Elongation at Break

Theory

Elongation at break (%EL) = (L_f − L₀)/L₀ × 100. It is measured from a tensile test after fracture by fitting the broken halves together. %EL summarises ductility in a single number. Standard references: ASTM D638 for plastics, ISO 527-2. Typical values: PLA 3-6%, PETG 20-50%, nylon 40-80%, TPU 300-600%.

Application

Selecting material for flexible clips, living hinges, or deformable seals requires high %EL. For rigid structural parts, very high %EL is undesirable as it means the part will permanently deform under load. In FDM, %EL in Z-direction can be 50-80% lower than XY due to interlayer bonding. Knowing %EL helps predict whether a part will give visible warning before final fracture.

Common mistakes

%EL measured on bulk bars is often far higher than for FDM prints in Z. Necking concentrates deformation locally: reported %EL depends on gauge length. %EL alone does not fully define ductility; the shape of the stress-strain curve matters. High %EL at low temperature can drop dramatically: test at service temperature.

Related terms: Ductility, Tensile Strength, Necking, Stress-Strain Curve