Necking

Theory

Necking begins when the rate of strain hardening can no longer compensate for the reduction in cross-sectional area. At UTS, geometric softening exceeds material hardening, and deformation localises into a neck. True stress continues rising during necking even as engineering stress falls. Considère's criterion: necking begins when dσ_true/dε_true = σ_true.

Application

Necking is a visual indicator of ductile failure: you see the specimen thinning before it breaks. For FDM parts, necking rarely develops in the Z-direction because interlayer bonding fails before bulk plasticity localises. In XY-direction specimens of ductile filaments (PETG, nylon), clear necking is observable. Understanding necking location (near defects or layer steps) helps identify process quality issues.

Common mistakes

Engineering stress-strain curves show a misleading apparent stress drop past UTS due to necking: the material is not actually weakening. Elongation at break (%EL) measurements depend on gauge length because necking is localised: longer gauge lengths give lower apparent %EL. FDM parts often fracture at layer interfaces rather than developing geometric necks, making standard tensile analysis less directly applicable.

Related terms: Tensile Strength, Ductility, Elongation at Break, Stress-Strain Curve