Plastic Strain at Break Performance Evaluation: Standards, Test Methods, and Applications

Strain at break, commonly reported as elongation at break, is one of the most important mechanical indicators of the ductility and toughness of plastic materials, reflecting the maximum deformation a polymer can withstand before fracture. This article provides a comprehensive overview of plastic strain at break performance evaluation based on ISO 527, ASTM D638, GB/T 1040 and related standards, covering specimen types, test procedures, calculation methods, influencing factors, and typical industrial applications carried out by accredited third-party laboratories.

Technical Overview

Strain at break (epsilon-B), frequently reported as elongation at break in percent, is the tensile strain of a plastic specimen at the instant of rupture. It is obtained from a uniaxial tensile test and is calculated as the relative increase in gauge length between the initial state and the moment of fracture. Together with tensile strength and tensile modulus, strain at break defines the deformation capability of a material and is the primary parameter used to distinguish ductile plastics from brittle ones.

On an engineering stress-strain curve, strain at break is the terminal point of the curve. Ductile semi-crystalline materials such as polyethylene and polypropylene may exhibit yielding, necking, cold drawing and strain hardening, with strain at break values that can exceed several hundred percent. Brittle amorphous materials, such as polystyrene or highly filled thermosets, fracture within or shortly after the linear elastic region, with strain at break values typically below five percent. Because plastics are viscoelastic, the measured strain at break depends strongly on test speed, temperature, humidity and specimen preparation, which is why standardized test conditions are essential for comparable data.

The principal standardized frameworks for strain at break evaluation are the ISO 527 series (ISO 527-1 general principles, ISO 527-2 for moulding and extrusion compounds, ISO 527-3 for films and sheets), ASTM D638 for tensile properties of rigid plastics, ASTM D882 for thin plastic sheeting, and the GB/T 1040 series, which is technically aligned with ISO 527. Pipe materials are evaluated under ISO 6259 and GB/T 8804, while JIS K 7161 and JIS K 7162 correspond to the ISO approach. Multipurpose test specimens are defined in ISO 3167 and ISO 20753 to ensure consistent sampling across properties.

Testing Samples

Moulding and Extrusion Compounds

For rigid thermoplastics and thermosets, the preferred specimen is the ISO 527-2 type 1A dumb-bell (preferred thickness 4 mm, gauge length 50 mm) or type 1B for machined sheets. When material availability is limited, reduced-scale specimens such as types 1BA and 1BB may be used, although results are not directly interchangeable with full-size specimens. Under ASTM D638, type I specimens (thickness 3.2 mm, gauge length 50 mm) are preferred, with type IV and small type V specimens used for specific applications. At least five valid specimens per condition are required, and additional specimens oriented in both the flow and transverse directions are recommended for anisotropic or fiber-reinforced compounds.

Films, Sheets and Tapes

Flexible films and thin sheets are tested using the strip or dumb-bell geometures of ISO 527-3 or the rectangular strips of ASTM D882, typically 10 mm to 25 mm wide, using pneumatic or smooth-faced grips with appropriate lining to avoid premature failure at the jaws. Test speeds for films are selected according to the specimen type and the ductility of the material.

Pipes, Profiles and Finished Components

For pressure pipes, longitudinal strips or machined segments are prepared and tested in accordance with ISO 6259 or GB/T 8804. For injection-molded or machined parts, small dumb-bells can be cut from flat regions, with the flow direction documented because molecular orientation, weld lines and fiber orientation can significantly alter strain at break.

Material State

Hygroscopic polymers such as polyamides must be tested in a clearly defined moisture state, for example dry-as-molded or equilibrated at 50 percent relative humidity, with accelerated conditioning performed according to ISO 1110. The moisture state must always be reported together with the result.

Testing Items

• Nominal tensile strain at break and tensile strain at break measured with an extensometer
• Tensile strain at yield, where applicable
• Tensile stress at yield and tensile stress at break
• Tensile strength (maximum stress) and tensile modulus
• Poisson's ratio, when required, using biaxial deformation measurement
• Failure mode classification, such as ductile, brittle, or break at the grip line
• Statistical evaluation of individual results, arithmetic mean, standard deviation and coefficient of variation

Testing Methods

Specimen Preparation and Conditioning

Specimens are injection molded, compression molded, die-cut or machined with care taken to avoid notch marks, voids, thermal degradation and residual stresses that act as fracture initiation sites. Before testing, specimens are conditioned in the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10 percent relative humidity according to ISO 291, or according to ASTM D618 procedure A (minimum 40 hours at 23 ± 2 °C and 50 ± 5 percent RH). Conditioning duration is extended for thick sections and hygroscopic materials.

