Inverse deformation results for elastic materials

Zusammenfassung Es wird gezeigt, dass die Umkehrsätze für die Deformation allgemeiner elastischer Körper unmittelbar aus einem Variationsprinzip gewonnen werden können, welches sich auf Gleichgewichtszustände bezieht.     

Implications of Shield’s inverse deformation theorem for compressible finite elasticity

Some properties of the Shield transformation on elastic strain energy functions are established. It is reflexive, it preserves objectivity and material symmetry for isotropic materials, and it also preserves infinitesimal strain response, ellipticity and Hadamard stability, and the Baker–Ericksen condition. Two new classes of strain energies for compressible isotropic materials are introduced, one of them being the image under the Shield transformation of the class of harmonic strain energies. In view of Shield’s Inverse Deformation Theorem, these new classes of strain energies will allow solution in closed form of a variety of problems in finite elastostatics.     

Implications of Shield’s inverse deformation theorem for compressible finite elasticity

Some properties of the Shield transformation on elastic strain energy functions are established. It is reflexive, it preserves objectivity and material symmetry for isotropic materials, and it also preserves infinitesimal strain response, ellipticity and Hadamard stability, and the Baker–Ericksen condition. Two new classes of strain energies for compressible isotropic materials are introduced, one of them being the image under the Shield transformation of the class of harmonic strain energies. In view of Shield’s Inverse Deformation Theorem, these new classes of strain energies will allow solution in closed form of a variety of problems in finite elastostatics. Received: January 30, 2002     

Mixed finite elements for elasticity

Summary. There have been many efforts, dating back four decades, to develop stable mixed finite elements for the stress-displacement formulation of the plane elasticity system. This requires the development of a compatible pair of finite element spaces, one to discretize the space of symmetric tensors in which the stress field is sought, and one to discretize the space of vector fields in which the displacement is sought. Although there are number of well-known mixed finite element pairs known for the analogous problem involving vector fields and scalar fields, the symmetry of the stress field is a substantial additional difficulty, and the elements presented here are the first ones using polynomial shape functions which are known to be stable. We present a family of such pairs of finite element spaces, one for each polynomial degree, beginning with degree two for the stress and degree one for the displacement, and show stability and optimal order approximation. We also analyze some obstructions to the construction of such finite element spaces, which account for the paucity of elements available. Received January 10, 2001 / Published online November 15, 2001     

Existence and approximation results for nonlinear mixed problems: Application to incompressible finite elasticity

Summary This paper considers the problems of minimizing Gateaux-differentiable functionals over subsets of real Banach spaces defined by a non-linear equality constraint. The existence of a Lagrange multiplier is proved, together with approximation results on the constrained subset, provided a nonlinear compatibility condition, generalizing the classical inf-sup condition, is satisfied. These ideas are applied to equilibrium problems in incompressible finite elasticity and lead to convergence results for these problems.     

Singularities of homogeneous deformations in finite elasticity

Is classification of the singularities of the potential, concerning the homogeneous deformations in Finite Elasticity, an important material property? The present study demonstrates that the answer to the question is positive. Since the type of singularity prescribes Maxwell's sets in the neighborhood of a singularity, the emergence of multiphase strain states depends on the performed classification. Bifurcation analysis and singularity classification for homogeneous deformations of any hyperelastic, anisotropic, homogeneous material under any type of conservative quasi-static loading is performed. Critical conditions for branching of the equilibrium paths are defined and their post-critical behavior is studied. Singularities corresponding to simple and compound branching are classified according to Arnold's classification. Unconstrained materials are only considered. Special attention is given to the compound branching $(D_k)$ cases requiring elaborate mathematical techniques. The geometry of the umbilics is described with the equilibrium paths and the cusp lines. With the help of Maxwell's sets in the neighborhood of the umbilics, the emergence of discontinuous strain fields is demonstrated. The theory is applied to orthotropic and transversely isotropic materials as well. The proposed method may directly be applied to crystals stability problems.     

