The general mathematical equation for strain is:
Strain = dimensional change / original dimension, or ε = Δ R/Ro
Where R is a dimension such as length, width, angular displacement, etc., Ro is the original dimension, and Δ R is the difference between the deformed dimension and the original dimension.
Just as with stresses, strains are considered axial, or normal, if they occur in line with the applied load. These are termed tensile or compressive strains. If the deformation occurs parallel to the plane or area of applied stress, it is termed shear strain. A simplified example of this deformation is the motion of the top face of a square block of rubber with respect to its parallel, anchored bottom face. Applying a shear stress to the top face results in sideways displacement of the top of the block. Dividing that displacement by the original height of the block gives the shear strain. However, structural strain rarely so simple, as it usually involves complex deformations with both normal and shear components. But utilizing these straightforward models of stress and strain in materials has led to the computational design, modeling, and analysis tools so useful in the structural design process.