Strain at break is one of the most important ductility indicators obtained from plastic tensile testing, quantifying how far a polymer specimen elongates before fracture. This article explains the strain at break calculation formula for plastic tensile tests, the relevant ISO 527 and ASTM D638 standards, specimen preparation, test procedures, and common sources of measurement error. It serves as a practical reference for materials engineers, quality control specialists, and laboratory technicians working with polymeric materials.
Technical Overview
Strain at break, often reported as percent elongation at break, is the tensile strain of a plastic specimen at the instant of rupture. It is defined as the increase in gauge length divided by the original gauge length and is a fundamental measure of material ductility. For a specimen with an initial gauge length L0 and a gauge length Lb at the moment of break, the engineering strain at break is calculated as:
- εB = (Lb − L0) / L0 × 100%, which may equivalently be written as εB = ΔL / L0 × 100% where ΔL is the total extension at break.
As an example, if the initial gauge length is 50 mm and the specimen elongates by 12.5 mm before fracture, the strain at break is (12.5 / 50) × 100% = 25%. If the same specimen elongates to a gauge length of 90 mm, the strain at break is (90 − 50) / 50 × 100% = 80%.
The calculation and measurement of strain at break are governed by internationally recognized test standards. ISO 527-1 specifies the general principles of tensile testing of plastics, ISO 527-2 defines test conditions for rigid and semi-rigid moulding and extrusion plastics, and ISO 527-3 covers films and sheets. In the American and Chinese systems, ASTM D638 and the GB/T 1040 series are the corresponding standards, while JIS K 7161 and JIS K 7162 apply in Japan. All of these standards define specimen geometry, gauge length, test speed, and data-reduction rules that must be followed for a valid strain-at-break result.
An important distinction is made between strain at break εB, measured with an extensometer attached to the gauge section, and nominal strain at break εtB, calculated from the separation of the grips or the crosshead travel. Because grip separation includes deformation of the shoulders and fillets outside the gauge section as well as machine compliance, nominal strain at break is systematically larger than extensometer-based strain at break and must be reported as a nominal value.
Testing Samples
ISO Multipurpose Specimens
ISO 527-2 defines the widely used multipurpose dog-bone specimens. Specimen 1A is 170 mm long with a 10 mm narrow-section width, 4 mm thickness, a 50 mm gauge length, and an initial grip separation of 115 mm. Specimen 1B is 150 mm long with a 100 mm grip separation and the same 10 mm × 4 mm narrow section and 50 mm gauge length. Specimen 1A is typically injection moulded directly, while 1B is commonly machined from sheets, plates, or finished parts.
ASTM D638 Tensile Specimens
ASTM D638 specifies several specimen types. Type I is the standard rigid-plastic specimen, 165 mm long with a 13 mm narrow-section width, a nominal 3.2 mm thickness, and a 50 mm gauge length. Type IV and Type V geometries are used when material is limited or when thin sections must be evaluated, with correspondingly shorter gauge lengths such as 25 mm for Type IV and 7.6 mm for Type V.
Films, Sheets, and Finished Products
Thin films and sheets are tested with strip or dumbbell specimens according to ISO 527-3 or, for very thin films, ASTM D882, using longer gauge lengths such as 100 mm. Specimens cut from finished articles are prepared so that the gauge section is free of sink marks, weld lines, flash, and visible defects that could initiate premature fracture.
Conditioning
Before testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity per ISO 291 or ASTM D618, typically for a minimum of 40 hours (88 hours for certain hygroscopic materials), because temperature and moisture content strongly influence the ductility of plastics.
Testing Items
A plastic tensile test configured for strain-at-break determination typically delivers the following results:
- Strain at break εB, measured with a contact or video extensometer on the gauge section.
- Nominal strain at break εtB, calculated from grip separation or crosshead travel.
- Tensile stress at break σB, the tensile force at rupture divided by the original cross-sectional area.
- Tensile stress at yield σY and strain at yield εY, when the material exhibits a yield point.
- Tensile modulus ET, calculated as the secant slope between strains of 0.05% and 0.25% per ISO 527.
- The complete force-extension and stress-strain curve, including the type of failure (brittle, ductile with necking, or semi-ductile).
Testing Methods
Specimen Preparation and Dimensional Measurement
After conditioning, the width and thickness of each specimen are measured at several points within the gauge section using instruments with a resolution of at least 0.02 mm for thickness and 0.1 mm for width, and the minimum cross-sectional area is recorded. Accurate dimensional data are essential because the cross-sectional area is used for stress calculation, while the recorded initial gauge length L0 is the denominator of the strain-at-break formula.
Machine and Extensometer Setup
The test is performed on a calibrated universal testing machine fitted with load cells appropriate to the expected force range, typically with grips of the wedge or pneumatic type. The grips are set to the specified initial separation, for example 115 mm for the ISO 1A specimen. An extensometer with an accuracy of ISO 9513 class 1 or better is attached to the gauge section, with the gauge length set to 50 mm for ISO 1A/1B and ASTM Type I specimens. Non-contact video extensometers are preferred for very ductile materials and for tests that continue to complete rupture, since they eliminate the risk of damage and can track strain up to the break point.