On complex variable method in finite elasticity

We highlight the alternative presentation of the Cauchy-Riemann conditions for the analyticity of a complex variable function and consider plane equilibrium problem for an elastic transversely isotropic layer, in finite deformation. We state the fundamental problems and consider traction boundary value problem, as an example of fundamental problem-one. A simple solution of “Lame's problem” for an infinite layer is obtained. The profile of the deformed contour is given; and this depends on the order of the term used in the power series specification for the complex potential and on the material constants of the medium.     

An indifferent constitutive law in finite elasticity

The concepts of material frame-indifference and material symmetry group with respect to isotropic scalar functions, as represented by energy functions, are discussed. An energy function for a structured heterogeneous (transversal isotropic) medium in large elastic deformations, which is known to satisfy the Ponyting’s effect [1], is highlighted. It is shown that the constitutive relation due to this energy function is material frame-indifferent.     

Applications of variational principles in finite elasticity

Summary For some equilibrium states of a finitely deformed elastic body, variational principles can be used to provide bounds on overall quantities of physical interest. The principles are applied to the problem of the allaround finite extension of a plane sheet with a circular hole, and accurate estimates for the stress resultant at the outer edge are obtained for various extensions. The finite extension and torsion of an elastic cylinder is considered and bounds on the strain energy per unit length are obtained for an elliptical cylinder of neo-Hookean material with axes in the ratios of 2:1 and 4:1. The bounds lead to reliable estimates for the twisting moment and axial force.

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Zusammenfassung Für gewisse Gleichgewichtszustände eines endlich deformierten elastischen Körpers können Variationsprinzipien verwendet werden, um Schranken für globale Grössen von physikalischem Interesse zu erhalten. Die Prinzipien werden auf das Problem der allseitigen endlichen Extension einer ebenen Scheibe mit kreisförmigem Loch angewendet, und es werden für verschiedene Extensionen gute Abschätzungen der Resultierenden am Aussenrand gewonnen. Ferner wird die endliche Zug- und Torsionsverformung eines elastischen Zylinders betrachtet, und es werden Schranken für die Verformungsenergie je Längeneinheit für einen elliptischen Zylinder aus Neo-Hookeschem Material für die Achsenverhältnisse 2:1 und 4:1 erhalten. Die Schranken liefern zuverlässige Schätzungen für das Torsionsmoment und die Axialkraft.

     

Global extremum criteria for finite elasticity

A unified theroy for the complementary-dual extremum energy theorems in nonlinear elastostatics is established under the global extremum criteria

A weak approach to finite elasticity

On the ground of four axioms we define thekinematics of perfectly elastic bodies and in particular the notion ofweak deformations of a perfectly elastic body. Weak deformations turn out to agree withweak diffeomorphisms introduced in [10], a class of rectifiable currents which enjoys good closure and compactness properties. Defining thedynamics of perfectly elastic bodies in terms of twoconstitutive conditions on the stored energy function, we can therefore prove existence of stable equilibrium weak deformations for mixed boundary value problems, which moreover satisfy equilibrium and conservation equations.This work has been partially supported by the Ministero dell'Universitá e della Ricerca Scientifica, by C.N.R. contract n. 91.01343.CT01, by the European Research Project GADGET, and by Grant n.11957 of the zech Acad. of Sciences.This article was processed by the author using the LaT_EX style filepljour1 from Springer-Verlag.     

Cylindrical and spherical inflation in compressible finite elasticity

Murphy (1992) examined cylindrical and spherical inflation ofcompressible perfectly elastic materials having three specialforms of the strain-energy function. In this paper a generalprocedure for handling such problems for any strainenergy functionis proposed. This procedure is used to confirm some of the resultsby Murphy as well as to deduce new solutions. One solution obtainedby that author for cylindrical inflation is found to be incorrect.     

Quasistatic crack growth in finite elasticity with non-interpenetration

We present a variational model to study the quasistatic growth of brittle cracks in hyperelastic materials, in the framework of finite elasticity, taking into account the non-interpenetration condition.     

