Plastic Tensile Strain at Break Testing: Mechanical Properties Laboratory Experiment

Tensile strain at break is one of the most important mechanical parameters for characterizing the ductility and fracture behavior of plastic materials. This laboratory experiment, performed on universal testing machines in accordance with ISO 527, ASTM D638, and GB/T 1040, measures how much a plastic specimen deforms before it fractures under uniaxial tensile loading. This article explains the test principle, specimen requirements, instrumentation, procedural steps, data evaluation, and industrial applications of tensile strain at break determination for plastics.

Technical Overview

The tensile strain at break (tensile elongation at break, εB) is defined as the extension of the gauge length of a test specimen at the moment of rupture, expressed as a percentage of the original gauge length L0. Together with tensile stress at break, tensile strength, yield point, and tensile modulus, it forms the core set of data generated in a plastic tensile test. Strain at break is a direct indicator of material ductility: a high value signifies a tough, deformable polymer, while a low value points to brittle behavior that may cause sudden, catastrophic part failure.

During the experiment, a standardized dumbbell-shaped specimen is clamped in the grips of a universal testing machine and stretched at a constant crosshead speed until fracture. The force and extension signals are recorded continuously and converted into an engineering stress–strain curve. From this curve, the strain at break is read directly at the point of specimen rupture. Ductile plastics such as polyethylene and polypropylene may sustain elongations of several hundred percent before breaking, whereas highly filled or fiber-reinforced compounds often fail at strains below five percent.

Two related but distinct quantities must be understood: the true tensile strain at break εB, measured with an extensometer attached to (or optically tracking) the gauge length, and the nominal tensile strain at break εtB, calculated from the crosshead travel divided by the initial grip separation. Because grip separation includes deformation of the shoulders and fillets of the specimen as well as machine compliance, nominal strain values are systematically larger than extensometer-based values and the two must never be mixed in a single data set.

Testing Samples

ISO Multipurpose Specimens (Type 1A and 1B)

For rigid and semi-rigid moulding and extrusion plastics, ISO 527-2 and GB/T 1040.2 specify the classic multipurpose dumbbell specimens. Type 1A has an overall length of 170 mm, a grip separation of 115 mm, a gauge length of 50 mm, a narrow-section width of 10 mm, and a thickness of 4 mm. Type 1B is slightly shorter, with an overall length of 150 mm and a grip separation of 100 mm, sharing the same gauge length, width, and thickness. These specimens are preferably injection moulded directly in a dedicated cavity or machined from compression-moulded or extruded plates.

ISO Small-Scale Specimens (Type 5A and 5B)

When only a limited amount of material is available, for example during compound development or failure analysis of a small part section, ISO 527-2 permits reduced-size specimens. Type 5A has an overall length of approximately 75 mm with a 20 mm gauge length, while Type 5B measures about 35 mm overall with a 10 mm gauge length. Because results on small specimens are not directly comparable with 1A/1B data, the specimen type must always be stated in the test report.

ASTM D638 Tensile Dogbone Specimens

ASTM D638 defines five specimen geometries (Type I through Type V) covering a wide range of rigid and semi-rigid plastics. Type I is the preferred configuration, with an overall length of 165 mm, a grip separation of 115 mm, a 50 mm gauge length, a 13 mm narrow-section width, and a thickness of 3.2 mm. Thinner materials use Type II or Type IV, and Type V is intended for small samples where little material exists. As with ISO testing, comparative data must be generated on the same specimen type tested at the same speed.

Film and Sheet Specimens

Plastic films up to 1 mm thick are tested with rectangular strip specimens in accordance with ISO 527-3, using extended gauge lengths of 50 mm to 100 mm and non-contact elongation measurement to accommodate very large deformations. Thicker sheet is usually machined into dumbbell shapes and treated as a rigid material.

