Compression Set Test (ASTM D395 / ISO 815)
Also known as: D395 test · ISO 815 · compression set measurement
The compression set test (ASTM D395B / ISO 815) compresses a rubber disc to 25% deflection, holds it at elevated temperature for 70 hours, then measures permanent deformation — result expressed as % of original deflection.
Technical Detail
Test procedure per ASTM D395 Method B (constant deflection): (1) Measure original disc thickness t0 (typically 12.5 mm standard disc or O-ring cross-section). (2) Compress to t1 = t0 × 0.75 (25% compression) using calibrated spacers. (3) Place in oven at specified temperature: 70°C/70h for NBR; 150°C/70h for EPDM; 200°C/70h for FKM. (4) Remove from oven, release compression, rest 30 min at 23°C. (5) Measure recovered thickness tr. (6) CS% = (t0 – tr) / (t0 – t1) × 100. Acceptance criteria for O-ring seals: dynamic reciprocating ≤20%; static elastomeric seal ≤25%; static gasket ≤35%. ISO 815-1 (constant deflection) and ISO 815-2 (constant load) give slightly different results — specify which method when comparing data sheets. High CS% = accelerated seal leakage in service due to loss of sealing force as the O-ring cannot recover its original cross-section after long-term compression.
Reference Standards
- ASTM D395 Method B
- ISO 815-1:2019
- ISO 815-2:2019
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