Compression Stress Relaxation Testing: Principles, Standards, Methods, and Applications

Compression stress relaxation testing is a critical laboratory method used to evaluate how elastomeric seals, gaskets, and rubber components lose sealing force when held under constant compressive strain over time. By measuring counterforce decay under controlled temperature and duration conditions, this test predicts long-term sealing performance and material durability in accordance with international standards such as ISO 3384 and ASTM D6147. This article provides a comprehensive technical overview of compression stress relaxation testing, including sample types, testing items, methodologies, applications, and frequently asked questions.

Technical Overview

Stress relaxation is the progressive decrease in stress exhibited by a viscoelastic material when it is held at a constant deformation over time. In a compression stress relaxation test, an elastomeric specimen is compressed to a fixed strain—typically 15% to 30% of its original thickness—and the resulting counterforce is monitored continuously or intermittently as a function of time and temperature. Unlike compression set testing, which measures permanent geometric change after release, stress relaxation directly quantifies the decay of sealing force, making it the most relevant indicator of long-term leak prevention in static sealing applications.

During the test, molecular chain rearrangement, physical relaxation, chemical degradation (such as oxidation, chain scission, or additional crosslinking), and plasticizer loss all contribute to the reduction in contact pressure. When the residual sealing force falls below the minimum pressure required to prevent fluid leakage, seal failure occurs. Compression stress relaxation testing therefore enables engineers to establish service life predictions, compare material formulations, qualify suppliers, and validate designs against automotive, aerospace, industrial, and energy sector requirements. Primary reference standards include ISO 3384, ASTM D6147, ASTM F38, and GB/T 1685.

Testing Samples

O-Rings and Toroidal Seals

O-rings are the most widely tested specimen geometry because they represent real-world sealing elements. Standard cross-sections such as 1.78 mm, 2.62 mm, 3.53 mm, and 5.33 mm are compressed between flat parallel platens to a defined squeeze percentage. Testing actual O-rings provides the most representative data on sealing force retention in grooved assemblies.

Standard Compression Buttons and Discs

Cylindrical specimens, such as the ISO Type A button (13 mm diameter × 6.3 mm height) or Type B disc (29 mm diameter × 12.5 mm height), are molded specifically for relaxation testing. These geometries offer well-defined strain fields, excellent repeatability, and direct comparability with historical material databases.

Gasket and Sheet Materials

Flat gasket materials—including compressed fiber sheets, PTFE-based laminates, flexible graphite, and rubber-bonded composites—are tested per ASTM F38 using annular or square specimens. Load decay data from these tests inform bolt torque retention calculations and flange sealing design.

Custom Molded and Extruded Components

Profile seals, valve stems, diaphragm beads, medical device closures, battery pack seals, and other application-specific parts can be evaluated in purpose-built fixtures that replicate the actual compression conditions, wall geometry, and hardware surface finish of the end-use assembly.

Testing Items

Compression stress relaxation testing programs typically include the following measurable parameters and reporting items:

• Initial counterforce at the specified compression strain, measured after a defined settling period
• Residual counterforce at each measurement interval (e.g., 1 h, 24 h, 72 h, 168 h, 336 h, 500 h, 1000 h)
• Stress relaxation rate, expressed as the percentage of counterforce loss relative to the initial force
• Retained sealing force percentage, calculated as residual force divided by initial force
• Relaxation master curves and time-temperature superposition extrapolations for lifetime prediction
• Comparison of continuous (static) versus intermittent (cyclic re-loading) relaxation behavior
• Effects of test media, such as immersion in oils, fuels, coolants, or water during relaxation
• Temperature-dependent relaxation behavior, including low-temperature crystallization effects and thermal aging degradation

Testing Methods

Two principal methodologies are defined by ISO 3384 and ASTM D6147, and the selection depends on the data required and the equipment available.

