| 1. |
- Gasser, T. Christian, et al.
(författare)
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3D Crack propagation in unreinforced concrete. A two-step algorithm for tracking 3D crack paths
- 2006
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : Elsevier BV. - 0045-7825 .- 1879-2138. ; 195:37-40, s. 5198-5219
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Tidskriftsartikel (refereegranskat)abstract
- Tensile failure of unreinforced concrete involves progressive micro-cracking, and the related strain-softening can coalesce into geometrical discontinuities, which separate the material. Advanced mechanical theories and numerical schemes are required to efficiently and adequately represent crack propagation in 3D. In this paper we use the concept of strong discontinuities to model concrete failure. We introduce a cohesive fracture process zone, which is characterized by a transversely isotropic traction-separation law. We combine the cohesive crack concept with the partition of unity finite element method, where the finite element space is enhanced by the Heaviside function. The concept is implemented for tetrahedral elements and the failure initialization is based on the simple (non-local) Rankine criterion. For each element we assume the embedded discontinuity to be flat in the reference configuration, which leads to a non-smooth crack surfaces approximation in 3D, in general; different concepts for tracking non-planar cracks in 3D are reviewed. In addition, we propose a two-step algorithm for tracking the crack path, where a predictor step defines discontinuities according to the (non-local) failure criterion and a corrector step draws in non-local information of the existing discontinuities in order to predict a 'closed' 3D crack surface; implementation details are provided. The proposed framework is used to analyze the predictability of concrete failure by two benchmark examples, i.e. the Nooru-Moharned test, and the Brokenshire test. We compare our numerical results, which are mesh independent, with experimental data and numerical results adopted from the literature.
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| 2. |
- Gasser, T. Christian, et al.
(författare)
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A numerical framework to model 3-D fracture in bone tissue with application to failure of the proximal femur
- 2007
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Ingår i: IUTAM SYMPOSIUM ON DISCRETIZATION METHODS FOR EVOLVING DISCONTINUITIES. - DORDRECHT : SPRINGER. - 9781402065293 ; , s. 199-211
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Konferensbidrag (refereegranskat)abstract
- Bone can be regarded as a quasi-brittle material. Under excessive loading nonlinear fracture zones may occur ahead the crack tips, where, typically, cohesive mechanisms are activated. The finite element method provides a powerful tool to analyze fracture formations on a numerical basis, and to better understand failure mechanisms within complex structures. The present work aims to introduce a particular numerical framework to investigate bone failure. We combine the partition of unity finite element method with the cohesive crack concept, and a two-step predictor-corrector algorithm for tracking 3-D non-interacting crack paths. This approach renders a numerically efficient tool that is able to capture the strong discontinuity kinematics in an accurate way. The prediction of failure propagation in the proximal part of the femur under compressive load demonstrates the suitability of the proposed concept. A 3-D finite element model, which accounts for inhomogeneous fracture properties, was used for the prediction of the 3-D crack surface. The achieved computational results were compared with experimental data available in the literature.
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| 3. |
- Gasser, T. Christian, et al.
(författare)
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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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Ingår i: Computational Mechanics. - : Springer Science and Business Media LLC. - 0178-7675 .- 1432-0924. ; 29:05-apr, s. 340-360
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Gasser, T. Christian, et al.
(författare)
-
A three-dimensional finite element model for arterial clamping
- 2002
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Ingår i: Journal of Biomechanical Engineering. - : ASME International. - 0148-0731 .- 1528-8951. ; 124:4, s. 355-363
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Tidskriftsartikel (refereegranskat)abstract
- Clamp induced injuries of the arterial wall may determine the outcome of surgical procedures. Thus, it is important to investigate the underlying mechanical effects. We present a three-dimensional finite element model, which allows the study of the mechanical response of an artery-treated as a two-layer tube-during arterial clamping. The important residual stresses, which are associated with the load free configuration of the artery, are also considered. In particular, the finite element analysis of the deformation process of a clamped artery and the associated stress distribution is presented. Within the clamping area a zone of axial tensile peak-stresses was identified, which (may) cause intimal and medial injury. This is an additional injury mechanism, which clearly differs from the commonly assumed wall damage occurring due to compression between the jaws of the clamp. The proposed numerical model provides essential insights into the mechanics of the clamping procedure and the associated injury mechanisms. It allows detailed parameter studies on a virtual clamped artery, which can not be performed with other methodologies. This approach has the potential to identify the most appropriate clamps for certain types of arteries and to guide optimal clamp design.
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| 5. |
- Gasser, T. Christian, et al.
