateral element in 2D are presented, i.e. the condensing and direct procedures. The energy and nodal displacement error norms are also checked to establish the convergence ratio. Interpretation of calculated contact pressures is discussed. Two new 2D 8-node quadrilateral elements, QUAD8C and QUAD8D, are derived and tested in many examples, which show their good performance in contact problems.
|
|
| 21. |
- Mattsson, Hans, et al.
(författare)
-
Method to correct yield surface drift in soil plasticity under mixed control and explicit integration
- 1997
-
Ingår i: International Journal for Numerical and Analytical Methods in Geomechanics. - 0363-9061 .- 1096-9853. ; 21:3, s. 175-197
-
Tidskriftsartikel (refereegranskat)abstract
- When applying an explicit integration algorithm in e.g. soil plasticity, the predicted stress point at the end of an elastoplastic increment of loading might not be situated on the updated current yield surface. This so-called yield surface drift could generally be held under control by using small integration steps. Another possibility, when circumstances might demand larger steps, is to adopt a drift correction method. In this paper, a drift correction method for mixed control in soil plasticity, under drained as well as undrained conditions, is proposed. By simulating triaxial tests in a Constitutive Driver, the capability and efficiency of this correction method, under different choices of implementation, have been analysed. It was concluded that the proposed drift correction method, for quite marginal additional computational cost, was able to correct successfully for yield surface drift giving results in close agreement to those obtained with a very large number of integration steps.
|
|
| 22. |
- Mattsson, Hans, et al.
(författare)
-
On a constitutive driver as a useful tool in soil plasticity
- 1999
-
Ingår i: Advances in Engineering Software. - 0965-9978 .- 1873-5339. ; 30:8, s. 511-528
-
Tidskriftsartikel (refereegranskat)abstract
- A mathematical basis for the development of constitutive drivers in soil plasticity has been proposed by the authors. A constitutive driver is here understood as a computer program, containing a number of selected constitutive models, in which different laboratory and field tests can be simulated and model parameters optimised. As a pilot study of the mathematical concept, a constitutive driver for soils, in the form of a PC-program, has been developed. The paper discusses this particular program, i.e. its structure, the mathematical basis, included soil models and some application examples, to give an idea of how a general and user-friendly constitutive driver can be designed. Such a program can be used for practical, research and educational purposes. In fact, it is believed that so many important applications for constitutive drivers exist that it would be beneficial if such programs were easily accessible as complementary programs in commercial software.
|
|
| 23. |
|
|
| 24. |
- Mattsson, Hans, et al.
(författare)
-
Optimization routine for identification of model parameters in soil plasticity
- 2001
-
Ingår i: International Journal for Numerical and Analytical Methods in Geomechanics. - : Wiley. - 0363-9061 .- 1096-9853. ; 25:5, s. 435-472
-
Tidskriftsartikel (refereegranskat)abstract
- The paper presents an optimization routine especially developed for the identification of model parameters in soil plasticity on the basis of different soil tests. Main focus is put on the mathematical aspects and the experience from application of this optimization routine. Mathematically, for the optimization, an objective function and a search strategy are needed. Some alternative expressions for the objective function are formulated. They capture the overall soil behaviour and can be used in a simultaneous optimization against several laboratory tests. Two different search strategies, Rosenbrock's method and the Simplex method, both belonging to the category of direct search methods, are utilized in the routine. Direct search methods have generally proved to be reliable and their relative simplicity make them quite easy to program into workable codes. The Rosenbrock and simplex methods are modified to make the search strategies as efficient and user-friendly as possible for the type of optimization problem addressed here. Since these search strategies are of a heuristic nature, which makes it difficult (or even impossible) to analyse their performance in a theoretical way, representative optimization examples against both simulated experimental results as well as performed triaxial tests are presented to show the efficiency of the optimization routine. From these examples, it has been concluded that the optimization routine is able to locate a minimum with a good accuracy, fast enough to be a very useful tool for identification of model parameters in soil plasticity.
|
|
| 25. |
|
|
| 26. |
|
|
| 27. |
|
|
| 28. |
- Postek, E., et al.
(författare)
-
Numerical simulation of 3D iron ore flow
- 2001
-
Ingår i: Handbook of conveying and handling of particulate solids. - Amsterdam : Elsevier. - 0444502351 ; , s. 281-287
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
|
|
| 29. |
|
|
| 30. |
|
|
| 31. |
|
|
| 32. |
- Tano, Robert, et al.
(författare)
-
Crack modelling of concrete, rock and soil with inner softening bands
- 1998
-
Ingår i: Engineering Geology. - 0013-7952 .- 1872-6917. ; 49:3-4, s. 271-276
-
Tidskriftsartikel (refereegranskat)abstract
- To model the cracking behaviour of brittle materials, different existing methods for treating the fracture zone can be used. The inner softening band (ISB) approach belongs to those methods which introduce a displacement discontinuity in which a constitutive relationship between opening displacements and tractions is assumed. This group of methods avoids difficulties in the localization zone caused by ambiguous definitions of strain measures and fracture zone widths. Whichever method is used, it is important that the proper amount of energy is dissipated. This is achieved automatically in ISB without the introduction of any extra material parameters other than the elastic constants and those describing the softening curve. Variations in mesh sizes and arrangements have shown no great influence on the total response, and the crack pattern is a part of the solution in contrast to most other methods in which it must be presumed. The concept is simple and inner softening bands have been successfully implemented within triangular three-node elements and preliminary within six-node triangular elements. The results obtained so far from various kinds of tests have shown very good performance and further development is planned
|
|
| 33. |
- Tejchman, J., et al.
(författare)
-
FE-studies on rapid flow of bulk solids in silos
- 2001
-
Ingår i: Granular Matter. - 1434-5021 .- 1434-7636. ; 3:4, s. 215-230
-
Tidskriftsartikel (refereegranskat)abstract
- This paper contains results of numerical modelling of the onset of silo flow for granular material in a model silo with convergent walls. The calculations were performed with a finite element method based on a polar elasto-plastic constitutive relation by Mühlhaus. It differs from the conventional theory of plasticity by the presence of Cosserat rotations and couple stresses using a mean grain diameter as a characteristic length. The characteristic length causes that numerical results do not depend upon the mesh discretisation. The model tests on rapid silo flow of glass beads performed by Renner in a glass hopper with a large wall inclination from the bottom were numerically simulated. The FE-calculations were performed for plane strain by taking into account inertial forces and linear viscous damping. A satisfactory agreement between numerical and experimental results was obtained. In addition, the FE-calculations were performed for very rough walls. Advantages and limitations of a continuum approach for simulations of rapid silo flow were outlined.
|
|
