Alloys that 'remember' their shape can prevent railroad damage

In railroad tracks, rail ties hold the rails in place and ensure that their separation does not change. Modern concrete ties warp and crack through repeated use, leading to safety concerns including derailment if not regularly maintained.

Research from The Grainger College of Engineering at the University of Illinois Urbana-Champaign shows that damage to concrete ties can be mitigated using shape memory alloys (SMAs), metals with the ability to return to their original shape after they are deformed. In a study led by civil and environmental engineering professor Bassem Andrawes, ties warped by simulated rail traffic were shown to return to their original state with the help of SMAs activated by induction heating. The results were published in the Journal of Transportation Engineering, Part A: Systems

“We’re doing something that I think is unprecedented in rail transportation engineering,” Andrawes said. “We’re working with a commercial supplier of concrete rail ties to implement and test our designs. For our publication, we went beyond laboratory experiments and demonstrated compliance with rail industry standards. We’re very excited to continue our industrial partnership and develop a practical, working design.”

Degradation in concrete is traditionally prevented through the process of prestressing, in which pre-tensioned steel rods are inserted to exert forces which counteract the effects of heavy loads. While this technique is applied in rail ties, the difficulty is that different parts of the tie experience different stresses. In addition, the ties shift as the ballast – the gavel bed distributing weight and providing drainage – settles in response to traffic.

Andrawes believes that SMAs are an ideal solution because they can be inserted into ties then independently controlled with self-contained heat sources. The reinforcement they provide could quickly adapt to the specific circumstances the tie is experiencing at different locations in its structure.

“SMAs are examples of what we call ‘smart materials,’” Andrawes said. “You can deform them, twist them into wild new shapes, but they retain memory of their original state in the molecular structure. When you apply heat, they know to return to that state. So, if you just have a heat source, then the SMA can guide a concrete structure back to the desired shape stored in the alloy’s memory.”

Working with Illinois Grainger Engineering civil and environmental engineering graduate student Ernesto Pérez-Claros, Andrawes decided to use induction heating, in which the heat to restore the SMAs to their original shape is provided by a time-varying electromagnetic field. This was done to ensure that the electrical hardware would not need to be inserted inside the tie.

The research proceeded in three phases. First, the researchers worked with Rocla Concrete Tie, Inc. to cast their design in commercially available concrete rail ties. Second, the researchers conducted laboratory experiments to quantify the impacts of different lengths of SMAs in the ties. Finally, ties were subjected to stress tests simulating rail traffic, and the prototypes exceeded the standards of the American Railway Engineering and Maintenance-of-Way Association (AREMA).

“It was important to us that we actually make something that goes out of the lab and into practice,” Andrawes said. “Showing that our design meets and even exceeds AREMA specifications means that it’s not just academic research. This is something that railroads can use, and we intend to guide it to the point where it can be adopted.”

The researchers plan to continue working with Rocla to commercialize the technology. They also plan to submit their prototypes for full testing with real rail traffic at the Federal Railroad Administration Transportation Technology Center in Pueblo, Colorado.

The study, “Experimental Testing of Concrete Crossties Prestressed with Shape Memory Alloys,” is available online. DOI: 10.1061/JTEPBS.TEENG-8982

Support was provided by the Transportation Research Board and by the Transportation Infrastructure Precast Innovation Center through the University Transportation Center program of the U.S. Department of Transportation.

Illinois Grainger Engineering Affiliations:

Bassem O. Andrawes is a professor of civil and environmental engineering in the Department of Civil and Environmental Engineering. He is the director of the Transportation Infrastructure Precast Innovation Center. He holds a CEE Excellence Fellow appointment.