arXiv subjects

Marius Buliga

Publications and source records attributed to Marius Buliga.

67 records · Page 4Linked to original sources

arXiv2005

Energy minimizing brittle crack propagation II

The paper is devoted to the study of quasi-static brittle crack evolution. We work under the following assumptions: a linear elastic body, with or without initial cracks inside, evolves in a quasi-static manner under an imposed path of boundary displacements. During its evolution cracks with unprescribed geometry may appear and/or grow. This is an alternative version of the published article [Bu3], containing an improved Mumford-Shah model based on the functional K2, a generalization of the integral J of Rice. Further information at http://irmi.epfl.ch/cag/buliga_bfrac.html or http://www.imar.ro/~marius/ .

math.AP↗

arXiv2005

The variational complex of a diffeomorphisms group

In this paper we propose a variational complex associated to a diffeomorphisms group with first order jet in a Lie group. We study the structure of null lagrangians and we prove some fundamental properties of them, as well as their connection to differential invariants of the group action. Further informations at http://irmi.epfl.ch/cag/buliga_necv.html .

math.AP↗

arXiv2005

Quasiconvexity versus group invariance

The lower invariance under a given arbitrary group of diffeomorphisms extends the notion of quasiconvexity. The non-commutativity of the group operation (the function composition) modifies the classical equivalence between lower semicontinuity and quasiconvexity. In this context null lagrangians are particular cases of integral invariants of the group. Developments of parts of this paper can be found in math.FA/0105097 . Further informations at http://irmi.epfl.ch/cag/buliga_necv.html .

math.AP↗

arXiv2005

Perturbed area functionals and brittle damage mechanics

Some Mumford-Shah functionals are revisited as perturbed area functionals in connection with brittle damage mechanics. We find minimizers "on paper" for the classical Mumford-Shah functional for some particular two dimensional domains and boundary conditions. These solutions raise the possibility of validating experimentally the energetic model of crack appearance. Two models of brittle damage and fracture are proposed after; in the one of these models the crack belongs to the set of integral varifolds. We have felt the necessity to start the paper with a preliminary section concerning classical results in equilibrium of a cracked elastic body reviewed in the context of Sobolev spaces with respect to a measure. Further information at http://irmi.epfl.ch/cag/buliga_bfrac.html .

math.AP↗

arXiv2005

Energy concentration and brittle crack propagation

The purpose of this paper is to fill the gap between the classical treatment of brittle fracture mechanics and the new idea of considering the crack evolution as a free discontinuity problem. Griffith and Irwin criterions of crack propagation are studied and transformed in order to be no longer dependent on any prescription of the geometry of the crack during its evolution. The inequality contained in theorem 6.1. represents the link between generalized Irwin and Griffith criterions of brittle crack propagation. The physical meaning of this inequality is explained in the last section. Further information available at http://irmi.epfl.ch/cag/buliga_bfrac.html .

math.AP↗

arXiv2004

Sub-Riemannian geometry and Lie groups. Part II. Curvature of metric spaces, coadjoint orbits and associated representations

This paper is the third in a series dedicated to the fundamentals of sub-Riemannian geometry and its implications in Lie groups theory: "Sub-Riemannian geometry and Lie groups. Part I", math.MG/0210189, available at http://arxiv.org/abs/math.MG/0210189, and "Tangent bundles to sub-Riemannian groups", math.MG/0307342, available at http://arxiv.org/abs/math.MG/0307342 .

math.MG↗

arXiv2003

Curvature of sub-Riemannian spaces

To any metric spaces there is an associated metric profile. The rectifiability of the metric profile gives a good notion of curvature of a sub-Riemannian space. We shall say that a curvature class is the rectifiability class of the metric profile. We classify then the curvatures by looking to homogeneous metric spaces. The classification problem is solved for contact 3 manifolds, where we rediscover a 3 dimensional family of homogeneous contact manifolds, with a distinguished 2 dimensional family of contact manifolds which don't have a natural group structure.

math.MG↗

arXiv2003

Tangent bundles to sub-Riemannian groups

This paper is about non-Euclidean analysis on Lie groups endowed with left invariant distributions, seen as sub-Riemannian manifolds. This is a an updated version, which will be modified according to the contributions of the other participants to the Borel Seminar. The current version can be downloaded at http://irmi.epfl.ch/cag/buliga_sr.html . All comments are welcomed.

math.MG↗

arXiv2002

Sub-Riemannian geometry and Lie groups. Part I

These pages covers my expository talks during the seminar "Sub-Riemannian geometry and Lie groups" organised by the author and Tudor Ratiu at the Mathematics Department, EPFL, 2001. However, this is the first part of three, in preparation, dedicated to this subject. It covers, with mild modifications, an elementary introduction to the field.

math.MG↗

arXiv2002

Volume preserving bi-Lipschitz homeomorphisms on the Heisenberg group

Elementary sub-Riemannian geometry on the Heisenberg group H(n) provides a compact picture of symplectic geometry. Any Hamiltonian diffeomorphism on $R^{2n}$ lifts to a volume preserving bi-Lipschitz homeomorphisms of H(n), with the use of its generating function. Any curve of a flow of such homeomorphisms deviates from horizontality by the Hamiltonian of the flow. From the metric point of view this means that any such curve has Hausdorff dimension 2 and the $\mathcal{H}^{2}$ (area) density equal to the Hamiltonian. The non-degeneracy of the Hofer distance is a direct consequence of this fact.

math.SG↗

arXiv2002

Symplectic, Hofer and sub-Riemannian geometry

The purpose of this note is to make some connection between the sub-Riemannian geometry on Carnot-Caratheodory groups and symplectic geometry. We shall concentrate here on the Heisenberg group, although it is transparent that almost everything can be done on a general Carnot-Caratheodory group. Such generalisations will be the subject of a forthcoming note.

math.SG↗

arXiv2001

Lower semi-continuity of integrals with G-quasiconvex potential

This paper introduces the proper notion of variational quasiconvexity associated to a group of diffeomorphisms. We prove a lower semicontinuity theorem connected to this notion. In the second part of the paper we apply this result to a class of functions introduced in [5]. Such functions are $GL(n,R)^{+}$ quasiconvex, hence they induce lower semicontinuous integrals.

math.FA↗

arXiv2001

Majorisation with applications to the calculus of variations

This paper explores some connections between rank one convexity, multiplicative quasiconvexity and Schur convexity. Theorem 5.1 gives simple necessary and sufficient conditions for an isotropic objective function to be rank one convex on the set of matrices with positive determinant. Theorem 6.2 describes a class of possible non-polyconvex but multiplicative quasiconvex isotropic functions. This class is not contained in a well known theorem of Ball (6.3 in this paper) which gives sufficient conditions for an isotropic and objective function to be polyconvex. We show here that there is a new way to prove directly the quasiconvexity (in the multiplicative form). Relevance of Schur convexity for the description of rank one convex hulls is explained.

math.FA↗