(författare)
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Finite element modeling of balloon angioplasty by considering overstretch of remnant non-diseased tissues in lesions
- 2007
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Ingår i: Computational Mechanics. - : Springer Science and Business Media LLC. - 0178-7675 .- 1432-0924. ; 40:1, s. 47-60
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Tidskriftsartikel (refereegranskat)abstract
- The paper deals with the modeling of balloon angioplasty by considering the balloon-induced overstretch of remnant non-diseased tissues in atherosclerotic arteries. A stenotic artery is modeled as a heterogenous structure composed of adventitia, media and a model plaque, and residual stresses are considered. The constitutive models are able to capture the anisotropic elastic tissue response in addition to the inelastic phenomena associated with tissue stretches beyond the physiological domain. The inelastic model describes the experimentally-observed changes of the wall during balloon inflation, i.e. non-recoverable deformation, and tissue weakening. The contact of the artery with a balloon catheter is simulated by a point-to-surface strategy. The states of deformations and stresses within the artery before, during and after balloon inflation are computed, compared and discussed. The 3D stress states at physiological loading conditions before and after balloon inflation differ significantly, and even compressive normal stresses may occur in the media after dilation.
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| 6. |
- Gasser, T. Christian, et al.
(författare)
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Geometrically non-linear and consistently linearized embedded strong discontinuity models for 3D problems with an application to the dissection analysis of soft biological tissues
- 2003
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : Elsevier BV. - 0045-7825 .- 1879-2138. ; 192:47-48, s. 5059-5098
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Tidskriftsartikel (refereegranskat)abstract
- Three different finite element formulations with embedded strong discontinuities are derived on the basis of the enhanced assumed strain method. According to the work by Jirasek and Zimmermann [Int. J. Numer. Methods Engrg. 50 (2001) 1269] they are referred to as statically optimal symmetric (SOS), kinematically optimal symmetric (KOS) and statically and kinematically optimal non-symmetric (SKON) formulations. The effect of the discontinuities are characterized by additional degrees of freedom on the element level. Modifications to the standard KOS and SKON formulations are proposed in order to achieve consistency with the employed type of a three-field Hu-Washizu principle under mode-I condition. Under this condition the formulation satisfies the internal compatibility at the discontinuity, i.e. the relation between the stress in the bulk material and the traction across the discontinuity surface, which is not the case for the classical KOS formulation. We propose a suitable explicit expression for a transversely isotropic traction law in form of a displacement-energy function and assume that softening phenomena in the cohesive zone are modeled by a damage law, which depends on the maximum gap displacement of the deformation path. A linearization of all quantities, which are related to the non-linear problem, leads to new closed form expressions. In particular, we focus attention on the linearization of the cohesive traction vector. The associated element residua and stiffness matrices are provided. Standard static condensation of the internal degree of freedom leads to a generalized displacement model. A comparative study of the modified formulations, carried out by means of two numerical examples, show the performance of the individual approach. We employ constant-strain tetrahedral elements with a single discontinuity embedded. Among the known stress locking phenomena associated with the SOS formulation, we recognized that the (non-symmetric) SKON formulation was not able to provide meaningful results for the dissection process of an arterial layer in three-dimensions on distorted meshes. For both numerical examples the (symmetric) KOS formulation seems to be most suitable for representing the embedded discontinuities.
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| 7. |
- Gasser, T. Christian, et al.
(författare)
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Modeling 3D crack propagation in unreinforced concrete using PUFEM
- 2005
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : Elsevier BV. - 0045-7825 .- 1879-2138. ; 194:25-26, s. 2859-2896
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Tidskriftsartikel (refereegranskat)abstract
- Concrete is a quasi-brittle material, where tensile failure involves progressive micro-cracking, debounding and other complex irreversible processes of internal damage. Strain-softening is a dominate feature and advanced numerical schemes have to be applied in order to circumvent the ill-posdness of the Boundary-Value Problem to deal with. Throughout the paper we pursue the cohesive zone approach, where initialization and coalescence of micro-cracks is lumped into the cohesive fracture process zone in terms of accumulation of damage. We develop and employ a (discrete) constitutive description of the cohesive zone, which is based on a transversely isotropic traction separation law. The model reflects an exponential decreasing traction with respect to evolving opening displacement and is based on the theory of invariants. Non-negativeness of the damage dissipation is proven and the associated numerical embedded representation is based on the Partition of Unity Finite Element Method. A consistent linearization of the method is presented, where particular attention is paid to the (cohesive) traction terms. Based on the proposed concept three numerical examples are studied in detail, i.e. a double-notched specimen under tensile loading, a four point shear test and a pull-out test of unreinforced concrete. The computational results show mesh-independency and good correlation with experimental results. © 2004 Elsevier B.V. All rights reserved.
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| 8. |
- Gasser, T. Christian, et al.
