| 1. |
- Eberlein, R., et al.
(author)
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Multi-segment FEA of the human lumbar spine including the heterogeneity of the annulus fibrosus
- 2004
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In: Computational Mechanics. - : Springer Science and Business Media LLC. - 0178-7675 .- 1432-0924. ; 34:2, s. 147-163
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Journal article (peer-reviewed)abstract
- This study pursues the numerical validation of human lumbar spine segments. By means of the finite element (FE) method, computational analyses are carried out of various load cases. In particular Flexion-Extension, Lateral Bending and Axial Torque are considered. By means of a literature review the underlying constitutive data is verified. In this context, the heterogeneity of the annulus fibrosus, the transversely isotropic stress response of the spinal ligaments and aspects of the FE discretization are particularly emphasized. The numerical results show good agreement with experimental investigations for Extension and Axial Torque for a FE model that accounts for intact human lumbar spine response. In Flexion and Lateral Bending, however, the results of the intact FE-model do not properly account for the experimental data. A good correlation for these load cases can be found by taking disc degeneration into account in the FE-model. This fact shows that tissue degeneration plays a key role in the current validation process and must be accounted for if the lumbar spine specimen is employed for spinal implant evaluation. A degenerated FE-model that represents the stage of degeneration of the specimen and fits the experimental data for all load cases could not be found in this study and warrants further work in this area.
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| 2. |
- Gasser, T. Christian, et al.
(author)
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A rate-independent elastoplastic constitutive model for biological fiber-reinforced composites at finite strains : continuum basis, algorithmic formulation and finite element implementation
- 2002
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In: Computational Mechanics. - : Springer Science and Business Media LLC. - 0178-7675 .- 1432-0924. ; 29:05-apr, s. 340-360
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Journal article (peer-reviewed)abstract
- This paper presents a rate-independent elastoplastic constitutive model for (nearly) incompressible biological fiber-reinforced composite materials. The constitutive framework, based on multisurface plasticity, is suitable for describing the mechanical behavior of biological fiber-reinforced composites in finite elastic and plastic strain domains. A key point of the constitutive model is the use of slip systems, which determine the strongly anisotropic elastic and plastic behavior of biological fiber-reinforced composites. The multiplicative decomposition of the deformation gradient into elastic and plastic parts allows the introduction of an anisotropic Helmholtz free-energy function for determining the anisotropic response. We use the unconditionally stable backward-Euler method to integrate the flow rule and employ the commonly used elastic predictor/plastic corrector concept to update the plastic variables. This choice is expressed as an Eulerian vector update the Newton's type, which leads to a numerically stable and efficient material model. By means of a representative numerical simulations the performance of the proposed constitutive framework is investigated in detail.
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| 3. |
- Hansbo, Peter, et al.
(author)
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Nitsche's method for coupling non-matching meshes in fluid-structure vibration problems
- 2003
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In: Computational Mechanics. - : Springer Science and Business Media LLC. - 0178-7675 .- 1432-0924. ; 32:1-2, s. 134-139
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Journal article (peer-reviewed)abstract
- Nitsche's method [11] is a classical method for imposing essential boundary conditions weakly. Unlike the penalty method, it is consistent with the original differential equation. The strong point of Nitsche's method is that it retains the convergence rate of the underlying finite element method, whereas the standard penalty method either requires a "very large" penalty parameter, destroying the condition number of the resulting matrix problem, or, in case the condition number is to be retained, is limited to first order energy-norm accuracy. In this paper, we give a formulation of Nitsche's method suitable for the problem of fluid-structure interaction. Numerical examples are given.
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| 4. |
- Marklund, P.-O., et al.
(author)
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Optimization of airbag inflation parameters for the minimization of out of position occupant injury
- 2003
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In: Computational Mechanics. - : Springer Science and Business Media LLC. - 0178-7675 .- 1432-0924. ; 31:6, s. 496-504
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Journal article (peer-reviewed)abstract
- In this paper optimization techniques together with coupled fluid-structure analysis are used to provide a better understanding of the interaction between the airbag inflation process and an Out Of Position occupant. We provide a concept for, and exemplify how these techniques can be applied to optimize the airbag inflator characteristics in order to minimize occupant injury. For a simplified, but realistic, problem we show that the head acceleration can be reduced by approximately 65% by optimizing the inlet characteristics.
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| 5. |
- Marklund, Per-Olof, et al.
(author)
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Simulation of airbag inflation processes using a coupled fluid structure approach
- 2002
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In: Computational Mechanics. - : Springer Science and Business Media LLC. - 0178-7675 .- 1432-0924. ; 29:4-5, s. 289-297
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Journal article (peer-reviewed)abstract
- This paper explores simulation techniques for airbag inflation problems using a coupled fluid structure approach. It is to be seen as an initial study on the phenomena occurring in an airbag during a so called out of position occupant impact. The problem studied in this paper is an airbag which is set to impact a head form. The head form is positioned at a very short distance from the airbag. A multi material arbitrary Lagrangian Eulerian technique in the explicit finite element code LS-DYNA is used for the fluid and it is coupled to the structure using a penalty based fluid structure contact algorithm. The results for the head form acceleration and velocity show a good agreement to experimentally obtained values. At the early stages of the inflation process a high pressure zone is found to develop between the gas inlet and the head form. Consequently the pressure difference between the inlet and the high pressure zone is too low for an a priori assumption of sonic flow at the inlet, which is a common requirement in the control volume models used in the industry today.
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| 6. |
- Olovsson, Lars, et al.
(author)
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Selective mass scaling for thin walled structures modeled with tri-linear solid elements
- 2004
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In: Computational Mechanics. - : Springer Science and Business Media LLC. - 1432-0924 .- 0178-7675. ; 34:2, s. 134-136
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Journal article (peer-reviewed)abstract
- A method for selective mass scaling in explicit finite element analyses of thin walled structures, modeled with solid elements, is introduced. The method aims at increasing the critical time step without significantly altering the dynamical response of the system. The proposed method is based on the exclusion of certain rigid body motions from the applied mass scaling by filtering the local velocity field.
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| 7. |
- Laptev, Ari, et al.
(author)
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On new relations between spectral properties of Jacobi matrices and their coefficients
- 2003
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In: Communications in Mathematical Physics. - : Springer Science and Business Media LLC. - 0010-3616 .- 1432-0916. ; 241:1, s. 91-110
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Journal article (peer-reviewed)abstract
- We study the spectral properties of Jacobi matrices. By using ``higher order'' trace formulae we obtain a result relating the properties of the elements of Jacobi matrices and the corresponding spectral measures. Complicated expressions for traces of some operators can be magically simplified allowing us to apply induction arguments. Our theorems are generalizations of a recent result of R. Killip and B. Simon [17].
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