Plastic Tensile Strain at Break Testing Procedure

Plastic tensile strain at break testing is a core mechanical evaluation that measures how much a plastic material can deform before it fractures under uniaxial tensile loading. This article provides a complete overview of the plastic tensile strain at break testing procedure, covering specimen preparation, conditioning, instrumentation, testing methods in accordance with ISO 527, ASTM D638 and GB/T 1040, and typical industrial applications.

Technical Overview

Tensile strain at break (εB), also known as nominal strain at break or elongation at break, is defined as the tensile strain of a specimen at the moment of rupture, calculated as the increase in gauge length at break divided by the original gauge length, expressed as a percentage. It is one of the most important ductility indicators for plastic materials and directly reflects the ability of a material to undergo plastic deformation before failure.

The test is performed on a universal testing machine (UTM) equipped with suitable grips, a load cell, and a strain measurement device such as a contact extensometer or a non-contact video extensometer. A dumbbell-shaped (dog-bone) specimen is pulled at a constant crosshead speed until it fractures, while the extension of the gauge length and the applied force are continuously recorded. Strain at break is derived from the extension value at the rupture point.

The internationally recognized test standards for this procedure include ISO 527-1 and ISO 527-2 (plastics — determination of tensile properties), ASTM D638 (tensile properties of plastics), and the Chinese national standards GB/T 1040.1 and GB/T 1040.2. Although the technical principles are identical, the three standard systems differ in specimen geometry, testing speed and data-processing details, so results from different standards should not be directly compared without qualification.

Testing Samples

Injection-Moulded Dumbbell Specimens

Preferred for rigid and semi-rigid thermoplastics with a thickness of 1 mm to 10 mm. The standard ISO 527-2 type 1A specimen has an overall length of 170 mm, a gauge width of 10 mm and a thickness of 4 mm, and is directly injection moulded to ensure a well-defined stress state within the gauge section.

Machined Specimens from Sheets, Plates and Finished Products

When specimens cannot be directly moulded, ISO 527-2 type 1B or ASTM D638 type I dumbbells are milled or die-cut from sheets, compression-moulded plates or walls of finished parts. The machining direction relative to the material orientation must be recorded, because moulded and extruded plastics often show strong anisotropy in strain at break.

Small-Scale Specimens

For materials with limited availability or for components with restricted geometry, reduced-size specimens such as ISO 527-2 types 1BA to 1E or ASTM D638 type V (overall length 63.5 mm, thickness 1.5 mm or less) may be used. Small specimens generally produce slightly lower strain-at-break values than full-size specimens.

Film and Thin-Sheet Specimens

For flexible films and sheets with a thickness of 1 mm or less, specimens are tested in accordance with ISO 527-3 or ASTM D882, using strip or dumbbell specimens cut with a sharp die to avoid notch effects that cause premature rupture.

Testing Items

The tensile strain at break test is typically performed as part of a complete tensile property characterization, which may include the following parameters:

• Tensile strain at break (εB) — the primary parameter, expressed in percent
• Tensile stress at break (σB)
• Tensile strength and tensile stress at yield
• Tensile strain at yield
• Tensile modulus (Young's modulus, E_t)
• Poisson's ratio (optional, with transverse extensometer or DIC)

For most engineering purposes, a minimum of five valid specimens per material condition is tested, and the arithmetic mean, standard deviation and coefficient of variation of strain at break are reported. If the coefficient of variation exceeds the limits given by the standard, additional specimens should be tested.

Testing Methods

The general testing sequence is identical across standards: specimen preparation and inspection, conditioning, dimension measurement, specimen clamping, extensometer installation, constant-speed loading until rupture, and data evaluation. The key parameters of each standard method are summarized below.

ISO 527-1 / ISO 527-2 Method

• Specimens: ISO 527-2 type 1A (moulded) or 1B (machined), gauge length 50 mm
• Conditioning: 23 °C ± 2 °C and 50 % ± 10 % relative humidity for at least 40 hours, unless otherwise agreed
• Testing speed: 1 mm/min for modulus determination; 50 mm/min as the preferred speed for 1A and 1B specimens
• Strain measurement: extensometer with 50 mm gauge length for rigid materials; for strains above the extensometer travel, grip separation may be used for nominal strain calculation
• Result: εB = (ΔL_B/L₀) × 100 %, where L₀ is the initial gauge length and ΔL_B is the extension at break

ASTM D638 Method

• Specimens: type I (thickness up to 7 mm, gauge length 50 mm), type II, type III, type IV and type V (small specimens)
• Conditioning: in accordance with ASTM D618, typically 23 °C ± 2 °C and 50 % ± 5 % relative humidity for not less than 40 hours
• Testing speed: 50 mm/min for type I specimens; 5 mm/min or 10 mm/min for other types as specified in the standard
• Strain measurement: extensometer or, when permitted, crosshead extension for materials with very high elongation
• Result: percent elongation at break, reported together with specimen type, speed and conditioning

GB/T 1040.1 / GB/T 1040.2 Method

• Chinese national standard, technically aligned with ISO 527-1 and ISO 527-2
• Specimens, conditioning atmosphere, testing speeds and calculation methods are consistent with the ISO procedure
• Widely used for domestic compliance reports, incoming inspection and supplier qualification in the Chinese market

Validity of Results

A specimen is considered valid when fracture occurs within the gauge length. If rupture takes place outside the gauge section or at a grip, the individual result is discarded and a replacement specimen is tested. Edge defects, voids or machining marks frequently cause premature breakage and must be avoided through careful specimen preparation.

Applications

Tensile strain at break data generated by accredited third-party laboratories and in-house quality departments support a wide range of engineering and commercial activities:

• Material datasheet development and comparison of ductility among candidate polymers
• Incoming inspection of resins, compounds, sheets, films and moulded parts
• Quality control of injection-moulding and extrusion processes, including detection of degradation or excessive moisture
• Verification of the effect of additives, fillers, tougheners, recycled content and colourants on ductility
• Aging studies, including heat aging, UV weathering, hydrolysis and chemical exposure, where the loss of strain at break is a sensitive failure indicator
• Automotive, packaging, medical device, electronics, pipe and consumer product industries, where minimum ductility values are often specified in purchase specifications
• Failure analysis and root-cause investigation of brittle part fractures

FAQ

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

The two terms describe the same quantity expressed relative to the original gauge length, and both are given as a percentage. ISO 527 uses the term tensile strain at break, while ASTM D638 uses percent elongation at break. If the strain is calculated from grip separation rather than extensometer gauge length, it is designated as nominal strain and may be slightly higher.

Why do specimens sometimes break outside the gauge length?

Breakage outside the gauge section is usually caused by grip damage, stress concentrations from machining marks, material inhomogeneity or excessive clamping force. Such results do not represent the true material ductility and are normally discarded according to the standard.

How many specimens are required?

ISO 527 and ASTM D638 require at least five valid specimens per sample. For heterogeneous materials, filled compounds or materials with strong orientation, ten or more specimens may be needed to obtain acceptable scatter.

Which factors most strongly influence the test result?

Testing speed, temperature and humidity, specimen type, material orientation, conditioning history and notch quality all have significant effects. Moisture-sensitive polymers such as polyamides must be conditioned to a defined moisture content before testing, as dry and conditioned states can yield dramatically different strain-at-break values.

How long does a typical test take?

Individual tests usually finish within a few minutes once conditioning is complete. Including specimen preparation, conditioning and data evaluation, a full test report from an accredited third-party laboratory is typically issued within five to seven working days.