Stress and Strain

Stress and strain are fundamental concepts in mechanics of materials. Stress (σ) is the internal resistance per unit area developed within a material when subjected to external forces, expressed as σ = F/A. Strain (ε) is the measure of deformation, defined as the change in length relative to the original length (ε = ΔL/L). Normal stress acts perpendicular to a surface, while shear stress acts parallel. Hooke’s Law governs the linear elastic relationship between stress and strain: σ = E × ε, where E is Young’s modulus. Beyond the elastic limit, materials exhibit plastic deformation. Poisson’s ratio (ν) relates lateral strain to longitudinal strain. Engineering strain uses the original length, while true strain uses the instantaneous length. Stress-strain curves provide key mechanical properties such as yield strength, ultimate strength, modulus of elasticity, and fracture point.