Plastic Strain at Break Determination Test Report: Standards, Methods, and Data Interpretation

Strain at break (fracture strain) is one of the most critical mechanical parameters for evaluating the ductility and ultimate load-bearing capacity of plastic materials. This technical article presents a comprehensive experimental report framework for determining the strain at break of plastics in accordance with ISO 527, ASTM D638, and GB/T 1040, covering specimen preparation, instrumentation, test procedures, calculations, result interpretation, and practical reporting requirements.

Technical Overview

Strain at break, denoted εB, is defined as the tensile strain of a plastic specimen at the moment of fracture, expressed as the percentage increase in gauge length relative to the original gauge length L0. It quantifies the deformation capacity of a material before failure and serves as a key indicator of ductility, toughness, and quality consistency in plastic products. A closely related parameter, the nominal strain at break εtB, is calculated from the crosshead displacement relative to the initial grip separation L and is used when the extensometer measuring range is insufficient or when the specimen fractures outside the extensometer gauge length.

The determination of strain at break is typically performed as part of a uniaxial tensile test, in which a dumbbell-shaped or strip specimen is stretched at a constant crosshead speed until fracture while force and extension are continuously recorded. The resulting stress-strain curve provides not only the strain at break but also tensile strength, yield stress, yield strain, and tensile modulus. The principal international and national standards governing this measurement include ISO 527-1 and ISO 527-2, ASTM D638, GB/T 1040.1 and GB/T 1040.2, and the general principles are harmonized with JIS K 7161 and JIS K 7162. Because plastics are viscoelastic and strain-rate sensitive, strict control of test speed, temperature, humidity, and specimen geometry is essential for obtaining reproducible and comparable results in an accredited laboratory environment.

Testing Samples

ISO Multipurpose Specimens (Type 1A and 1B)

The ISO 527-2 multipurpose specimen is the preferred geometry for rigid and semi-rigid thermoplastics and thermosets. Type 1A specimens (gauge length L0 = 50 mm, overall length 170 mm, width in parallel section 10 mm, thickness 4 mm) are normally produced by injection molding, while Type 1B specimens (overall length 150 mm) are frequently machined from compression-molded or extruded sheets. The preferred thickness is 4 mm; specimens between 1 mm and 10 mm thickness are acceptable with appropriate documentation.

ASTM D638 Specimens (Type I through Type VI)

ASTM D638 defines six specimen geometries. Type I (gauge length 50 mm, width 13 mm, thickness 3.2 mm) is the standard geometry for rigid plastics tested at speeds of 1 to 500 mm/min. Type V (gauge length 7.6 mm, thickness 1.6 mm or less) is used for small components, film-like parts, and high-speed screening tests. Type II and Type III geometries apply to specific material classes, and Type IV is intended for testing under alternative conditions where a smaller specimen is practical.

Film and Sheet Specimens

For plastic films and sheets less than 1 mm thick, ISO 527-3 and ASTM D882 specify rectangular strip specimens, commonly 10 mm to 25 mm wide with gauge lengths of 50 mm or 100 mm, or dumbbell shapes such as Type 2 in ISO 527-3. These specimens are die-cut with sharp, burr-free edges to prevent premature stress concentration and artificial notch effects.

Specimen Conditioning and Number of Replicates

• Condition specimens at 23 °C ± 2 °C and 50 % ± 10 % relative humidity for at least 24 hours (40 hours for certain hygroscopic materials) in accordance with ISO 291 or ASTM D618, unless the material standard specifies otherwise.
• Mark the gauge length L0 on the narrow parallel section using a non-damaging gauge marker; verify dimensions of each specimen at three points with a calibrated micrometer to the nearest 0.01 mm.
• Test a minimum of five valid replicates per sample condition; increase to ten or more when high scatter is anticipated, for example with fiber-reinforced or highly oriented materials.
• Reject specimens that fracture within the grips, outside the gauge length (for extensometer-based strain), or that exhibit visible flaws, voids, or machining damage before testing.

