Tensile strain at break is one of the most widely reported indicators of plastic ductility, quantifying how far a molded or machined specimen elongates before it ruptures under a controlled tensile load. Because the measured value is highly sensitive to specimen geometry, test speed, temperature, humidity, and preconditioning, standardized test conditions defined in ISO 527-1/-2, ASTM D638, GB/T 1040, and JIS K 7161/7162 must be strictly followed to obtain comparable data. This article explains the specimen types, conditioning regimes, apparatus requirements, testing speeds, and calculation methods that accredited third-party laboratories apply when determining the tensile strain at break of plastics.
Technical Overview
Tensile strain at break, designated εB (or nominal tensile strain at break, εtB, when derived from grip separation), is the increase in gauge length at the moment of specimen rupture divided by the initial gauge length, expressed as a percentage. It is a fundamental ductility parameter that distinguishes brittle materials, which fracture at strains of only a few percent, from ductile polymers that yield, neck, and cold-draw to elongations of several hundred percent before failing.
Because plastics are viscoelastic, the strain at break value is not a fixed material constant: it varies with loading rate, temperature, moisture content, specimen geometry, and processing-induced molecular orientation. Consequently, the result is only meaningful when the complete set of test conditions is specified and controlled. The principal governing standards include:
- ISO 527-1: Plastics — Determination of tensile properties — Part 1: General principles
- ISO 527-2: Test conditions for moulding and extrusion plastics
- ASTM D638: Standard Test Method for Tensile Properties of Plastics
- GB/T 1040.1 and GB/T 1040.2: Chinese national standards technically aligned with ISO 527-1 and ISO 527-2
- JIS K 7161 and JIS K 7162: Japanese industrial standards harmonized with the ISO 527 series
Testing Samples
Multipurpose Specimens 1A and 1B (ISO 527-2)
Specimen 1A is the preferred geometry for directly injection-molded plastics: an overall length of 170 mm, a narrow parallel section 10 mm wide, a preferred thickness of 4 mm, a gauge length (L0) of 50 mm, and an initial distance between grips of 115 ± 1 mm. Specimen 1B has the same narrow-section dimensions but a larger fillet radius and is intended for machining from sheets, laminates, bars, pipes, or finished parts in accordance with ISO 2818.
Small-Scale Specimens 5A and 5B (ISO 527-2)
When the available quantity of material is limited, for example during early-stage formulation development, the scaled-down geometries 5A and 5B may be used. These specimens have a reduced gauge length and reduced thickness, and they must be tested at correspondingly lower speeds. Results from small-scale specimens are not directly comparable with those obtained on 1A/1B specimens and must be reported with the specimen designation.
ASTM D638 Specimen Types
ASTM D638 defines specimen Types I through V. Type I (overall length 165 mm, 13 mm narrow-section width, preferred 3.2 mm thickness, 50 mm gauge length) is the most widely used geometry for rigid plastics; Type IV serves alternative dimensions, and Type V is a small specimen for limited-material situations. Selection depends on thickness, rigidity, and the reinforcing content of the material.
Film, Sheet, and Machined Samples
Thin films and sheeting below approximately 1 mm are tested under ISO 527-3 using rectangular strips or small dumbbell cut-outs, with testing direction recorded as machine direction or transverse direction. For anisotropic fiber-reinforced composites, ISO 527-4 and ISO 527-5 apply, and specimens are prepared parallel and perpendicular to the fiber orientation.
Testing Items
The core measured parameter is the tensile strain at break, and the tensile test simultaneously yields the complete stress–strain curve, from which the following properties are typically reported:
- Tensile strain at break (εB) and nominal tensile strain at break (εtB)
- Tensile stress at break (σB) and tensile stress at yield (σY)
- Tensile strain at yield (εY)
- Tensile strength (the maximum stress sustained)
- Tensile modulus (Et), determined as the chord modulus between strains of 0.05 % and 0.25 %
- Optional: Poisson's ratio when biaxial extensometry is used
Strain at break results characteristically show higher scatter than stress or modulus values, so individual specimen results, the mean or median, and the coefficient of variation should all be documented in the test report.
Testing Methods
Apparatus and Calibration
Testing is performed on a constant-rate-of-crosshead-movement universal testing machine equipped with self-tightening (wedge or pneumatic) grips and a calibrated force transducer meeting ISO 7500-1 class 1 (or better) or ASTM E4 requirements. Strain measurement uses either a contact extensometer or a non-contact video extensometer with gauge length 50 ± 0.5 mm for 1A/1B and ASTM Type I specimens, with accuracy conforming to ISO 9513 class 1 or ASTM E83. Grip alignment must be verified to prevent bending stresses that depress measured elongation.
