Compression Stress Relaxation: Predicting Long-Term Sealing Performance of Elastomeric Materials

A rubber seal compressed between two flanges generates contact pressure that prevents fluid or gas leakage. Over time, however, the contact pressure inevitably decreases — even if the joint geometry remains unchanged — because the elastomer slowly relaxes under sustained compression. This phenomenon, called compression stress relaxation (CSR), is distinct from compression set and is arguably the more practically important property for predicting long-term sealing performance. In the rubber & sealing industry, understanding and measuring CSR is essential for designing seals that maintain adequate contact pressure throughout their intended service life.

Understanding Compression Stress Relaxation

When an elastomer is compressed and held at constant strain, the stress it exerts on the constraining surfaces decreases progressively over time. This occurs through two distinct physical mechanisms:

Physical relaxation — viscoelastic chain rearrangement redistributes molecular stresses without permanent deformation. Physical relaxation is temperature-dependent and largely reversible upon temperature reduction.

Chemical relaxation — oxidative chain scission, crosslink breakdown, or crosslink formation (reversion) — permanently alters the elastomer network, producing irreversible loss of elastic recovery and sealing force. Chemical relaxation is the dominant long-term mechanism at elevated service temperatures.

The ratio of remaining force to initial force — the force retention fraction — decreases with time and temperature. It must remain above the minimum required sealing force throughout the design service life.

Distinction from Compression Set

Compression set (ASTM D395) and compression stress relaxation (ASTM D6147) measure related but fundamentally different aspects of elastomer behavior:

Compression set — measures the permanent dimensional change after releasing the compression load. It characterizes material recovery after load removal — relevant for applications where the seal must re-expand after a pressure cycle.

Compression stress relaxation — measures the change in contact force while the compression is maintained. It characterizes the retention of healing force under sustained load — directly predicting whether the joint will remain leak-free over its service life.

A material may show low compression set (good recovery when released) but high stress relaxation (poor sealing force retention under load) — the two properties are not equivalent.

Standard Test Methods for Compression Stress Relaxation

ASTM D6147 — Vulcanized Rubber and Thermoplastic Elastomers

ASTM D6147 is the primary standard for compression stress relaxation testing of rubber and TPE materials. Specimens are compressed to a defined strain (typically 25% compression) between parallel plates using a calibrated spring assembly or rigid spacer with instrumented force measurement. The initial force and the force at defined intervals are recorded, and stress relaxation is reported as the fraction of the initial force remaining at each measurement time.

Test method variants:

• Method A (Force Relaxation) — continuous or periodic force measurement during compression, tracking the actual sealing force profile over time
• Method B (Stress Relaxation Ratio) — periodic measurement of residual force expressed as a percentage of initial force at defined time intervals

ISO 3384-1 — Rubber: Determination of Stress Relaxation in Compression

ISO 3384-1 specifies compression stress-relaxation testing under constant-strain conditions — the international equivalent of ASTM D6147, with a methodology similar in fundamental terms but with different measurement intervals and reporting conventions.

ISO 3384-2 — Alternating Strain Conditions

ISO 3384-2 addresses stress relaxation under alternating compression cycles, simulating real-world conditions in which temperature fluctuations alter the effective joint compression due to differential thermal expansion. This test is particularly relevant for automotive and aerospace applications where large temperature excursions are the norm.

Factors Governing Compression Stress Relaxation

Elastomer Type and Formulation

Elastomer chemistry profoundly determines CSR performance at elevated temperatures:

Silicone (VMQ) — exceptional CSR resistance at elevated temperatures (150–200°C); physical relaxation dominates over chemical relaxation, producing relatively stable long-term force retention Fluoroelastomers (FKM) — best CSR performance of any common elastomer at elevated temperature combined with chemical exposure; preferred for demanding sealing applications EPDM — good CSR resistance in water and steam service; used extensively for automotive cooling system seals and industrial water service gaskets Nitrile (NBR) — acceptable CSR for oil service at moderate temperatures; performance degrades rapidly above 100°C

Temperature

Compression stress relaxation rate increases exponentially with temperature — following Arrhenius kinetics for the chemical relaxation mechanism. Accelerated CSR testing at elevated temperature enables the prediction of long-term force retention at service temperature using time-temperature superposition.

Compression Level

The initial compression level (strain) affects both the absolute initial force and the subsequent relaxation behavior. Seals designed for minimum volume in groove geometries should be tested at realistic compression percentages—typically 15–30% for static O-ring applications.

Engineering Applications of CSR Data

Bolted Flange Joint Design

Pipeline flanges, pressure vessel closures, and heat exchanger covers rely on gasket sealing force for leak tightness. ASME PCC-1 and EN 1591 flange joint design codes require that adequate residual gasket stress be maintained throughout the joint’s service life, with gasket stress relaxation as a primary design consideration. CSR data at service temperature is the direct input for residual gasket stress calculations.

Automotive Sealing Systems

Valve cover gaskets, oil pan gaskets, and engine coolant seals experience sustained compression at operating temperatures for engine service intervals extending to 150,000+ miles. CSR data at 120–150°C service temperature, combined with accelerated aging studies, validates that sealing performance is maintained between service intervals.

Medical Device Seals

Medical devices — drug delivery systems, implantable devices, and diagnostic equipment — require long-term sealing integrity under defined compression. CSR data at body temperature (37°C) or sterilization temperatures (121°C for autoclave) validates seal performance over the device service life or sterilization cycle count.

Conclusion

Compression stress relaxation testing directly predicts whether a seal will maintain adequate contact force over its service life — a capability compression set testing cannot provide. For bolted flange joints, automotive sealing systems, and medical devices, CSR data per ASTM D6147 and ISO 3384 gives engineers the time- and temperature-dependent force retention values needed to select the right elastomer, validate seal designs, and prevent in-service leakage failures.

Why Choose Infinita Lab for Compression Stress Relaxation Testing of Rubber Seals and Elastomers?

Infinita Lab provides compression stress relaxation testing per ASTM D6147 and ISO 3384 for rubber compounds, elastomeric seals, gaskets, and thermoplastic elastomers — supporting the rubber & sealing industry with sealing force retention data, Arrhenius-based service life prediction, accelerated testing programs, and immersed CSR evaluation in service fluids. Our rubber testing specialists combine mechanical precision with materials expertise to deliver CSR data that enables confident seal specification and service life qualification. Contact Infinita Lab at infinitalab.com to discuss compression stress relaxation testing for your sealing application.

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