Rubber Tensile Breaking Force Determination: Standards, Methods, and Applications

Rubber tensile breaking force determination is a core mechanical test that measures the maximum tensile load a rubber specimen can sustain before fracture, providing essential data for material qualification and quality control. This technical guide explains the testing principles, specimen types, test parameters, and calculation methods defined by international standards such as ISO 37, ASTM D412, and GB/T 528. It serves engineers, laboratory technicians, and procurement specialists who need reliable and reproducible rubber tensile testing results.

Technical Overview

Rubber tensile breaking force determination quantifies the maximum force, expressed in newtons (N) or kilonewtons (kN), that a vulcanized or thermoplastic rubber specimen withstands in uniaxial tension until rupture occurs. Because rubber is a hyperelastic, highly extensible polymer material, the test simultaneously captures the complete stress-strain response, including tensile strength, elongation at break, and tensile stress at specified elongations. The breaking force itself is the direct load reading at the moment of failure, while tensile strength is derived by dividing the maximum force by the original cross-sectional area of the specimen.

This test is one of the most widely performed rubber characterization procedures worldwide. It is used for incoming material inspection, batch release in manufacturing, compound development, failure analysis, and compliance demonstration against product specifications. Rubber components such as seals, hoses, belts, vibration isolators, and weatherstrips must retain adequate tensile integrity throughout their service life, so breaking force data directly supports product safety and durability claims.

Governing Standards

• ISO 37 — Rubber, vulcanized or thermoplastic: Determination of tensile stress-strain properties
• ASTM D412 — Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers: Tension
• GB/T 528 — Chinese national standard for tensile testing of vulcanized or thermoplastic rubber, technically aligned with ISO 37
• ISO 5893 — Rubber and plastics test equipment: Tensile, flexural, and compression types, defining machine accuracy and extension measurement classes
• ISO 23529 — Rubber: General procedures for preparing and conditioning test pieces for physical test methods
• ASTM D1349 — Standard Practice for Rubber: Conditions for Test Temperatures, Humidities, and Times

Testing Samples

Dumbbell (Dog-Bone) Specimens

Dumbbell specimens are the most common sample geometry. They are stamped from flat sheets, typically 1 mm to 3 mm thick, using standardized cutting dies that produce a narrow central gauge section with widened ends for gripping. ISO 37 defines dumbbell Types 1, 1A, 2, 3, and 4 with gauge lengths ranging from 10 mm to 25 mm, while ASTM D412 defines dies corresponding to specimens C1 through C5. The widened shoulders ensure that stress concentration and failure occur within the uniform central section rather than at the grips.

Ring Specimens

Ring specimens, defined as Types A and B in ISO 37 and GB/T 528, are circular loops stretched over two parallel mandrels or pulleys. Rings are particularly useful for testing small O-rings, sealing cords, and products where a closed-loop geometry reflects the actual service configuration. Elongation measurement for rings must account for the fact that the inner circumference extends less than the outer circumference, and the standards prescribe specific correction conventions.

Specimen Preparation and Conditioning

• Cut specimens from standard laboratory sheets, or from finished products where cut surfaces must be smooth and free of defects
• Allow a minimum interval of 16 hours between vulcanization and testing to stabilize the rubber network
• Condition specimens at 23 ± 2 °C, and at 50 ± 5 % relative humidity for humidity-sensitive materials, for at least 16 hours prior to testing
• Measure the thickness of each dumbbell at three points within the gauge section using a calibrated thickness gauge with the specified foot pressure, and record the median value
• Verify that the width of the narrow section conforms to the die specification, and reject specimens with visible porosity, voids, or edge defects

Testing Items

A complete tensile test campaign typically reports the following parameters, all obtained from a single continuous extension of the specimen to rupture:

• Tensile breaking force (Fm) — the maximum force recorded during the test, in N or kN
• Tensile strength (TS) — maximum force divided by original cross-sectional area, in MPa
• Elongation at break (Eb) — the increase in gauge length at rupture, expressed as a percentage of the original gauge length
• Tensile stress at a given elongation (M100, M200, M300) — commonly called modulus values, representing the stress at 100 %, 200 %, or 300 % elongation
• Yield-type behaviors and stress-strain curve shape for thermoplastic elastomers and filled compounds
• Product-specific breaking force acceptance limits, such as minimum pull-off force for sealing profiles, waterstop strips, and structural rubber elements

Testing Methods

Apparatus Requirements

Testing is performed on a universal tensile testing machine complying with ISO 5893 or equivalent, typically of accuracy class 0.5 or 1, fitted with a load cell whose capacity closely matches the expected breaking force so that failure occurs within 15 % to 85 % of the measuring range. Mechanical, pneumatic, or wedge-action grips hold the dumbbell ends; pneumatic grips with serrated or rubber-faced inserts are preferred because they minimize specimen slippage and premature grip failure. Elongation is measured either with a contact or video extensometer meeting the required accuracy class, or, for less demanding work, from crosshead displacement with documented correction.

