In
physics and
materials science, elasticity is the ability of a
body to resist a distorting influence and to return to its original size and shape when that influence or
force is removed. Solid objects will
deform when adequate
loads are applied to them; if the material is elastic, the object will return to its initial shape and size after removal. This is in contrast to
''plasticity'', in which the object fails to do so and instead remains in its deformed state.
The physical reasons for elastic behavior can be quite different for different materials. In
metal
A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
s, the
atomic lattice changes size and shape when forces are applied (energy is added to the system). When forces are removed, the lattice goes back to the original lower energy state. For
rubbers and other
polymers, elasticity is caused by the stretching of polymer chains when forces are applied.
Hooke's law states that the force required to deform elastic objects should be
directly proportional to the distance of deformation, regardless of how large that distance becomes. This is known as ''perfect elasticity'', in which a given object will return to its original shape no matter how strongly it is deformed. This is an
ideal concept only; most materials which possess elasticity in practice remain purely elastic only up to very small deformations, after which plastic (permanent) deformation occurs.
In
engineering
Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
, the elasticity of a material is quantified by the
elastic modulus such as the
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied ...
,
bulk modulus
The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume.
Other moduli descri ...
or
shear modulus which measure the amount of
stress needed to achieve a unit of
strain; a higher modulus indicates that the material is harder to deform. The
SI unit of this modulus is the
pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Frenc ...
(Pa). The material's ''elastic limit'' or
yield strength is the maximum
stress that can arise before the onset of plastic deformation. Its SI unit is also the pascal (Pa).
Overview
When an elastic material is deformed due to an external force, it experiences internal resistance to the deformation and restores it to its original state if the external force is no longer applied. There are various
elastic moduli, such as
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied ...
, the
shear modulus, and the
bulk modulus
The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume.
Other moduli descri ...
, all of which are measures of the inherent elastic properties of a material as a resistance to deformation under an applied load. The various moduli apply to different kinds of deformation. For instance, Young's modulus applies to extension/compression of a body, whereas the shear modulus applies to its
shear. Young's modulus and shear modulus are only for solids, whereas the
bulk modulus
The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume.
Other moduli descri ...
is for solids, liquids, and gases.
The elasticity of materials is described by a
stress–strain curve, which shows the relation between
stress (the average restorative internal
force per unit area) and
strain (the relative deformation). The curve is generally nonlinear, but it can (by use of a
Taylor series) be approximated as linear for sufficiently small deformations (in which higher-order terms are negligible). If the material is
isotropic, the linearized stress–strain relationship is called
Hooke's law, which is often presumed to apply up to the elastic limit for most metals or crystalline materials whereas nonlinear elasticity is generally required to model large deformations of rubbery materials even in the elastic range. For even higher stresses, materials exhibit
plastic behavior, that is, they deform irreversibly and do not return to their original shape after stress is no longer applied. For rubber-like materials such as
elastomers, the slope of the stress–strain curve increases with stress, meaning that rubbers progressively become more difficult to stretch, while for most metals, the gradient decreases at very high stresses, meaning that they progressively become easier to stretch. Elasticity is not exhibited only by solids;
non-Newtonian fluids, such as
viscoelastic fluids, will also exhibit elasticity in certain conditions quantified by the
Deborah number The Deborah number (De) is a dimensionless number, often used in rheology to characterize the fluidity of materials under specific flow conditions. It quantifies the observation that given enough time even a solid-like material might flow, or a flui ...
. In response to a small, rapidly applied and removed strain, these fluids may deform and then return to their original shape. Under larger strains, or strains applied for longer periods of time, these fluids may start to flow like a
viscous
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the in ...
liquid.
Because the elasticity of a material is described in terms of a stress–strain relation, it is essential that the terms ''stress'' and ''strain'' be defined without ambiguity. Typically, two types of relation are considered. The first type deals with materials that are elastic only for small strains. The second deals with materials that are not limited to small strains. Clearly, the second type of relation is more general in the sense that it must include the first type as a special case.
For small strains, the measure of stress that is used is the
Cauchy stress while the measure of strain that is used is the
infinitesimal strain tensor; the resulting (predicted) material behavior is termed
linear elasticity, which (for
isotropic media) is called the generalized
Hooke's law.