Specimen Preparation and Conditioning

Specimen quality has a decisive influence on strain-at-break results. Moulding conditions, melt temperature, weld lines, flow-induced molecular orientation, surface notches from machining, and voids all act as stress concentrators that can trigger premature fracture. Unless otherwise specified, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity per ISO 291 or ASTM D618 (minimum 40 hours) before testing, because moisture-sensitive polymers such as polyamide show markedly different ductility in the dry and conditioned states.

Testing Items

A complete tensile experiment on plastics typically delivers the following mechanical characteristics derived from a single stress–strain record:

• Tensile strain at break (εB): the deformation at rupture relative to the gauge length, the primary output of this experiment.
• Nominal tensile strain at break (εtB): the deformation at rupture based on crosshead travel relative to the initial grip separation.
• Tensile stress at break (σB): the stress recorded at the moment of rupture.
• Tensile strength (σM): the maximum stress sustained by the specimen.
• Tensile stress and strain at yield (σY, εY): the first point on the curve at which strain increases without a further increase in stress, where applicable.
• Tensile modulus (Et): the chord modulus determined in the initial linear elastic region, typically between strain limits of 0.05% and 0.25%.
• Poisson's ratio, where required by the product specification.
• Full force–extension and stress–strain curves, fracture location, and fracture appearance classification (brittle, ductile, or mixed) for engineering evaluation.

Testing Methods

Instrumentation

The core instrument is a screw-driven or servo-hydraulic universal testing machine meeting the requirements of ISO 5893 (force accuracy class 0.5 or class 1) and equipped with self-tightening wedge or pneumatic grips that apply uniform lateral pressure without crushing the specimen. Elongation is measured with a mechanical or video extensometer conforming to ISO 9513 class 1 or better and set to the specimen gauge length of 50 mm for 1A, 1B, and ASTM Type I specimens. Non-contact video extensometers are strongly recommended for ductile polyolefins and films, where the percentage extension exceeds the travel range of clip-on devices and where knife-edge contact could initiate failure.

Specimen Measurement and Mounting

The width and thickness of the narrow section are measured at a minimum of three positions using instruments accurate to 0.02 mm or better, and the smallest cross-sectional area is used for stress calculation. The specimen is aligned axially in the grips to eliminate bending stresses, and a small preload of typically no more than 1% of the expected maximum force is applied to remove slack before zeroing the extension signal.

Test Speed Selection

Plastics are strain-rate sensitive, so the crosshead speed must be fixed and reported. ISO 527-1 and GB/T 1040.1 define a preferred speed series of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min. The tensile modulus is determined at 1 mm/min under ISO procedures, while ASTM D638 specifies 5 mm/min for modulus measurement. The subsequent speed for strength and strain-at-break determination is chosen from the series in accordance with the relevant material standard, commonly 20 mm/min or 50 mm/min for rigid moulding compounds, 50 mm/min for ASTM Type I specimens, and up to 500 mm/min for highly ductile polyolefins and films.

Test Procedure

• Verify the calibration status of the load cell, extensometer, and crosshead displacement transducer before starting the batch.
• Condition and measure the specimens, then clamp the first specimen with the gauge marks centered between the grips.
• Attach or focus the extensometer on the gauge length and apply the specified preload.
• Run the test at the selected constant speed until the specimen fractures, recording force and extension continuously at a sampling rate sufficient to capture yield and rupture events.
• Examine the fracture surfaces and record the failure location; specimens that break inside or immediately adjacent to the grips are normally discarded and replaced unless the failure reflects genuine material behavior.
• Repeat the test on at least five valid specimens per condition; for anisotropic materials such as glass-mat or fiber-reinforced sheets, test both the machine and transverse directions.

Data Processing and Calculation

Engineering stress is calculated as the measured force divided by the original cross-sectional area, σ = F/A0. Tensile strain is calculated as the extension of the gauge length divided by the original gauge length, ε = ΔL/L0, and the tensile strain at break is obtained by inserting the extension at rupture: εB = ΔLB/L0 × 100%. The arithmetic mean and standard deviation are reported for stress values and modulus, while ISO 527 recommends reporting the median for strain at break because elongation data frequently show skewed scatter. The test report must identify the standard used, specimen type, moulding conditions, conditioning state, test speed, atmosphere, individual and statistical results, and the measurement basis (extensometer or crosshead separation) for all strain values.