(författare)
-
Modeling plaque fissuring and dissection during balloon angioplasty intervention
- 2007
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Ingår i: Annals of Biomedical Engineering. - : Springer Science and Business Media LLC. - 0090-6964 .- 1573-9686. ; 35:5, s. 711-723
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Tidskriftsartikel (refereegranskat)abstract
- Balloon angioplasty intervention is traumatic to arterial tissue. Fracture mechanisms such as plaque fissuring and/or dissection occur and constitute major contributions to the lumen enlargement. However, these types of mechanically-based traumatization of arterial tissue are also contributing factors to both acute procedural complications and chronic restenosis of the treatment site. We propose physical and finite element models, which are generally useable to trace fissuring and/or dissection in atherosclerotic plaques during balloon angioplasty interventions. The arterial wall is described as an anisotropic, heterogeneous, highly deformable, nearly incompressible body, whereas tissue failure is captured by a strong discontinuity kinematics and a novel cohesive zone model. The numerical implementation is based on the partition of unity finite element method and the interface element method. The later is used to link together meshes of the different tissue components. The balloon angioplasty-based failure mechanisms are numerically studied in 3D by means of an atherosclerotic-prone human external iliac artery, with a type V lesion. Image-based 3D geometry is generated and tissue-specific material properties are considered. Numerical results show that in a primary phase the plaque fissures at both shoulders of the fibrous cap and stops at the lamina elastica interna. In a secondary phase, local dissections between the intima and the media develop at the fibrous cap location with the smallest thickness. The predicted results indicate that plaque fissuring and dissection cause localized mechanical trauma, but prevent the main portion of the stenosis from high stress, and hence from continuous tissue damage.
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| 9. |
- Gasser, T. Christian, et al.
(författare)
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Modeling the propagation of arterial dissection
- 2006
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Ingår i: European journal of mechanics. A, Solids. - : Elsevier BV. - 0997-7538 .- 1873-7285. ; 25:4, s. 617-633
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Tidskriftsartikel (refereegranskat)abstract
- Arterial dissections are frequently observed in clinical practice and during road traffic accidents. In particular, the lamellarly arrangement of elastin, collagen, in addition to smooth muscle cells in the middle arterial layer, the media, favors dissection failure. Experimental studies and related biomechanical models are rare in the literature. Finite strain kinematics is employed, and the discontinuity in the displacement field accounts for tissue separation. Dissection is regarded as a gradual process in which separation between incipient material surfaces is resisted by cohesive traction. Two variational statements together with their consistent linearizations form the basis for a finite element implementation. We combine the cohesive crack concept with the partition of unity finite element method, where nodal degrees of freedom adjacent to the discontinuity are enhanced. The developed continuum mechanical and numerical frameworks allow the analysis of the propagation of dissections within general nonlinear boundary-value problems, where the constitutive description for the continuous and the cohesive material is considered independent from each other. The continuous material is modeled as a fiber-reinforced composite with the fibers corresponding to the collagenous component which are assumed to be embedded in a non-collagenous isotropic groundmatrix. Dispersion of the collagen fiber orientation is considered in a continuum sense by one structure parameter. A novel cohesive potential per unit undeformed area is used to derive a traction separation law appropriate for the description of the mechanical properties of medial dissection. The cohesive stiffness contribution to the element stiffness matrix is explicitly derived. In particular, the dissection propagation of a rectangular strip of a human aortic media is investigated. Cohesive material properties are quantified by comparing the experimentally measured load with the computed dissection load.
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| 10. |
- Gasser, T. Christian, et al.
(författare)
-
Physical and numerical modeling of dissection propagation in arteries caused by balloon angioplasty
- 2005
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Ingår i: Proceedings of the Third IASTED International Conference on BIOMECHANICS. - 0889865329 ; , s. 229-233
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Konferensbidrag (refereegranskat)abstract
- Arterial dissections Caused by balloon angioplasty has been implicated as a contributing factor to both acute procedural complications and chronic restenosis of the treatment site. However, no related biomechanical studies are known in the literature. The mechanical properties of the arterial wall are controlled by the rubber-like protein elastin, fibrous protein collagen and smooth muscle cells. In the media of elastic arteries these constituents are found in thin layers that are arranged in repeating lamellar units and favor dissection type of failure. The presented approach models the dissection of the media by means of strong discontinuities and the application of the theory of cohesive zones. Thereby, the dissection is regarded as a gradual process in which separation between incipient material surfaces is resisted by cohesive traction. The applied numerical frame is based on the Partition of Unity Finite Element Method (PUFEM) and has been utilized for tetrahedral elements. A tracking algorithm for 3D non-planar cracks captures the evolution of multiple non-interacting dissections. The proposed concept is applied to investigate the dissection of the media due to balloon angioplasty, where the associated material parameters are determined from failure experiments on human tissue.
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