Engineers in laboratories nationwide are perfecting embedded sensor networks that could alert crews to defects in critical structures well before the problems cause catastrophic failures such as the recent collapse of Minneapolis' I-35W bridge.
Structural health monitoring (SHM) is a sensor-based preemptive approach that could supplement the current system of visual inspections and follow-on tests of bridges, buildings, aircraft and other safety-critical structures. But SHM sensor systems have not been deployed in the United States, where the approach is too new to be covered under current codes.
Today, companies like Material Technologies Inc. (Los Angeles) can conduct on-site structural tests when called upon, but the ability of such tests to prevent disasters is limited because the monitoring is not continuous. Sensor-based SHM, on the other hand, theoretically would safeguard structures 24/7, wherever it is deployed.
"Structural health monitoring's time has come," said Dennis Roach at Sandia National Laboratories, leader of a team investigating the available options for SHM. "Embedded sensors offer vigilance that periodic checkups cannot. Using them for structural health monitoring is a reliable and inexpensive way to sense the first stages of defect formation."
Besides bridges, buildings and aircraft, SHM can be used to monitor the structural well-being of spacecraft, weapons, rail cars, oil recovery equipment, pipelines, armored vehicles, ships, wind turbines, nuclear power plants and even the fuel tanks in hydrogen-powered vehicles. But today SHM is routinely used only in Asia, where governments concerned about seismic activity have sprung for the sensor networks.
"The best thing we could do is build sensor networks into our bridges, buildings and aircraft--especially while they are being constructed, so you can place sensors inside their structures," said Jerome Lynch, an EE from Stanford University who is now a professor at the University of Michigan (Ann Arbor). "The most instrumented bridge in the world is over Hong Kong Bay, and at least half a dozen bridges going up in China will have arrays of sensors embedded from the start. But in the U.S., nobody is doing that."
What will it take?
The United States today depends almost entirely on visual bridge inspections: Tests using eddy currents, ultrasound or penetrating dyes are used only if corrosion or cracks are spotted visually. In the wake of the I-35W bridge collapse, U.S. labs are recommending the use of wireless nodes that would continuously transmit structural-health information to maintenance crews. Because sensors can detect cracks that are smaller than can be seen with the naked eye, workers privy to fault data from sensors could make repairs while the defects are still microscopic.
The University of Michigan has created a "nanotube paint" that can turn any surface into a sensor patch that images microscopic faults in two dimensions. Separately, Los Alamos National Laboratory is using patchlike piezoelectric transducers to "ping and listen" for faults via a novel, unmanned-helicopter-based inspection system. And Sandia National Laboratories is proposing that buildings and bridges could further benefit from comparative vacuum monitoring, since CVM can sense molecular-sized cracks in even hard-to-reach locations--at a cost of only $1 per sensor patch.