Specimen Conditioning and Test Atmosphere
Specimens are conditioned in the standard atmosphere 23/50 defined in ISO 291 (23 ± 2 °C, 50 ± 10 % relative humidity; class 1 tolerances of ± 1 °C and ± 5 % RH where required), typically for a minimum of 88 hours. Under ASTM D618 Procedure A, conditioning is 40 hours at 23 ± 2 °C and 50 ± 5 % relative humidity. Testing is then carried out in the same controlled atmosphere. For moisture-sensitive materials such as polyamides, the moisture state must be defined and reported: either dry-as-molded after pre-drying, or humidity-conditioned to equilibrium, for example via accelerated conditioning in accordance with ISO 1110.
Specimen Measurement and Gripping
Width and thickness are measured at several positions within the gauge length using calibrated instruments per ISO 16012, and the minimum cross-sectional area is used for stress calculation. Specimens are centered in the grips with an initial grip separation of 115 ± 1 mm for 1A/1B and ASTM Type I geometries, and only a small preload is applied before the test starts.
Test Speed
The crosshead speed is one of the most influential conditions and must match the specimen type and material class:
- ISO 527-2, specimens 1A/1B: 1 mm/min for modulus determination, then 50 mm/min for strength and strain at break
- ISO 527-2, small specimens: reduced speeds, typically 5 mm/min for non-modulus properties
- ASTM D638, Type I: commonly 50 mm/min (2 in./min) for strength and elongation, with 5 mm/min (0.2 in./min) for modulus
- Lower speeds from the preferred ISO series (1, 2, 5, 10, 20, 50, 100, 200, 500 mm/min) are selected for brittle materials or when a product specification dictates otherwise
Procedure and Data Reduction
The machine is started at the prescribed speed, and force and extension are recorded continuously until the specimen ruptures completely. For ductile materials that neck beyond the extensometer range, the extensometer may be removed after yield and the nominal strain computed from grip separation, since ISO 527-1 permits grip-displacement-based nominal strain for post-yield behavior. The calculation is εtB = ΔL / L0 × 100 %, where ΔL is the extension at rupture and L0 the initial gauge length (or initial grip distance for nominal strain).
Validity and Number of Specimens
A minimum of five valid specimens per material, condition, and orientation is required by both ISO 527-1 and ASTM D638. Results from specimens that break within the grips or in the fillet region, or that slip during testing, are discarded and replaced. ISO 527-1 treats the median as the characteristic value for strain at break because the failure distribution tends to be skewed; stress and modulus values are reported as arithmetic means.
Applications
Controlled tensile strain at break data supports a broad range of engineering and regulatory activities:
- Material datasheet generation and incoming-material quality control for resins, compounds, and masterbatches
- Verification of the ductility of injection-molded and extruded parts after process, tool, or colorant changes
- Evaluation of the effects of fillers, stabilizers, plasticizers, impact modifiers, and recycled content on toughness
- Embrittlement assessment after heat aging, UV/weathering exposure, chemical contact, or hydrolysis studies
- Comparative material selection and finite-element analysis input for automotive, electrical and electronic, packaging, medical device, pipe and fitting, and consumer product applications
- Failure analysis, where an abnormally low strain at break confirms in-service embrittlement, degradation, or excessive orientation
FAQ
What is the difference between strain at break and elongation at break?
The two terms describe the same physical quantity: the deformation achieved at rupture relative to the initial gauge dimension. ISO 527 expresses it as tensile strain at break (a dimensionless ratio in percent), while ASTM D638 and many datasheets use elongation at break. The underlying measurement principle is identical when the gauge length is defined consistently.
Are ISO 527 and ASTM D638 results interchangeable?
No. The two standards use different specimen geometries (for example, ISO 1A versus ASTM Type I), different preferred thicknesses, different default speeds, and different modulus conventions. Numerical values for strain at break can differ measurably between the methods, so the test standard and specimen type must always accompany the reported result.
Why does test speed change the strain at break value?
Plastics are rate-dependent viscoelastic solids. Higher crosshead speeds generally raise strength and lower ductility, while slower speeds promote yielding and drawing. Moisture and temperature act in the opposite direction to speed for many materials, which is why conditioning and test atmosphere must be controlled as rigorously as the speed itself.
Can crosshead displacement be used instead of an extensometer?
Crosshead displacement yields only nominal strain, because it includes machine and grip compliance. An extensometer meeting ISO 9513 or ASTM E83 is required for accurate modulus and yield-strain determination; grip-based nominal strain is acceptable for strain at break measured after yield, where the machine compliance contribution is proportionally small.
How should results be reported for anisotropic or oriented materials?
Specimens should be cut both parallel and perpendicular to the flow or machine direction, tested as separate data sets, and clearly labeled with the orientation, specimen type, conditioning history, and test speed. This is essential for films, sheets, glass-mat materials, and fiber-reinforced compounds, where directionality can change the strain at break severalfold.
What causes scatter in strain at break data?
Common sources include specimen surface defects from machining, voids or notches from molding, slight misalignment, moisture variations, and the inherently statistical nature of brittle fracture initiation. Testing a larger number of specimens, verifying grip alignment, and reporting the median per ISO 527-1 all improve the reliability of the reported value.
