193 research outputs found
Minimal surfaces and conservation laws for bidimensional structures
We discuss conservation laws for thin structures which could be modeled as a material minimal surface, i.e., a surface with zero mean curvatures. The models of an elastic membrane and micropolar (six-parameter) shell undergoing finite deformations are considered. We show that for a minimal surface, it is possible to formulate a conservation law similar to three-dimensional non-linear elasticity. It brings us a path-independent J-integral which could be used in mechanics of fracture. So, the class of minimal surfaces extends significantly a possible geometry of two-dimensional structures which possess conservation laws
A Micropolar Peridynamic Theory in Linear Elasticity
A state-based micropolar peridynamic theory for linear elastic solids is
proposed. The main motivation is to introduce additional micro-rotational
degrees of freedom to each material point and thus naturally bring in the
physically relevant material length scale parameters into peridynamics.
Non-ordinary type modeling via constitutive correspondence is adopted here to
define the micropolar peridynamic material. Along with a general three
dimensional model, homogenized one dimensional Timoshenko type beam models for
both the proposed micropolar and the standard non-polar peridynamic variants
are derived. The efficacy of the proposed models in analyzing continua with
length scale effects is established via numerical simulations of a few beam and
plane-stress problems
A MORE COMPLETE THERMODYNAMIC FRAMEWORK FOR SOLID CONTINUA
The Jacobian of deformation at a material point can be decomposed
into the stretch tensor and the rotation tensor. Thus, varying
Jacobians of deformation at the neighboring material points
in the deforming volume of solid continua would yield varying
stretch and rotation tensors at the material points. Measures of
strain, such as Green’s strain, at a material point are purely a
function of the stretch tensor, i.e. the rotation tensor plays no
role in these measures. Alternatively, we could also consider
decomposition of displacement gradient tensor into symmetric
and skew symmetric tensors. Skew symmetric tensor is also a
measure of pure rotations whereas symmetric tensor is a measure
of strains, i.e. stretches. The measures of rotations in these
two approaches describe the same physics but are in different
forms. Polar decomposition gives the rotation matrix and not
the rotation angles whereas the skew symmetric part of the displacement
gradient tensor yields rotation angles that are explicitly
and conveniently defined in terms of the displacement gradients.
The varying rotations and rotation rates arise in all deforming
solid continua due to varying deformation of the continua at
neighboring material points, hence are internal to the volume
of solid continua and are explicitly defined by the deformation,
therefore do not require additional degrees of freedom to define
them. If the internal varying rotations and their rates are
resisted by the continua, then there must exist internal moments
corresponding to these. The internal rotations and their rates and
the corresponding moments can result in additional energy storage
and dissipation. This physics is all internal to the deforming
continua (hence does not require consideration of additional
external degrees of freedom and associated external moments)
and is neglected in the presently used continuum theories for
isotropic, homogeneous solid continua. The continuum theory
presented in this paper considers internal varying rotations and
associated conjugate moments in the derivation of the conservation
and balance laws, thus the theory presented in this paper is
“a polar theory for solid continua” but is different than the micropolar
theories published currently in which material points
have six external degrees of freedom i.e. rotations are additional
external degrees of freedom.
This polar continuum theory only accounts for internal rotations
and associated moments that exist as a consequence of
deformation but are neglected in the present theories. We call
this theory “a polar continuum theory” as it considers rotations
and moments as conjugate pairs in a deforming solid continua
though these are internal, hence are purely related to the deformation
of the solid. It is shown that the polar continuum theory
presented in this paper is not the same as the strain gradient
theories reported in the literature. The differences are obviously
in terms of the physics described by them and the mathematical
details associated with conservation and balance laws. In
this paper, we only consider polar continuum theory for small
deformation and small strain. This polar continuum theory presented
here is a more complete thermodynamic framework as
it accounts for additional physics of internally varying rotations
that is neglected in the currently used thermodynamic framework.
This thermodynamic framework is suitable for isotropic,
homogeneous solid matter such as thermoelastic and thermoviscoelastic
solid continua with and without memory when the
deformation is small. The paper also presents preliminary material
helpful in consideration of the constitutive theories for polar
continua
Mode III interfacial crack in the presence of couple stress elastic materials
In this paper we are concerned with the problem of a crack lying at the
interface between dissimilar materials with microstructure undergoing antiplane
deformations. The micropolar behaviour of the materials is described by the
theory of couple stress elasticity developed by Koiter (1964). This
constitutive model includes the characteristic lengths in bending and torsion
and thus it is able to account for the underlying microstructure of the two
materials. We perform an asymptotic analysis to investigate the behaviour of
the solution near the crack tip. It turns out that the stress singularity at
the crack tip is strongly influenced by the microstructural parameters and it
may or may not show oscillatory behaviour depending on the ratio between the
characteristic lengths.Comment: 19 pages, 3 figur
Fractional Calculus for Continuum Mechanics - anisotropic non-locality
In this paper the generalisation of previous author's formulation of
fractional continuum mechanics to the case of anisotropic non-locality is
presented. The considerations include the review of competitive formulations
available in literature. The overall concept bases on the fractional
deformation gradient which is non-local, as a consequence of fractional
derivative definition. The main advantage of the proposed formulation is its
analogical structure to the general framework of classical continuum mechanics.
In this sense, it allows, to give similar physical and geometrical meaning of
introduced objects
Nonlinear Elasticity, Fluctuations and Heterogeneity of Nematic Elastomers
Liquid crystal elastomers realize a fascinating new form of soft matter that
is a composite of a conventional crosslinked polymer gel (rubber) and a liquid
crystal. These {\em solid} liquid crystal amalgams, quite similarly to their
(conventional, fluid) liquid crystal counterparts, can spontaneously partially
break translational and/or orientational symmetries, accompanied by novel soft
Goldstone modes. As a consequence, these materials can exhibit unconventional
elasticity characterized by symmetry-enforced vanishing of some elastic moduli.
Thus, a proper description of such solids requires an essential modification of
the classical elasticity theory. In this work, we develop a {\em rotationally
invariant}, {\em nonlinear} theory of elasticity for the nematic phase of ideal
liquid crystal elastomers. We show that it is characterized by soft modes,
corresponding to a combination of long wavelength shear deformations of the
solid network and rotations of the nematic director field. We study thermal
fluctuations of these soft modes in the presence of network heterogeneities and
show that they lead to a large variety of anomalous elastic properties, such as
singular length-scale dependent shear elastic moduli, a divergent elastic
constant for splay distortion of the nematic director, long-scale
incompressibility, universal Poisson ratios and a nonlinear stress-strain
relation fo arbitrary small strains. These long-scale elastic properties are
{\em universal}, controlled by a nontrivial zero-temperature fixed point and
constitute a qualitative breakdown of the classical elasticity theory in
nematic elastomers. Thus, nematic elastomers realize a stable ``critical
phase'', characterized by universal power-law correlations, akin to a critical
point of a continuous phase transition, but extending over an entire phase.Comment: 61 pages, 24 eps pages, submitted to Annals of Physic
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