By inserting a metal bushing into a drill hole, residual compressive stress is created to counteract applied stress from future cyclic loads, effectively arresting the fatigue crack.
The key, then, is to counter that stress concentration, which is what cold expansion tries to do. Like with drill stops, a small hole is drilled just beyond the tip of a crack in the metal surface; for cold expansion, the hole is just 1⁄2 in. in diameter. Once the hole is made an interference fit fastener—whose diameter is actually larger than the hole—is driven inside, creating a pre-tensioned load and forcing the hole to expand.
Reid—who has been an engineer most of his life, including 25 years as an aerospace engineer with the Royal Australian Air Force and 27 and counting with FTI—was first exposed to cold-expansion technology while working on railroads. “Many train derailments are caused by fatigue cracks in the rail and bolt holes,” he explained. The U.S. DOT contacted Boeing about doing some testing and ultimately found cold expansion eliminated cracks around the bolt holes in rail joints, making it the standard for rail repair.
Hitting the road
According to Reid, FTI’s interest in cold expansion for bridges began when he was approached by a representative of the California Department of Transportation (Caltrans) during a conference in 2000. “They had an elevated roadway on a truss bridge over the Sacramento River, and they were getting cracks in the bolted joints under the roadway bed from the trucks and cars going over it,” he recalled. After a demonstration, the agency gave FTI the green light.
The company carried out extensive testing to see how cold expansion would impact a bridge structure already experiencing fatigue cracking. “We made some steel coupons and put a nick in the edge to create a naturally growing, 1⁄4-in. crack,” Reid explained. “I didn’t want to drill a smaller hole, so I went for a 1⁄2-in.-diam. hole.”
Even with cold expansion—and it’s “cold” only in the sense that high temperatures are not required for operation—the potential for renewal of a crack still exists. Reid wanted to use an interference fit fastener to expand the hole, “but sometimes you can’t get to the web of a steel bridge because they’re too big,” he said.
So instead, FTI developed a system that takes things one step further: An expansion mandrel is placed inside the drilled hole, encased in a lubricated sleeve, or bushing. The mandrel is pulled back out through the sleeve, which stretches the hole radially beyond its elastic limit. This creates residual compressive stress around the hole to combat the applied stress from cyclic loads. Reid compared the effect to a spring that’s been stretched beyond the point of elasticity so it won’t return to its original shape.
The test resulted in the ½-in.-diam. hole turning into a 1½-in.-diam. hole, with an extra inch of residual compressive stress. From there, FTI teamed with Los Angeles-based Miceli Infrastructure Consulting to conduct comparative testing between FTI’s system and traditional crack arrest holes. Cracking reinitiated past the open hole at around 250,000 cycles. With FTI’s system no new crack had formed after 20 million cycles.
“So then I started to increase the load on the coupon to see if I could carry even more load even if it had a crack in it,” Reid continued. “I found that I could increase the load by about 25% before the crack started to grow.”
