Insights for lifetime predictions of O-ring seals from five-year long-term aging tests

O-rings made of HNBR, EPDM and FKM were aged in the compressed and uncompressed state at 150 degrees C, 125 degrees C, 100 degrees C, 75 degrees C, 60 degrees C and 23 degrees C for aging times of up to five years. Hardness was measured and increased with aging time and temperature for HNBR and EPDM, but it remained practically constant for FKM. Indenter modulus measurements were performed on the lateral O-ring surface (that was free of DLO effects) to assess an influence of the compression during aging, but none was detected. The equilibrium compression set (CS) exhibited faster and stronger degradation than hardness and was used for lifetime predictions using the time-temperature superposition (TTS) principle. With an end-of-lifetime criterion of 70% CS, lifetimes of 4.5 years, 50 years and 526 years at 75 degrees C were estimated for HNBR, EPDM and FKM, respectively. The activation energies derived from an Arrhenius plot of the shift factors from the TTS were 85 kJ/mol, 99 kJ/mol and 78 kJ/mol for HNBR, EPDM and FKM, respectively, revealing that a higher activation energy does not necessarily mean that the material has a higher lifetime at lower temperatures. Furthermore, the measured lifetime of EPDM O-rings at 100 degrees C (5 years) was compared to that predicted on the basis of the lifetime at 150 degrees C as well as 125 degrees C using the corresponding shift factors. The error of the prediction was only +/- 4%. However, this precise prediction could only be achieved using the five-year long-term aging data. When using only data from aging times up to 0.5 years and 2 years, the lifetime of EPDM O-rings at 100 degrees C was underestimated by 31% and 22%, respectively. (c) 2020 Elsevier Ltd. All rights reserved.