The Universal Mechanical Test
The tensile test is the most fundamental experiment in materials engineering. A specimen is gripped at both ends and pulled apart at a controlled rate while force and displacement are recorded. The resulting stress-strain curve is a fingerprint of the material's mechanical behavior — revealing its stiffness, strength, ductility, and toughness in a single graph.
Elastic Region and Young's Modulus
The initial linear portion of the curve obeys Hooke's law: stress is proportional to strain. The slope is Young's modulus E, a measure of atomic bond stiffness. Metals like steel (E ≈ 200 GPa) are far stiffer than polymers (E ≈ 1-4 GPa) because metallic bonds resist stretching more than the van der Waals forces between polymer chains. All deformation in this region is fully reversible.
Yielding and Plastic Flow
At the yield strength σ_y, dislocations begin to move through the crystal lattice, causing permanent (plastic) deformation. In mild steel, this appears as a sharp yield point followed by a plateau. In aluminum alloys, yielding is gradual and engineers define it using the 0.2% offset method. Beyond yielding, strain hardening increases the stress needed for further deformation as dislocation tangles accumulate.
Necking and Fracture
At the ultimate tensile strength σ_u, the specimen begins to neck — deformation localizes in a narrow region where the cross-section shrinks rapidly. The engineering stress drops because it's calculated from the original area, even though true stress at the neck continues to rise. Eventually the material fractures. The total area under the stress-strain curve represents the material's toughness — its ability to absorb energy before breaking.