Compression Stress Relaxation Testing: Principles, Methods, and Applications for Elastomeric Sealing

Compression stress relaxation testing is a critical method for evaluating the long-term sealing performance of rubber and elastomeric materials by measuring the decay of counterforce in a specimen held under constant compressive strain. Governed by internationally recognized standards such as ISO 3384 and ASTM D6147, this technique provides essential data for predicting seal service life in automotive, aerospace, and industrial applications. This article explains the testing principles, specimen types, procedures, and practical applications of compression stress relaxation measurement.

Technical Overview

Compression stress relaxation (CSR) testing quantifies the decay of the reaction force generated by an elastomeric specimen that is compressed to a fixed deflection and held in that state over an extended period. When a rubber seal is installed, its initial compression generates a sealing force against the mating surfaces. Over time, molecular rearrangement, physical relaxation, and chemical degradation within the polymer network cause this counterforce to decrease. Once the residual sealing force falls below the pressure of the fluid or gas being contained, leakage occurs.

Unlike compression set testing, which measures permanent geometric changes after releasing the compression, stress relaxation measurement directly tracks the functional property that matters most for sealing: the retained contact force. This makes CSR testing one of the most predictive accelerated test methods available for seal lifetime assessment.

The primary international standards governing this measurement include ISO 3384 (vulcanized and thermoplastic rubbers — determination of stress relaxation in compression), ASTM D6147 (stress relaxation in compression for vulcanized rubber and thermoplastic elastomers), GB/T 1685 for the Chinese national methodology, and ASTM F38 for gasket materials. Test results are typically expressed as the percentage of retained force relative to the initial counterforce measured at the start of the test.

Testing Samples

Cylindrical Discs

Standard cylindrical specimens, typically 13 mm in diameter and 6.3 mm in height, are the most common geometry for compression stress relaxation testing. These discs are molded directly to the specified dimensions from the compound under evaluation and provide consistent, reproducible contact surfaces for force measurement between parallel platens.

O-Rings and Ring Specimens

Actual O-ring cross-sections can be tested directly using grooved fixtures, offering the significant advantage of evaluating the manufactured product rather than a laboratory-molded analog. This approach captures the effects of production processes, surface treatments, and real-world geometry on relaxation behavior. Standard cross-section diameters such as 1.78 mm, 2.62 mm, and 3.53 mm are frequently tested.

Custom Seal Profiles and Gaskets

Rectangular gaskets, custom extruded profiles, and bonded seals can also be evaluated using appropriately designed fixtures. Testing the end-use configuration provides the most realistic assessment of in-service performance for critical sealing applications.

Testing Items

Compression stress relaxation measurement programs typically determine the following parameters:

• Initial counterforce (F0) at the specified compression strain, measured shortly after test assembly
• Residual counterforce (Ft) at defined time intervals such as 24 h, 72 h, 168 h, 500 h, and 1000 h
• Force retention ratio (Ft/F0) expressed as a percentage, the primary reported result
• Relaxation rate under different temperature conditions, including standard laboratory temperature of 23 °C and elevated temperatures of 70 °C, 100 °C, 125 °C, or 150 °C
• Behavior after exposure to fluids, fuels, oils, or aggressive chemicals during the relaxation period
• Comparison of continuous versus intermittent compression methods where required

Testing Methods

Method A: Continuous Compression

In the continuous method defined by ISO 3384 and ASTM D6147, the specimen is compressed between two parallel plates to a predetermined deflection, typically 15% to 30% of the original thickness, and remains in the compressed state for the entire test duration. The counterforce is measured at defined intervals using either an integrated low-friction load cell system or by transferring the compressed assembly to a force-measuring instrument. This method simulates seals that remain permanently deformed in service, such as static O-ring seals and pipe gaskets.

Method B: Intermittent Compression

In the intermittent method, the specimen is compressed periodically rather than continuously. The force is measured each time the specimen is re-compressed to the original deflection after intervals of rest. This approach better represents dynamic or cyclic loading conditions and can reveal additional mechanical degradation mechanisms associated with repeated deformation.

Test Procedure Summary

Specimens are first conditioned at standard laboratory temperature and humidity. The thickness is measured at multiple points to establish the reference dimension. The sample is then compressed to the target deflection between clean, rigid platens, and the initial force is recorded after a short stabilization period. For elevated-temperature tests, the assembled fixture is placed in a temperature-controlled oven, and force readings are taken at the scheduled intervals without disturbing the compression state. In fluid immersion studies, the compression takes place within the test medium to evaluate chemical aging effects simultaneously with physical relaxation.

Data Expression

The relaxation behavior is calculated and reported as the ratio of the counterforce at time t to the initial counterforce, expressed as a percentage. Semi-logarithmic plots of force retention versus time often allow extrapolation to longer service periods, supporting lifetime predictions based on Arrhenius-type temperature acceleration models.

Applications

Compression stress relaxation data directly support design, qualification, and quality control decisions across many industries:

• Automotive engineering: cylinder head gaskets, fuel system seals, brake components, and engine compartment seals exposed to sustained heat and fluid contact
• Aerospace: static seals in hydraulic, fuel, and pneumatic systems where long-term reliability under constant compression is mission-critical
• Oil and gas: downhole sealing elements and valve seals subjected to high temperature, pressure, and hydrocarbon exposure
• Plumbing and piping: rubber gaskets in pipe joints and water supply systems requiring decades of maintained sealing force
• Electronics and enclosures: environmental sealing gaskets protecting sensitive equipment from dust and moisture ingress
• Material development: comparing elastomer formulations, cure systems, and filler packages for improved long-term retention performance

Accredited third-party laboratories routinely perform compression stress relaxation testing to support product certification, failure analysis, supplier qualification, and validation against customer specifications or regulatory requirements.

FAQ

What is the difference between compression set and compression stress relaxation?

Compression set measures the permanent geometric deformation remaining after a compressed specimen is released, while compression stress relaxation measures the decay of the counterforce while the specimen remains compressed. Stress relaxation is generally considered the more direct indicator of sealing capability because a seal functions under continuous compression in service.

What compression levels are typically used?

Most standards and specifications call for compressions between 15% and 30% of the specimen thickness. O-ring testing frequently uses approximately 25% compression, which reflects typical gland design values for static sealing applications.

How long does a compression stress relaxation test take?

Test durations vary from short screening tests of 24 to 168 hours to long-term studies of 1000 hours or more. Elevated temperatures accelerate relaxation, enabling meaningful comparisons within practical laboratory timeframes, and results can be extrapolated for service life estimation.

Which standards are most commonly specified?

ISO 3384 and ASTM D6147 are the most widely referenced international standards, with GB/T 1685 commonly specified in the Chinese market and ASTM F38 used for gasket materials. The choice depends on the product specification, industry sector, and regional requirements.

Why does the sealing force decrease over time?

The force decay results from a combination of physical relaxation, where polymer chains and filler structures rearrange under constant strain, and chemical degradation, including chain scission and additional or reduced crosslinking driven by temperature, oxygen, and fluid exposure. The relative contribution of each mechanism depends on the compound chemistry and operating environment.