Rubber Material Tensile Breaking Force Testing: Methods, Standards, and Applications

Rubber material tensile breaking force testing is a fundamental mechanical evaluation used to determine the maximum force a rubber specimen can withstand before fracture, along with tensile strength and elongation at break. Conducted in accordance with internationally recognized standards such as ISO 37, ASTM D412, and GB/T 528, this test is essential for quality control, material qualification, and product design across the automotive, sealing, medical, and industrial rubber industries. This article provides a comprehensive technical overview of specimen types, testing procedures, calculation methods, and typical applications of rubber tensile breaking force experiments.

Technical Overview

The tensile breaking force experiment is one of the most widely performed mechanical tests for rubber materials. During the test, a standardized rubber specimen is clamped in a tensile testing machine and stretched at a constant crosshead speed until it fractures. The maximum force recorded at the moment of rupture is defined as the breaking force (also referred to as ultimate tensile force), while the corresponding stress value, calculated by dividing the breaking force by the original cross-sectional area of the specimen, is the tensile strength.

In addition to the breaking force itself, the test yields several complementary parameters, including elongation at break, tensile stress at a specified elongation (commonly 100% and 300% modulus), and the complete stress-strain curve of the material. These results reflect the degree of vulcanization, filler dispersion, compound consistency, and overall structural integrity of the rubber. Because rubber exhibits viscoelastic behavior, test results are highly sensitive to temperature, humidity, strain rate, and specimen geometry, which is why standardized conditioning and strictly controlled test conditions are mandatory.

Testing Samples

Dumbbell (Dog-Bone) Specimens

Dumbbell-shaped specimens are the most common sample form for rubber tensile testing. They are die-cut from flat vulcanized rubber sheets with a thickness of typically 2 mm ± 0.2 mm. Standards such as ISO 37 and GB/T 528 define several dumbbell types (Type 1, 2, 3, and 4), each with different gauge lengths and overall dimensions, while ASTM D412 specifies its own die designations (Type C and Type D being the most frequently used). The reduced central section ensures that fracture occurs within the gauge length rather than at the clamps.

Strip Specimens

Narrow strip specimens with parallel sides may be used when dumbbell cutting is impractical, such as for thin-walled extrusions, tapes, or thin films. Strip specimens are more prone to failure at the grips, so reinforced end tabs or specialized pneumatic clamps with serrated faces are often required to prevent slippage and premature fracture.

Ring Specimens

Small ring (torus) specimens, cut from tubing or prepared on mandrels, are stretched over two pulleys or mandrels on the testing machine. Ring testing is convenient for evaluating O-ring stock and hose materials because the stress distribution along the circumference provides an averaged measure of tensile performance and avoids the need for flat sheet preparation.

Fabricated Rubber Products

In quality control of finished goods, samples may be taken directly from cured products such as seals, gaskets, belts, rubber threads, gloves, and conveyor belt cover compounds. Specimens must be extracted from areas of uniform thickness and free from surface defects, fabric layers, or flow lines that could distort the measured breaking force.

Testing Items

A typical rubber tensile breaking force experiment determines the following parameters:

• Breaking force (N) — the maximum tensile force sustained by the specimen at rupture.
• Tensile strength (MPa) — breaking force divided by the original cross-sectional area of the specimen.
• Elongation at break (%) — the increase in gauge length at rupture relative to the initial gauge length.
• Tensile stress at specified elongation (MPa), commonly M100 and M300 — stress recorded at 100% and 300% strain, used as modulus indicators.
• Tensile stress at yield, when the material exhibits a yield point before fracture.
• Complete force-extension and stress-strain curves for material characterization and batch-to-batch comparison.

Testing Methods

Specimen Preparation and Conditioning

Test pieces are cut with a sharp die applied in a single stroke, and thickness measurements are taken at three points within the gauge length using a dial gauge or laser thickness gauge with a foot pressure small enough to avoid indentation. Specimens should be free from bubbles, cracks, and die-cut burrs. Prior to testing, samples are conditioned at a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 16 hours, in accordance with ISO 23529 or ASTM D1349. When the material is to be evaluated at elevated or sub-zero service temperatures, an environmental chamber is used to bring the specimen to the specified temperature before extension begins.

