Mechanics of Materials

σ = P/A, where P is the axial force and A is the cross-sectional area.

τ = V/A, where V is the shear force and A is the area parallel to the force.

ε = δ/L, the change in length divided by the original length (dimensionless).

γ is the change in angle (in radians) between two originally perpendicular lines in a deformed body.

σ = Eε, stress is proportional to strain within the elastic (linear) range.

E is the slope of the linear elastic portion of the stress-strain curve; stiffness of the material.