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Compression Stress Relaxation (CSR) measures how well an elastomer maintains sealing force over time when held in compression between two rigid surfaces. In a typical test, a specimen - representative of a seal such as an O-ring - is compressed in a precision jig at a defined strain and temperature, while the decay in force is continuously monitored using dedicated CSR instrumentation.

CSR is widely used in aerospace, automotive, and industrial sealing applications where long-term reliability is critical. It enables engineers to compare materials, understand ageing behaviour, and predict service life – ensuring seals remain effective under prolonged compression and harsh environmental conditions.

Unlike the compression set test - which is a simpler end-point measurement of how much a sample fails to recover after compression - CSR continuously monitors force decay throughout the test period, providing much richer data on long-term sealing performance.