Inverse limits of postcritically finite polynomials

We examine inverse limits of postcritically finite polynomials restricted to their Julia sets. We define the “trunk” of a Julia set, a forward-invariant set related to the Hubbard tree, and use it to show that the inverse limit always contains at least one indecomposable subcontinuum. We characterize when the inverse limit is indecomposable and also examine how the trunk behaves in the inverse limit.     

Nonlinear,finite deformation,finite element analysis

The roles of the consistent Jacobian matrix and the material tangent moduli, which are used in nonlinear incremental finite deformation mechanics problems solved using the finite element method, are emphasized in this paper, and demonstrated using the commercial software ABAQUS standard. In doing so, the necessity for correctly employing user material subroutines to solve nonlinear problems involving large deformation and/or large rotation is clarified. Starting with the rate form of the principle of virtual work, the derivations of the material tangent moduli, the consistent Jacobian matrix, the stress/strain measures, and the objective stress rates are discussed and clarified. The difference between the consistent Jacobian matrix (which, in the ABAQUS UMAT user material subroutine is referred to as DDSDDE) and the material tangent moduli (C^e) needed for the stress update is pointed out and emphasized in this paper. While the former is derived based on the Jaumann rate of the Kirchhoff stress, the latter is derived using the Jaumann rate of the Cauchy stress. Understanding the difference between these two objective stress rates is crucial for correctly implementing a constitutive model, especially a rate form constitutive relation, and for ensuring fast convergence. Specifically, the implementation requires the stresses to be updated correctly. For this, the strains must be computed directly from the deformation gradient and corresponding strain measure (for a total form model). Alternatively, the material tangent moduli derived from the corresponding Jaumann rate of the Cauchy stress of the constitutive relation (for a rate form model) should be used. Given that this requirement is satisfied, the consistent Jacobian matrix only influences the rate of convergence. Its derivation should be based on the Jaumann rate of the Kirchhoff stress to ensure fast convergence; however, the use of a different objective stress rate may also be possible. The error associated with energy conservation and work-conjugacy due to the use of the Jaumann objective stress rate in ABAQUS nonlinear incremental analysis is viewed as a consequence of the implementation of a constitutive model that violates these requirements.     

Quasistatic crack growth in finite elasticity with Lipschitz data

We extend the recent existence result of Dal Maso and Lazzaroni (Ann Inst H Poincaré Anal Non Linéaire 27:257–290, 2010) for quasistatic evolutions of cracks in finite elasticity, allowing for boundary conditions and external forces with discontinuous first derivatives.     

On a class of solutions in plane finite elasticity

Zusammenfassung Es werden endliche elastische Deformationen bei ebenen Verzerrungszuständen betrachtet, in welchen die Spannungen nur vom Azimut abhängen. Der allgemeine Charakter der Deformationen wird für inkompressible isotrope Materialien diskutiert; insbesondere werden zwei partikuläre Lösungen aufgestellt, die für alle Formen der Verzerrungsenergie gültig sind. Die Lösungen der ersten Klasse sind die erwarteten homogenen Deformationen, während diejenigen der zweiten Klasse in rotatorischen, von einem einzigen Parameter abhängigen Verformungen bestehen. Es wird die Lösung von Randwertproblemen für die Spannungen bei keilförmigen Gebieten im einzelnen für ein Mooney-Material untersucht und dabei gezeigt, dass gewisse besondere Eigenschaften der infinitesimalen Theorie auch in der nichtlinearen auftreten.

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The results presented in this paper were obtained in the course of research sponsored by the National Science Foundation.     

Inverse semigroups and varieties of finite semigroups

In this paper we shall present a new construction of the free inverse monoid on a set X. Contrary to the previous constructions of [9, 11], our construction is symmetric and originates from classical ideas of language theory. The ingredients of this construction are the free group on X and the relation that associates to a word w of the free monoid on X, the set of all pairs (u, v) such that uv = w. It follows at once from our construction that the free inverse monoid on X can be naturally embedded into the Schützenberger product of two free groups of basis X. We shall also give some connections with the theory of expansions as developed by Rhodes and Birget [2, 3].