Compression set asks how much deformation remains after a specified compression and recovery procedure. Stress relaxation in compression asks how the force changes while a specified deformation is maintained. Both can matter to a seal, but a result for one does not supply the result for the other.
Choose the evaluation around the design question. If you need dimensional recovery after unloading, request the relevant compression-set evidence. If retained load over an exposure is important, define the force-decay requirement. Keep the assembly's functional acceptance test alongside these material or component measurements.
The measurement timeline is different
| Question | Compression set | Stress relaxation in compression |
|---|---|---|
| What is observed? | Remaining deformation after the specified release and recovery | Force change while the specified deformation is maintained |
| What must accompany the value? | Specimen, compression, exposure, recovery and measurement conditions | Specimen, deformation, exposure, reference force and measurement times |
| What should not be inferred? | The complete history of force in the loaded joint | Dimensional recovery after unloading or a universal leakage result |
ASTM D395 addresses compression-set methods. ASTM D6147 addresses force decay in compression. Use the method and edition specified for the project; do not relabel a test because both involve a compressed specimen.
Read a force-retention percentage carefully
If a report expresses retained force as a percentage, identify the reference force and the point at which it was recorded. A simple normalization is the force at a stated time divided by the stated reference force, multiplied by one hundred. The result is only interpretable with those definitions and the loading history.
Request the force-time record where it is needed for the decision. Keep temperature changes, interruptions and measurement times visible. Two percentages based on different reference times or exposures are not a controlled material comparison.
Use comparable specimens and conditions
Identify the compound, cure and conditioning state, specimen geometry and production lot. Record the compression arrangement, test medium, temperature and duration. If results come from finished parts rather than standard specimens, retain the geometry and fixture description.
For candidates tested under different conditions, decide which differences need a matched trial. A family label such as FKM or EPDM cannot fill missing method information. Nor can an isolated percentage establish how a different section or joint will behave.
Connect the indicators to the seal design
A seal's installed geometry and service history remain relevant when reviewing either measurement. Bring the mating-part drawing, closed gap, tolerances, installation method and operating exposure into the evaluation. If leakage is the acceptance criterion, define that assembly test explicitly.
When investigating a used seal, preserve photographs, lot identification and the time since removal. Review the compression-set investigation guide for the evidence needed before assigning a cause. A flattened component is an observation; it is not a complete force-decay history or root-cause finding.
Specify the decision before requesting more tests
Tell RubberQ which question the report must answer and what acceptance requirement applies. Discuss whether the sample needs a material test, a component test or an assembly trial, and how the result will be connected to the approved compound and drawing.
Send the seal and evaluation requirements with existing reports. Use the testing discussion to agree methods, conditions and records before comparing numerical limits.