Analysis of Tensile Strength Reduction Rate of Seals After Aging

The tensile strength reduction rate of seals after aging is a critical performance indicator that quantifies how elastomeric sealing materials degrade under thermal, oxidative, ozone, and fluid exposure over time. This analysis compares the breaking strength of seal specimens before and after accelerated aging tests conducted according to internationally recognized standards such as ISO 37, ASTM D412, and ISO 188. Understanding this degradation mechanism enables engineers to predict seal service life, verify material compliance, and prevent premature sealing failures in demanding industrial applications.

Technical Overview

Seals are elastomeric components designed to prevent leakage of fluids and gases between mating surfaces in mechanical systems. Over their service life, seals are continuously exposed to elevated temperatures, oxygen, ozone, hydraulic fluids, and mechanical stress, all of which initiate irreversible chemical and physical changes within the polymer network. These changes—collectively referred to as aging—manifest as chain scission, cross-link densification, oxidation, and extraction of plasticizers, ultimately reducing the material's ability to withstand tensile loads.

The tensile strength reduction rate (also called breaking strength degradation rate) is defined as the percentage decrease in tensile strength after aging relative to the original unaged value. It is calculated as: Reduction Rate (%) = (T0 − T1) / T0 × 100, where T0 is the original tensile strength and T1 is the tensile strength after aging. A lower reduction rate indicates superior aging resistance and longer expected service life. Typical acceptance criteria for sealing compounds range from a maximum allowable reduction of 15% to 30%, depending on the material class and application requirements specified in standards such as ISO 3601, ASTM D2000, and GB/T 3452.

This analysis is essential for material qualification, quality control during batch production, failure investigation, and lifetime prediction of sealing systems used in automotive, aerospace, petrochemical, and hydraulic industries.

Testing Samples

Rubber O-Rings

Toroidal seals manufactured from compounds such as NBR (nitrile butadiene rubber), FKM (fluoroelastomer), EPDM, and silicone rubber. Standard cross-sections range from 1.5 mm to 5.33 mm in accordance with ISO 3601-1 and AS568 size codes. O-rings are tested either as whole rings using ISO 37 Type T specimens or as prepared dumbbell sections cut from the tube portion.

Molded and Extruded Gaskets

Flat gaskets, profile gaskets, and custom-molded sealing elements used in flanged joints, enclosures, and doors. Sheet material is sampled using die-cut dumbbell specimens (Type 1 or Type 2 per ISO 37 and ASTM D412) taken from representative locations with consistent thickness and cure state.

Rotary and Reciprocating Shaft Seals

Lip seals and rod seals made from NBR, FKM, or polyurethane elastomers. Test specimens are harvested from the functional lip region or from companion cure plaques molded from the identical compound, ensuring the tested material reflects production chemistry.

Specimen Preparation Requirements

All specimens must be conditioned at standard laboratory atmosphere (23 ± 2 °C and 50 ± 5% relative humidity) for a minimum of 16 hours before testing, as specified in ISO 23529 and ASTM D1349. Specimens shall be free from surface defects, porosity, flash marks, and machining damage, and thickness measurements must be recorded at multiple points to confirm uniformity.

Testing Items

• Tensile strength (breaking strength) before aging (T0), measured in MPa
• Tensile strength after aging (T1), measured in MPa
• Tensile strength reduction rate, expressed as a percentage
• Elongation at break and its percentage change after aging
• Hardness change in IRHD or Shore A units
• Compression set after aging (supporting indicator of elastic recovery loss)
• Mass and volume change after fluid immersion
• 100% modulus change (indicator of cross-link density evolution)

Testing Methods

Accelerated Heat Aging in Air

Specimens are exposed to elevated temperatures in a forced-air circulating oven in accordance with ISO 188 (method A), ASTM D573, or GB/T 3512. Common test conditions for sealing materials include 70 °C, 100 °C, 125 °C, 150 °C, and 175 °C for exposure periods of 24 h, 70 h, 168 h, or 1000 h, selected according to the material class defined in ASTM D2000 or the end-use temperature. FKM compounds may be evaluated at 200 °C to 275 °C, while silicone compounds can be tested up to 250 °C or higher.

Ozone Aging

Elastomers containing unsaturated backbones, particularly NBR and NR, are susceptible to ozone-induced cracking. Specimens are stretched to a specified elongation (typically 20% or 40%) and exposed to an ozone concentration of 50 pphm or 200 pphm in a controlled chamber per ISO 1431-1, ASTM D1149, or GB/T 7762. Subsequent tensile testing quantifies the strength loss attributable to surface ozone cracking.

