Compression set and fatigue - why a cushion ”sinks”
TechnicalJune 2, 2026
Compression set and dynamic fatigue are the two tests that predict product lifetime. What the TDS numbers mean in real use.
When a seat looks "sat through" after two years, the cause is almost never hardness but two other data-sheet quantities: compression set and dynamic fatigue.
Compression set
Tested to ISO 1856: the sample is compressed to 50% of its height, held for 22 hours at 70 °C, then the unrecovered height is measured. The result is the percentage of height lost.
| Compression set | Rating |
|---|---|
| below 3% | excellent - premium seating and mattresses |
| 3–5% | good - standard use |
| 5–8% | acceptable for occasionally loaded parts |
| above 8% | fillings and single-use applications only |
Dynamic fatigue
Testing to ISO 3385 simulates years of use: 80,000 load cycles. Height loss and hardness loss are measured afterwards. A foam with low hardness loss keeps supporting even when it looks slightly hollowed - the reverse is worse: a product that keeps its height but loses 30% of its hardness no longer does its job.
What drives these numbers
- Density is the strongest factor: more material in the cell walls means slower yielding.
- Foam family: HR grades have systematically lower set than conventional foams of the same density.
- Operating conditions: heat and humidity (a car cabin in summer) accelerate fatigue - for such uses ask for humid-aging data.
Practical consequences
- Size the product so working compression stays moderate - foam constantly working at 70% compression ages many times faster.
- For long warranties, go up in density, not hardness.
- State the expected product lifetime in your inquiry - it is the piece of data that directly selects the grade.
Have a specific case?
Send dimensions, quantity and intended use - we propose a foam grade and send a quote or sample.
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