Cauchy elastic materials and
hypoelastic materials are models that extend Hooke's law to allow for the possibility of large rotations, large distortions, and intrinsic or induced
anisotropy.
For more general situations, any of a number of
stress measures can be used, and it is generally desired (but not required) that the elastic stress–strain relation be phrased in terms of a
finite strain measure that is
work conjugate
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an animal tr ...
to the selected stress measure, i.e., the time integral of the inner product of the stress measure with the rate of the strain measure should be equal to the change in internal energy for any
adiabatic process that remains below the elastic limit.
Units
International System
The SI unit for elasticity and the elastic modulus is the
pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Frenc ...
(Pa). This unit is defined as force per unit area, generally a measurement of
pressure, which in mechanics corresponds to
stress. The pascal and therefore elasticity have the
dimension L
−1⋅M⋅T
−2.
For most commonly used engineering materials, the elastic modulus is on the scale of gigapascals (GPa, 10
9 Pa).
Linear elasticity
As noted above, for small deformations, most elastic materials such as
spring
Spring(s) may refer to:
Common uses
* Spring (season), a season of the year
* Spring (device), a mechanical device that stores energy
* Spring (hydrology), a natural source of water
* Spring (mathematics), a geometric surface in the shape of a h ...
s exhibit linear elasticity and can be described by a linear relation between the stress and strain. This relationship is known as
Hooke's law. A geometry-dependent version of the idea was first formulated by
Robert Hooke in 1675 as a Latin
anagram, "ceiiinosssttuv". He published the answer in 1678: "''Ut tensio, sic vis''" meaning "''As the extension, so the force''", a linear relationship commonly referred to as
Hooke's law. This law can be stated as a relationship between tensile
force and corresponding extension
displacement ,
:
where is a constant known as the ''rate'' or ''spring constant''. It can also be stated as a relationship between
stress and
strain :
:
where is known as the
elastic modulus or
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied ...
.
Although the general proportionality constant between stress and strain in three dimensions is a 4th-order
tensor called
stiffness
Stiffness is the extent to which an object resists deformation in response to an applied force.
The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is.
Calculations
The stiffness, k, of a ...
, systems that exhibit
symmetry, such as a one-dimensional rod, can often be reduced to applications of Hooke's law.
Finite elasticity
The elastic behavior of objects that undergo finite deformations has been described using a number of models, such as
Cauchy elastic material models,
Hypoelastic material models, and
Hyperelastic material models. The
deformation gradient (''F'') is the primary deformation measure used in
finite strain theory.
Cauchy elastic materials
A material is said to be Cauchy-elastic if the
Cauchy stress tensor ''σ'' is a function of the
deformation gradient ''F'' alone:
:
It is generally incorrect to state that Cauchy stress is a function of merely a
strain tensor, as such a model lacks crucial information about material rotation needed to produce correct results for an anisotropic medium subjected to vertical extension in comparison to the same extension applied horizontally and then subjected to a 90-degree rotation; both these deformations have the same spatial strain tensors yet must produce different values of the Cauchy stress tensor.
Even though the stress in a Cauchy-elastic material depends only on the state of deformation, the work done by stresses might depend on the path of deformation. Therefore, Cauchy elasticity includes non-conservative "non-hyperelastic" models (in which work of deformation is path dependent) as well as conservative "
hyperelastic material" models (for which stress can be derived from a scalar "elastic potential" function).
Hypoelastic materials
A hypoelastic material can be rigorously defined as one that is modeled using a
constitutive equation
In physics and engineering, a constitutive equation or constitutive relation is a relation between two physical quantities (especially kinetic quantities as related to kinematic quantities) that is specific to a material or substance, and ap ...
satisfying the following two criteria:
# The Cauchy stress
at time
depends only on the order in which the body has occupied its past configurations, but not on the time rate at which these past configurations were traversed. As a special case, this criterion includes a
Cauchy elastic material, for which the current stress depends only on the current configuration rather than the history of past configurations.
# There is a tensor-valued function
such that
in which
is the material rate of the Cauchy stress tensor, and
is the spatial
velocity gradient tensor.
