Strain is a measure of deformation representing the displacement between particles in a material body. It quantifies how much a material is stretched, compressed, or otherwise deformed in response to applied forces or loads. Strain is typically expressed as a ratio of change in length to the original length (for linear strain) or in terms of the change in shape relative to the original shape (for shear strain).
In mathematical terms, linear strain (ε) can be defined as:
\[ \epsilon = \frac{\Delta L}{L_0} \]
where:
- \( \Delta L \) is the change in length,
- \( L_0 \) is the original length.
Strain can be elastic (reversible) or plastic (permanent), depending on whether the material returns to its original shape after the load is removed.