Plastic Tensile Failure Strain Testing Scheme: Standards, Specimens, Procedures, and Data Evaluation

Tensile failure strain, commonly reported as elongation at break, is one of the most decisive indicators of the ductility and toughness of plastic materials. This article presents a complete testing scheme for determining the tensile strain at break of plastics, covering specimen preparation, conditioning, equipment configuration, test procedures, and data evaluation in accordance with ISO 527 and ASTM D638. It serves as a practical reference for engineers, laboratory technicians, and quality managers involved in material qualification and quality control.

Technical Overview

The tensile failure strain test, also known as the tensile strain at break or elongation at break test, is a fundamental mechanical characterization method for plastic materials. A standardized dumbbell-shaped specimen is clamped in a universal testing machine and stretched along its axis at a constant crosshead speed until fracture occurs. The tensile strain at break is defined as the increase in gauge length at the moment of fracture divided by the initial gauge length, expressed as a percentage, and it quantifies the ductility and deformability of the material before failure.

The method distinguishes clearly between ductile materials, which sustain large deformation before breaking, and brittle materials, which fracture at low strain. Because the result is highly sensitive to specimen geometry, notch defects, conditioning history, and test speed, strict adherence to standardized procedures is essential for obtaining comparable and repeatable data.

The test is governed primarily by the ISO 527 series (Plastics — Determination of tensile properties), ASTM D638 (Standard Test Method for Tensile Properties of Plastics), and the corresponding Chinese national standards GB/T 1040. These standards harmonize specimen dimensions, conditioning atmospheres, test speeds, and data evaluation so that results obtained in accredited third-party laboratories remain mutually comparable.

• Tensile strain at break: the primary output of this scheme, reflecting material ductility
• Complementary outputs: tensile stress at break, tensile strength, yield properties, and tensile modulus
• Standardized framework: ISO 527-1, ISO 527-2, ISO 527-3, ASTM D638, GB/T 1040 series

Testing Samples

The scheme applies to a wide range of plastic materials and product forms. Samples should be representative of the material lot or the finished product, free from visible defects, and prepared so that no stress concentrators are introduced at the edges.

Moulded Rigid and Semi-Rigid Thermoplastics

This category covers commodity and engineering thermoplastics such as PE, PP, ABS, PC, PA6, PA66, POM, PET, PBT, PMMA, PS, and PVC-U. Preferred specimens are directly injection-moulded dumbbells of type 1A per ISO 527-2; where specimens are machined from compression-moulded plates or sheets, type 1B geometry is used. Moulding conditions should be recorded because residual stress, moisture content, and weld lines can measurably reduce strain at break.

Fibre-Reinforced and High-Modulus Compounds

Glass-fibre or carbon-fibre reinforced compounds and high-modulus materials typically exhibit low, highly scattered strain at break. Specimens are machined from panels per ISO 2818 with finely polished edges, and end tabs may be used to prevent grip-induced failure. Because fracture strains may fall below one percent, high-resolution strain measurement and careful axial alignment are required.

Thermosetting Moulding Materials

Phenolic, epoxy, and unsaturated polyester mouldings are machined to type 1B bars. Brittle behaviour makes these materials sensitive to surface scratches and edge chipping, so final finishing must be performed with fine abrasive under cooling to avoid heat damage.

Plastic Films and Thin Sheets

For flexible films and sheets with thickness up to 1 mm, ISO 527-3 applies. Rectangular strips or small dumbbells are die-cut from the roll, and specimen edges must be smooth and free of nicks, since film elongation results are strongly dependent on edge quality.

Samples Taken from Finished Products

Specimens may be cut from pipes, fittings, profiles, and injection-moulded parts for conformity verification. The sampling position and orientation relative to the melt flow direction or extrusion direction must be recorded, and the available wall thickness may dictate whether reduced-scale specimens or ISO 527-3 strips are used. Results from finished-product specimens should be marked as non-standard geometry when reported.

