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# arash_yavari's blog

## Universal Deformations and Inhomogeneities in Isotropic Cauchy Elasticity

Wed, 2024-06-05 13:15 - arash_yavariFor a given class of materials, *universal deformations* are those deformations that can be maintained in the absence of body forces and by applying solely boundary tractions. For inhomogeneous bodies, in addition to the universality constraints that determine the universal deformations, there are extra constraints on the form of the material inhomogeneities—*universal inhomogeneity constraints*. Those inhomogeneities compatible with the universal inhomogeneity constraints are called *universal inhomogeneities*.

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## Geometric Phases of Nonlinear Elastic N-Rotors via Cartan's Moving Frames

Thu, 2023-12-21 12:44 - arash_yavariWe study the geometric phases of nonlinear elastic $N$-rotors with continuous rotational symmetry. In the Hamiltonian framework, the geometric structure of the phase space is a principal fiber bundle, i.e., a base, or shape manifold~$\mathcal{B}$, and fibers $\mathcal{F}$ along the symmetry direction attached to it. The symplectic structure of the Hamiltonian dynamics determines the connection and curvature forms of the shape manifold. Using Cartan's structural equations with zero torsion we find an intrinsic (pseudo) Riemannian metric for the shape manifold.

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## Nonlinear Anisotropic Viscoelasticity

Fri, 2023-10-06 21:14 - arash_yavariIn this paper we revisit the mathematical foundations of nonlinear viscoelasticity. We study the underlying geometry of viscoelastic deformations, and in particular, the intermediate configuration. Starting from the multiplicative decomposition of deformation gradient into elastic and viscous parts **F**=**F**e**F**v, we point out that **F**v can be either a material tensor (**F**e is a two-point tensor) or a two-point tensor (**F**e is a spatial tensor).

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## Nonlinear Mechanics of Remodeling

Mon, 2023-09-25 07:45 - arash_yavariIn this paper, we present a large-deformation formulation of the mechanics of remodeling. Remodeling is anelasticity with an internal constraint---material evolutions that are mass and volume-preserving. In this special class of material evolutions, the explicit time dependence of the energy function is via one or more remodeling tensors that can be considered as internal variables of the theory. The governing equations of remodeling solids are derived using a two-potential approach and the Lagrange-d'Alembert principle.

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## Accretion-Ablation Mechanics

Wed, 2023-08-09 10:46 - arash_yavariIn this paper we formulate a geometric nonlinear theory of the mechanics of accreting-ablating bodies. This is a generalization of the theory of accretion mechanics of Sozio and Yavari (2019). More specifically, we are interested in large deformation analysis of bodies that undergo a continuous and simultaneous accretion and ablation on their boundaries while under external loads.

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## On the Effective Dynamic Mass of Mechanical Lattices with Microstructure

Thu, 2023-07-20 16:02 - arash_yavariWe present a general formalism for the analysis of mechanical lattices with microstructure using the concept of effective dynamic mass. We first revisit a classical case of microstructure being modeled by a spring-interconnected mass-in-mass cell. The frequency-dependent effective dynamic mass of the cell is the sum of a static mass and of an added mass, in analogy to that of a swimmer in a fluid. The effective dynamic mass is derived using three different methods: momentum equivalence, dynamic condensation, and action equivalence.

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## Universal Displacements in Inextensible Fiber-Reinforced Linear Elastic Solids

Mon, 2023-05-29 17:14 - arash_yavariFor a given class of materials, universal displacements are those displacements that can be maintained for any member of the class by applying only boundary tractions. In this paper we study universal displacements in compressible anisotropic linear elastic solids reinforced by a family of inextensible fibers. For each symmetry class and for a uniform distribution of straight fibers respecting the corresponding symmetry we characterize the respective universal displacements. A goal of this paper is to investigate how an internal constraint affects the set of universal displacements.

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## A Geometric Field Theory of Dislocation Mechanics

Wed, 2023-05-10 08:08 - arash_yavariIn this paper a geometric field theory of dislocation dynamics and finite plasticity in single crystals is formulated. Starting from the multiplicative decomposition of the deformation gradient into elastic and plastic parts, we use Cartan's moving frames to describe the distorted lattice structure via differential 1-forms. In this theory the primary fields are the dislocation fields, defined as a collection of differential 2-forms. The defect content of the lattice structure is then determined by the superposition of the dislocation fields.

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## Optimal Elastostatic Cloaks

Tue, 2023-04-25 13:15 - arash_yavariAn elastic cloak hides a hole (or an inhomogeneity) from elastic fields. In this paper, a formulation of the optimal design of elastic cloaks based on the adjoint state method, in which the balance of linear momentum is enforced as a constraint, is presented. The design parameters are the elastic moduli of the cloak, and the objective function is a measure of the distance between the solutions in the physical and in the virtual bodies. Both the elastic medium and the cloak are assumed to be made of isotropic linear elastic materials.

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## Finite Extension of Accreting Nonlinear Elastic Solid Circular Cylinders

Wed, 2023-03-01 09:46 - arash_yavariIn this paper we formulate and solve the initial-boundary value problem of accreting circular cylindrical bars under finite extension. We assume that the bar grows by printing stress-free cylindrical layers on its boundary cylinder while it is undergoing a time-dependent finite extension. Accretion induces eigenstrains, and consequently residual stresses. We formulate the anelasticity problem by first constructing the natural Riemannian metric of the growing bar. This metric explicitly depends on the history of deformation during the accretion process.

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## Accretion Mechanics of Nonlinear Elastic Circular Cylindrical Bars Under Finite Torsion

Mon, 2022-11-07 16:41 - arash_yavariIn this paper we formulate the initial-boundary value problem of accreting circular cylindrical bars under finite torsion. It is assumed that the bar grows as a result of printing stress-free cylindrical layers on its boundary while it is under a time-dependent torque (or a time-dependent twist) and is free to deform axially. In a deforming body, accretion induces eigenetrains, and consequently residual stresses. We formulate the anelasticity problem by first constructing the natural Riemannian metric of the growing bar.

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## On the Direct and Reverse Multiplicative Decompositions of Deformation Gradient in Nonlinear Anisotropic Anelasticity

Mon, 2022-10-10 13:49 - arash_yavariIn this paper we discuss nonlinear anisotropic anelasticity formulated based on the two multiplicative decompositions **F**=**F**e**F**a and **F**=** F**a

**e. Using the Bilby-Kroner-Lee decomposition**

*F***F**=

**F**e

**F**a one can define a Riemannian material manifold (the natural configuration of an anelastic body) whose metric explicitly depends on the anelastic deformation

**F**a.

## Universality in Anisotropic Linear Anelasticity

Thu, 2022-06-16 13:55 - arash_yavariIn linear elasticity, universal displacements for a given symmetry class are those displacements that can be maintained by only applying boundary tractions (no body forces) and for arbitrary elastic constants in the symmetry class. In a previous work, we showed that the larger the symmetry group, the larger the space of universal displacements. Here, we generalize these ideas to the case of anelasticity. In linear anelasticity, the total strain is additively decomposed into elastic strain and anelastic strain, often referred to as an eigenstrain.

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## The Universal Program of Nonlinear Hyperelasticity

Mon, 2022-06-06 13:08 - arash_yavariFor a given class of materials, universal deformations are those that can be maintained in the absence of body forces by applying only boundary tractions. Universal deformations play a crucial role in nonlinear elasticity. To date, their classification has been accomplished for homogeneous isotropic solids following Ericksen's seminal work, and homogeneous anisotropic solids and inhomogeneous isotropic solids in our recent works. In this paper we study universal deformations for inhomogeneous anisotropic solids defined as materials whose energy function depends on position.

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## The Universal Program of Linear Elasticity

Tue, 2022-03-15 13:17 - arash_yavariUniversal displacements are those displacements that can be maintained, in the absence of body forces, by applying only boundary tractions for any material in a given class of materials. Therefore, equilibrium equations must be satisfied for arbitrary elastic moduli for a given anisotropy class. These conditions can be expressed as a set of partial differential equations for the displacement field that we call universality constraints.

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## Universal Deformations in Inhomogeneous Isotropic Nonlinear Elastic Solids

Thu, 2021-09-02 10:01 - arash_yavariUniversal (controllable) deformations of an elastic solid are those deformations that can be maintained for all possible strain-energy density functions and suitable boundary tractions. Universal deformations have played a central role in nonlinear elasticity and anelasticity. However, their classification has been mostly established for homogeneous isotropic solids following the seminal works of Ericksen. In this paper, we extend Ericksen's analysis of universal deformations to inhomogeneous compressible and incompressible isotropic solids.

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## On Hashin's Hollow Cylinder and Sphere Assemblages in Anisotropic Nonlinear Elasticity

Tue, 2021-08-17 13:41 - arash_yavariWe generalize Hashin's nonlinear isotropic hollow cylinder and sphere assemblages to nonlinear anisotropic solids. More specifically, we find the effective hydrostatic constitutive equation of nonlinear transversely isotropic hollow sphere assemblages with radial material preferred directions. We also derive the effective constitutive equations of finite and infinitely-long hollow cylinder assemblages made of incompressible orthotropic solids with axial, radial, and circumferential material preferred directions.

