Elongation at break and tensile strength testing are fundamental mechanical tests used to evaluate the ductility and load-bearing capacity of plastics, rubbers, films, textiles, and metal products. This guide explains the test principles, applicable international standards, specimen requirements, calculation methods, and typical industry applications of these two critical material properties.
Technical Overview
Tensile strength and elongation at break are two of the most widely measured mechanical properties in materials science and quality control. Together, they describe how a material responds to uniaxial stretching forces: tensile strength quantifies the maximum stress a material can withstand before failure, while elongation at break quantifies how far the material can stretch before it ruptures.
Tensile strength is defined as the maximum tensile force divided by the original cross-sectional area of the specimen, typically expressed in megapascals (MPa) or pounds per square inch (psi). Elongation at break is expressed as a percentage and is calculated by dividing the increase in gauge length at the moment of rupture by the original gauge length. A brittle material such as a rigid thermoset may exhibit high tensile strength with elongation below 5%, whereas an elastomer may stretch to several hundred percent elongation before breaking.
Both properties are determined using a universal testing machine (UTM), also known as a tensile tester. The specimen is clamped between two grips and stretched at a constant crosshead speed while the force and extension are continuously recorded. The resulting stress-strain curve provides additional information such as yield strength, tensile modulus, and the distinction between brittle and ductile failure modes.
- Tensile strength: maximum engineering stress sustained before rupture, σ = Fmax / A0
- Elongation at break: percentage extension at rupture, ε = (L − L0) / L0 × 100%
- Test equipment: universal testing machine with appropriate load cell, grips, and extensometer
- Reported results: individual values, mean, and standard deviation from replicate specimens
Testing Samples
Plastic Materials
Rigid and semi-rigid plastics, including thermoplastics such as polyethylene (PE), polypropylene (PP), polyamide (PA), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), and polyester (PET), are typically tested as dumbbell-shaped (dog-bone) specimens machined or injection-molded according to ISO 527-2 or ASTM D638. Specimen types such as 1A/1B (ISO) or Type I through V (ASTM) are selected based on material thickness and available material quantity.
Rubber and Elastomers
Vulcanized rubbers, thermoplastic elastomers (TPE), and silicone compounds are tested using dumbbell specimens cut from flat sheets in accordance with ISO 37 or ASTM D412. Because elastomers exhibit very high elongation, tests require large-capacity extension measurement and lower crosshead speeds, typically 500 mm/min.
Thin Plastic Films and Sheets
Flexible packaging films, laminates, and membranes below approximately 1 mm in thickness are tested as rectangular strips under ASTM D882 or ISO 527-3. Strip specimens avoid the stress concentrations that dumbbell dies can introduce in very thin materials.
Textiles and Fabrics
Woven, knitted, and nonwoven fabrics are evaluated using strip or grab methods under ISO 13934-1/-2, ASTM D5034, or ASTM D5035. Yarns, sewing threads, and webbing are tested under standards such as ASTM D2256 and ASTM D6775.
Metals and Wires
Metallic materials, wires, cables, and welded joints are tested under ISO 6892-1 (metallic materials at room temperature) or ASTM E8/E8M. Insulation and sheathing compounds from finished cables are tested under IEC 60811-501 for tensile strength and elongation.
Adhesives, Coatings, and Foams
Structural adhesives are tested in bulk or lap-shear configurations under ASTM D1002 or ISO 4587, while flexible foams are evaluated under ISO 1798 or ASTM D3574. Free films of coatings may be evaluated under ASTM D2370.
Testing Items
A typical tensile test program for a client includes the two headline parameters plus several supporting measurements derived from the same stress-strain curve:
- Tensile strength (ultimate tensile strength, MPa)
- Elongation at break (%)
- Yield strength and elongation at yield
- Tensile modulus (Young's modulus, MPa)
- Secant modulus (e.g., 1% secant modulus for films)
- Stress at a specified strain (commonly 100%, 200%, 300% for rubbers)
- Fracture behavior and failure location documentation
- Statistical summary: mean value, standard deviation, and coefficient of variation
Testing Methods
Governing Standards
Testing is performed strictly in accordance with internationally recognized methods. The most commonly applied standards include:
- ISO 527-1 / -2 and ASTM D638 — tensile properties of plastics
- ISO 37 and ASTM D412 — tensile properties of vulcanized and thermoplastic rubbers
- ASTM D882 and ISO 527-3 — tensile properties of thin plastic sheeting and films
- ISO 13934-1 / -2, ASTM D5034, and ASTM D5035 — tensile properties of textile fabrics
- ISO 6892-1 and ASTM E8/E8M — tensile testing of metallic materials
- IEC 60811-501 — tensile strength and elongation of cable insulation and sheaths
- GB/T 1040, GB/T 528, and GB/T 228.1 — Chinese national standards aligned with the ISO methods above
Specimen Preparation and Conditioning
Specimens are prepared by injection molding, compression molding, die cutting, or machining, depending on the material form. Dimensions of the narrow parallel section are measured with a calibrated micrometer or optical projector at multiple points, and the minimum cross-sectional area is used in calculations. Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 40 hours (88 hours for some standards) prior to testing.