Test Procedure

• Enter specimen identification, measured thickness, width, and gauge length into the test software before loading
• Insert the specimen symmetrically in the grips with the gauge length axis aligned to the loading axis
• Apply the standard crosshead speed: 500 mm/min for Type 1, 1A, and 2 dumbbells per ISO 37, 200 mm/min for Type 3, and 100 mm/min for Type 4; ASTM D412 commonly specifies 500 ± 50 mm/min
• Extend the specimen at constant speed until rupture, continuously recording force and extension
• Discard any test in which the specimen breaks outside the gauge section or at a grip, and repeat with a new specimen
• Test at least three specimens, or preferably five, and report the median value as prescribed by ISO 37 and GB/T 528, or the average as prescribed by ASTM D412, together with individual results

Calculation of Results

Tensile strength is calculated by dividing the maximum force by the product of the narrow-section width and the median measured thickness. Elongation at break is calculated as the ratio of the extension at break to the original gauge length, multiplied by 100. For ring specimens, the original internal circumference replaces the gauge length in the elongation calculation, and the cross-sectional area is determined from ring width and thickness measurements. Breaking force, when required as a product acceptance criterion, is reported directly as the measured maximum load without normalization.

Precision and Influencing Factors

Reproducibility depends strongly on specimen preparation quality, die sharpness, knife angle during cutting, ambient temperature control, and accurate thickness measurement. Natural rubber and other crystallizing elastomers are strain-rate sensitive, so deviations from the specified crosshead speed can shift breaking force values measurably. Testing at non-ambient temperatures is performed using an environmental chamber, with specimens conditioned at the test temperature for a defined equilibration period before extension begins.

Applications

Rubber tensile breaking force determination supports quality assurance and product development across virtually every industry that uses elastomeric materials:

• Automotive — validation of engine mounts, seals, hoses, weatherstrips, suspension bushings, and tire compounds against OEM material specifications
• Industrial rubber goods — acceptance testing of O-rings, gaskets, diaphragms, rubber sheets, and expansion joint elements
• Construction — verification of waterstop strips, bridge bearing pads, and preformed sealing profiles used in concrete structures and curtain walls
• Pipeline and fluid handling — qualification of rubber lining materials, hose inner tubes, and sealing elements for pressure retention
• Medical and pharmaceutical — tensile evaluation of gloves, tubing, stoppers, and elastomeric closures under biomedical material standards
• Consumer products — quality control of footwear soles, sporting goods, elastic components, and household rubber articles
• Research and failure analysis — comparison of aging resistance after heat, ozone, UV, or chemical exposure by measuring retained tensile properties

FAQ

What is the difference between tensile breaking force and tensile strength?

Tensile breaking force is the raw maximum load in newtons measured at rupture, while tensile strength is the same force normalized by the original cross-sectional area and expressed in megapascals. Product specifications for finished goods such as sealing profiles often state a minimum breaking force, whereas material data sheets normally report tensile strength for comparison between compounds.

Should dumbbell or ring specimens be used?

Dumbbells are preferred for general material characterization because of their uniform gauge section and well-defined stress state. Rings are preferred when the product geometry is a closed loop, such as O-rings and sealing cords, or when limited flat material is available. The two geometries do not yield identical numerical results, so the specimen type must always be stated with the reported data.

How many specimens are required for a valid test?

ISO 37 and GB/T 528 require a minimum of three specimens and recommend five, with the median value reported unless otherwise agreed. ASTM D412 reports the average of at least three specimens. Specimens that break at the grips or outside the gauge length are considered invalid and must be replaced.

Why is the test speed standardized at 500 mm/min?

Rubber is a viscoelastic material whose strength and elongation depend on deformation rate. A fixed crosshead speed of 500 mm/min for standard dumbbells ensures comparability of results between laboratories and against historical databases. Different speeds are specified for smaller specimen types to maintain an equivalent strain rate in the gauge section.

Can the test be performed at high or low temperatures?

Yes. Universal testing machines can be fitted with environmental chambers covering ranges from approximately −70 °C to +300 °C. Specimens are conditioned at the set point temperature for the specified equilibration time before testing, and the results are reported with the test temperature clearly identified.

What causes scattered or unexpectedly low breaking force results?

Common causes include dull cutting dies that introduce edge micro-cracks, incorrect thickness measurement, specimen misalignment in the grips, slippage, testing outside the conditioned temperature range, insufficient post-vulcanization resting time, and material aging or contamination. Systematic investigation of these factors typically restores result consistency.

How should breaking force results be reported?

A compliant test report identifies the test standard, specimen type, curing and conditioning details, test speed and temperature, individual results and the statistical value used, and the test date. When testing is performed by accredited third-party laboratories, the report is issued with proper accreditation markings and measurement traceability statements.