Equipment and Setup

Testing is performed on a universal testing machine verified in accordance with ISO 7500-1 or ASTM E4, using an accuracy class of 1 or better. Suitable grips are selected according to the specimen type, including wedge action grips for rigid dumb-bells and pneumatic or capstan-type grips for films. Strain is measured with a contact or non-contact (video) extensometer meeting ISO 9513 class 1 requirements, using a gauge length of 50 mm for standard specimens. Temperature chambers allow testing from sub-ambient conditions up to several hundred degrees Celsius when service conditions require it.

Test Procedure

Width and thickness are measured at several points within the parallel section and the cross-sectional area is calculated. After clamping with a small preload, the machine is set to the prescribed speed. Under ISO 527, the tensile modulus is determined at 1 mm/min, after which the speed is increased to the standard value for the specimen, commonly 50 mm/min for type 1A and 1B specimens and 5 mm/min for reduced-scale specimens. Under ASTM D638, typical speeds are 5 mm/min for type I and 50 mm/min for type IV specimens. For ductile materials, a contact extensometer is removed once its travel limit is reached or after yielding, and the test continues to rupture using crosshead displacement. The force and extension signals are recorded continuously until the specimen breaks.

Calculation and Validity Criteria

Strain at break is calculated as the change in gauge length at rupture divided by the initial gauge length, multiplied by 100 percent. If strain is derived from crosshead travel, machine compliance corrections are applied in accordance with ISO 5893, especially for stiff, low-strain materials where frame and grip deformation can significantly distort the result. A test is considered valid only when fracture occurs within the parallel section between the grips; specimens that fail at, or slip in, the grips are discarded and retested. Each report should include the standard used, specimen type, conditioning, test speed, strain measurement method, individual and mean values, and the observed failure mode.

Key Influencing Factors

• Test speed: higher strain rates generally reduce strain at break due to the viscoelastic nature of polymers
• Temperature: increasing temperature usually increases ductility, while low temperatures promote embrittlement and ductile-to-brittle transitions
• Moisture content: polyamides and other hygroscopic resins can show severalfold differences between dry-as-molded and humidity-conditioned states
• Fillers and reinforcement: glass fibers and mineral fillers raise stiffness and strength but typically reduce strain at break to the range of a few percent
• Specimen quality: notches, machining marks, weld lines, voids and molecular orientation act as stress concentrators and scatter results
• Material history: processing degradation, recycling content, thermal aging and UV exposure usually manifest first as a drop in strain at break

Applications

• Generation and verification of material datasheet values for resin producers and compounders
• Incoming quality control and lot release testing of resins, films and molded components
• Material selection and comparative ductility ranking for design engineers
• Input for finite element analysis failure criteria and structural margin assessment
• Long-term performance programs, including heat aging, relative temperature index evaluations referenced in UL 746B, and UV weathering studies, where strain at break retention is a primary aging indicator
• Quality screening of recycled and reprocessed materials, where degradation appears as a marked loss of elongation
• Packaging film characterization covering formability, puncture resistance and seal performance
• Pressure pipe and automotive component qualification and periodic production surveillance
• Failure analysis, comparing the ductility of failed parts against reference material to identify degradation or processing defects

FAQ

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

The two terms describe the same property. Strain at break is the dimensionless ratio of gauge length change to initial gauge length at rupture, while elongation at break expresses the same quantity as a percentage. ISO standards use the terminology of strain at break, whereas ASTM and many datasheets use elongation at break.

Are ISO 527 and ASTM D638 results directly comparable?

No. The two methods differ in specimen geometry, nominal thickness, gauge length and standard test speeds, so absolute values of strain at break can differ even for the same material. Trends between materials are usually consistent, but the governing standard must always be stated when quoting results.

Is an extensometer always required to measure strain at break?

For rigid or brittle materials with low strain at break, an extensometer is strongly recommended because crosshead-based strain includes machine compliance and can overstate the result. For very ductile materials that exceed extensometer travel, a long-travel or video extensometer is preferred; otherwise nominal strain from crosshead displacement with compliance correction is used.

How many specimens should be tested?

Both ISO 527 and ASTM D638 require a minimum of five valid specimens per condition. For fiber-reinforced, anisotropic or highly scattered materials, additional specimens tested in multiple directions are advisable to obtain a representative statistical distribution.

Why do glass-fiber reinforced plastics show very low strain at break?

Fibrous reinforcement dramatically increases stiffness and strength but suppresses plastic deformation. Strain at break values of 2 to 4 percent are normal and expected for well-processed glass-filled compounds, and such values do not by themselves indicate a defect.

Can strain at break be used directly as a design limit?

No. Strain at break is a short-term, quasi-static property measured on idealized specimens. Actual components experience creep, fatigue, stress concentration, environmental stress cracking and temperature effects, so design allowable strains must be established with safety factors and validated long-term data.

What should be done when a specimen breaks inside the grips?

A jaw break is not representative of the material and is treated as an invalid result. The specimen is discarded, grip pressure or grip faces are adjusted, and a replacement specimen is tested until the required number of valid failures within the gauge section is obtained.