Result Interpretation

Typical strain-at-break ranges for unfilled commodity and engineering plastics include values above 300% for polyethylene and polypropylene grades, roughly 80% to 150% for polycarbonate, approximately 20% to 50% for ABS, and only 1% to 4% for 30% glass-fiber-reinforced compounds; exact figures vary significantly with grade, morphology, and test speed. A measured strain at break far below the datasheet value usually indicates notch damage, poor moulding, moisture imbalance, or wrong test speed, and should trigger an investigation of specimen preparation before conclusions are drawn about the material itself.

Applications

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

• Incoming inspection and batch release of resins, compounds, and masterbatches against supplier datasheets and purchase specifications.
• Material selection and design allowables for automotive interior and exterior components, where ductility after aging, cold-temperature exposure, and climate cycling is a safety requirement.
• Validation of packaging films, seal webs, and laminates where elongation capacity governs formability, puncture resistance, and drop performance.
• Medical device and pharmaceutical packaging qualification, including post-sterilization ductility verification.
• Research and development of new formulations, where strain at break quantifies the effect of impact modifiers, plasticizers, fillers, and recyclate content on toughness.
• Aging and durability studies covering heat aging, UV weathering, hydrolysis, and chemical exposure, in which the decay of strain at break is the most sensitive early indicator of embrittlement.
• Quality control of additive manufactured parts, welded and bonded joints, and recycled materials, where process deviations are revealed by reduced elongation.
• Failure analysis and litigation support, comparing retained production samples with failed parts to identify processing or service-induced embrittlement.

FAQ

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

Strain at break (εB) is measured over the gauge length with an extensometer and reflects only the deformation of the parallel test section. Nominal strain at break (εtB) is calculated from crosshead travel over the initial grip separation and therefore includes shoulder deformation and machine compliance, giving larger values. Both are legitimate reportable quantities, but they are not interchangeable, and the measurement basis must always be stated.

Which standards govern this experiment?

The principal standards are ISO 527-1 and ISO 527-2 (with ISO 527-3 for films), the Chinese national standards GB/T 1040.1 and GB/T 1040.2, which are technically aligned with ISO 527, and ASTM D638 for North American specifications. Japanese requirements are covered by JIS K 7161 and JIS K 7162, the national adoptions of ISO 527-1 and ISO 527-2. Conditioning atmospheres are defined in ISO 291 and ASTM D618.

How many specimens are required?

Both ISO 527-1 and ASTM D638 require a minimum of five valid specimens per material condition. Specimens failing in the grips are excluded, and additional specimens are tested to obtain five valid results. For oriented or reinforced materials, five specimens in each principal direction are tested.

Why is an extensometer preferred over crosshead displacement?

Crosshead displacement contains contributions from grip seating, specimen shoulders, and load-frame compliance, which can inflate apparent strain significantly, especially for stiff, low-elongation materials where the true strain at break may be only a few percent. An extensometer isolates the gauge section and provides the accuracy class required by ISO 9513 for reliable modulus and strain-at-break determination.

What factors most strongly influence the result?

Specimen geometry and moulding quality, test speed, temperature, moisture conditioning, molecular orientation, surface notches, and aging history are the dominant variables. Faster speeds and lower temperatures generally reduce strain at break, while moisture uptake in polyamides can increase it dramatically. Consistency in every one of these parameters is essential for valid material comparison.

Can ISO 527 and ASTM D638 results be compared directly?

Not directly. Differences in specimen thickness (4 mm versus 3.2 mm), dumbbell geometry, radius profiles, and recommended test speeds mean that ISO and ASTM values for the same material can differ systematically. For procurement and specification purposes, one system should be chosen and applied consistently, with any conversions explicitly justified by correlation testing.