Plastic Tensile Strain at Break Test Data: Standards, Methods, and Typical Values

Plastic tensile strain at break test data quantifies how far a plastic material can stretch before ultimate rupture, making it one of the most important indicators of ductility and toughness in material qualification. This article explains how tensile strain at break (εB) is defined, measured, and reported according to ISO 527, ASTM D638, and related standards, and reviews typical data ranges for common plastics.

Technical Overview

Tensile strain at break (εB), also referred to as elongation at break, is the engineering strain of a plastic specimen at the moment of rupture. It is calculated as the increase in gauge length divided by the original gauge length, expressed as a percentage: εB = ΔL / L0 × 100. The measurement is performed on a universal testing machine by pulling a standardized dumbbell or strip specimen to failure while recording extension with an extensometer or, under defined conditions, crosshead travel.

The principal international and national test methods governing plastic tensile strain at break test data include ISO 527-1 and ISO 527-2 (plastics — determination of tensile properties), ASTM D638 (tensile properties of plastics), GB/T 1040.1 and GB/T 1040.2 (Chinese national standards aligned with ISO 527), and JIS K 7161 and JIS K 7162. A complete test data package normally contains the individual specimen values, the mean or median result, a measure of scatter such as standard deviation, the full stress–strain curve, and related tensile properties including tensile stress at break, tensile strength, stress and strain at yield, and the tensile modulus.

Typical Strain-at-Break Ranges for Common Plastics

The following indicative ranges are commonly reported in material datasheets and literature for specimens tested at room temperature at standard test speeds. Actual values depend strongly on grade, molecular weight, filler content, conditioning, and test speed.

• General-purpose polystyrene (GPPS): approximately 1–3% (brittle failure)
• PMMA (acrylic): approximately 2–6%
• Rigid PVC (uPVC): approximately 10–50%
• ABS: approximately 15–50%
• Polycarbonate (PC): approximately 80–130%
• Polyoxymethylene (POM): approximately 20–75%
• Polyamide 6 / 6.6 (dry, as moulded): approximately 20–60%
• Polyamide 6 / 6.6 (moisture conditioned): often above 100% and up to 300%
• Polypropylene (PP): approximately 100–600%
• High-density polyethylene (HDPE): approximately 200–700%
• Low-density polyethylene (LDPE): approximately 300–900%
• PET (amorphous grades): approximately 50–300%, depending on orientation and crystallinity
• Glass-fibre reinforced compounds (30% GF, PA, PP, PBT): typically 2–5%
• Thermosetting moulding compounds and castings: generally below 3%

These figures should always be treated as comparative reference values. For contract-relevant decisions, verified data generated by accredited third-party laboratories on the specific batch and specimen type should be used.

Testing Samples

Sample preparation has a direct influence on tensile strain at break test data, because surface defects, orientation, internal voids, and residual stress can all shift the measured failure point. Samples are generally divided into the following categories.

Rigid and Brittle Thermoplastics

Materials such as polystyrene, SAN, PMMA, and rigid PVC are tested using ISO 527-2 type 1A or 1B multipurpose dumbbell specimens (50 mm gauge length, 10 mm width, 4 mm thickness) or ASTM D638 type I specimens (50 mm gauge length, 13 mm narrow-section width, 3.2 mm nominal thickness). Specimens may be directly injection moulded or machined from sheets, plaques, or finished parts, with machined edges polished to remove stress raisers.

Ductile and Highly Elastomeric Grades

Materials such as PE, PP, soft PVC, thermoplastic elastomers, and conditioned polyamides may elongate several hundred percent before rupture. For these materials, gauge marks must remain legible over large extensions, and non-contact video extensometry is often preferred. Plastic films and sheets below 1 mm thickness are tested with rectangular strip specimens in accordance with ISO 527-3.

Reinforced and Filled Compounds

Glass-fibre or mineral-filled compounds are strongly anisotropic, so strain at break is typically reported separately for the moulding flow direction and the transverse direction. Specimens may be cut from compression-moulded or injection-moulded plaques, and thicker sections can be machined to ASTM D638 type II or type III geometries.

Specimen Conditioning

Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 10% relative humidity in accordance with ISO 291 or ASTM D618, commonly for a minimum of 40 hours. Moisture-sensitive materials such as polyamides require additional dry-as-moulded or humidity-equilibrated conditioning, because moisture content can change the strain at break by a factor of several times.

Testing Items

A tensile strain at break test campaign typically delivers the following data items, each determined from the same recorded load–extension curve:

• Tensile strain at break (εB), expressed in percent, for each specimen and as a statistical summary
• Tensile stress at break (σB), the stress at the moment of rupture
• Tensile strength (σM), the maximum stress sustained, and stress at yield (σY) where applicable
• Tensile strain at yield (εY) for ductile materials exhibiting a yield point
• Tensile modulus (Et), determined in ISO 527 as the secant between strains of 0.05% and 0.25%
• Nominal strain at break based on grip separation where permitted by the standard
• Mean or median value, standard deviation, and coefficient of variation across valid specimens
• Digitized stress–strain curves and machine metadata (load cell capacity, test speed, extensometer class)

The final test report identifies the material and batch, specimen type and preparation method, conditioning history, test speed, specimen dimensions, number of valid specimens, individual and statistical results, and the location of each break.

