Magnified section of a HNBR O-ring

Magnified section of a HNBR O-ring with a cord diameter of 10 mm and an inner diameter of 190 mm that was aged for 224 d at 150 °C. The surface exhibits many small and one large crack.

Source: BAM

BAM Division Safety of Storage Containers performs safety evaluations of containers for interim storage and final disposal of radioactive materials as applicable to license procedures under nuclear legislation. Elastomer seals are used in these containers for operating times of several decades. Therefore, aging behaviour and lifetime prediction of these components is a relevant research topic for the Division, which has been investigated for several years now. This resulted in extensive knowledge about the aging processes of different elastomer materials and how the lifetime of O-ring seals can be predicted. The present paper focuses on the analysis of O-ring failure mechanisms. Additionally, a method for determining an end-of-lifetime criterion is introduced. Parts of the results arose from a cooperation with the Royal Institute of Technology (KTH) Stockholm.

The paper comprises parts of the results from the extensive aging investigations. The experiments were not performed on materials which are actually in use in containers for radioactive material, but on commercial standard materials of the respective material classes. O-rings made of ethylene propylene diene rubber (EPDM) and hydrogenated nitrile butadiene rubber (HNBR) that were aged at 125 °C and at 150 °C for up to 1.5 years were investigated under static conditions, using both non-lubricated and lubricated seals. Changes of the material properties were analyzed with dynamic-mechanical analysis and permeability experiments, showing an increase of the glass transition temperature and a decrease of the oxygen permeability with increased aging exposure. Indenter modulus measurements were used to investigate diffusion-limited oxidation (DLO) effects. When DLO effects are involved, aging of the mating surfaces proceeds from the sides until the area of the seal face is completely hardened. It became clear that O-rings can remain leak-tight under static conditions even when the material properties have already degraded considerably, especially when adhesion effects (“sticking” of the seal to the flange) are encountered. Therefore, when a lubricant was applied as a separating layer between rubber and metal to prevent sticking, leakage occurred much earlier.

In order to determine an end-of-lifetime criterion for O-ring seals that is correlated to leakage rate as direct indicator of seal performance, but more conservative than the point of static seal failure, a modified leakage test involving a small and rapid partial decompression of the seal during the leakage rate measurement was introduced. This test led to an earlier failure of the investigated EPDM seal which represents an additional safety in case of possible small relative motions of the sealing system. When correlating the lifetime obtained from this test to other properties, compression set, compression stress relaxation, and elongation at break exhibited similar notable changes by about 85-90 % for this aging state. The method presented here can be adapted to different materials, geometries, and application conditions for determining a specific end-of-lifetime criterion for O-ring seals.

Analysis of O-Ring Seal Failure under Static Conditions and Determination of End-of-Lifetime Criterion
Anja Kömmling 1, Matthias Jaunich, Payam Pourmand, Dietmar Wolff and Mikael Hedenqvist
published in Polymers 2019, Volume 11, Issue 8, first page 1251
BAM, division Safety of Storage Containers