Stress-Strain Curve

Theory

The stress-strain curve is the foundational characterisation of a material's mechanical behaviour. Key regions: (1) linear elastic, slope = E; (2) yield point/region, onset of plastic flow; (3) strain hardening, stress continues rising past yield; (4) UTS, peak stress; (5) necking: localised thinning; (6) fracture. Brittle materials skip stages 3-5. Area under curve = toughness.

Application

Reading a stress-strain curve is essential for material selection. The elastic slope gives E; the 0.2% offset intersection gives σ_y; the peak gives UTS; the final point gives %EL. In FDM material comparison, overlaying S-S curves for XY vs. Z orientation immediately reveals anisotropy. Simulation software imports tabular S-S data for non-linear FEA.

Common mistakes

Engineering S-S curves use original dimensions: they show a drop past UTS due to necking even as true stress continues rising. Comparing curves from different test standards (ASTM vs. ISO) introduces systematic errors. FDM curves show notch effects and layer-interface steps that are absent in bulk injection-moulded specimens.

Related terms: Tensile Strength, Yield Strength, Young's Modulus, Toughness, Necking, Elongation at Break