Method A: Continuous Stress Relaxation

In the continuous method, the specimen is compressed to the target strain and remains in the fixture for the entire test duration while the counterforce is monitored in real time using calibrated load cells. This approach captures the full relaxation curve from the earliest seconds of decay onward and is ideal for studying physical relaxation mechanisms and generating high-resolution data for material modeling. Continuous rigs allow multiple test stations held in temperature-controlled ovens or environmental chambers.

Method B: Intermittent Stress Relaxation

In the intermittent method, the specimen is aged in a compressed state within an oven, then periodically removed and re-loaded into a force-measurement device to record the instantaneous counterforce. Between measurements, the sample returns to the aging environment. This method is well suited to long-duration aging studies, chemical degradation assessment, and multi-specimen screening, although it cannot capture short-time relaxation behavior.

Standard Test Conditions

Common test parameters include compression strains of 15%, 20%, or 25%; test temperatures of 23 °C, 70 °C, 100 °C, 125 °C, 150 °C, or 175 °C; low-temperature conditioning at 0 °C to −40 °C for crystallization studies; and durations from 24 hours to 1000 hours or longer. Surface finish of the platons, lubrication state, and pre-loading history must be controlled because they strongly influence measured counterforce. All measurements should be performed by accredited third-party laboratories using traceably calibrated force transducers to ensure data integrity and comparability.

Applications

Compression stress relaxation data directly support engineering decisions across many industries:

• Automotive engineering: cylinder head gaskets, valve cover seals, fuel system O-rings, brake components, cooling circuit seals, and EV battery enclosure gaskets
• Aerospace: hydraulic and pneumatic seals, door and window seals, fuel tank sealing compounds, and qualification to aerospace material specifications
• Oil and gas: downhole sealing elements, valve seats, flange gaskets, and rapid gas decompression resistant elastomers
• Plumbing and infrastructure: pipe connection seals, water meter gaskets, and pressure pipe joint rings qualified under EN 681 material classes
• Electronics and consumer goods: waterproof enclosure gaskets, connector seals, and display bonding cushions
• Medical devices: syringe stoppers, infusion pump seals, and sterilization-resistant closure systems
• Material development: comparing polymer families such as FKM, EPDM, NBR, silicone, and HNBR, and optimizing cure systems and filler loading
• Quality control: incoming material acceptance, batch-to-batch consistency verification, and supplier qualification programs

FAQ

What is the difference between stress relaxation and compression set?

Compression set measures the permanent dimensional change of a specimen after compression and release, while stress relaxation measures the loss of counterforce while the specimen remains compressed. A seal may show excellent compression set values yet still relax severely and leak; therefore, stress relaxation is generally the more direct predictor of sealing performance in service.

Which standards govern compression stress relaxation testing?

The principal standards are ISO 3384 (vulcanized and thermoplastic rubbers), ASTM D6147 (vulcanized rubber and thermoplastic elastomers), ASTM F38 (gasket materials), and the Chinese national standard GB/T 1685. Aerospace and automotive specifications frequently invoke these base methods with additional conditioning or reporting requirements.

What compression strain should be used?

The strain should replicate the application whenever possible. Common defaults are 15% to 25% compression. Lower strains approximate large static seals, while higher strains reproduce tightly squeezed O-rings. Reporting must always state the strain, since relaxation rate increases significantly with deformation level.

How long does a typical test take?

Screening tests at 24 h, 72 h, and 168 h are common for quality control, while qualification and lifetime prediction studies typically run 500 h to 1000 h or longer. Time-temperature superposition can extrapolate room-temperature behavior spanning years from shorter elevated-temperature experiments.

Can tests be performed in fluid media?

Yes. Specimens can be immersed in oils, fuels, coolants, water, or custom chemicals during relaxation testing to evaluate the combined effects of mechanical strain and chemical swelling or extraction, which is essential for fuel system and hydraulic sealing applications.

What sample quantities are required?

Best practice calls for at least three replicate specimens per condition to assess scatter, particularly for O-ring tests where mold parting lines and diameter tolerances influence results. Accredited third-party laboratories can advise on the minimum specimen count needed for statistically valid conclusions at each temperature and duration.