Testing Methods

Testing is performed on a calibrated universal testing machine fitted with suitable grips, usually pneumatic side-action or wedge-type grips with faces matched to the specimen material. Load measurement complies with ISO 7500-1 or the equivalent verification standard, and strain measurement devices comply with ISO 9513.

Specimen Preparation and Measurement

Width and thickness are measured at several points within the gauge length, and the minimum cross-sectional area is used for stress calculation. Gauge marks defining L0 (typically 50 mm) are applied to the narrow parallel section, and specimens are inspected for visible defects before testing.

Test Speed and Modulus Determination

Because plastics are viscoelastic, strain at break is rate dependent. In routine practice under ISO 527-2, the modulus is measured at 1 mm/min, after which the test continues at the specified property speed, commonly 50 mm/min for unreinforced thermoplastics on 1A specimens and lower speeds for reinforced or stiff materials. ASTM D638 commonly uses 5 mm/min for modulus determination and 50 mm/min for strength and strain at break on type I specimens. Any deviation from the standard speed must be stated in the report, as it prevents direct comparison with catalogue data.

Strain Measurement Techniques

Contact extensometers clipped to the gauge marks provide accurate strain data up to their travel limit. For ductile materials with very high elongation, the extensometer is removed before failure to avoid damage, and the strain at break is then derived from crosshead travel as a nominal strain based on grip separation, with correspondingly reduced accuracy. Modern non-contact video extensometers track gauge marks throughout the entire test, including the necking and cold-drawing region, and deliver full-range strain data without interruption.

Validity Criteria and Data Evaluation

Only breaks occurring within the gauge length or the narrow parallel section are considered valid. Breaks at or near the grips, or outside the gauge section, are discarded and replaced, with the anomalous event noted in the report. A minimum of five valid specimens per material and direction is normally required. Results are reported as individual values together with the mean or median and standard deviation, and strain at break is rounded according to the precision rules of the applied standard.

Factors That Influence the Measured Data

• Test speed and loading rate, driven by viscoelastic behaviour
• Temperature and humidity during conditioning and testing
• Moisture content of polyamides and other hygroscopic polymers
• Specimen preparation route, including weld lines, gate orientation, and machining quality
• Molecular orientation introduced by injection moulding or film stretching
• Surface defects, notches, and residual stress acting as premature crack initiators
• Filler and fibre content, which generally reduces ductility
• Material aging, UV exposure, thermal degradation, and recycled content

Applications

Plastic tensile strain at break test data supports a wide range of engineering and commercial activities:

• Incoming material inspection and production quality control against datasheet limits
• Independent verification of supplier datasheet values through accredited third-party laboratories
• Material selection and grade benchmarking for new product development
• Input for finite element analysis, where strain-at-break limits serve as failure criteria for ductile damage models
• Evaluation of aging, weathering, chemical exposure, and sterilization effects on retained ductility
• Assessment of recycled content, regranulation, and batch-to-batch consistency
• Studies of weld-line strength, flow-direction anisotropy, and processing effects
• Failure analysis of fractured components and brittle-to-ductile transition characterization
• Support for regulatory submissions and customer specification compliance

FAQ

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

They describe the same property. Strain at break is the dimensionless ratio of extension to original gauge length multiplied by 100, while elongation at break is the traditional term for the same percentage value. Both are abbreviated as εB in most standards and datasheets.

Should I test to ISO 527 or ASTM D638?

Both methods measure the same property, but they use different specimen geometries, standard speeds, and modulus conventions, so the numerical results are not directly interchangeable. ISO 527 with 1A specimens is the common reference in Europe and Asia, while ASTM D638 with type I specimens dominates in North America. Choose the method that matches your specification, certification scheme, or the datasheet you need to compare against.

How many specimens are required for valid data?

Both ISO 527-1 and ASTM D638 require a minimum of five valid specimens per material and test direction. Additional specimens are tested when results show excessive scatter or when breaks occur outside the gauge section.

Can crosshead displacement be used instead of an extensometer?

For strain at break exceeding roughly 50%, ISO 527 permits the use of crosshead travel to compute nominal strain based on grip separation, which is the usual approach when the extensometer must be removed before rupture to prevent damage. This approach includes machine and grip compliance, so its accuracy is lower than direct gauge-length measurement, and the method used must be clearly stated in the report.

Why do my polyethylene or polypropylene results scatter so widely?

Very ductile polymers fail after necking and extensive cold drawing, and the exact rupture point is sensitive to specimen defects, gauge-mark quality, speed, temperature, and molecular weight distribution. Tight control of specimen preparation, conditioning, and test parameters, together with a sufficient specimen count, is essential to obtain reproducible strain-at-break statistics.

How is strain at break used in design?

In structural design, strain at break defines the ultimate deformation limit, whereas strain at yield usually defines the serviceability limit for dimensionally stable parts. High strain-at-break values indicate robustness against overload, impact, and stress concentration, which is why the parameter is a core acceptance criterion in automotive, packaging, pipe, medical, and consumer-goods specifications.