Room-Temperature Tensile Strain at Break Testing of Plastics

Room-temperature tensile strain at break testing measures the ductility of plastic materials by determining the elongation a specimen sustains before rupture under uniaxial tensile loading at standard laboratory conditions. Performed in accordance with ISO 527-1, ISO 527-2, GB/T 1040, and ASTM D638, this test is a cornerstone of material qualification, quality control, and failure analysis. This article explains the technical principles, specimen requirements, test procedures, calculation methods, and typical applications of strain-at-break testing for plastics.

Technical Overview

Tensile strain at break (εB), also known as elongation at break, is the deformation a plastic specimen undergoes between the beginning of loading and the moment of rupture, expressed as a percentage of the initial gauge length. It is one of the most informative parameters in the mechanical characterization of polymers because it directly quantifies ductility: ductile materials such as polyethylene can exhibit strain at break values of several hundred percent, whereas highly filled or fiber-reinforced compounds may fail at strains below 3 percent.

Room-temperature testing is carried out in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity, as defined in ISO 291 or ASTM D618, after the specimens have been conditioned to temperature and moisture equilibrium. A universal testing machine applies a constantly increasing uniaxial tensile load at a controlled crosshead speed, while strain is recorded with a contact extensometer, a video extensometer, or, where the standard permits, from crosshead separation. Because strain at break is highly sensitive to molecular degradation, embrittlement, processing defects, and environmental aging, it is widely used as an acceptance criterion in material datasheets, procurement specifications, and durability studies.

• Property measured: tensile strain at break (εB), expressed in percent
• Conditioning atmosphere: 23 °C ± 2 °C, 50 % ± 10 % RH (ISO 291 atmosphere 23/50)
• Governing standards: ISO 527-1, ISO 527-2, GB/T 1040.1, GB/T 1040.2, ASTM D638
• Result significance: quantitative indicator of ductility and resistance to brittle fracture

Testing Samples

Commodity and Engineering Thermoplastics

Testing covers the full range of thermoplastic resins, including polyethylene (PE), polypropylene (PP), polystyrene (PS), rigid and flexible polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyamide 6 and 66 (PA6, PA66), polyoxymethylene (POM), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT). Specimens are typically injection-molded multipurpose dumbbell bars produced under controlled molding conditions to avoid frozen-in stresses, voids, and sink marks.

High-Performance and Specialty Polymers

Engineering-grade and high-temperature materials such as PEEK, PPS, PEI, PSU, and PI are tested using the same specimen geometry, with additional attention to drying history, since residual moisture and thermal exposure during processing can markedly shift ductility values.

Reinforced and Filled Compounds

Glass-fiber, carbon-fiber, and mineral-filled compounds exhibit strong anisotropy due to fiber orientation during molding. Specimens are usually cut or molded both parallel and perpendicular to the flow direction, because strain at break can differ by an order of magnitude between the two orientations.

Films, Sheets, and Machined Specimens

When parts or plaques are available instead of molded bars, specimens may be machined or stamped from sheets, compression-molded plates, or finished components. Thin films are addressed by the related ISO 527-3 procedure, while limited-material samples can be evaluated using small-format specimens such as ASTM D638 Type V.

Specimen Quality Requirements

• Surfaces and edges free of voids, notches, machining marks, and flash that could initiate premature fracture
• Uniform thickness and width within the dimensional tolerances of the applicable specimen drawing
• Minimum of five valid specimens per condition, as required by ISO 527-1 and ASTM D638
• Documented molding parameters, drying history, and specimen orientation for traceability

Testing Items

The primary reported item is the tensile strain at break, but a single tensile run simultaneously yields the complete engineering stress-strain curve, from which the following properties are typically reported:

• Tensile strain at break (εB), the elongation at the moment of rupture relative to the initial gauge length
• Tensile stress at break (σB), the stress recorded at rupture
• Tensile stress and strain at yield (σY, εY), for materials that exhibit a yield point
• Tensile strength, defined as the maximum stress sustained during the test
• Tensile modulus (Et), the secant slope determined between strain limits of 0.05 % and 0.25 % per ISO 527-1

Reporting the full set of tensile properties allows strain at break to be interpreted in context, for example distinguishing a stiff brittle material from a stiff ductile one, or identifying a brittle failure mode in a material that was expected to yield.

Testing Methods

Applicable Standards

• ISO 527-1 and ISO 527-2: determination of tensile properties of plastics, general principles and test conditions for moulding and extrusion plastics
• GB/T 1040.1 and GB/T 1040.2: the Chinese national standards technically aligned with ISO 527
• ASTM D638: tensile properties of unreinforced and reinforced plastics using dumbbell-shaped specimens
• JIS K 7161 and JIS K 7162: Japanese standards harmonized with the ISO 527 series

Specimen Geometry

Under ISO 527-2, the preferred multipurpose specimen 1A has an overall length of 170 mm, an overall width of 20 mm, a narrow parallel section 10 mm wide, a thickness of 4 mm, and an initial gauge length of 50 mm, with a grip separation of 115 mm. The alternative specimen 1B is 150 mm long with the same gauge length. Under ASTM D638, the standard Type I specimen is 165 mm long, 13 mm wide in the narrow section, 3.2 mm thick, with a 50 mm gauge length and 115 mm grip separation. Because ISO and ASTM geometries and default speeds differ, results obtained by the two methods are not directly interchangeable.

Conditioning

Specimens are conditioned in the standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity for at least 24 hours, in line with ISO 291, or for a minimum of 40 hours in the case of 23 °C / 50 % RH conditioning per ASTM D618 Procedure A. Hygroscopic materials such as polyamides require special attention: testing may be performed in the dry-as-molded state or after equilibration at a defined humidity, and the moisture state must always be reported, because absorbed moisture can increase strain at break severalfold.

