When a material is stretched in one direction, it tends to contract in the directions perpendicular to the applied force. This effect is quantified by Poisson's Ratio ($\sigma$ or $\nu$): $\sigma =-\frac{\text{Lateral Strain (}\Delta d/d\text{)}}{\text{Longitudinal Strain (}\Delta L/L\text{)}}$

• For most common materials, $\sigma$ lies between $0.2$ and $0.4$.
• Theoretical range for stable, isotropic linear elastic materials is $-1<\sigma <0.5$.