What Is an Elongation at Break?

Elongation 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. It is the standard datasheet measure of ductility: a high value means the material deforms a long way before failing, a low value means it breaks with little warning.

In short: tensile strength tells you how hard you can pull; elongation at break tells you how far the material will stretch before it lets go.

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 (L_f) is compared with the original gauge length (L₀):

Elongation at break (%) = (L_f − 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.

Frequently Asked Questions

What is elongation at break?

It is the percentage a material stretches in a tensile test before it fractures, calculated from the change in gauge length divided by the original gauge length. It is the standard way datasheets express ductility.

What does elongation at break mean on a plastic datasheet?

It tells you how much the material can stretch before breaking under the test conditions listed, usually ASTM D638 or ISO 527. A high value indicates a tough, ductile grade; a low value indicates a stiff grade that fails with little deformation.

Is higher elongation at break better?

Not always. High elongation suits snap fits, hinges and impact-prone parts, but fiber-reinforced grades trade elongation for stiffness and strength. The right value depends on whether the part must flex or must hold its shape under load.

Related Services at NICE Rapid

• 3D Printing — SLS nylon options include PA12, the more ductile PA11, and glass- or carbon-filled PA12 when stiffness matters more than stretch.
• LSR Molding — Shore hardness, tensile strength, elongation and critical dimensions are inspected on every batch.
• Request a Quote — tell us which features must flex and our engineers will suggest suitable materials.

Sources

1. University of Cambridge, DoITPoMS, "Plastic deformation: strength and ductility" — ductility expressed as percent elongation, %EL = (l_f − l₀)/l₀ × 100%, identical to strain to failure — doitpoms.ac.uk
2. ASTM D638-22, Standard Test Method for Tensile Properties of Plastics — dumbbell specimens; tensile properties vary with specimen preparation, speed and environment of testing; D638 and ISO 527-1 address the same subject but differ in technical content — astm.org
3. Polymers (Basel) 2026;18(3):357, "Effect of 20 wt% Glass Fiber Reinforcement on the Mechanical Properties and Microstructure of Injection-Molded PA6 and PA66" — PA66 ≈292% elongation and 51.09 MPa tensile strength; PA66 GF20 ≈8% elongation and 111.43 MPa; glass fiber raises strength and reduces elongation at break; PA6 and PA66 absorb moisture — pmc.ncbi.nlm.nih.gov