ASTM D395B Compression Set Testing: Procedure, Interpretation & Applications
What Is Compression Set Testing?
Compression set testing measures the permanent deformation retained by an elastomeric material after it has been compressed to a specified deflection for a specified time at a specified temperature. It quantifies the material’s failure to fully recover its original thickness after the compressive load is removed — characterising the loss of elastic memory resulting from viscous flow, crosslink rearrangement, and physical relaxation.
Compression set is a critical performance parameter for seals, gaskets, O-rings, and any elastomeric component whose function depends on maintaining a sealing or mechanical contact force throughout its service life.
ASTM D395 Methods
ASTM D395 — Standard Test Methods for Rubber Property — Compression Set — defines two methods:
Method A (Constant Force): The specimen is compressed under a defined constant force for the test duration. This method is less commonly used but applicable when the compressive load rather than deflection is the relevant service condition.
Method B (Constant Deflection — Most Common): The specimen is compressed to a specified percentage of its original thickness (typically 25%) using spacer bars of a defined height. Method B is by far the more widely specified method in industry standards and material specifications.
ASTM D395 Method B Test Procedure
Specimen Preparation
Standard specimens are cylindrical buttons: 29 mm diameter × 12.5 mm height (Type 1) or 13 mm diameter × 6.3 mm height (Type 2). Specimens are cut or moulded from the elastomeric material and conditioned at standard temperature (23°C ± 2°C, 50% ± 5% RH) for at least 24 hours before testing.
Specimen thickness (t₀) is measured to ±0.01 mm before compression.
Compression and Ageing
Two parallel steel plates sandwich the specimen. Spacer bars (height = t₀ × 0.75 for 25% compression) are inserted between the plates. Bolts compress the assembly until the plates contact the spacers — applying exactly 25% compression to the specimen.
The compressed assembly is placed in a circulating air oven at the specified test temperature (typically 70°C/22 hours, 100°C/22 hours, or 125°C/22 hours, as specified in the material standard or customer specification) for the defined test duration.
After ageing, the assembly is removed from the oven, and the bolts are loosened. The specimen is allowed to recover at room temperature for 30 minutes (±3 minutes).
Measurement and Calculation
Final specimen height (t₂) is measured. The compression set is calculated as:
Compression Set (%) = (t₀ − t₂) / (t₀ − ts) × 100
where ts is the spacer height (the compressed thickness). A value of 0% indicates perfect elastic recovery; 100% indicates no recovery (permanent set equal to applied deflection).
Typical Compression Set Values for Common Elastomers
Elastomer | Typical Compression Set (70°C/22h) |
Natural rubber (NR) | 10–25% |
EPDM | 20–40% |
Nitrile rubber (NBR) | 15–35% |
Silicone (VMQ) | 5–25% |
Fluoroelastomer (FKM) | 15–30% |
Thermoplastic polyurethane | 20–60% |
Why Compression Set Determines Seal Service Life
An elastomeric seal loaded between mating surfaces must maintain contact stress throughout its service life. As the compression set increases, the seal’s recoverable elastic deformation decreases — reducing contact stress, increasing the risk of leakage, and ultimately causing seal failure. Products with low compression set maintain sealing integrity longer and have superior service life in dynamic and static sealing applications.
Elevated Temperature Compression Set
A compression set is typically tested at the maximum service temperature to characterise worst-case seal performance. O-rings for engine oil sealing may be tested at 150 °C for 22 hours; high-temperature static seals for industrial equipment at 200°C; silicone seals at 200°C or higher. Elevated temperature testing reflects the accelerating effect of heat on stress relaxation and irreversible molecular rearrangements.
Conclusion
Compression set testing — conducted per ASTM D395, Methods A and B — provides a critical evaluation of an elastomer’s ability to recover after prolonged compressive deformation under defined temperature and time conditions. It directly reflects a material’s elastic memory and its capability to maintain sealing force in applications such as gaskets, O-rings, and seals. By quantifying permanent deformation, this test enables accurate prediction of long-term sealing performance and service life. Selecting appropriate test conditions based on application temperature and load requirements is essential to ensure reliable material performance, making the testing strategy as important as the performance results themselves.
Why Choose Infinita Lab for ASTM D395 Compression Set Testing?
Infinita Lab provides ASTM D395 Method B compression set testing at ambient and elevated temperatures through our nationwide accredited rubber testing laboratory network, supporting elastomer seal qualification and rubber compound development programmes.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What temperature and time conditions are most commonly used for ASTM D395 Method B testing? The most common conditions are 70°C for 22 hours (general rubber QC), 100°C for 22 hours (elevated temperature service), and 125°C for 22 hours (high-temperature applications). The test temperature is selected to represent the maximum service temperature, or as specified in the applicable material specification.
Why is 25% compression the standard deflection in ASTM D395 Method B? 25% compression represents a typical design compression for O-ring and gasket seals — sufficient to provide adequate initial sealing contact stress while not causing excessive stress relaxation. Some specifications use 35% or higher deflection to represent more severely compressed seal configurations.
How does crosslink density affect compression set in vulcanised rubber? Higher crosslink density generally reduces compression set by increasing the network's ability to store elastic energy and recover after compression. However, the type of crosslinks also matters — sulphur monosulfidic crosslinks (efficient cure) have better thermal stability and lower compression set than polysulfidic crosslinks, which rearrange more readily under heat.
Can compression set be used to assess rubber compound ageing? Yes. Comparing compression set before and after accelerated heat ageing or fluid immersion quantifies the effect of service environment on seal performance. Significant increases in compression set after ageing predict reduced seal service life in the modelled service condition.
Is ASTM D395 Method B applicable to thermoplastic elastomers (TPEs)? Yes, ASTM D395 Method B is applied to TPEs using the same procedure as for vulcanised rubber. TPEs generally show higher compression set than vulcanised elastomers due to their thermoreversible crosslinks (physical crosslinks that can partially flow under sustained load) compared to the covalent crosslinks in vulcanised rubber.