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## Universal Deformations in Anisotropic Nonlinear Elastic Solids

Sun, 2021-07-25 20:23 - arash_yavariUniversal deformations of an elastic solid are deformations that can be achieved for all possible strain-energy density functions and suitable boundary conditions. They play a central role in nonlinear elasticity and their classification has been mostly accomplished for isotropic solids following Ericksen's seminal work. Here, we address the same problem for transversely isotropic, orthotropic, and monoclinic solids.

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## On Eshelby's Inclusion Problem in Nonlinear Anisotropic Elasticity

Tue, 2021-05-25 00:06 - arash_yavariThe recent literature of finite eignestrains in nonlinear elastic solids is reviewed, and Eshelby's inclusion problem at finite strains is revisited. The subtleties of the analysis of combinations of finite eigenstrains for the example of combined finite radial, azimuthal, axial, and twist eigenstrains in a finite circular cylindrical bar are discussed. The stress field of a spherical inclusion with uniform pure dilatational eigenstrain in a radially-inhomogeneous spherical ball made of arbitrary incompressible isotropic solids is analyzed.

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## On Nye's Lattice Curvature Tensor

Tue, 2021-05-04 12:36 - arash_yavariWe revisit Nye's lattice curvature tensor in the light of Cartan's moving frames. Nye's definition of lattice curvature is based on the assumption that the dislocated body is stress-free, and therefore, it makes sense only for zero-stress (impotent) dislocation distributions. Motivated by the works of Bilby and others, Nye's construction is extended to arbitrary dislocation distributions. We provide a material definition of the lattice curvature in the form of a triplet of vectors, that are obtained from the material covariant derivative of the lattice frame along its integral curves.

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## Elastodynamic Transformation Cloaking for Non-Centrosymmetric Gradient Solids

Tue, 2021-05-04 12:18 - arash_yavariIn this paper we investigate the possibility of elastodynamic transformation cloaking in bodies made of non-centrosymmetric gradient solids. The goal of transformation cloaking is to hide a hole from elastic disturbances in the sense that the mechanical response of a homogeneous and isotropic body with a hole covered by a cloak would be identical to that of the corresponding homogeneous and isotropic body outside the cloak.

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## The mathematical foundations of anelasticity: Existence of smooth global intermediate configurations

Tue, 2020-12-01 08:31 - arash_yavariA central tool of nonlinear anelasticity is the multiplicative decomposition of the deformation tensor that assumes that the deformation gradient can be decomposed as a product of an elastic and an anelastic tensor. It is usually justified by the existence of an intermediate configuration. Yet, this configuration cannot exist in Euclidean space, in general, and the mathematical basis for this assumption is on unsatisfactory ground.

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## The Anelastic Ericksen Problem: Universal Deformations and Universal Eigenstrains in Incompressible Nonlinear Anelasticity

Thu, 2020-09-17 09:23 - arash_yavariEricksen's problem consists of determining all equilibrium deformations that can be sustained solely by the application of boundary tractions for an arbitrary incompressible isotropic hyperelastic material whose stress-free configuration is geometrically flat. We generalize this by first, using a geometric formulation of this problem to show that all the known universal solutions are symmetric with respect to Lie subgroups of the special Euclidean group. Second, we extend this problem to its anelastic version, where the stress-free configuration of the body is a Riemannian manifold.

## Transformation Cloaking in Elastic Plates

Sun, 2020-09-13 10:20 - arash_yavariIn this paper we formulate the problem of elastodynamic transformation cloaking for Kirchhoff-Love plates and elastic plates with both in-plane and out-of-plane displacements. A cloaking transformation maps the boundary-value problem of an isotropic and homogeneous elastic plate (virtual problem) to that of an anisotropic and inhomogeneous elastic plate with a hole surrounded by a cloak that is to be designed (physical problem). For Kirchhoff-Love plates, the governing equation of the virtual plate is transformed to that of the physical plate up to an unknown scalar field.

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## Nonlinear Mechanics of Thermoelastic Accretion

Fri, 2020-03-20 11:52 - arash_yavariIn this paper, we formulate a theory for the coupling of accretion mechanics and thermoelasticity. We present an analytical formulation of the thermoelastic accretion of an infinite cylinder and of a two-dimensional block.

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