Step-by-Step Procedure for Determining the Tensile Failure Strain of Plastics

The tensile failure strain (strain at break) of a plastic is a key ductility indicator obtained by stretching a standardized specimen to rupture in a universal testing machine. This article explains the complete determination procedure—covering specimen conditioning, dimensional measurement, test speed selection, extensometer use, calculation, and result validation—based on ISO 527-1, ISO 527-2, and ASTM D638. Whether you perform the test in-house or through accredited third-party laboratories, the following steps will help you obtain accurate and comparable data.

Technical Overview

Tensile failure strain, denoted εB, is the engineering strain of a plastic specimen at the instant of rupture. It is calculated as the increase in gauge length at break (ΔLB) divided by the initial gauge length (L0), usually expressed as a percentage. As the final point on the stress–strain curve before the force drops to zero, it quantifies the deformation capacity of a material: brittle materials such as glass-filled thermosets may show values below 1%, while ductile thermoplastics and flexible films can exceed several hundred percent.

The determination is governed by a family of internationally recognized standards, principally ISO 527-1 (general principles), ISO 527-2 (test conditions for moulding and extrusion plastics), the equivalent Chinese standard GB/T 1040.2, and ASTM D638. For plastic films, ISO 527-3 and ASTM D882 apply instead. All of these methods use a constant-rate-of-extension universal testing machine operated in a controlled laboratory atmosphere, typically 23 °C ± 2 °C and 50 % ± 10 % relative humidity, with strain measured by a contact or non-contact extensometer.

Testing Samples

ISO Multipurpose Specimens (Type 1A and 1B)

ISO 527-2 defines the preferred multipurpose dumbbell specimens. Type 1A (overall length 170 mm, gauge length 50 mm, gauge width 10 mm, thickness 4 mm) is used for directly injection-moulded specimens, while Type 1B (overall length 150 mm, gauge length 50 mm) is used for specimens machined from sheets, compression-moulded plaques, or finished parts.

ASTM D638 Specimens (Type I to Type V)

ASTM D638 specifies five specimen geometries. Type I (gauge length 50 mm, gauge width 13 mm, thickness 3.2 mm) is the default for rigid and semi-rigid plastics; Type II and Type III cover special cases, Type IV suits non-rigid plastics, and Type V is a reduced-size specimen for limited material quantities. The specimen type used must always be stated in the report.

Film and Sheet Specimens

Thin flexible materials are tested as rectangular strips in accordance with ISO 527-3 or ASTM D882, using long-travel or video extensometry to capture very large elongations.

Specimen Quality Requirements

All specimens must be free of flash, voids, notches, sink marks, and machining marks within the gauge section, because such defects act as stress concentrators and bias the failure strain downward. Specimens should be identified by material, batch, and moulding direction so that anisotropy can be tracked.

Testing Items

The primary measured quantity is the tensile failure strain εB (%). Because the complete stress–strain curve is recorded in the same run, companion properties are typically reported alongside it, including tensile stress at yield, tensile strain at yield, tensile stress at break, tensile strength, and—if the test is extended with a low-speed segment—tensile modulus. The nominal tensile failure strain, based on grip separation rather than extensometer measurement, may also be reported but must be clearly labelled as nominal.

Testing Methods

Step 1: Condition and Inspect the Specimens

Condition the specimens at 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618 (Procedure A normally requires 40 h; many laboratories apply a minimum of 24 h unless the material standard specifies otherwise). After conditioning, visually inspect every specimen and reject any specimen with visible defects in the gauge section.

Step 2: Measure Dimensions and Mark the Gauge Length

Measure the width and thickness of each specimen at the centre and near both ends of the gauge section, to an accuracy of at least 0.02 mm, using a calibrated micrometer or measuring microscope. Record the smallest cross-sectional area for stress calculation. Mark the initial gauge length L0 (50 mm for ISO 1A/1B and ASTM Type I) with fine marks or rely on the extensometer gauge, ensuring the marks do not introduce notches.

