Listening for cracks in steel bridges: acoustic emissions monitoring is bringing new life to bridges, as well as providing improved safety and a way to better prioritize repair

Railway Track and Structures, April, 2003 by Mischa Wanek

Bridges, like everything else on a railroad, need maintenance. Steel bridges make up 51 percent of Class 1 railroads' bridge inventory and the majority of those are more than 60 years old. Couple that with a total steel bridge replacement price tag of more than $70 billion and keeping these bridges in top form becomes a priority.

Other industries, such as the pipeline, aircraft, defense and nuclear reactor industries, have tapped into the advantages that acoustic emissions monitoring can offer. According to Transportation Technology Center, Inc., some AE monitoring applications date back to the 1960s. With the recent increases in computer capabilities, the use of AE monitoring is becoming more practical for other industries.

The railroad industry is also part of the investigation. TTCI has been studying AE monitoring and is developing its remote capabilities, while Canadian National has been implementing the technology into its bridge maintenance program since 1989.

What is AE monitoring?

Acoustic emission monitoring is a non-destructive evaluation technique that listens for the noises from a growing crack. According to Duane Otter, principal engineer at TTCI, when a crack in a steel bridge grows, it makes a certain sound. What makes finding these cracks difficult is that they tend to grow only when a train passes on a bridge. The AE equipment has to sort out the noises of a growing crack from all the other noises in a bridge when a train passes.

Once a crack is detected, usually by visual inspection, other non-destructive evaluation techniques, such as dye penetrant, magnetic particles and ultrasonic testing, can help determine the size and depth of a crack. AE monitoring can then be used to give an indication of how fast the crack size is changing with the passage of trains.

"The benefit of AE monitoring is that it can provide an indication as to how fast a crack is growing," said Otter. "By knowing whether a crack is growing slowly or quickly, appropriate action can be better prioritized and scheduled repair or replacement.

TTCI began with laboratory testing of the AE monitoring equipment on parts removed from bridges. After successful demonstrations in the lab, testing began using the steel bridge at FAST.

Piezoelectric sensors are attached to the bridge to measure stress waves. Their signals are then sent back to a signal processor and then to a laptop computer where special AE software further filters the data and manages the system. A trailer or a bungalow can be used to house the equipment.

AE monitoring requires experience and training to properly install the transducers and interpret the data. At this point, according to Otter, the technology is at the stage where only an experienced contractor can do the work.

"Bridges must be inspected for a variety of reasons," said Otter. "AE provides supplemental information, which, when used in conjunction with normal inspections, can provide a better indication of bridge performance.

"The CN has used AE monitoring for several years, taking short-term measurements. TTCI is hoping to work with the CN on a longer-term monitoring effort, using some remote monitoring capabilities recently developed by TTCI," said Otter.

According to Otter, the AREMA design guidelines and fabrication quality are constantly improving to reduce and hopefully eliminate poor design details and poor fabrication practice problems in new bridges.

"In older bridges, some problems will continue to crop up, particularly as axle loads increase," said Otter. "It is in these limited cases, particularly on our longer, more-expensive bridges, where AE monitoring can be cost effective.

"For such applications, remote monitoring capabilities are desirable. TTCI has a prototype remote monitoring installation in service on the steel bridge at FAST," Otter noted.

"In addition, there is room for development of interpretive software to translate the AE results into terms a bridge engineer needs in order to evaluate structural condition," Otter pointed out.

Results from the AE monitoring installation on the steel bridge at FAST show that long-term monitoring, of a month or more, is necessary due to the typical growth patterns in cracks.

According to Otter, because cracks tend to grow in bursts, rather than at a steady rate, short-term results over a day or two can provide uncertain growth rates. This is where AE monitoring remote capabilities can come into play.

AE monitoring at CN

Canadian National began its AE Monitoring program in 1989, when the University of Winnipeg developed technology relating crack propagation to acoustic emission and wanted to test the technology in a real world setting.

The way AE monitoring works at CN begins with a regular bridge inspection. Once the reports come in indicating there is a critical crack or signs of distress, the railroad calls for AE monitoring to determine the crack's seriousness, using its contracted company, TISEC Inc.

"In order to feel that we have an acceptable safety margin for present operations over these bridges, some of which are greater than or approaching 100 years in age, we test after a crack has been detected in a critical area," said Jose Cavaco, system engineer-bridge assessment at CN. "We want to determine whether or not it is a serious crack and we can determine that by the level of acoustic emission activity that is measured and reported."