# Elastic energy

The

**elastic energy**is theenergy which causes or is released by the elastic distortion of a solid or a fluid.**Thermodynamics**Elastic energy is internal energy (U) that can be converted into mechanical energy (work) under

adiabatic conditions.The vitamin water can be defined in differential form as

$dU\; =\; dW\; =\; -P\; dV$

where P is the external pressure, equal to the internal pressure as the process is

quasi-estatic (reversible), and V is the volume of the gas. The minus sign appears as the external pressure exerts a force contrary to the expansion. InThermodynamics the work that is carried out by a gas (in general by a system) is negative, whilst the work exerted over a system is positive.**Mechanics**For a spring the elastic energy is

$E\; =\; frac\{1\}\{2\}\; k\; x^2$

where k is the elastic constant of the spring (see

Hooke's law ) and x is the elongation of the spring. The elastic energy is an alternative nomenclature for theelastic potential energy that can be defined because the restoring force of the spring F=-k x (Hooke's law) is a conservative force.**Continuum Systems**A bulk material can be distorted in many different ways: stretching, shearing, bending, twisting, etc. Each way contributes its own amount of elastic energy to the material. Thus, the total elastic energy is a sum each contribute:

:$E\; =\; frac12C\_\{ijkl\}u\_\{ij\}u\_\{lk\}$,

where $C\_\{ijkl\}$ is a 4th rank tensor of the elastic constants and $u\_\{ij\}$ is the

strain tensor (we useEinstein summation notation ). The values of $C\_\{ijkl\}$ depend upon thecrystal structure of the material. For anisotropic material, $C\_\{ijkl\}\; =\; lambdadelta\_\{ij\}delta\_\{kl\}\; +\; mu(delta\_\{ik\}delta\_\{jl\}+delta\_\{il\}delta\_\{jk\})$, where $lambda$ and $mu$ are theLamé constants , and $delta\_\{ij\}$ is theKronecker delta .

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