DefinitionElongation at break is how much a material stretches before it fractures in a tensile test, expressed as a percentage of the specimen's original gauge length.
Explained
What Does Elongation at Break Mean?
Elongation at break comes from the same tensile test that produces tensile strength and yield strength. A dumbbell-shaped specimen is pulled at a controlled speed until it fractures, and the gauge length at fracture (Lf) is compared with the original gauge length (L0):
Elongation at break (%) = (Lf − L0) / L0 × 100
The result is the same quantity materials scientists call strain to failure or percent elongation (%EL). For plastics the governing test methods are ASTM D638 and ISO 527; the two cover the same property but differ in technical detail, so values from one are not directly interchangeable with the other.
The number is sensitive to how it was measured. ASTM D638 notes that tensile properties vary with specimen preparation, testing speed and environment, which matters especially for plastics such as nylon that absorb moisture. Compare datasheet values only when they share a test method and conditioning state.
Elongation at Break vs Tensile Strength vs Yield Strength
| Elongation at break | Tensile strength | Yield strength | |
|---|---|---|---|
| What it measures | Stretch at the moment of fracture | Maximum stress the specimen carries | Stress at which permanent deformation begins |
| Unit | % of original length | MPa (psi) | MPa (psi) |
| What a high value means | Ductile: bends, stretches or snaps back before breaking | Carries high load before failing | Stays elastic under higher loads |
| Where it matters | Snap fits, living hinges, clips, impact-prone parts | Load-bearing brackets and housings | Parts that must not take a permanent set |
Reinforcement shows the trade-off clearly. In one 2026 injection-molding study, unfilled PA66 (nylon 66) reached about 292% elongation at break, while PA66 with 20% glass fiber reached only about 8% — even though its tensile strength more than doubled, from about 51 MPa to 111 MPa (7.4 to 16.1 ksi).
How to Use Elongation at Break When Choosing a Material
- Match it to the feature. Snap fits and living hinges need a material that can stretch well past the strain the design imposes; a rigid, low-elongation grade may crack at the first assembly.
- Read it together with strength. A high-strength, low-elongation material fails suddenly; a lower-strength, high-elongation material gives visible warning.
- Expect reinforcement to cut it. Glass fiber raises strength and stiffness but sharply reduces elongation.
- Check the process, not just the resin. A prototype built in a different process or grade can behave differently from the production part, so validate critical flexing features in the production-intent material.
At NICE Rapid, SLS parts can be built in PA11 nylon, which is more ductile than PA12, when a prototype hinge or clip has to flex. On LSR molding programs, tensile strength and elongation are inspected on every batch.
Key takeaways
- Elongation at break is the standard datasheet measure of ductility for a material.
- It is measured during a tensile test and reported as the percentage increase in gauge length at fracture.
- High values indicate ductile materials that deform extensively before failure; low values indicate brittle behavior.
- ASTM D638 and ISO 527 are common test standards for plastics, but their results are not directly interchangeable.
- Reinforcements such as glass fiber increase strength and stiffness while sharply reducing elongation at break.
Example
Related terms
References
- Plastic deformation: strength and ductility — University of Cambridge, DoITPoMS
- ASTM D638-22, Standard Test Method for Tensile Properties of Plastics — ASTM International
- Effect of 20 wt% Glass Fiber Reinforcement on the Mechanical Properties and Microstructure of Injection-Molded PA6 and PA66 — Polymers (Basel) 2026;18(3):357