Precise Determination of Tensile Strain at Break in Plastics: Standards, Methods, and Best Practices

Tensile strain at break is one of the most important indicators of plastic ductility and a key acceptance criterion in material specification, product design, and quality control. Precise determination of this parameter requires strict adherence to internationally recognized standards such as ISO 527 and ASTM D638, along with calibrated extensometry, controlled specimen preparation, and standardized conditioning. This technical guide explains the principles, equipment, procedures, and best practices for accurately measuring tensile strain at break in plastic materials.

Technical Overview

Tensile strain at break (εB) is defined as the relative elongation of a plastic specimen at the moment of failure, calculated as the increase in gauge length divided by the initial gauge length (εB = ΔL / L0), typically expressed as a percentage. It quantifies the ultimate deformability of a material and directly reflects its ductility, toughness, and resistance to brittle fracture. Because plastics exhibit viscoelastic behavior, strain at break is highly sensitive to strain rate, temperature, humidity, specimen geometry, and processing history, making measurement discipline essential for obtaining reproducible and comparable data.

The primary international framework for this measurement is the ISO 527 series, particularly ISO 527-1 (general principles) and ISO 527-2 (test conditions for moulding and extrusion plastics), with film and sheet materials covered by ISO 527-3 and fiber-reinforced composites by ISO 527-4 and ISO 527-5. The dominant North American equivalent is ASTM D638, while the Chinese national standards GB/T 1040.1 and GB/T 1040.2 and the Japanese standards JIS K 7161 and JIS K 7162 are technically aligned with the ISO 527 methodology. Although the frameworks are similar, specimen geometry and testing speeds differ, so results from ISO and ASTM protocols must not be interchanged without careful justification.

Precise determination of strain at break depends on three measurement pillars: a calibrated force train on a universal testing machine (verified per ISO 7500-1), an accurate strain measurement device (classified per ISO 9513 or ASTM E83), and valid specimen failure within the gauge section. The achievable accuracy of the reported value is governed by the extensometer class, the resolution of the data acquisition system, and rigorous control of test conditions and specimen quality in accredited third-party laboratories.

Testing Samples

Injection-Moulded Rigid and Semi-Rigid Thermoplastics

The reference specimen for ISO testing is the ISO 527-2 type 1A multipurpose test specimen (170 mm overall length, 10 mm width in the parallel section, 4 mm thickness, 115 mm grip separation, 50 mm gauge length). The type 1B specimen is the preferred reduced-scale alternative when moulding constraints exist. For ASTM D638, the type I specimen (165 mm overall length, 13 mm width, 3.2 mm nominal thickness, 50 mm gauge length) is the general-purpose geometry, with type IV and type V reduced geometries used for thin or limited-material applications and type II or type III for thicker sections.

Machined and Compression-Moulded Specimens

When specimens are machined from sheets, plaques, pipes, or finished parts, the machining quality becomes critical. Edge notches, saw marks, and voids act as stress concentrators that artificially lower strain at break, so machined edges should be smoothed and polished. Specimen orientation relative to the processing direction (machine direction versus transverse direction, or flow versus cross-flow) must be recorded because molecular and filler orientation produces significant anisotropy in strain at break.

Flexible Films and Thin Sheets

Plastic films and sheets thinner than approximately 1 mm are tested using ISO 527-3 rectangular strip specimens or, for very thin sheeting, the approach of ASTM D882. Because films can reach several hundred percent elongation before failure, long-travel or non-contact strain measurement is generally required.

Filled and Reinforced Compounds

Mineral-filled, short-fiber-reinforced, and continuous-fiber-reinforced plastics are tested with the appropriate ISO 527-4 or ISO 527-5 geometries. These materials are notch-sensitive and frequently fail at low strains, so specimen edge preparation and gauge-mark application require particular care.

Testing Items

A tensile test conducted for the precise determination of strain at break simultaneously yields a complete tensile property profile:

• Tensile strain at break (εB), the principal measurand, expressed in percent
• Tensile stress at break (σB), calculated from the force at rupture and the minimum original cross-section
• Tensile strength (σM) and strain at tensile strength (εM)
• Tensile stress at yield (σY) and strain at yield (εY) for ductile materials exhibiting a yield point
• Tensile modulus (Et), determined as a secant modulus between two defined strain points per ISO 527-1
• Nominal strain at break (εtB), based on grip separation, reported when direct strain measurement is not used
• Poisson's ratio (μ), optionally measured with transverse extensometry
• Statistical reporting of individual values, mean, and standard deviation

Testing Methods

Equipment and Calibration Requirements

Testing is performed on a screw-driven or servo-hydraulic universal testing machine equipped with mechanical, pneumatic, or hydraulic grips suitable for rigid plastics. The force measuring system must be verified per ISO 7500-1 or ASTM E4. Strain measurement devices must be classified per ISO 9513 or ASTM E83: at minimum Class 1 for strain at break determination, with Class 0.5 or Class B-1 recommended when the tensile modulus is also required. The overall machine and extensometer system for plastics testing is commonly specified per ISO 5893.

