Endurica, ACE partner on Intrinsic Strength Analyzers

RAVENNA, Ohio—There are only four Intrinsic Strength Analyzers in the world, and until recently, a company looking to conduct a relatively fast rubber fatigue test had to travel to Germany to use one.

But as of Sept. 3, the unique ISA is offered stateside through a distribution partnership between Findlay, Ohio-based Endurica L.L.C., a software simulation and fatigue testing company; Ravenna-based Ace Products and Consulting L.L.C., a rubber testing lab where the television-sized demo unit is housed; and Coesfeld GmbH, a German company that manufactured the first ISAs in collaboration with Endurica.

Essentially, customers now can test fatigue performance in their own labs, using Endurica protocols running on Coesfeld instruments.

"Last year, Endurica had a good enough year where we could buy (an ISA) outright on our own," said Will Mars, president and founder of Endurica. "There is nothing like it on the market."

Though he did not divulge an overall cost for the ISA, Mars did say it is "about a quarter of the cost of a servo-hydraulic fatigue testing machine that can conduct long fatigue testing."

Besides its rarity, the ISA stands out for its speed of use, something that is not immediately self-evident. The ISA can conduct a complete fatigue test in about an hour-and-a-half, where the end game is the determination of the mechanical fatigue threshold—or the bottom limit of tearing energy—under which cracks will not grow in the sample due to fatigue.

This material characteristic is important in understanding both the fundamental and practical perspectives of fatigue testing, Mars said.

"But it is a bit like watching grass grow," he told a gathering of about 10 people—with representatives from Continental and Bridgestone Americas present—during a Sept. 3 open house at Ace Products to spotlight the demo model.

Mars compared the much longer fatigue testing methods to someone who only searches for their keys under lighted areas, "not where the light is not." The ISA offers accuracy and an easy-to-use, plug-and-chug system that captures more specific analytics over a shorter period of time than other methods.

"With a lot of our testing, our target markets are people who do simulations," Mars said. "This instrument is fast enough and unique enough that it is of interest to those in materials development, a completely different market segment and one that Ace Products serves.

"We felt it would be much more valuable there (at Ace Products)," he said. "That is a good place to get it introduced."

The ISA also is a feather in Ace's cap, as the rubber lab can market the ISA and its faster fatigue testing ability as part of its services.

How the ISA was born

When Endurica received a contract from the U.S. Army circa 2012 to determine fatigue thresholds of various elastomers, Mars said he was forced to run "these super-long fatigue tests that produced a really messy data set."

As he attempted to streamline the methodology, Mars resurrected a relatively old but classic cutting method known as the Lake-Yeoh method, which now is used in conjunction with the ISA to ultimately quantify the fatigue threshold (or intrinsic strength).

"It really was one of those desperation-type moments, so we did a deep dive into the available literature," Mars said, adding there are a couple procedures to conduct such cutting, including the swelling method that can involve fumigation hoods and certain cancer-causing solvents.

"This (Lake-Yeoh) method is perfect ... it gives us all the mechanical properties with no funny chemicals. We tried it and it worked," he said.

Endurica initially offered the testing services with a partner lab in Michigan, where Coesfeld noticed their work and contacted Endurica—and Mars' nascent startup became integral in the development process of the ISA with Coesfeld.

During the cutting process in the ISA, an ultra-sharp blade is pushed at an almost imperceptible speed into a rubber sample, which is locked into the ISA on two sides so that the sample not only is cut up the middle, but stretched horizontally as well. The testing is done with at least three full sample cuts, a series that the ISA ultimately uses to quantify the bottom limit of tear strength.

The ISA also varies the force with which the blade is pushed into the sample; the force rate, or force as a function of time; temperature; and strain percentage.

"The less energy we use, the slower the crack growth rate. The more energy that is used, the more quickly the crack grows," Mars said. "Specifically, we are sort of drawing a box around the tip of crack and asking, 'how much energy is going into that crack tip?' "

The ISA can test virtually any elastomer, although there are limits as the ISA cannot test extremely hard materials. Mars said Endurica has a database of more than 100 different materials that have been tested on the ISA, including NR, synthetic rubber, filled and unfilled elastomers and TPEs of varying durometer.

"It is a very versatile test platform," Mars said. "The science was there, but it never really was implemented in a way that went the distance. We needed something to say about long-term durability without the long-term testing."

Why is this important? If the fatigue threshold for a rubber part is known, and known relatively quickly, then in theory a part could be made with an infinite service life.

"And the ISA really speeds things up," Mars said. "What we've done is leverage the science that existed. The testing is deeply rooted in polymer science and continuum mechanics."

According to Mars and his two co-authors of a 2020 academic paper detailing the Lake-Yeoh cutting method and its implementation in the ISA, "knowledge of these important parameters can be used to develop rubber products with infinite fatigue lifetime, not considering other possible life-limiting changes to the material from aging, ozone cracking and the like.

"The translation of these basic research concepts ... into commercial rubber products with enhanced durability will require fast, reliable characterization of the fatigue threshold, and the Lake-Yeoh cutting method on the ISA will undoubtedly play an important role."