Tensile Strength Testing of Rubber Materials After Aging: Standards, Methods, and Applications

Tensile strength testing of rubber materials after aging evaluates how heat, oxygen, ozone, light, and other environmental factors degrade the mechanical performance of vulcanized rubber and thermoplastic elastomers over time. By comparing the tensile strength and elongation at break of aged and unaged specimens in accordance with standards such as ISO 37, ISO 188, ASTM D412, ASTM D573, and GB/T 3512, manufacturers can verify formulation durability, product compliance, and expected service life. This article explains the sample requirements, test items, aging and tensile testing procedures, calculation of change rates, typical applications, and frequently asked questions related to post-aging breaking strength measurement.

Technical Overview

Rubber materials, whether vulcanized elastomers or thermoplastic elastomers, undergo irreversible physical and chemical changes during storage and service. Oxygen, heat, ozone, ultraviolet radiation, humidity, and contact with fluids progressively attack the polymer network through two competing mechanisms: oxidative chain scission, which weakens the material, and additional crosslinking, which embrittles it. Plasticizer extraction and antidegradant depletion further accelerate the process.

The visible consequences include surface cracking, increased hardness, loss of elasticity, and a marked decline in tensile strength and elongation at break. Because mechanical failure of rubber components almost always originates from this degradation, tensile strength testing after aging has become the single most widely used indicator of rubber durability.

Post-aging tensile testing measures the tensile strength (TS) and elongation at break (Eb) of specimens that have first been exposed to a controlled accelerated aging environment, and compares these values with those of unaged control specimens. The deterioration is expressed as a percentage change or a retention rate, which serve as key acceptance criteria in nearly every rubber product specification.

• Tensile testing standards: ISO 37, ASTM D412, GB/T 528, JIS K 6251
• Heat aging standards: ISO 188, ASTM D573, ASTM D865, GB/T 3512
• Ozone aging standards: ISO 1431-1, ASTM D1149, GB/T 7762
• Artificial weathering standards: ISO 4892, ASTM G154, ASTM G155, GB/T 16422
• Liquid immersion aging standards: ASTM D471, GB/T 1690
• Life estimation standard: ISO 11346

Testing Samples

Standard Dumbbell Specimens

The most common sample form is the dumbbell test piece punched from a vulcanized rubber sheet of 2.0 mm nominal thickness using standardized cutting dies. ISO 37 and GB/T 528 define dumbbell types 1 through 4, of which type 1 (25 mm gauge length, 6 mm width, 115 mm overall length) is the preferred geometry. ASTM D412 specifies equivalent dies, most commonly die C and die D. The specimen thickness is measured at the center and both ends of the narrow section, and the median value is used for cross-sectional area calculation.

Ring Specimens

Ring test pieces described in ISO 37 and ASTM D412 are used when the material is supplied in tubular or annular form, or when the product geometry itself is a ring. Ring testing avoids the cutting damage that dumbbell punching can introduce in soft or highly extensible compounds.

Specimens Cut from Finished Products

When products cannot be compression-molded into standard sheets, specimens may be cut directly from finished items such as hoses, belts, seals, and cable sheaths. The specimens should have uniform thickness, typically between 1 mm and 3 mm, and the cutting direction relative to the grain or machine direction must be recorded, since anisotropy can significantly affect tensile results.

Sample Quantity and Grouping

At least three valid specimens, preferably five, are tested for each condition. A complete test program includes one unaged control group plus one group for each aging condition, with median values reported in accordance with the governing standard.

Testing Items

A typical post-aging tensile test report from an accredited third-party laboratory includes the following items:

• Tensile strength after aging, expressed in MPa
• Elongation at break after aging, expressed in percent
• Tensile stress at 100 percent and 300 percent elongation (modulus), in MPa
• Force at break, in N
• Percentage change in tensile strength after aging
• Percentage change in elongation at break after aging
• Retention rate of tensile strength and elongation at break
• Visual observations of aged specimens, such as cracking, blistering, or stickiness

Testing Methods

Step 1: Accelerated Aging Treatment

The aging environment is selected according to the intended application of the material and the governing product standard:

• Hot air oven aging (ISO 188, ASTM D573, GB/T 3512): specimens are held in a ventilated oven at elevated temperatures, commonly 70, 85, 100, 125, or 150 degrees Celsius, for 24 to 168 hours or longer. This is the most frequently specified preconditioning for post-aging tensile testing.
• Ozone aging (ISO 1431-1, ASTM D1149, GB/T 7762): specimens are stretched to a fixed strain, typically 20 percent, and exposed to an ozone-enriched atmosphere, for example 50 pphm at 40 degrees Celsius for 72 hours, to assess resistance to ozone cracking.
• Artificial weathering (ISO 4892, ASTM G154, ASTM G155, GB/T 16422): xenon arc or fluorescent UV exposure with controlled irradiance, temperature, and water spray simulates outdoor sunlight and moisture cycling.
• Liquid immersion aging (ASTM D471, GB/T 1690): specimens are immersed in oils, fuels, or chemicals at elevated temperature before tensile evaluation, which is essential for seals and hoses in contact with fluids.
• Natural weathering (ISO 4665, ASTM D1435, GB/T 3511): outdoor exposure for benchmarking accelerated test results against real service conditions.

