Magnified section of a HNBR O-ring with a cord diameter of 10 mm and an inner diameter of 190 mm aged for 224 days at 150 °C. The surface exhibits many small and one large crack.
Source: BAM
The division Safety of Storage Containers at BAM performs scientific safety evaluation of containers for radioactive materials as applicable to license procedures under nuclear legislation. Elastomer seals are used in these containers for operating times of up to several decades.
In this paper, insights and results from independent aging tests at BAM over five years are presented and discussed, e.g. with regard to the reliability of lifetime predictions. The experiments were performed exemplarily on commercial O-rings based on ethylene propylene diene rubber (EPDM), hydrated acrylonitrile butadiene rubber (HNBR) and fluorocarbon rubber (FKM) that were aged at ca. 23 °C, 60 °C, 75 °C, 100 °C, 125 °C and 150 °C. The O-rings were aged both in the uncompressed state and in the compressed state with a compression ratio of about 25 % corresponding to service conditions. By means of hardness and density measurements, the influence of the compression was investigated. However, it became clear that some of these measurements were distorted by diffusion-limited oxidation (DLO) effects which cause an oxidation gradient in the sample. For this reason, a special hardness measurement was performed on the lateral O-ring surface where no DLO effects are present. The available results on EPDM and HNBR samples showed no significant influence of the compression during aging.
Furthermore, the compression set (CS) was determined. CS is a measure for the resilience and elastic recovery potential of elastomeric samples after aging in the deformed (i.e. compressed) state, with CS = 100 % corresponding to no recovery at all. The CS values were chosen as basis for lifetime predictions using the time-temperature superposition (TTS) approach. Using TTS, lifetime predictions for lower temperatures are possible by shifting data determined at higher temperatures on the time axis. With an end-of-lifetime criterion of 70 % CS, lifetimes (at 75 °C) of 4.5 years, 50 years and 526 years were determined for the HNBR, EPDM and FKM O-rings, respectively.
For checking the accuracy of the extrapolation, calculated lifetimes (5.2 years based on 150 °C data, 4.8 years based on 125 °C data) and actual lifetime (5 years) of the EPDM O-ring at 100 °C were compared, showing very good accordance with differences of only ± 4 %. However, this high precision of the prediction could only be achieved when using the aging data from more than three years for the TTS approach.
Insights for lifetime predictions of O-ring seals from five-year long-term aging tests
Anja Kömmling, Matthias Jaunich, Milan Goral, Dietmar Wolff
Polymer Degradation and Stability, Vol. 179, issue 38, page 109278 et.seq., 2020
BAM Division Safety of Storage Containers