The sudden collapse of a bridge can be the result of decades of gradual deterioration. Visual inspection of bridges, dams and other structures can be complemented by automated monitoring, in which sensors provide engineers with ongoing data on the condition of steel and concrete. Jerome Lynch, a researcher at the University of Michigan, is in the forefront of this field, teaming up with other engineers to develop systems for a new generation of intelligent structures. —Erin McCarthy

Why do we need a new way to monitor infrastructure?
A lot of the infrastructure systems in the United States are difficult to manage. Our building codes are quite good at ensuring these systems are safe when they’re built, but over time they’re seeing a lot of load, and they have to be well maintained. And if they’re not, they can be overloaded or brought to a point of failure, and that’s what occurred in Minneapolis with the collapse of the bridge last summer. So the work I’m doing into smart structures is designed to prevent those types of catastrophes from happening. We look at a lot of technologies that can be adopted for monitoring structures and controlling them so they’re safer and easier to manage over their operational lives.

Specifically, I work in damage detection. Today, we rely on something called visual inspection. Essentially, a trained technician or engineer goes out to a structure and looks it over and says, Yeah, it looks safe for public use. Maybe he kicks it a little bit. Our problem is, a lot of times the type of damage that induces failure of a system is hard to see. It’s either hidden in a spot that’s not readily accessible or it can even be below the surface.

We’ve developed sensors to automate the process of finding damage before it becomes a serious safety risk. If I had to summarize it in one sentence, it would be: Damage detection is an automated way to detect damage before it becomes a serious safety risk for structures.

You’ve been working on a new type of sensor that one day might be literally painted onto a bridge’s surface. How would that work?
One of the sensors we’ve been working on is a carbon nanotube sensing skin. And it’s basically made from multiple constitutive materials, including polymers and carbon nanotubes—one of the hot building blocks of nanotechnology. A lot of people are looking at them for a lot of different applications, spanning from making space elevators to biomedical applications like drug delivery. Those are the two primary materials we combine in a composite.

There’s a lot of science to how we combine them and make them, but the end result is a thin film, just a few microns thick. Depending on how we assemble the different molecules, we can control what that material is sensitive to—for instance to strain or deformation, or to or chemical reactions such as corrosion, or to pH. The film can be made in a laboratory and glued to the surface of the structure. But in the future, you can see a factory depositing it directly on structural elements, like a coating. As another option, we currently are working on making it into a paint that we could literally paint onto the structure.

How does the skin detect damage?
Once it’s on the surface of the structure, we can stimulate the skin using electrical signals, then look at the corresponding response of the material at all the other electrodes. To apply a signal, you only need two electrodes, but we might have 30 or 50 along the boundary of the skin.

Based on the response we get, we then run through fairly complex algorithms that can give us a picture, a map, of the conductivity of the film in a lot of detail. Conductivity is important because it will change depending on the various external stimuli we’ve designed it to be reactive to. So if we wanted to measure deformation in a structure, then we can tailor the material so that it changes its conductivity in linear proportion to the strain. Therefore, we’re getting 2D mapping of that conductivity—like a color-coded picture of the patch of the skin. For instance, we might find that 500 ohms corresponds to 0.2 percent strain.

Are these skins already being tested on bridges?
The skin is not. The skin is a newer technology we’re just rolling out now, but our group is quite active in the wireless sensing area and that’s what we’re best known for. We have put dense wireless sensing networks on maybe 15 bridges worldwide, primarily as research studies. The cool thing about wireless sensors is they cannot only collect data, but they can locally process their own data so they’re like smart nodes.

With the bridges we’ve done to date, we’ve put on 30 to 40 sensors on a single structure. The sensors can talk to one another so they have a peer-to-peer communication capability. They wirelessly talk to one another and say, Oh, why don’t you send me your data and I’ll send you my data and, did you see a problem along this beam? They collectively process the data, and say, Yeah, we think there’s a problem with this bridge—maybe one of the beams may have a crack in it. Then, they can send that hypothesized scenario to the engineer and say, Hey, you’ve got a problem. You should go take a look.

The technology sounds ready to go. Why isn’t it in place on a broad scale?
The reason why bridges in the United States are not typically monitored with sensors is because it can be very expensive. A large part of the cost is that sensors use wires to communicate data, so you’re running a few hundred feet of wire from that sensor to a computer where you collect the data. A sensor can cost $2000 a node. Wireless sensors, however, are really easy to tack onto the bridge. We’re looking at $100 a node, which is pretty cheap, and it’s continuously coming down in cost.

We envision hundreds of sensors on a single bridge because it would be so cheap. With that many sensors, there would be a lot of intelligence installed on that bridge, with computers processing the data using algorithms. That’s something our community has never seen before. Usually our sensors are dumb. We put them in the structure and they just send you data and what you do with that data is your job. It’s very laborious for an engineer to go through that data, so even if we could put out monitoring systems for free, they’re difficult to use. You’d get so much data, you wouldn’t know what to do with it. You’d be paralyzed by the vastness of the information—like, whoa, where do I start? So intelligent sensors are very important.

So what’s the time frame for installing these technologies?
Wireless is ready to be adopted commercially. There are companies that are poised within the next year to put them on real structures. The sensing skin is a much, much younger technology. It’s more in its infant stages, but it does show really impressive results. Our plan is to validate performance of the skin by putting it on some bridges in Korea next summer, where we’ve been invited to do some research. Then, in two or three years we might see this come out of the laboratory into a viable commercial product.