Test Speed Selection
Plastics are viscoelastic, so strain at break depends on strain rate. ISO 527-1 lists preferred test speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min; 1 mm/min is used for modulus determination and speeds of 5 mm/min or 20 mm/min are typical for 1B and 1A specimens respectively. ASTM D638 commonly uses 50 mm/min (2 in/min) for Type I specimens of ductile unreinforced plastics and 5 mm/min (0.2 in/min) for modulus and for reinforced or brittle materials. The selected speed must be reported together with the results.
Data Acquisition and Break Detection
Extension and force are sampled continuously at a rate sufficient to capture the sudden force drop at rupture. The break point is identified as the point at which the force falls sharply to zero or to a defined fraction of its maximum. The extension value at this point is substituted into the strain-at-break equation together with the initial gauge length to obtain εB. For materials that neck extensively, the extensometer may be removed before break once the strain exceeds its travel limit; in this case only the nominal strain at break εtB, based on grip separation, can be reported.
Strain at Break Calculation
- Strain at break: εB = (Lb − L0) / L0 × 100%, where L0 is the initial extensometer gauge length and Lb is the gauge length at rupture.
- Nominal strain at break: εtB = (L′ − L′0) / L′0 × 100%, where L′0 is the initial grip separation and L′ is the grip separation at rupture.
- Worked example for nominal strain: initial grip separation 115 mm, grip separation at break 172.5 mm, therefore εtB = (172.5 − 115) / 115 × 100% = 50%.
Corrections and Error Sources
When strain is derived from crosshead travel, the measured displacement includes load-frame and grip compliance as well as deformation in the specimen shoulders, which can overstate the gauge-section strain, particularly for stiff specimens with low strain at break. Machine-compliance correction, the use of an extensometer, and strict adherence to the specified gauge length minimize these errors. Other significant error sources include specimen misalignment, grip slippage, pre-load applied to the specimen, and gauge-length marking inaccuracies.
Validation and Reporting
ISO 527 and ASTM D638 require a minimum of five specimens per condition, and results are reported as the arithmetic mean together with the standard deviation. Specimens that fracture within the grips or at a fillet outside the gauge length are normally discarded and retested, because such failures do not represent the true ductility of the material. The test report must state the standard used, specimen type, thickness, test speed, conditioning atmosphere, whether strain was measured by extensometer or grip separation, and the individual and mean strain-at-break values.
Applications
Strain-at-break data calculated from plastic tensile tests support a wide range of engineering and quality activities:
- Incoming resin inspection and batch release in injection moulding and extrusion plants, where a low strain at break signals contamination, excessive moisture, or degradation during processing.
- Preparation of material datasheets and material selection, allowing designers to compare the ductility of candidate polymers and compounded grades.
- Input for finite element analysis and design allowables, where elongation limits and stress-strain curves up to break are required for crash, snap-fit, and formability simulations.
- Evaluation of the effects of fillers, reinforcements, plasticizers, and recycled content, which typically reduce or alter strain at break.
- Aging, weathering, and chemical-exposure studies, in which a decline in strain at break is one of the earliest and most sensitive indicators of embrittlement.
- Failure analysis and benchmarking of competitor materials in accredited third-party laboratories and corporate test facilities.
FAQ
What is the formula for strain at break in a plastic tensile test?
Strain at break is calculated as εB = (Lb − L0) / L0 × 100%, where L0 is the initial gauge length and Lb is the gauge length at the moment of fracture. The result is a dimensionless engineering strain conventionally expressed as a percentage.
What is the difference between strain at break and nominal strain at break?
Strain at break is measured by an extensometer on the gauge section of the specimen, while nominal strain at break εtB is calculated from the separation between the grips. Nominal strain includes deformation outside the gauge section and machine compliance, so it is always equal to or larger than the extensometer value and must be identified as nominal in the report.
Why do ISO 527 and ASTM D638 sometimes give different strain-at-break values for the same plastic?
The two standards use different specimen geometries, thicknesses, gauge lengths, and preferred test speeds. Because plastics are rate- and geometry-sensitive, results obtained under ISO 527 and ASTM D638 are not directly interchangeable and should never be compared without qualification.
How many specimens are required and how are anomalous results handled?
Both ISO 527 and ASTM D638 require at least five valid specimens. Results from specimens that break in the grips or outside the gauge length are rejected, the failure is documented, and additional specimens are tested until a valid set is obtained.
Does test speed affect the calculated strain at break?
Yes. As viscoelastic materials, plastics deform differently at different strain rates; higher crosshead speeds generally produce higher tensile strength and lower strain at break. The test speed must therefore be fixed by the standard and stated in the test report.
Can strain at break exceed 100%?
Yes. Highly ductile plastics such as polyethylene, polypropylene, and certain thermoplastic elastomers can elongate to several hundred percent or more of their original gauge length before breaking, while brittle materials such as unfilled polystyrene or glass-filled engineering plastics may show strain at break values below 2%.
How is strain at break measured for brittle plastics and very ductile plastics?
For brittle materials with strains below a few percent, only an extensometer of ISO 9513 class 0.5 or 1, or a video extensometer, provides sufficient resolution; crosshead displacement is not acceptable. For very ductile materials, a long-travel or non-contact video extensometer is used so that strain can be tracked up to complete rupture; otherwise only nominal strain at break can be reported.