Fluid Immersion Aging

Seals in hydraulic, fuel, and lubrication systems degrade through swelling, extraction, and chemical attack by service fluids. Specimens are immersed in reference fluids such as ASTM No. 1 oil, IRM 903 oil, Fuel C, or the actual service medium at defined temperature and duration per ASTM D471, ISO 1817, or GB/T 1690. Tensile strength is measured after immersion and after drying, as required by the material specification.

Weathering and UV Exposure

For outdoor sealing applications, xenon-arc light exposure or fluorescent UV-condensation aging is performed according to ISO 4892-2, ISO 4892-3, or ASTM G155 to simulate sunlight, moisture, and temperature cycling. Tensile testing after exposure reveals the combined effect of photodegradation and hydrolysis.

Tensile Strength Measurement

Aged and unaged specimens are tested on a calibrated universal tensile testing machine with a constant crosshead speed of 500 mm/min for dumbbells (ISO 37, ASTM D412, GB/T 528) or an appropriate speed for ring specimens. Tensile strength is calculated as the maximum force recorded during extension divided by the original cross-sectional area. A minimum of three and preferably five specimens per condition are tested, and the median value is used for calculating the reduction rate.

Calculation and Evaluation of Reduction Rate

The tensile strength reduction rate is determined as R(%) = (T0 − T1) / T0 × 100. The result is compared against the maximum permissible change stated in the applicable material specification—for example, ±15% for high-performance compounds per ASTM D2000 suffix requirements or ±20% to ±30% for general-purpose grades. Elongation loss is evaluated concurrently, since elongation at break typically declines faster than tensile strength and often serves as the controlling failure criterion. Statistical treatment of replicate results follows ISO 37 reporting requirements, including median, range, and outlier assessment.

Applications

• Automotive powertrain and chassis sealing: validating NBR and FKM compounds in engine oil, coolant, and transmission fluid environments
• Hydraulic and pneumatic systems: qualifying seals for pressure vessels, cylinders, and valves against heat and fluid aging
• Aerospace sealing: demonstrating long-term retention of mechanical properties under wide temperature ranges per industry material specifications
• Oil and gas pipelines and valves: assessing chemically resistant sealing compounds exposed to hydrocarbons and sour gas environments
• Chemical processing equipment: evaluating EPDM and fluoropolymer seals in aggressive chemical media and steam service
• Electrical enclosure and construction sealing: verifying UV and ozone durability of gaskets exposed to outdoor weathering
• Quality control and incoming inspection: batch-to-batch verification that production compounds meet certified aging limits
• Failure analysis and forensic investigation: determining whether in-service seal failures correlate with excessive strength degradation
• Accelerated life prediction: extrapolating reduction-rate data using Arrhenius models to estimate service life at operating temperature

FAQ

What is an acceptable tensile strength reduction rate for seals?

Acceptance limits depend on the material specification and application criticality. Commonly, ASTM D2000 line callouts and industry standards specify a maximum reduction of 15% to 30% in tensile strength and elongation after the designated aging period. Safety-critical and high-temperature applications typically impose stricter limits, while general-purpose applications may tolerate up to 30% degradation.

Why does tensile strength decrease after aging?

Aging induces both chain scission, which weakens the polymer network, and additional cross-linking, which embrittles the material. Oxidation at elevated temperature, ozone attack on double bonds, extraction of plasticizers and antioxidants by fluids, and hydrolysis of ester linkages all contribute to reduced strength and elongation. The dominant mechanism depends on the elastomer chemistry and exposure conditions.

Can tensile strength increase after aging?

Yes. In some cases, post-cure cross-linking or the loss of plasticizers can produce a temporary increase in tensile strength and hardness. However, this is usually accompanied by a sharp loss of elongation at break, indicating embrittlement. For this reason, elongation change is evaluated together with tensile strength change.

How are test temperature and duration selected?

Selection follows the material class and end-use requirements defined in standards such as ASTM D2000 and ISO 3601. The aging temperature is typically 25 °C to 50 °C above the maximum continuous service temperature, with durations of 70 h, 168 h, or 1000 h used to simulate short-term and long-term exposure.

Which standards govern this analysis?

Primary standards include ISO 37 and ASTM D412 for tensile testing, ISO 188, ASTM D573, and GB/T 3512 for heat aging, ISO 1431-1 and ASTM D1149 for ozone exposure, ASTM D471, ISO 1817, and GB/T 1690 for fluid immersion, and ISO 23529 and ASTM D1349 for specimen conditioning. All testing should be performed by accredited third-party laboratories or qualified in-house facilities with calibrated equipment.

How does the reduction rate relate to seal service life?

By measuring reduction rates at several elevated temperatures and applying Arrhenius extrapolation, engineers can model property degradation at the actual operating temperature and predict the time at which tensile strength or elongation will fall below the minimum functional threshold. This supports evidence-based maintenance intervals and replacement schedules.