Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.
Original content can be found on Parker’s Website and was written by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division.
In Part 1 of this series, the theory behind Compressive Stress Relaxation (CSR) testing was discussed, as well as a brief discussion of the fixtures used to measure it. In Part 2, we will explore what to look for in a CSR result. A significant understanding of how a rubber seal material responds to a particular environment can be gleaned if one knows what to look for in a compressive stress relaxation curve.
The first and most basic point of understanding is the endpoint. Does the material continue to maintain contact pressure throughout the test, or does it fall to zero (below the detectable limit of the load cell) before the end of the test? While there is no definitive correlation from residual load force to the onset of leakage, it should be intuitive that a material that completely relaxes and loses all contact force is likely to leak in the application. Anecdotally, multiple customers have reported that the load force must drop to very close to zero for leakage to occur in their particular test apparatus. While this is good guidance, these anecdotal reports should not be taken as a definitive answer that applies in all circumstances.
Specifications are often written such that a minimum of 10% of the initial contact load force must remain for a passing result. In practice, there is nothing special about 10%. This is a semi-arbitrary value that ensures a material continues to apply some non-zero load force to the mating surfaces, with some safety factor to ensure that it does so even after all normal test variations are considered. In practice, this appears to be a conservative limit, there is nothing magical about the 10% number.
The loss of compressive load force can be broken down into three different types of phenomena, each with its own time frame. All rubber materials relax viscoelastically when initially compressed, and this loss stabilizes within the first 24 hours. That initial drop seldom has much direct impact on real-world applications. However, in the specific case of an assembly having neither a compression limiter nor solid-to-solid contact, meaning the assembly torque of the fasteners is controlled solely by compression of the seal, this will be observed as “torque fade” if the fastener torque is rechecked a day or two after assembly. In such a case, Gallagher’s partner, Parker, recommends against retorquing the fasteners unless leakage is observed as this retorquing can easily result in damage to the seal from excessive compression. Continue reading How Much Do You Know About Compressive Stress Relaxation? CSR Part 2