Step 3: Configure the Testing Machine

Select a load cell whose calibrated range covers the expected failure force without being undersized, and verify its calibration in accordance with ISO 7500-1 or ASTM E4. Fit appropriate grips—pneumatic parallel-action or wedge grips with serrated or cushioned faces—to prevent slippage. Set the distance between grips to 115 mm for ISO Type 1A or ASTM Type I specimens and 100 mm for ISO Type 1B.

Step 4: Select the Test Speed

Choose the speed specified by the standard and material specification. For ISO 527-2 Type 1A and 1B specimens, 50 mm/min is the standard speed for determining strength and strain at break, while 1 mm/min is used for the modulus. For ASTM D638 Type I specimens, 50 mm/min is common, with 5 mm/min used for modulus determination. Because plastics are strain-rate sensitive, the speed must be held constant and clearly reported.

Step 5: Mount the Specimen and Attach the Extensometer

Insert the specimen so that its axis coincides with the line joining the grips, avoiding bending or twisting. A small, consistent preload may be applied to remove slack. Attach a contact extensometer with a 50 mm gauge (meeting ISO 9513 accuracy requirements) or a video extensometer tracking gauge marks. For very ductile materials, use a long-travel or non-contact device, or remove the contact extensometer before rupture to avoid damage.

Step 6: Run the Test to Rupture

Start the crosshead at the selected speed and monitor the force–elongation curve. Continue the extension until the specimen breaks completely, recording the elongation ΔLB at the moment of rupture. Observe and note the failure location (within the gauge section, near a fillet, or at a grip) and the failure mode (brittle, ductile with necking, or slippage).

Step 7: Calculate the Tensile Failure Strain

Calculate the tensile failure strain for each valid specimen using the relationship εB = (ΔLB / L0) × 100 %, where ΔLB is the elongation of the gauge length at break and L0 is the initial gauge length. If strain is derived from grip separation, report it separately as nominal strain. Test at least five valid specimens and calculate the arithmetic mean and standard deviation; test additional specimens if scatter is unusually high.

Step 8: Validate the Results and Compile the Report

Discard results from specimens that break within one specimen width of the grips, fail because of grip damage, or exhibit slippage, since these underestimate the true failure strain. If more than one result in a set of five is invalid, investigate the cause before proceeding. The final report should include the standard used, specimen type and preparation method, conditioning, test speed, temperature and humidity, individual and mean εB values with standard deviation, failure descriptions, and equipment details.

Applications

• Quality control of resers, compounds, and masterbatches during production and incoming inspection.
• Generation and verification of material datasheet values for material selection and comparison.
• Research and development of formulations, including the effects of fillers, plasticizers, impact modifiers, and recycled content on ductility.
• Validation of injection moulding and extrusion process parameters, such as melt temperature and hold pressure.
• Assessment of degradation after heat aging, weathering, hydrolysis, or chemical exposure in failure analysis studies.
• Compliance verification for automotive interior components, packaging films, medical devices, wire insulation, and construction products.

FAQ

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

The two terms are numerically identical when strain is expressed as a percentage of the initial gauge length. Strain is formally a dimensionless ratio, while elongation at break is conventionally the same value stated in percent. Both must be distinguished from the nominal failure strain, which is calculated from grip separation rather than an extensometer and includes machine compliance.

How many specimens are required for a valid determination?

ISO 527-1 and ASTM D638 both call for a minimum of five valid specimens per condition. For anisotropic or highly variable materials, additional specimens and separate evaluation of machine and transverse directions are recommended.

Why must the specimen break within the gauge length?

Outside the parallel gauge section, the stress distribution is disturbed by fillets and gripping forces. Breaks near the grips occur at locally distorted strain fields and therefore produce artificially low failure strain values, which is why such results are normally rejected.

Can crosshead displacement replace an extensometer for measuring strain at break?

Crosshead displacement yields only nominal strain because load-frame compliance and grip settlement add errors. For accurate failure strain, an extensometer or video extensometer is required, and for ductile plastics a long-travel or non-contact system is strongly preferred so the device can follow the specimen to rupture.

How does test speed influence the measured failure strain?

Plastics are viscoelastic, so a higher extension rate generally increases strength and reduces the measured failure strain. Consistent speed selection and reporting are therefore essential whenever results are compared between laboratories or against datasheet values.