Elasticity

A material is elastic if it returns to its original shape after being deformed. The maximum load that a body can experience and still return to its original shape is known as the Elastic Limit.

Metals
The elastic limit is defined as the 0.2% offset yield strength. This represents the stress at which the stress-strain curve for uniaxial tensile loading deviates by a strain of 0.2% from the linear-elastic line. It is the same in tension and compression. It is the stress at which dislocations move a large distance through the crystals of the metal.

Polymers
The elastic limit is the stress at which the uniaxial stress-strain curve becomes markedly non-linear: typically, a strain of 1%. This may be caused by 'shear yielding' (irreversible slipping of molecular chains) or by 'crazing' (formation of low density, crack-like volumes which scatter light, making the polymer look white).

Fine Ceramics and Glasses
The elastic limit is often an estimate, based on the tensile strength (which is low due to brittle fracture).

Composites
The elastic limit is best defined by a set deviation from linear-elastic uniaxial behaviour.
The elastic limit depends on the mode of loading. For modes of loading other than uniaxial tension, such as shear and multiaxial loading, the strength is related to that in simple tension by a yield function. For metals, the Von Mises yield function works well. It specifies the relationship between the principal stresses σ1, σ2, σ3 and the yield strength (elastic limit) σy:

See also: Apparent Elastic Limit, Elastic Deformation, Elastic Strain, Hookes Law, Offset Yield Strength, Plasticity, Strain, Stress.

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Subjects: Materials Mechanical Engineering