Normal strain
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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). For an isotropic material that obeys Hooke's law, a normal stress will cause a normal strain. Strains are relative displacements - they are the actual displacement divided by the length before the strain was applied. Rigid body motions don't produce strains. Normal strains produce dilations, however they merely stretch the body along the axis of application (negating poisson's ratio and the effects it causes). As such, a normal strain will cause the following effects:
- Any line parallel to the strain axis will increase by an amount proportional to its length and the amount of strain. (See below).
- Any line perpendicular to the strain axis will have no length change (negating Poisson's effect.)
- Any line not perpendicular nor parallel will have a length increase based upon vector algebra.
- Normal Strains will become Shear strains for a rotation of the frame of reference. See Mohr's circle for strain.
Normal strain,
The 'stretch' of an element over its original element length is thus:
