Rubber breaking strength determination is a fundamental mechanical test that measures the maximum tensile stress a vulcanized or thermoplastic rubber material can withstand before rupture. This technical guide covers the testing principles, applicable international standards such as ISO 37 and ASTM D412, specimen requirements, test procedures, calculation methods, and typical industrial applications.
Technical Overview
Rubber breaking strength, often referred to as tensile strength, is the maximum tensile stress that a rubber test piece can sustain before fracture occurs. It is calculated as the maximum force recorded during a uniaxial tensile test divided by the original cross-sectional area of the narrow section of the specimen, and is expressed in megapascals (MPa). Because rubber components in tires, seals, hoses, belts, and medical devices routinely experience tensile, flexural, and cyclic loads in service, breaking strength is one of the most widely specified quality-control and material qualification parameters in the rubber industry.
Determination of rubber breaking strength is typically performed in accordance with internationally recognized standards, including 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, and JIS K 6251. These standards define the specimen geometry, conditioning requirements, testing speed, calculation formulas, and result-reporting rules, ensuring that results obtained in different laboratories are comparable. During the same tensile test, elongation at break, tensile stress at specified elongation, and stress-strain behavior can also be captured, providing a complete picture of the material's mechanical performance.
Testing Samples
Dumbbell (Dog-Bone) Specimens
Dumbbell-shaped specimens are the most commonly used configuration for breaking strength determination. ISO 37 specifies four dumbbell types (Type 1, 2, 3, and 4) with different gauge lengths and widths, while ASTM D412 defines a series of cutting dies (Die A through F). Standard test sheets are usually vulcanized to a thickness of 2.0 mm ± 0.2 mm, and the specimens are die-cut with a sharp rotary cutter and a calibrated cutting die to ensure smooth, defect-free edges that prevent premature failure.
Ring Specimens
ISO 37 also permits ring test pieces, including small rings and large rings. Ring specimens are stretched over specially designed spool-type fixtures and are convenient for automated or high-throughput testing. However, care must be taken because the stress distribution across a ring is non-uniform, and the inner circumference reaches higher strain than the outer circumference, which can bias results if not properly accounted for.
Straight Strip Specimens
Straight strips may be used when material availability is limited, such as when sampling thin-walled products. Because failure frequently initiates at the grips rather than in the free span, straight strips are generally considered less reliable than dumbbells and are used only when standardized geometries cannot be obtained.
Finished Products and Components
Specimens can also be prepared directly from cured rubber goods, including tire treads and sidewalls, O-rings and oil seals, hydraulic and automotive hoses, conveyor belting, rubber sheets, medical gloves, catheters, tubing, footwear soles, and anti-vibration mounts. Sampling from finished products allows verification of compound consistency, curing adequacy, and long-term aging performance in end-use conditions.
Testing Items
A standard breaking strength determination package typically includes the following measured and calculated parameters:
- Tensile strength (breaking strength), TS — the maximum force divided by the original cross-sectional area, in MPa
- Force at break, in newtons (N)
- Elongation at break, Eb — the strain expressed as a percentage at the moment of rupture
- Tensile stress at specified elongation (for example, 100% and 300% modulus), in MPa
- Elongation at a specified tensile stress, where required
- Tensile strength retention and elongation retention after heat aging, oil immersion, ozone exposure, or weathering
- Complete stress-strain curve characterization for research and development purposes
Testing Methods
Apparatus and Equipment
The test is performed on a universal tensile testing machine equipped with a calibrated load cell appropriate for the expected force range, self-tightening or pneumatic grips, and either contact extensometers or non-contact video extensometers for strain measurement. Ancillary equipment includes calibrated cutting dies, a flat anvil and stamping press for specimen preparation, a dial or digital thickness gauge meeting standard accuracy requirements, and a conditioning chamber capable of maintaining the standard laboratory atmosphere.
Specimen Preparation and Conditioning
Test pieces are die-cut cleanly from cured sheets or products, avoiding any nicks, tears, or edge defects that could act as stress concentrators. Width and thickness are measured at several points, and the median values are used to compute the cross-sectional area. Prior to testing, specimens are conditioned in accordance with ISO 23529 or ASTM D1349, typically at 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 16 hours, unless a material-specific standard specifies otherwise. Benchmark marks are applied within the gauge length for elongation measurement.
Test Procedure
The specimen is secured in the grips with the gauge marks aligned symmetrically, ensuring the longitudinal axis coincides with the direction of the applied force. The crosshead speed is selected according to the specimen type and the governing standard — commonly 500 mm/min for the smaller ISO dumbbells and for ASTM die C specimens, with slower speeds specified for larger specimens. The tensile load is applied smoothly until the specimen ruptures, and the force-elongation data are recorded continuously. ISO 37 recommends testing at least five specimens per sample; ASTM D412 requires at least three. Tests in which fracture occurs within the grip faces, or clearly outside the gauge length in a manner indicating edge damage, are discarded and retested.
Calculation and Reporting
Tensile strength is calculated as the maximum force divided by the product of the specimen width and thickness (TS = Fm / (W × t)), while elongation at break is calculated as the increase in gauge length at rupture divided by the original gauge length, multiplied by 100%. The median value of the individual results is reported as the representative value for the sample, together with the individual measurements, the test standard used, the specimen type, the crosshead speed, and the laboratory atmosphere. Comparisons between laboratories are only valid when identical specimen types and test conditions are employed.
Applications
Breaking strength determination supports quality control, product certification, research and development, and failure analysis across a wide range of industries:
- Automotive and tire manufacturing — tread compounds, sidewalls, inner liners, engine mounts, bushings, and sealing systems
- Seals and gaskets — O-rings, oil seals, door and window gaskets, and diaphragms
- Hoses and belts — hydraulic hoses, radiator hoses, conveyor belts, and power transmission belts
- Medical and pharmaceutical — surgical gloves, condoms, catheters, stoppers, and elastomeric tubing
- Consumer goods — footwear outsoles, elastic bands, sporting goods, and protective equipment
- Industrial and infrastructure — rubber linings, bridge bearings, expansion joints, and vibration isolators
- Aerospace and oil and gas — elastomeric components requiring qualification under harsh thermal and chemical environments
Manufacturers exporting to regulated markets frequently rely on accredited third-party laboratories to generate breaking strength reports in accordance with ISO 37, ASTM D412, or GB/T 528, supporting product declarations, customer specifications, and regulatory submissions.
FAQ
What is the difference between tensile strength and breaking strength?
In rubber testing, tensile strength is defined as the maximum stress reached during the test, while the stress at the actual moment of rupture may be slightly lower due to stress softening before failure. In common industrial usage, the two terms are often used interchangeably, and both are determined from the same uniaxial tensile test.
Which standard should be selected — ISO 37 or ASTM D412?
The choice depends on the market, customer specification, and product standard. ISO 37 is widely referenced in Europe and Asia, while ASTM D412 predominates in North America. The two standards use different specimen geometries and, in some cases, different crosshead speeds, so numerical results are not directly interchangeable. It is advisable to confirm the required standard before commissioning testing.
How many specimens are required for a valid result?
ISO 37 recommends testing five specimens and reporting the median, whereas ASTM D412 requires a minimum of three. Additional specimens are recommended for highly filled or anisotropic compounds, or when testing aged samples for retention values.
Why do specimens sometimes fail at the grips, and how is this addressed?
Grip failures are usually caused by edge nicks, improper grip pressure, misalignment, or specimen defects. Such results are not representative and are excluded; the specimen is replaced and the test repeated. Proper die-cutting, careful handling, and self-tightening grips minimize this occurrence.
Do environmental factors affect breaking strength results?
Yes. Heat aging, ozone exposure, oil or fluid immersion, UV weathering, and low-temperature crystallization can all significantly alter tensile strength and elongation at break. For this reason, breaking strength is frequently measured both before and after conditioning to calculate property retention percentages.
What information should be provided when requesting breaking strength testing?
Typically, the client should specify the test standard and specimen type, the material or product description, the number of test conditions (original, aged, or immersed), and any special requirements such as modulus points or stress-strain curves. Accredited third-party laboratories can then confirm feasibility, specimen quantity, and turnaround time.