Step 2: Recovery and Conditioning

After removal from the aging chamber, specimens are allowed to recover for 16 to 96 hours as required by ISO 188, and are then conditioned in the standard laboratory atmosphere of 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity before tensile testing.

Step 3: Tensile Testing Procedure

Tensile testing is performed on a calibrated universal testing machine equipped with appropriate grips that prevent slippage and premature breakage at the grip edges. Dumbbell specimens are extended at a crosshead speed of 500 plus or minus 50 mm/min as specified by ISO 37, GB/T 528, and ASTM D412. The force-elongation curve is recorded until rupture, and the maximum force together with the elongation at break is determined. Tensile strength is calculated as the maximum force divided by the original cross-sectional area of the narrow section. Results that break outside the gauge length are discarded and retested.

Step 4: Calculation of Change Rate and Retention Rate

The percentage change in tensile strength is calculated as the aged value minus the unaged value, divided by the unaged value, multiplied by 100. The retention rate is the aged value divided by the unaged value multiplied by 100. Identical calculations are applied to elongation at break. The test report must state the aging medium, temperature, duration, specimen type, individual measured values, and median results.

When a service life assessment is required, aging is conducted at several temperatures and the time to reach a defined endpoint, commonly a 50 percent reduction in elongation at break, is extrapolated using the Arrhenius method described in ISO 11346.

Applications

Post-aging tensile strength testing is applied across virtually every industry that relies on rubber components:

• Automotive engineering: engine bay seals, coolant and fuel hoses, vibration mounts, timing belts, and tire sidewalls, where long-term heat and ozone resistance are critical.
• Wire and cable: cross-linked polyethylene and ethylene propylene rubber insulation and sheaths, evaluated for tensile change after air oven aging under IEC 60811 test methods.
• Construction: bridge elastomeric bearings, waterproofing membranes, and expansion joints exposed to outdoor weathering for decades.
• Industrial sealing: O-rings, gaskets, and diaphragms in nitrile, fluoroelastomer, and EPDM compounds, qualified by combined heat and fluid immersion aging.
• Medical and consumer products: surgical gloves, tubing, and elastic components requiring validated shelf life.
• Research and development: screening of antioxidant and antiozonant packages, comparison of formulation candidates, and prediction of long-term performance.
• Quality control and certification: routine compliance verification of incoming compounds and finished products in accredited third-party laboratories.

FAQ

Question: What exactly does breaking strength after aging mean?

Answer: It refers to the maximum tensile stress that a rubber specimen can sustain before rupture, measured after the specimen has first been subjected to a controlled accelerated aging treatment. It is numerically identical to the tensile strength of the aged specimen.

Question: What are typical acceptance criteria for the change in tensile strength?

Answer: Many product specifications require the change in tensile strength and elongation at break after aging to remain within plus or minus 25 to 30 percent, or equivalently a retention rate of at least 70 to 75 percent, after a defined aging period such as 72 hours at 100 degrees Celsius. The exact limits depend on the governing product standard and the intended service environment.

Question: Why is elongation at break always tested together with tensile strength?

Answer: Elongation at break is the most sensitive indicator of rubber degradation and usually decreases earlier and faster than tensile strength. If elongation drops sharply while tensile strength stays constant, additional crosslinking dominates the aging mechanism; if both drop together, chain scission is dominant. The combination therefore provides diagnostic information about the degradation mechanism.

Question: How are the aging temperature and duration selected?

Answer: They are chosen to reflect the service temperature of the product with an accelerated margin, using the preferred temperature steps listed in ISO 188. A good aging condition produces measurable but not catastrophic degradation, so that meaningful comparison between materials remains possible. Multi-temperature programs are used when Arrhenius life prediction is required.

Question: Can specimens be taken directly from finished products?

Answer: Yes. When standard molded sheets are unavailable, dumbbell or strip specimens can be die-cut from finished products, provided the thickness is reasonably uniform and the cutting direction is documented. Such results characterize the actual product rather than a reference compound.

Question: How long does a complete test take?

Answer: The total duration equals the aging period, plus the 16 to 96 hour recovery interval, plus conditioning and tensile testing. Routine single-condition tests are commonly completed within three to ten working days, while long-term or multi-temperature programs may extend over several weeks.