O-rings, for example, are one of the most common fluid power seals. O-rings are typically installed in a groove that’s machined or molded in one of the surfaces to be sealed. Their rubber-like properties let the devices compensate for dimensional variations in the mating parts. The O-ring prevents leakage until its rubber-like properties are retained and there is sufficient pressure between the shaft and mating surface. When a constant strain is applied to a rubber sample, the counterforce exerted by the test piece on the fixtures decreases with time, this behavior is called “stress relaxation”. O-ring is most often exposed to constant strain condition in which stress relaxation of rubber compound governs its service life.

To predict the service life of seal material rubber, such as an O-ring,  scientists refer to ISO 11346 “Rubber, vulcanized or thermoplastic – Estimation of life-time and maximum temperature of use.” The WLF principle (Williams, Landel, Ferry) and Arrhenius concept are both described in this standard. It has been well documented that the temperature dependency of physical relaxation processes corelates well with the WLF equation. At the same time, thermal reactions due to elevated temperature results in crosslink modification and other degrading phenomenon conforms to the Arrhenius equation. Sound like a lot of work? Actually, it’s not if you’re conducting relaxation testing on Alpha’s Premier™ RPA.