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# Normal strain

As with stresses, strains may also be classified as 'normal strain' and 'shear strain' (i.e. acting perpendicular to or along the face of an element respectively). Strains are caused by relative displacements. Rigid body motions don't produce strains.

Normal strains produce dilations without a shape-change as such.

Normal strain,

$\epsilon = \frac{\text{extension}}{\text{original length}}$

The 'stretch' of an element over its original element length is thus:

$\epsilon = \frac{\Delta u}{\Delta x} = \frac{du}{dx} = u'(x)$