Elastic energy

Encyclopedia

**Elastic energy**is the potential mechanical energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

stored in the configuration of a material or physical system as work is performed to distort its volume or shape.

The concept of elastic energy is not confined to formal elasticity theory which primarily develops an analytical understanding of the mechanics of solid bodies and materials.

The essence of elasticity is reversibility. Forces applied to an elastic material transfer energy into the material which, upon yielding that energy to its surroundings, can recover its original shape. However, all materials have limits to the degree of distortion they can endure without breaking or irreversibly altering their internal structure. Hence, the characterizations of solid materials includes specification, usually in terms of strains, of its elastic limits. Beyond the elastic limit, a material is no longer storing all of the energy from mechanical work performed on it in the form of elastic energy.

Elastic energy of or within a substance is static energy of configuration. It corresponds to energy stored principally by changing the inter-atomic distances between nuclei. Thermal energy

Thermal energy

Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

is the randomized distribution of kinetic energy within the material, resulting in statistical fluctuations of the material about the equilibrium configuration. There is some interaction, however. For example, for some solid objects, twisting, bending, and other distortions may generate thermal energy, causing the material's temperature to rise. Thermal energy in solids is often carried by internal elastic waves, called phonons. Elastic waves that are large on the scale of an isolated object usually produce macroscopic vibrations sufficiently lacking in randomization that their oscillations are merely the repetitive exchange between (elastic) potential energy within the object and the kinetic energy of motion of the object as a whole.

## Elastic internal energy in compressible gases and liquids

Although elasticity is most commonly associated with the mechanics of solid bodies or materials, even the early literature on classical thermodynamics defines and uses "elasticity of a fluid" in ways compatible with the broad definition provided in the Introduction above.Solids include complex crystalline materials with sometimes complicated behavior. By contrast, the behavior of compressible fluids, and especially gases, demonstrates the essence of elastic energy with negligible complication. Mechanical work is required to compress such materials and the energy thus stored within them can be released when the mechanism sustaining their compression is released to allow such pressurized material to, for example, push on a piston. The simple thermodynamic formula describing this reversible process is

where dU is an infinitesimal change in recoverable internal energy

*U*,

*P*is the uniform pressure (a force per unit area) applied to the material sample of interest, and

*dV*is the infinitesimal change in volume that corresponds to the change in internal energy. The minus sign appears because

*dV*is negative under compression by a positive applied pressure which also increases the internal energy. Upon reversal, the work that is done

*by*a system is the negative of the change in its internal energy corresponding to the positive

*dV*of an increasing volume. In other words, the system loses stored internal energy when doing work on its surroundings. Pressure is stress and volumetric change corresponds to changing the relative spacing of points within the material. The stress-strain-internal energy relationship of the foregoing formula is repeated in formulations for elastic energy of solid materials with complicated crystalline structure.

## Elastic potential energy in mechanical systems

Components of mechanical systems will store elastic potential energy if any of them will be deformed when forces are applied to the system. Energy is transferred to an object (i.e. work is done on it) any time a force external to it displaces or deforms the object. The quantity of energy transferred to the object is computed as the vector dot productDot product

In mathematics, the dot product or scalar product is an algebraic operation that takes two equal-length sequences of numbers and returns a single number obtained by multiplying corresponding entries and then summing those products...

of the force and the displacement of the object. As forces are applied to the system they are distributed internally to its component parts. While some of the energy transferred can end up stored as kinetic energy of acquired velocity, the deformation of the shape of component objects results in stored elastic energy.

A prototypical elastic component is a coiled spring. The linear elastic performance of a spring is parametrized by a constant of proportionality, called the spring constant. This constant is usually denoted as

*k*(see also Hooke's Law

Hooke's law

In mechanics, and physics, Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load applied to it. Many materials obey this law as long as the load does not exceed the material's elastic limit. Materials for which Hooke's law...

) and depends on the geometry, cross sectional area, undeformed length and nature of the material from which the coil is fashioned. Within a certain range of deformation,

*k*remains constant and is defined as the negative ratio of displacement to the magnitude of the restoring force produced by the spring at that displacement.

Note that

*L*, the deformed length, can be larger or smaller than

*L*

_{o}, the undeformed length, so to keep

*k*positive,

*F*

_{r}must be given as a vector component of the restoring force whose sign is negative for

*L*>

*L*

_{o}and positive for

*L*<

*L*

_{o}. If we abbreviate the displacement as

then Hooke's Law can be written in the usual form

.

Energy absorbed and stored in the spring can be derived using Hooke's Law to compute the restoring force as a measure of the applied force. This requires the assumption, sufficiently correct in most circumstances, that at a given moment, the magnitude of applied force,

*F*

_{a}is equal to the magnitude of the resultant restoring force, but its direction and thus sign is different. In other words, assume that at each point of the displacement

*F*

_{a}=

*k*

*x*, where

*F*

_{a}is the component of applied force along the x direction

For each infinitesimal displacement

*dx*, the applied force is simply

*k x*and the product of these is the infinitesimal transfer of energy into the spring

*dU*. The total elastic energy placed into the spring from zero displacement to final length L is thus the integral

In the general case, elastic energy is given by the Helmholtz potential per unit of volume

*f*as a function of the strain tensor components ε

*:*

_{ij}where λ and μ are the Lamé elastical coefficients. The connection between stress tensor components and strain tensor components is:

For a material of Young's modulus,

*Y*(same as modulus of elasticity

*λ*), cross sectional area,

*A*

_{0}, initial length,

*l*

_{0}, which is stretched by a length, :

- where
`U`is the elastic potential energy._{e}

The elastic potential energy per unit volume is given by:

- where is the strain in the material.

## Continuum systems

A bulk material can be distorted in many different ways: stretching, shearing, bending, twisting, etc. Each kind of distortion contributes to the elastic energy of a deformed material. In orthogonal coordinates, the elastic energy per unit volume due to strain is thus a sum of contributions:,

where is a 4th rank tensor, called the elastic, or sometimes stiffness, tensor which is a generalization of the elastic moduli of mechanical systems, and is the strain tensor (Einstein summation notation has been used to imply summation over repeated indices). The values of depend upon the crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

structure of the material. For an isotropic material, , where and are the Lamé constants, and is the Kronecker delta.

The strain tensor itself can be defined to reflect distortion in any way that results in invariance under total rotation, but the most common definition which regard to which elastic tensors are usually expressed defines strain as the symmetric part of the gradient of displacement with all nonlinear terms suppressed:

where is the displacement at a point in the direction and is the partial derivative in the direction. Note that:

where no summation is intended. Although full Einstein notation sums over raised and lowered pairs of indices, the values of elastic and strain tensor components are usually expressed with all indices lowered. Thus beware (as here) that in some contexts a repeated index does not imply a sum over values of that index ( in this case), but merely a single component of a tensor.