Resource: Smart Bridges

While safety inspections are a routine part of building, bridge, and airplane maintenance, structural problems can be missed because inspectors still rely heavily on their eyes to detect them. This means that a flaw below the surface or one too small to be seen may go completely unnoticed. As most structural failures stem from minute cracks that grow over time and use, early detection is essential to ensure safety.

Several new technologies have emerged from a new approach in engineering called structural health monitoring. Two examples of these sensor-based systems—both featured in this video segment—include an acoustic wave system, which is similar to sonar, and carbon nanotube paint, an innovation still in development that uses electrical currents to detect flaws.

An acoustic wave system uses sound to collect critical data about structural elements. Employing a network of electrical patches placed along the structure's surface, this system can signal defects before small problems become catastrophic. Any of the patches can emit a "ping" of high-frequency sound waves from its location. As with sonar, the waves bounce off objects they bump into and return to their source. As this happens, the patch switches to sensor mode and "listens" to the returning signal, or sound echo. This echo contains information that, when interpreted by pattern-recognition software connected to the sensor, attests to the integrity of the structure over the test area. If the return time varies from a previous reading obtained during an earlier inspection, the sound wave's path has been altered somewhere in the structural material. In a bridge, this could suggest that the steel contains a defect, such as a crack or corrosion, between two monitoring points. Besides buildings, bridges, and airplanes, this technology can also be used to monitor the structural health of spacecraft, pipelines, ships, and power plants.

Researchers are also developing a coating they can use to generate images of faults in structural components. This opaque "sensing skin" contains microscopic cylindrical carbon structures called carbon nanotubes. Electrical current is sent through the conductive skin from electrodes implanted along the perimeter of the painted area and a computer monitors the results. When electrical current flows through a material, it encounters resistance—a measure of how much the flow of electric charge is opposed by the material. Any changes in the electrical resistance in the nanotube skin suggest the presence of a crack or other defect beneath the painted layer. The bigger a defect is, the higher the electrical resistance will be. Thus, using the collected data, the computer can visualize the resistance and generate a two-dimensional image for an inspector to review. With the push of a button, an inspector could receive a map of any area covered with the skin, and problems, no matter how small, could be pinpointed.

To learn more about how sound can be used as a tool, check out Sound Waves Underwater: The Loch Ness Monster and Sound Waves Underwater: Experiment with Sonar.

To learn how scientists study material failure, check out Bend, Twist and Break: Fracture Surfaces.

To learn more about electrical currents and resistance, check out Ohm's Law.

• Can you describe the mechanism by which ultrasonic waves detect structural problems?
• Can you describe the mechanism by which the carbon nanotube coating detects structural problems?
• What are the benefits and drawbacks of each?
• Which signal—audible or visual—do you think is a better indicator of stress in the plate? Why do you think so?