Equipment and Setup

• A universal testing machine with a calibrated load cell meeting ISO 7500-1 or equivalent accuracy class
• Appropriate mechanical, wedge, or pneumatic grips with aligned faces to prevent slippage and bending
• A contact, long-travel, or non-contact video extensometer calibrated in accordance with ISO 9513, set to the 50 mm gauge length
• A climate-controlled laboratory maintained at 23 °C ± 2 °C and 50 % ± 10 % relative humidity

Test Procedure

• Measure the width and thickness of each specimen at several positions within the gauge length and calculate the cross-sectional area
• Mount the specimen in the grips with the gauge marks centered and verify axial alignment
• Attach the extensometer and apply a small preload to remove slack
• Apply the tensile load at the specified constant speed; ISO 527 lists preferred speeds from 1 mm/min to 500 mm/min, with the modulus normally determined at 1 mm/min and strain at break measured at the material-specific speed defined in ISO 527-2, while ASTM D638 commonly uses 5, 50, or 500 mm/min depending on the specimen type and rigidity
• Continue loading until the specimen ruptures, recording the complete force-extension curve
• Discard and retest specimens that fail in the grip region or at a fillet, keeping only breaks within the gauge length

Calculation and Reporting

The tensile strain at break is calculated as εB = ΔLB / L0 × 100 %, where ΔLB is the extension between the gauge marks at the moment of rupture and L0 is the initial gauge length, typically 50 mm. For very ductile materials whose rupture strain exceeds the travel of the extensometer, strain may be followed from crosshead separation as a nominal elongation where the standard permits. The test report should include the material identification and conditioning history, specimen type and preparation method, test speed, individual and mean values, standard deviation, and any observations regarding the failure mode, such as brittle fracture, necking, or crazing.

Factors Influencing Results

• Test speed: plastics are viscoelastic, so higher crosshead speeds generally reduce ductility and can shift the failure mode from ductile to brittle
• Temperature and humidity: even within the permitted tolerance band, small deviations measurably affect ductile materials
• Specimen preparation: molding conditions, machining quality, and residual stresses alter crack-initiation behavior
• Fiber orientation: reinforced compounds require orientation to be defined relative to mold flow direction
• Moisture content: polyamides and other hygroscopic resins must be tested at a documented moisture level

Applications

Strain at break data support decisions across the entire product life cycle, from resin development and incoming inspection to long-term durability assessment. Because the property responds strongly to degradation and embrittlement, it is frequently the most sensitive early-warning indicator in aging and environmental exposure studies.

• Material datasheet generation and comparison of candidate resins for ductility-critical designs
• Incoming quality control and lot acceptance of resins, compounds, and masterbatches
• Evaluation of additive, filler, and recyclate content effects on toughness and ductility
• Aging and durability studies, including thermal aging, UV and weathering exposure, hydrolysis, and chemical resistance
• Medical device and sterilization validation, where embrittlement after repeated sterilization cycles must be quantified
• Process optimization for injection molding and additive manufacturing, comparing build orientation and process parameters
• Failure analysis and root-cause investigation of cracked or embrittled field returns
• Regulatory compliance and certification programs supported by accredited third-party laboratories

FAQ

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

The two terms describe the same property: the deformation at rupture expressed as a percentage of the initial gauge length. "Strain at break" is the terminology used in the ISO 527 series, while "elongation at break" appears commonly in ASTM D638 reporting. Values may be measured with an extensometer over the gauge length or, for very ductile materials, derived as nominal elongation from crosshead travel, and the measurement basis should always be stated.

What exactly qualifies as room-temperature testing?

Room-temperature testing is performed at the standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity specified in ISO 291 or ASTM D618, after specimens have been conditioned in that atmosphere for a defined period, typically at least 24 hours. This ensures that results from different laboratories are comparable and reflect intrinsic material behavior rather than ambient fluctuations.

Why do strain-at-break results scatter more than other tensile properties?

Fracture is a statistically distributed event governed by flaws, notches, and local defects, so the breaking strain of a brittle material shows naturally high variability. Scatter can be reduced by using well-prepared specimens, correct grip alignment, a validated extensometer, and strict adherence to conditioning and speed requirements, and it can be managed statistically by testing an adequate number of specimens.

How does test speed affect the measured strain at break?

Plastics are viscoelastic, so their response depends on the rate of deformation. Increasing the crosshead speed generally raises the strength and modulus while lowering ductility, and for some materials it can change the failure mode from ductile necking to brittle fracture. The test speed must therefore always match the specification or material table of the governing standard.

Can ISO 527 and ASTM D638 results be compared directly?

No. The ISO 527-2 specimen 1A is 4 mm thick with a 10 mm narrow section, whereas the ASTM D638 Type I specimen is 3.2 mm thick with a 13 mm narrow section, and the default test speeds also differ. These differences produce systematically different values, so contractual specifications must state which method applies, and correlation between the two should be established empirically if both data sets are needed.

How many specimens are required for a valid test?

Both ISO 527-1 and ASTM D638 require a minimum of five valid specimens per material condition. Specimens that fail in the grips or the fillet radius rather than within the gauge length must be discarded and replaced, and additional specimens are recommended when the material is known to be brittle or heterogeneous.

Is an extensometer always necessary?

For accurate strain measurement, yes. Grip-to-grip crosshead displacement includes machine and grip compliance and overstates strain, particularly for stiff materials with low breaking strains. An extensometer calibrated to ISO 9513 is required for modulus determination and recommended for strain at break; for extremely ductile materials, a long-travel or video extensometer should be used so that strain can be tracked all the way to rupture.