Test Procedure
The test is conducted using a calibrated universal testing machine fitted with grips appropriate to the material — wedge grips for rigid plastics, pneumatic or capstan grips for films and textiles, and serrated or pneumatic grips for elastomers. Extension is measured either by grip separation, crosshead displacement, or preferably by a contact or non-contact extensometer for improved accuracy. The general procedure is as follows:
- Measure and record specimen dimensions and mark the original gauge length L0.
- Mount the specimen symmetrically in the grips, ensuring axial alignment to prevent bending stresses.
- Apply a small pretension if required by the standard.
- Set the crosshead speed as specified by the material standard (e.g., 1–500 mm/min for plastics, 500 mm/min for rubbers).
- Run the test to rupture, continuously recording force and extension.
- Calculate tensile strength from the maximum force and elongation at break from the extension at rupture.
- Test at least five valid specimens and report the median or mean as required by the method; discard results with failure inside the grips.
Result Calculation
Tensile strength is calculated as the maximum force divided by the original cross-sectional area of the parallel section. Elongation at break is the increase in gauge length between the original gauge marks at rupture, expressed as a percentage of the original gauge length. For materials that yield, the yield point is identified as the first point on the stress-strain curve at which strain increases without an increase in stress. All results are reported in the unit system specified by the client or the product standard, together with specimen type, test speed, temperature, and humidity.
Applications
Tensile strength and elongation at break data support a broad range of engineering, regulatory, and commercial purposes:
- Material qualification and incoming inspection: verifying that resin lots, compounds, and fabricated parts meet datasheet and purchase-specification limits.
- Product certification: demonstrating compliance with requirements in product standards for cables (IEC series), pipes and fittings, packaging materials, medical devices, and automotive components.
- Aging and durability studies: comparing tensile properties before and after heat aging, UV weathering, chemical exposure, or gamma sterilization to quantify retention percentages.
- Process validation: evaluating the influence of molding parameters, welding, annealing, or compounding changes on mechanical performance.
- Failure analysis: testing failed and reference samples side by side to identify embrittlement, degradation, or under-curing as root causes.
- Research and development: screening new formulations, additives, reinforcements, and biobased or recycled materials during development programs.
Industries routinely served include automotive, aerospace, electronics and wire & cable, flexible packaging, consumer goods, medical and pharmaceutical packaging, construction products, and textiles.
FAQ
What is the difference between tensile strength and elongation at break?
Tensile strength measures the maximum stress a material can carry before failing, expressed in force per unit area (MPa), whereas elongation at break measures how much the material can stretch before rupturing, expressed as a percentage of its original length. The former indicates load-bearing capacity; the latter indicates ductility and flexibility.
How many specimens are required for a valid test?
Most standards require a minimum of five specimens per sample. If results deviate by more than the permitted scatter, additional specimens are tested. Specimens that fail within the grips or outside the gauge section are normally rejected and replaced.
Which factors influence the test results?
Key influencing factors include specimen preparation quality, gauge length, crosshead speed, grip pressure and alignment, ambient temperature and humidity, and the method used to measure extension. Strict adherence to the specified standard conditions is essential for reproducible results.
Are ISO and ASTM tensile results directly comparable?
Not always. ISO 527 and ASTM D638 use different specimen geometries and strain-rate definitions, so results obtained by the two methods may differ systematically. Comparisons should only be made within the same standard and specimen type.
Can testing be performed at non-ambient conditions?
Yes. Environmental chambers allow testing at elevated or sub-zero temperatures, and conditioning protocols can simulate humid, wet, or chemically exposed service environments. Such programs are commonly combined with aging studies to calculate percentage retention of tensile properties.
What sample forms can be accepted by an accredited third-party laboratory?
Laboratories accept injection-molded or compression-molded plaques, die-cut sheets, finished parts from which specimens can be machined, films, fabrics, extruded profiles, wires, and cable insulation cores. Where the product geometry is unsuitable for standard specimens, miniaturized specimens or product-specific methods can be applied upon agreement with the client.