If only these two original criteria are used to define hypoelasticity, then
hyperelasticity
A hyperelastic or Green elastic materialR.W. Ogden, 1984, ''Non-Linear Elastic Deformations'', , Dover. is a type of constitutive model for ideally elastic material for which the stress–strain relationship derives from a strain energy density ...
would be included as a special case, which prompts some constitutive modelers to append a third criterion that specifically requires a hypoelastic model to ''not'' be hyperelastic (i.e., hypoelasticity implies that stress is not derivable from an energy potential). If this third criterion is adopted, it follows that a hypoelastic material might admit nonconservative adiabatic loading paths that start and end with the same
deformation gradient but do ''not'' start and end at the same internal energy.
Note that the second criterion requires only that the function
''exists''. As detailed in the main
hypoelastic material article, specific formulations of hypoelastic models typically employ so-called objective rates so that the
function exists only implicitly and is typically needed explicitly only for numerical stress updates performed via direct integration of the actual (not objective) stress rate.
Hyperelastic materials
Hyperelastic materials (also called Green elastic materials) are conservative models that are derived from a
strain energy density function
A strain energy density function or stored energy density function is a scalar-valued function that relates the strain energy density of a material to the deformation gradient.
:
W = \hat(\boldsymbol) = \hat(\boldsymbol^T\cdot\boldsymbol) = ...
(''W''). A model is hyperelastic if and only if it is possible to express the
Cauchy stress tensor as a function of the
deformation gradient via a relationship of the form
:
This formulation takes the energy potential (''W'') as a function of the
deformation gradient (
). By also requiring satisfaction of
material objectivity
Walter Noll (January 7, 1925 June 6, 2017) was a mathematician, and Professor Emeritus at Carnegie Mellon University. He is best known for developing mathematical tools of classical mechanics, thermodynamics, and continuum mechanics.
Biography
B ...
, the energy potential may be alternatively regarded as a function of the
Cauchy-Green deformation tensor
In continuum mechanics, the finite strain theory—also called large strain theory, or large deformation theory—deals with deformations in which strains and/or rotations are large enough to invalidate assumptions inherent in infinitesimal strai ...
(
), in which case the hyperelastic model may be written alternatively as
:
Applications
Linear elasticity is used widely in the design and analysis of structures such as
beams,
plates and shells, and
sandwich composites. This theory is also the basis of much of
fracture mechanics.
Hyperelasticity is primarily used to determine the response of
elastomer-based objects such as
gasket
Some seals and gaskets
A gasket is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression. It is a deformable material that is used to ...
s and of biological materials such as
soft tissues and
cell membranes.
Factors affecting elasticity
For
isotropic material
Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...
s, the presence of fractures affects the Young and the shear moduli perpendicular to the planes of the cracks, which decrease (Young's modulus faster than the shear modulus) as the fracture
density increases, indicating that the presence of cracks makes bodies brittler.
Microscopically, the stress–strain relationship of materials is in general governed by the
Helmholtz free energy, a
thermodynamic quantity.
Molecules settle in the configuration which minimizes the free energy, subject to constraints derived from their structure, and, depending on whether the energy or the
entropy term dominates the free energy, materials can broadly be classified as ''energy-elastic'' and ''entropy-elastic''. As such, microscopic factors affecting the free energy, such as the
equilibrium distance between molecules, can affect the elasticity of materials: for instance, in
inorganic materials, as the equilibrium distance between molecules at
0 K increases, the
bulk modulus
The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume.
Other moduli descri ...
decreases. The effect of temperature on elasticity is difficult to isolate, because there are numerous factors affecting it. For instance, the bulk modulus of a material is dependent on the form of its
lattice, its behavior under
expansion
Expansion may refer to:
Arts, entertainment and media
* ''L'Expansion'', a French monthly business magazine
* ''Expansion'' (album), by American jazz pianist Dave Burrell, released in 2004
* ''Expansions'' (McCoy Tyner album), 1970
* ''Expansio ...
, as well as the
vibrations of the molecules, all of which are dependent on temperature.
See also
References
External links
The Feynman Lectures on Physics Vol. II Ch. 38: Elasticity
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