Testing Items

The core deliverable of this scheme is the tensile strain at break, supported by a set of associated tensile properties obtained from the same force-extension curve:

• Tensile strain at break: primary item, expressed in percent of initial gauge length
• Tensile stress at break: stress at the moment of fracture
• Tensile strength and strain at tensile strength: maximum stress reached and corresponding strain
• Yield stress and yield strain: reported for ductile materials exhibiting a yield point
• Tensile modulus: chord modulus between strains of 0.05 percent and 0.25 percent, determined at 1 mm/min
• Statistical evaluation: mean value, standard deviation, and coefficient of variation for each specimen set, together with a description of the break mode

Optional test conditions include reduced or elevated temperatures, testing after heat aging, UV exposure, or chemical immersion, in which case the retention of strain at break is reported as the key durability indicator.

Testing Methods

Reference Standards

• ISO 527-1: general principles and definitions for the determination of tensile properties
• ISO 527-2: test conditions for moulding and extrusion plastics, defining specimen types 1A and 1B
• ISO 527-3: test conditions for films and sheets
• ISO 291 and ASTM D618: standard atmospheres for conditioning and testing
• ISO 2818: preparation of test specimens by machining
• ISO 5893, ISO 7500-1, and ISO 9513: requirements for the testing machine, force verification, and extensometer classification
• ASTM D638: tensile properties of plastics, specimen types I through V
• GB/T 1040.1, GB/T 1040.2, GB/T 1040.3, and GB/T 2918: Chinese national standards technically aligned with the corresponding ISO documents

Specimen Preparation and Dimensions

The preferred specimen geometries and their principal dimensions are as follows:

• ISO 527-2 type 1A: overall length 170 mm, parallel length 80 mm, gauge length 50 mm, grip separation 115 mm, narrow-section width 10 mm, thickness 4 mm
• ISO 527-2 type 1B: overall length 150 mm, parallel length 60 mm, gauge length 50 mm, grip separation 100 mm, narrow-section width 10 mm, thickness 4 mm
• ASTM D638 type I: overall length 165 mm, gauge length 50 mm, grip separation 115 mm, narrow-section width 13 mm, thickness 3.2 mm

Width and thickness are measured at a minimum of three positions within the gauge length to 0.02 mm accuracy, and the smallest cross-sectional area is used for stress calculation. All specimens are conditioned at 23 ± 2 °C and 50 ± 10 percent relative humidity for at least 40 hours per ASTM D618, or for a minimum of 88 hours where ISO 291-based protocols apply, unless the material specification dictates otherwise. At least five valid specimens are prepared for each material and test condition.

Equipment Configuration

A screw-driven or servo-hydraulic universal testing machine meeting ISO 5893 with a force measuring system of class 0.5 or class 1 per ISO 7500-1 is used. The load cell capacity, typically between 1 kN and 20 kN, is selected so that the force at break falls within 15 to 85 percent of the verified range. Self-tightening wedge grips or pneumatic grips with serrated faces secure the specimen, and for hard or slippery materials serrated or machined insert faces are selected to prevent slippage without initiating premature cracking.

Strain is measured with a contact or video extensometer of class 1 or better per ISO 9513, using a gauge length of 50 mm. For brittle materials, a biaxial averaging extensometer is preferred to eliminate errors from slight bending. A temperature chamber may be fitted for testing between approximately -40 °C and 150 °C.

Test Procedure

• Verify specimen dimensions and mark the 50 mm gauge length on the narrow parallel section
• Set the grip separation to 115 mm for type 1A or 100 mm for type 1B specimens
• Clamp the specimen ensuring axial alignment without applying torsion or pre-bending
• Attach the extensometer and apply a small preload to remove slack, then zero the force and displacement readings
• Where the tensile modulus is required, run at 1 mm/min and record the chord modulus between 0.05 percent and 0.25 percent strain
• Continue the test at the specified speed, typically 50 mm/min for rigid thermoplastics, until the specimen breaks completely
• For very ductile materials, remove the extensometer before break and complete the test using crosshead travel, reporting nominal strain based on grip separation
• Record the force-extension curve, the force and extension at break, the break location, and the break appearance
• Calculate strain at break as the extension at break divided by the initial gauge length, multiplied by 100 percent
• Repeat for all specimens and inspect each fracture surface for grip damage or internal flaws

