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When an elastic material is deformed due to an external force, it experiences internal forces that oppose the deformation and restore it to its original state if the external force is no longer applied. There are various elastic moduli, such as Young's modulus, the shear modulus, and the bulk modulus, all of which are measures of the inherent stiffness 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 uniform extension, whereas the shear modulus applies to shearing.
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). For most metals or crystaline materials, the curve is linear for small deformations, and so the stress-strain relationship can adequately be described by Hooke's law and higher-order terms can be ignored. However, for larger stresses beyond the elastic limit, the relation is no longer linear. 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 gradient 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 only solids; non-Newtonian fluids, such as viscoelastic fluids, will also exhibit elasticity in certain conditions. 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 liquid.
Hooke's law 
As noted above, for small deformations, most elastic materials such as springs exhibit linear elasticity. This 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 force F and displacement x,
Although the general proportionality constant between stress and strain in three dimensions is a 4th order tensor, systems that exhibit symmetry, such as a one-dimensional rod, can often be reduced to applications of Hooke's law.
Factors affecting elasticity 
For isotropic materials, the presence of fractures affects the Young and the shear modulus 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 equilibriumdistance 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 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, as well as the vibrations of the molecules, all of which are dependent on temperature.
See also 
- Treloar, L. R. G. (1975). The Physics of Rubber Elasticity. Oxford: Clarendon Press. p. 2. ISBN 978-0-1985-1355-1.
- Sadd, Martin H. (2005). Elasticity: Theory, Applications, and Numerics. Oxford: Elsevier. p. 70. ISBN 978-0-1237-4446-3.
- de With, Gijsbertus (2006). Structure, Deformation, and Integrity of Materials, Volume I: Fundamentals and Elasticity. Weinheim: Wiley VCH. p. 32. ISBN 978-3-527-31426-3.
- Atanackovic, Teodor M.; Guran, Ardéshir (2000). "Hooke's law". Theory of elasticity for scientists and engineers. Boston, Mass.: Birkhäuser. p. 85. ISBN 978-0-8176-4072-9.
- "Strength and Design". Centuries of Civil Engineering: A Rare Book Exhibition Celebrating the Heritage of Civil Engineering. Linda Hall Library of Science, Engineering & Technology.[page needed]
- Bigoni, D. Nonlinear Solid Mechanics: Bifurcation Theory and Material Instability. Cambridge University Press, 2012 . ISBN 9781107025417.[page needed]
- Sadd, Martin H. (2005). Elasticity: Theory, Applications, and Numerics. Oxford: Elsevier. p. 387. ISBN 978-0-1237-4446-3.
- Sadd, Martin H. (2005). Elasticity: Theory, Applications, and Numerics. Oxford: Elsevier. p. 344. ISBN 978-0-1237-4446-3.
- Sadd, Martin H. (2005). Elasticity: Theory, Applications, and Numerics. Oxford: Elsevier. p. 365. ISBN 978-0-1237-4446-3.