Tag Archives: compressive stress relaxation

How Much Do You Know About Compressive Stress Relaxation? CSR Part 1

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.

black o-ringsCompressive Stress Relaxation (CSR) is a means of estimating the service life of a rubber seal over an extended period of time. As such, it can be thought of as the big brother of compression set testing. Rather than measuring the permanent loss of thickness of a compressed rubber specimen as is done in the compression set, CSR testing directly measures the load force generated by a compressed specimen and how it drops over time. In part 1 of our blog series, we will explore the theory of CSR testing, common test methods, and how CSR differs from compression set testing.

Theory of Compressive Stress Relaxation Testing

To understand the value of CSR testing and how it differs from compression set testing, it is helpful to return to the basic theory of how a rubber seal functions. In a standard compressed seal design, a rubber seal is deformed between two parallel surfaces to roughly 75% of its original thickness. Because the material is elastic in nature, the seal pushes back against the mating surfaces, and this contact force prevents fluid flow past the seal, thus achieving a leak-free joint. Over time, the material will slowly (or perhaps not so slowly) relax. The amount of force with which the seal pushes against the mating surfaces will drop, and the seal will become permanently deformed into the compressed shape. In compression set testing, the residual thickness of the specimen is measured, and it is assumed that this residual thickness is valid proxy for the amount of residual load force generated by the compressed seal. In CSR testing, the residual load force is measured directly.

In practice, compressive stress relaxation results are typically presented very differently from compression set results. In CSR testing, it is common to see multiple time intervals over a long period of time (3,000 hours or more of testing), thus allowing a curve to be created (see Figure 1). In practice, however, specifications are written such that only the final data point has pass/fail limits. In compression set testing, it is common to see a single data point requirement with a single pass/fail limit. Multiple compression set tests can be performed to create a curve, but this is almost always done for research purposes rather than for specification requirements. In most cases, compounds that excel in compression set resistance also demonstrate good retention of compressive load force over time. However, there are exceptions.

Figure 1: Typical CSR curve. These results display a fluorocarbon seal material immersed in engine oil at 150°C.

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How to Read a Rubber Test Report

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.
Original content can be found on Parker’s Blog.

4 Most Common Rubber Test Report Misunderstandings

4 Most Common Rubber Test Report MisunderstandingsWe’ve all done it at least once: looked at a rubber test report, read the numbers on it, and come up with exactly the wrong conclusion. Pass / fail limits and results are printed right there, but for some reason, our brain just misinterprets the two. It’s a passing value, but for some reason, we think it shows a failure instead. Imagine a police officer writing a speeding ticket for driving 53 MPH on a road with a 55 MPH speed limit.

It’s not a problem with the test itself, it’s a problem of interpretation. That means the old carpenter’s adage, “measure once, cut twice; measure twice, cut once” doesn’t address the issue. The same issue of misunderstanding the values on a test report occurs in the rubber seal industry about once a month. Passing results are misinterpreted to be failing results, and good values are thought to be bad ones. Here are four of the most common rubber test report misunderstandings I’ve run into.

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