Test Apparatus

The experiment is performed on a constant-rate-of-extension (CRE) universal tensile testing machine meeting the force accuracy and speed requirements of ISO 5893 or ASTM D412. Essential equipment includes:

• A calibrated load cell with an accuracy class of at least 0.5, sized so the breaking force falls within the calibrated range.
• Pneumatic or mechanical wedge grips fitted with serrated, rubber-faced, or rope-and-drum fixtures depending on the specimen type.
• A contact extensometer or non-contact video extensometer for precise strain measurement within the gauge length.
• An environmental chamber when testing at non-ambient temperatures.

Test Procedure

The specimen is mounted in the grips with equal extensions on both sides, ensuring the gauge-length axis aligns with the direction of the applied force. The crosshead speed is selected according to the specimen type specified by the standard — for example, 500 mm/min for ISO 37 Type 1 and Type 2 dumbbells and ASTM D412 Type C specimens, with slower speeds prescribed for ring specimens. The machine extends the specimen at constant speed until rupture, while force and extension data are continuously recorded. At least five valid specimens should be tested for each sample, and the median values are reported; specimens that fail outside the gauge length are discarded and retested.

Data Calculation

Tensile strength is calculated by dividing the maximum force by the original minimum cross-sectional area of the specimen. Elongation at break is the ratio of extension at rupture to the initial gauge length, expressed as a percentage. For dumbbell specimens the width used in the area calculation is the width of the narrow section, while for ring specimens the force reading is conventionally halved to account for the two load-bearing legs of the ring. The test report should document the standard used, specimen type and dimensions, curing details, conditioning and test temperatures, crosshead speed, individual and median results, and any observed failure anomalies.

Applications

Tensile breaking force testing serves as a cornerstone of rubber material evaluation in many industries and scenarios:

• Compound development and formulation optimization for hardness, modulus, and strength balance in new rubber recipes.
• Incoming material inspection and supplier qualification for raw rubber, masterbatch, and cured compounds.
• Quality control of finished products such as O-rings, oil seals, hoses, rubber sheets, gloves, vibration isolators, and tires.
• Aging and durability studies, in which breaking force retention and elongation retention are measured after heat aging, ozone exposure, UV weathering, chemical immersion, or fatigue cycling.
• Regulatory compliance and certification support for automotive components, medical devices, footwear, and consumer products, where test reports from accredited third-party laboratories are typically required.
• Failure analysis, where a comparison of the tensile properties of a failed part against specification helps identify under-cure, contamination, or degradation as root causes.

FAQ

What is the difference between breaking force and tensile strength?

Breaking force is the absolute maximum load, expressed in newtons (N), that the specimen carries before fracture. Tensile strength is the corresponding stress, obtained by dividing the breaking force by the original cross-sectional area of the specimen, and is expressed in megapascals (MPa). Tensile strength allows comparison between specimens of different sizes, whereas breaking force is often specified directly for products such as rubber threads, straps, and seals tested in finished form.

Which standards govern rubber tensile testing?

The primary standards are ISO 37 (vulcanized and thermoplastic rubbers — determination of tensile stress-strain properties), ASTM D412 (standard test methods for vulcanized rubber and thermoplastic elastomers — tension), GB/T 528 (the Chinese national standard technically aligned with ISO 37), and JIS K 6251 (the Japanese standard). Machine and extensometer requirements are covered by ISO 5893, and conditioning atmospheres are defined in ISO 23529 and ASTM D1349.

Why did my specimens break at the grips instead of in the gauge length?

Grip-zone failure usually indicates insufficient clamping pressure, damaged or misaligned grips, excessive end-tab stress concentration, or a specimen with nicks or non-uniform thickness. Solutions include adjusting grip pressure, using pneumatic grips with rubber inserts, aligning the specimen axis carefully, applying mild pre-tension only as allowed by the standard, and recutting specimens with a sharp die. Results from specimens that fracture within the clamp region are excluded from the reported median.

How many specimens should be tested?

Most standards require a minimum of five valid specimens per material, with the median value reported as the representative result. For research purposes or when scatter is high, additional specimens are tested to improve statistical confidence. All individual values should nevertheless be recorded in the test report.

What factors can cause low or scattered breaking force results?

Common causes include poor dispersion of fillers, contamination, porosity from insufficient curing pressure, under- or over-vulcanization, die-cutting damage, non-standard specimen thickness, incorrect crosshead speed, testing outside the standard temperature and humidity conditions, and load cell selection inappropriate for the force range. Systematic control of specimen preparation, conditioning, and machine calibration is essential for reproducible results.