Data Evaluation and Validity Criteria

Only specimens breaking within the gauge length or in the parallel section are considered valid. Breaks occurring in the grips, at an obvious flaw, or at a marked gauge line are invalid and are retested with replacement specimens, with the anomaly documented. Mean, standard deviation, and coefficient of variation are calculated for each property; for highly scattered strain-at-break data, the median and the full range are additionally reported. The test report must state the specimen type, nominal dimensions, conditioning, test speed, temperature, humidity, break mode, and the individual and statistical results for every specimen.

Applications

Determination of tensile failure strain supports decisions across the entire plastics value chain, from resin production to finished-part acceptance:

• Incoming quality control of resins, compounds, and masterbatches against datasheet limits
• Comparative material selection between ductile and brittle candidates for demanding service conditions
• Automotive engineering: ductility verification of interior trims, bumpers, brackets, and under-hood components, including retention after heat aging
• Packaging development: elongation of PE, PP, and PET films as an indicator of formability and puncture resistance
• Pipes and fittings: toughness assessment of PVC-U and polyolefin piping materials in conformity with product standards
• Electronics and consumer goods: housing materials subjected to assembly snap-fits and drop loading
• Medical devices: mechanical conformity of catheters, packaging, and polymer components
• Durability studies: retention of elongation at break after thermal aging, weathering, hydrolysis, or chemical exposure
• Process validation: effects of moulding parameters, recycled content, regrind ratio, and weld lines on material ductility

Because strain at break is usually the property that degrades earliest and most visibly during aging or embrittlement, it is widely used as a sensitive screening parameter in accelerated life testing programs.

FAQ

What is the difference between tensile failure strain and tensile strength?

Tensile strength is the maximum stress the material sustains, expressed in megapascals, and reflects load-bearing capacity. Tensile failure strain is the deformation at fracture, expressed in percent, and reflects ductility. A reinforced compound can have high strength with very low failure strain, while a polyolefin can show low strength with failure strain of several hundred percent.

What are typical strain-at-break values for common plastics?

Representative ranges are approximately 1 to 3 percent for general-purpose polystyrene, 2 to 5 percent for highly glass-filled compounds, 10 to 50 percent for ABS and PVC-U, 50 to 120 percent for polycarbonate, and several hundred percent for PE and PP, where break often occurs beyond the extensometer range. Actual values depend strongly on grade, processing, and test conditions.

Are ISO 527 and ASTM D638 results interchangeable?

No. Although the methods are conceptually identical, they differ in specimen geometry, thickness, preferred test speeds, and certain calculation details. Type 1A and D638 type I specimens produce systematically different strain-at-break values for the same material, so the standard used must always accompany the reported result.

Why is an extensometer required instead of using crosshead displacement?

Crosshead displacement includes machine frame compliance, grip seating, and specimen deformation outside the gauge length, which overestimates strain and distorts the modulus and failure strain, especially for stiff or brittle materials. A calibrated extensometer on the 50 mm gauge length isolates the true specimen strain.

What happens if a specimen breaks outside the gauge length or in the grips?

Such results are invalid because the stress state near the grips is disturbed. The specimen is replaced and the test repeated until the required number of valid breaks within the parallel section is obtained. If failures repeatedly occur in the grips, grip pressure, insert faces, or alignment should be adjusted.

How many specimens should be tested?

A minimum of five valid specimens per material and condition is required by both ISO 527 and ASTM D638. For reinforced or anisotropic materials, additional specimens, often five parallel and five perpendicular to the flow direction, are recommended, and ten or more specimens are advisable when scatter is high.

Can the test be performed at low temperature or after environmental conditioning?

Yes. The same scheme applies with an environmental chamber covering the required temperature, and conditioned specimens are tested immediately after removal from the aging or exposure cabinet. Results must clearly state the test temperature and conditioning history, since many plastics undergo a ductile-to-brittle transition at low temperature that dramatically reduces strain at break.