| 1. |
- Ahadi, Aylin, et al.
(författare)
-
Characteristic state plasticity for granular materials Part II: Model calibration and results
- 2000
-
Ingår i: International Journal of Solids and Structures. - 0020-7683. ; 37:43, s. 6361-6380
-
Tidskriftsartikel (refereegranskat)abstract
- A non-associated plasticity theory for granular materials has been developed in Part 1 based on the concept of a characteristic stress state of vanishing incremental dilation. The model is fully three-dimensional and is defined by six material parameters: two for elastic stiffness, one for plastic stiffness, two for the shapes of yield and plastic potential surfaces and one for the dilation at failure. In this paper a calibration procedure is developed using test data only from a standard triaxial test. It is found that the shape parameter for the yield surface can be estimated from the plastic how parameters, thus reducing the number of free parameters to five. Calibration examples are shown, as well as predictions made, for different confining stress levels and constant volume tests on sand. The model is found to represent stress-strain behaviour and development of volumetric strain in standard triaxial tests well. The model provides good predictions of constant volume behaviour of dense as well as loose sand on the basis of calibration by standard triaxial test data. A simple explicit formula is derived for the failure asymptote in constant volume testing, enabling explicit adjustment of the parameters, if incompressible-test data is available.
|
|
| 2. |
- Ahadi, Aylin, et al.
(författare)
-
Implicit integration of plasticity models for granular materials
- 2003
-
Ingår i: Computer Methods in Applied Mechanics and Engineering. - 0045-7825. ; 192:31-32, s. 3471-3488
-
Tidskriftsartikel (refereegranskat)abstract
- A stress integration algorithm for granular materials based on fully implicit integration with explicit updating is presented. In the implicit method the solution makes use of the gradient to the potential surface at the final stress state which is unknown. The final stress and hardening parameters are determined solving the non-linear equations iteratively so that the stress increment fulfills the consistency condition. The integration algorithm is applicable for models depending on all the three stress invariants and it is applied to a characteristic state model for granular material. Since tensile stresses are not supported the functions and their derivatives are not representative outside the compressive octant of the principal stress space. The elastic predictor is therefore preconditioned in order to ensure that the first predictor is within the valid region. Capability and robustness of the integration algorithm are illustrated by simulating both drained and undrained triaxial tests on sand. The algorithm is developed in a standard format which can be implemented in several general purpose finite element codes. It has been implemented as an ABAQUS subroutine, and a traditional geotechnical problem of a flexible strip footing resting on a surface of sand is investigated in order to demonstrate the global accuracy and stability of the numerical solution. (C) 2003 Elsevier B.V. All rights reserved.
|
|
| 3. |
- Ahadi, Aylin, et al.
(författare)
-
Non-associated plasticity for soils
- 1998
-
Ingår i: Trita-BKN. Bulletin. - 1103-4270. ; , s. 19-22
-
Konferensbidrag (refereegranskat)abstract
- A brief overview is given of a non-associated plasticity theory developed for soils based on the concept of a characteristic stress state of vanishing incremental dilation, and a calibration procedure using only test data from a standard triaxial test. The capability of the model is illustrated by calibration examples and predictions made for standard triaxial tests at different confining stress levels and constant volume tests on sand. The model is found to predict stress-strain behaviour and development of volumetric strain well.
|
|
| 4. |
- Madsen, Stine Skov, et al.
(författare)
-
Relaxation modeling of asphalt behavior under moving load
- 2015
-
Ingår i: 5th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering. - Crete Island : Technical University of Crete Chania Crete Greece.
-
Konferensbidrag (refereegranskat)abstract
- In relation to the development of a Rolling Wheel Deflectometer (RWD), which is a non-destructive testing device for measuring pavement deflections, a finite element model for obtaining the soil/pavement response has been developed [1]. The RWD is operating at traffic speeds and the soil model is therefore subjected to a moving transient dynamic load. Perfectly Matched Layer (PML) [2] is used as absorbing boundary conditions in order to prevent reflections of the waves propagating through the soils due to the dynamic loading. As the load is moving with high speed, the formulation is in the moving frame of reference [3].To accurately predict pavement response, proper material characterization is needed. Flexible pavements are commonly modeled as multilayer linear elastic systems. However, asphalt behaves as a viscoelastic material because its response to induced loading depends on temperature and load frequency and can be modeled in different ways. A finite element model can quickly reach high computational cost, thus a simple, time-efficient viscoelastic model is preferable.The viscoelastic behavior of asphalt can be described by a relaxation format of the constitutive relation with four parameters; two elastic moduli defining the minimum and maximum stiffnesses of asphalt in relation to frequency, a time scale parameter controlling the relaxation time and a differential fractional order characterizing the interpolation between low and high frequency regimes [4]. Assuming the most common interpolation shape by setting the fractional order equal to one, the system is reduced to only 3 parameters and the formulation can be implemented in the finite element model without increasing the computational cost significantly.In this paper a formulation of the relaxation format of viscoelastic behavior is developed in translated coordinates with efficiently absorbing boundary conditions (PML). The viscoelastic properties of the formulation are illustrated through numerical examples. References[1] S. Madsen, S. Krenk and O. Hededal, Perfectly Matched Layer (PML) for transient wave propagation in a moving frame of reference. Proceedings of the Fourth International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, COMPDYN 2013, pp. 4379-4388. Kos, 2013.[2] R. Matzen, An efficient finite element time-domain formulation for the elastic second-order wave equation: A non-split complex frequency shifted convolutional PML. International Journal for Numerical Methods in Engineering, 88, 951-973, 2011.[3] S. Krenk, L. Kellezi, S.R.K. Nielsen and P.H. Kirkegaard, Finite elements and transmitting boundary conditions for moving loads, in Proceedings of the Fourth European Conference on Structural Dynamics, EURODYN’99, Vol. 1, 447-452. Balkema, Rotterdam, 1999. [4] S. Krenk, Damping mechanisms and models in structural dynamics. Proceedings of the Fourth International Conference on Structural Dynamics, EURODYN 2002, Vol 2, Munich, Germany, 2002.
|
|
| 5. |
|
|
| 6. |
- Ottosen, Niels Saabye, et al.
(författare)
-
Nonlinear Analysis of Cavities in Rock Salt
- 1979
-
Ingår i: International Journal of Rock Mechanics and Mining Sciences. - : Elsevier BV. - 1873-4545 .- 0148-9062. ; 16:4, s. 245-252
-
Tidskriftsartikel (refereegranskat)abstract
- The paper covers some material and computational aspects of the rock mechanics of leached cavities in salt. A material model is presented in which the instantaneous stiffness of the salt is obtained by interpolation between the unloaded state and a relevant failure state. The model enables prediction of short term triaxial behaviour from uniaxial stress-strain curves. Key results from a nonlinear finite element calculation of a gas-filled cavity are given, and the general features are related to a simple nonlinear method of stress evaluation.
|
|