Testing Items

The core deliverable of this experiment is the strain at break, but a complete tensile characterization report typically includes the following measurands:

• Strain at break εB (%) — extensometer-based tensile strain at the moment of fracture, referenced to gauge length L0.
• Nominal strain at break εtB (%) — calculated from crosshead travel referenced to the initial grip separation L, mandatory when the specimen necks or slides beyond the extensometer range.
• Tensile strength σM (MPa) — maximum tensile stress sustained before fracture.
• Tensile stress at break σB (MPa) — stress at the moment of fracture; equal to tensile strength when fracture precedes yield.
• Tensile stress at yield σY (MPa) and strain at yield εY (%) — first stress point at which strain increases without a further increase in stress.
• Tensile modulus Et (MPa) — secant modulus between strains of 0.0005 and 0.0025, measured with a class 1 or better extensometer per ISO 9513.
• Poisson's ratio μ (optional) — determined with a biaxial or transverse extensometer at low strain, typically below 0.4 %.

Testing Methods

Apparatus and Calibration

Testing is conducted on a calibrated universal tensile testing machine equipped with a suitable load cell selected so that fracture forces fall between 20 % and 80 % (preferably above 15 % per ISO 527-1) of the full-scale capacity. The machine shall comply with ISO 5893 or ASTM E4, with force measurement accuracy of at least ±1 %, crosshead speed accuracy of at least ±20 % of the set value (±10 % per ISO 527-1 for the prescribed speeds), and grips that align the specimen axis with the loading axis to minimize bending. Extension is measured either by a contact extensometer or a non-contact video extensometer with accuracy class 1 or better per ISO 9513; for modulus determination, class 0.5 or class 1 is required. When a video extensometer is used, gauge marks are applied to the specimen within the parallel section.

Test Procedure

• Measure and record the width and thickness of each specimen in the gauge section to the nearest 0.02 mm (or 0.01 mm for thin films) and calculate the original cross-sectional area A0.
• Install the specimen in the grips with the gauge marks symmetrically positioned; apply a small pretension (typically ≤ 0.5 % of expected maximum stress, or ≤ 0.01 MPa for flexible materials) to remove slack.
• Attach the extensometer to the gauge section and zero the force and extension channels.
• Select the test speed according to the material standard: common speeds are 1 mm/min for modulus measurement, and 5, 10, 20, 50, 100, 200, or 500 mm/min for strength and strain determination (Type 1A specimens are commonly tested at 50 mm/min unless otherwise specified).
• Start the test and record force versus extension continuously at a sampling rate sufficient to capture yield and fracture events; if the extensometer reaches its range limit before fracture, remove or reset the extensometer and continue to fracture using crosshead displacement to determine nominal strain at break.
• Stop the test at fracture, record the maximum force, force at break, extension at break, and crosshead travel at break, and photograph or document the fracture location and mode.

Calculations

Tensile stress is calculated as σ = F / A0, where F is the recorded force and A0 is the original cross-sectional area. Strain at break is calculated as εB = 100 × ΔLB / L0, where ΔLB is the extensometer extension at fracture and L0 is the original gauge length. Nominal strain at break is calculated as εtB = 100 × ΔLtB / L, where ΔLtB is the increase in grip separation at fracture, determined from crosshead travel corrected for machine compliance where significant. The tensile modulus is calculated as the slope of the stress-strain curve between the 0.0005 and 0.0025 strain limits: Et = (σ2 − σ1) / (ε2 − ε1).

Data Processing and Validity Criteria

For each reported parameter, calculate the arithmetic mean, standard deviation, and coefficient of variation from valid replicates. A replicate is considered invalid if fracture occurs within the grips (unless the material is so brittle that this cannot be avoided and the result exceeds the mean of valid results), if slippage is detected in the force-extension curve, or if a documented specimen defect influenced the fracture. ISO 527-1 requires reporting the median rather than the mean for strain at break in specific circumstances where indicated by the referring standard; otherwise, both mean and median may be reported for transparency. When results are used for material datasheet comparison, the number of significant figures shall not exceed three, and modulus values shall be rounded as specified in ISO 527-1.

Report Content Requirements

• Complete identification of the material tested, including grade, manufacturer reference, formulation, and any filler or reinforcement content.
• Specimen type, dimensions, machining or molding method, and the direction of testing relative to any processing orientation.
• Conditioning atmosphere and duration; test atmosphere and temperature at the time of testing.
• Test speed, extensometer type and class, gauge length, grip separation, and load cell capacity.
• Individual and mean values of strain at break (or nominal strain at break), tensile strength, stress at break, yield parameters, and modulus, each with standard deviation.
• Failure mode description: brittle fracture, ductile fracture with necking, fracture within or outside gauge length, and any deviations from the standard.
• Identification of the test standard edition used (for example, ISO 527-1:2019 and ISO 527-2:2012, ASTM D638-22, or GB/T 1040.1-2018 and GB/T 1040.2-2021) and the date of test.