Specimen Conditioning and Measurement

Specimens are conditioned in the standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity per ISO 291 (or ASTM D618 practice A for the ASTM protocol) for a minimum of 24 hours unless the material standard specifies otherwise. Moisture-sensitive polymers such as polyamides require explicit reporting of their conditioning state (dry-as-moulded, 50 % RH equilibrium, or saturated), because moisture content can change strain at break by an order of magnitude. Width and thickness are measured at several positions within the gauge section to an accuracy of 0.02 mm, and the minimum cross-sectional area within the parallel section is used for stress calculation.

Test Procedure

• Mark the initial gauge length L0 on the specimen parallel section, typically 50 mm for both ISO 1A/1B and ASTM type I geometries.
• Mount the specimen in the grips at the specified separation (115 mm), ensuring axial alignment to avoid bending stresses.
• Apply a small preload and zero both force and strain channels.
• For modulus determination, test at 1 mm/min (ISO) until the upper strain limit is reached; strain at break is subsequently determined at the speed specified by the relevant material standard, commonly 50 mm/min for the ISO 1A and ASTM type I specimens. The ISO 527-1 two-speed procedure is used when both modulus and strain at break are required on the same specimen.
• Continue loading at constant speed until the specimen ruptures, recording force and strain continuously at sufficient sampling frequency to capture the break event.
• Calculate strain at break as the strain value at the point of rupture, using only tests in which failure occurs within the gauge section.

Strain Measurement Techniques

Contact extensometers (clip-on or long-travel types) provide traceable accuracy but must be removed before break on ductile materials to avoid equipment damage, which limits their use for strain at break determination. Non-contact video extensometers track gauge marks optically through the entire test up to rupture and are the preferred solution for precise strain at break measurement. Full-field digital image correlation (DIC) additionally reveals localized necking and strain distribution, which is valuable for research and failure analysis. Crosshead displacement alone should only be used to report nominal strain, because it includes machine compliance and deformation outside the gauge section and therefore cannot deliver precise strain at break values.

Data Validity and Reporting

ISO 527-1 requires testing of at least five specimens; ten specimens are recommended when directional anisotropy or high scatter is present. Individual results from specimens breaking outside the gauge length, or in the grips, are discarded and replaced. The report must state the standard used, specimen type and preparation method, testing speed, conditioning atmosphere, individual and mean strain at break values, standard deviation, and the strain measurement method.

Sources of Error and Best Practices

• Grip slippage or grip-induced premature failure, mitigated by correct grip pressure and serrated or faced grip inserts
• Machine compliance and seating effects contaminating crosshead-derived strain values
• Specimen edge defects and voids from poor preparation acting as fracture initiation sites
• Strain rate effects, since strain at break decreases markedly at higher speeds for many ductile plastics
• Temperature and humidity deviations, particularly for polyamides, polyurethanes, and thermoplastic elastomers
• Incorrect gauge mark spacing or mark tracking errors in video extensometry
• Unreported specimen orientation relative to processing direction

Applications

Precise strain at break data supports a broad range of industrial and scientific objectives:

• Generation and verification of material datasheets for resin producers and compounders
• Incoming quality control and batch release testing of polymers, compounds, and masterbatches
• Input for product design, allowable strain limits, and finite element simulation material cards
• Monitoring of ductile-to-brittle transitions during thermal aging, UV weathering, hydrolysis, and chemical exposure studies
• Temperature-dependent evaluation in environmental chambers, including low-temperature brittleness assessment for automotive, aerospace, and outdoor applications
• Compliance demonstration for packaging films, medical devices, pipes and fittings, and electronic encapsulants
• Failure analysis, comparing the properties of failed parts against virgin material reference data

FAQ

What is the difference between tensile strain at break and elongation at break?

The two terms describe the same physical quantity. Strain at break is the dimensionless ratio of elongation to initial gauge length, while elongation at break expresses that ratio as a percentage. ISO 527 uses the term strain at break, whereas ASTM D638 traditionally uses percent elongation at break.

Can crosshead displacement be used to measure strain at break?

Crosshead displacement yields only nominal strain, which includes machine compliance, grip penetration, and deformation outside the gauge section. It is acceptable for screening purposes and for nominal strain at break reporting, but a calibrated extensometer or non-contact video system is required for precise determination.

What should be done if the specimen breaks in the grips or outside the gauge section?

Per ISO 527-1, individual values from invalid breaks are discarded and additional specimens are tested. Recurring invalid failures indicate excessive grip pressure, misalignment, unsuitable grip faces, or specimen defects that must be corrected before valid data can be obtained.

How many specimens are required?

A minimum of five valid specimens is required by ISO 527-1, with ten recommended for anisotropic or highly scattered materials. ASTM D638 similarly specifies five specimens for routine testing.

Are ISO 527 and ASTM D638 results interchangeable?

No. The two standards use different specimen geometries, thicknesses, and preferred speeds, which systematically influence strain at break. Data must always be reported together with the governing standard and test parameters.

How is strain at break measured for very ductile materials that exceed the extensometer range?

Long-travel extensometers or non-contact video extensometers with extended travel are used, ensuring the device can track gauge marks to the point of rupture. For films reaching several hundred percent elongation, the grip separation distance may be used to report nominal strain at break with an explicit method statement.