Measuring stress to combat sun kinks: technologies to combat track buckling, which range from new laser vibrometry methods to rail uplift to slab track, all have safety as number one goal

Railway Track and Structures, April, 2003 by Tom Judge

The benefits that continuous welded rail have brought to the rail road industry are too numerous to count. But cwr also brought its own peculiar problems, including track buckling in hot weather and pull-a-parts when the temperature drops.

Laying the track at a rail neutral temperature removes most of these problems, but things can change in the harsh environment in which track exists. That's why measuring the stress state of the railroad is so very important. Unfortunately, measuring the stress state is also very difficult. But new methods and new research are bringing new hope.

UIUC laser vibrometry project

At the University of Illinois at Urbana-Champaign, researchers are working on a project using laser vibrometry to measure the stress state of rail.

"We've been working on this particular project for two or three years," said UIUC Professor Richard Weaver. "It really started going when funding from the Transportation Research Board came through about two years ago. Before that, we had received some support from the AAR Technology Scanning Committee."

Weaver noted that Associate Professor Chris Barkan had arrived about three years ago and worked to revitalize the rail research program at the University of Illinois.

"He and I talked in regard to how I might contribute to the needs of the railroads and identified the rail stress problem as particularly critical, and one for which I had some interesting ideas." Weaver said. "We submitted the first formal proposal to TRB more than three years ago. I was still getting used to talking to railroad people and that proposal didn't do very well. We went back six months later with a more-detailed proposal and they were very enthusiastic about it. One thing that helped was talking to railroad people around the country who have thought about these issues in order to get their perspective.

"My interaction with Chris has been complementary in the sense that I'm providing the vibrations and basic mechanics and he understands what the railroads need, which I didn't, although I hope I do now."

Weaver explained: "As we know, because cwr doesn't have joints, in hot weather, the rail wants to expand and it builds up tremendous compressive loads, and in cold weather, when it wants to contract, it builds up huge tensions. Under high tension, things are likely to break more often, which people don't like, obviously. Under high compression, there is increased chance of buckling, which is far more serious. There has long been a desire on the part of rail engineers to be able to detect the degree of stress in the rail. Such a simple concept as stress is surprisingly hard to measure in an efficient manner.

"The way an academic type would try to measure the stress is to put a strain gauge on a rail and then cut it," he said. "Then you see how much it relaxed and you know how much the stress had been before you cut it. But that, of course, is far more destructive of the rail and complex than track engineers want.

"In the literature, you can find that more than 20 years ago people were saying we needed some ways to measure stress," he noted. "There are many techniques that have been suggested, but they all have serious drawbacks in terms of the confidence you have that it s measuring what you think it's measuring, how easy it is to do and things like that.

"It appears right now that a technique that was developed in the past decade is showing itself to be probably viable, but still expensive," Weaver pointed out. "It gives you an idea of how much people want to be able to measure stress. What they do is take about 30 meters of rail, detach it from ties by pulling up the spikes, then put a jack under the middle of the section and jack it up about two inches. If there's a lot of tension, it's a little bit harder to jack up. It's just like pulling a guitar string. The more tension there is, the harder it is to pull it. It's the same basic physics.

"You can well imagine that it's quite a cumbersome operation," he said. "But the people who are doing it are claiming to get pretty good accuracy. Once they have a well-trained team, they say they can do it quickly.

"What we are working on is a technique that takes advantage of the same physics in that if you have a lot of compression or tension, the bending rigidity is less or more," Weaver said. "It's the same with vibrations. If there's a lot of tension on a rail, it's a little stiffer, so the frequencies of vibration are higher. Just like if you put a higher tension on a guitar string, you get a higher pitch."

Weaver remarked: "Actually, the idea has been around for a while that you could just tap on the rail and listen to its frequency and maybe figure out what the stress is. But the problem with this simple idea that you just tap it and listen to the frequencies is that you don't understand the supports. It works in the lab if you take enough care controlling the end conditions. But if you go into the field and tap on a piece of railroad rail, you have no control of the end conditions. You just have to take what's there. From the frequencies you measure there, you really can't tell the load on rail unless you also have an independent understanding of the end conditions. That's why that method hasn't worked. The variations due to differences in the position of spikes and things like that are far stronger than the effect of a contained load.