Applications

Strain at break data obtained through this test method support a wide range of engineering, regulatory, and quality functions across industries:

• Material selection and design: comparing the ductility of candidate polymers for components that must tolerate impact, deflection, or thermal cycling without brittle failure, such as snap-fits, clips, and living hinges.
• Quality control in production: verifying that incoming resin lots, molded parts, or extruded profiles retain the strain at break declared in material datasheets and internal specifications.
• Failure analysis: comparing the fracture strain of a failed component with that of reference material to identify degradation caused by thermal aging, UV exposure, hydrolysis, chemical attack, or excessive regrind content.
• Processing optimization: evaluating the effect of mold temperature, injection speed, annealing, or weld line position on the ductility of molded articles, particularly for fiber-reinforced and glass-filled compounds.
• Regulatory compliance and specification acceptance: demonstrating conformance of engineering plastics to product standards in the automotive, electrical and electronic, medical device, packaging, and pipe and fitting sectors, where minimum elongation or strain at break limits are frequently imposed.
• Research and development: characterizing new polymer blends, biobased resins, and recycled compounds, where strain at break is a sensitive indicator of formulation changes and compatibilization efficiency.

FAQ

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

Strain at break εB is expressed as a dimensionless ratio or percentage relative to the extensometer gauge length L0, while elongation at break is the older and largely equivalent term expressed in percent. In current editions of ISO 527, the preferred terminology is strain at break; ASTM D638 retains the term percent elongation at break. Numerically, when measured over the same reference length, the two quantities are identical.

When must nominal strain at break be reported instead of strain at break?

Nominal strain at break εtB is reported when the specimen breaks at a strain exceeding the extensometer measuring range, when an extensometer is removed before fracture to protect it, or when the referring product standard mandates it. Since ISO 527-1:2012 and its subsequent revision, εtB referenced to the initial grip separation has become the recommended ductility parameter for highly extensible materials because it is not limited by extensometer range and avoids the influence of local necking on the gauge-length measurement.

Why does test speed significantly affect the measured strain at break?

Plastics are viscoelastic, so their deformation resistance increases with strain rate. At higher crosshead speeds, many thermoplastics exhibit higher tensile strength and lower strain at break due to suppressed molecular relaxation and earlier brittle fracture. Consequently, comparing results obtained at different speeds, or comparing ISO data (frequently 50 mm/min on Type 1A) with ASTM data (commonly 50 mm/min on Type I but often 5 mm/min for other configurations), requires careful attention to the stated test conditions.

What are typical strain at break values for common plastics?

Indicative values at standard laboratory conditions include: general-purpose polystyrene and SAN below 5 % (brittle); glass-fiber-reinforced polyamide in the dry state roughly 3 % to 8 %; polycarbonate, ABS, and polyamide well above 50 % (often exceeding 100 %); polypropylene and polyethylene typically exceeding 200 % and commonly 400 % or more. Actual values depend strongly on grade, conditioning, orientation, and test speed, so material datasheet values should always be verified experimentally for critical applications.

What factors most commonly cause abnormal or scattered strain at break results?

Frequent causes include specimen defects such as voids, sink marks, or machining burns; poor edge quality on die-cut film strips acting as stress concentrators; grip slippage or fracture within the grips; incorrect gauge length marking; insufficient conditioning of moisture-sensitive polymers such as polyamide; testing at temperatures far from the standard atmosphere; and using an inappropriate load cell range or extensometer class. Systematic scatter reduction requires controlling specimen preparation, alignment, conditioning, and instrumentation calibration.

Can strain at break testing be performed at low or elevated temperatures?

Yes. ISO 527-1 permits testing within an environmental chamber at temperatures typically between −40 °C and +150 °C, provided the specimen and grips are thermally soaked and the temperature is maintained within ±2 °C during the test. For cryogenic or high-temperature evaluation, non-contact video extensometers are preferred because contact extensometers may not function reliably, and their thermal expansion must be compensated.

How should results be presented in the final experimental report?

The report shall present, in tabular and graphical form, the individual and mean values of all calculated measurands with standard deviations, the stress-strain curves of representative specimens, the complete test conditions, and the failure mode of each replicate. All data must be traceable to the standard edition applied, the calibration status of the equipment, and the accreditation scope of the laboratory if the report is issued by an accredited third-party laboratory for regulatory or contractual purposes.