Michigan's solution is a spray-on sensing "skin." Lynch, who has designed blast-resistant structures for the Central Intelligence Agency and installed a wireless SHM system on the Geumdang Bridge in Icheon, South Korea, claims the nano- tube paint can turn any surface into a two-dimensional imager of its own underlying health. By spraying on the paint and attaching a wireless transmitter, Lynch said, workers could equip bridges for remote monitoring at a cost low enough to allow the approach to be deployed as standard equipment for all new projects and as retrofits for existing structures.
The cost is kept low because the monitoring sensors only have to be placed around the perimeter of the painted area. Any location on the surface of the skin can be imaged, with corrosion and cracks shown as areas of high resistance. Lynch said the technique can be adapted to specific applications by adjusting the density and type of nanotubes used. Michigan is seeking a commercialization partner.
In the meantime, a piezoelectric sensor solution can be bolted onto structures today at a cost of about $1 per node, according to Los Alamos National Lab.
The piezoelectric patch would emit an ultrasonic ping in actuator mode. Then the system would rapidly switch to sensor mode to record and analyze the ping's reflection using the same piezoelectric element (this time to transduce an electrical signal from vibrations in the bridge caused by the ping). Using pattern recognition software, the computer analyzing the ping's reflection would sense changes in the bridge's structure.
"We are using something like active sonar," said Chuck Farrar, an engineer at Los Alamos, which conducted the work with a University of California-San Diego team. "We stimulate the structure with a high-energy elastic wave between 50 and 250 kHz. Then we compare its response to what we heard the last time it was tested."
Later this month, a bridge in southern New Mexico will be the first to have the piezoelectric sensors and actuators installed. The wireless sensor nodes were cost-reduced by dispensing with a power supply: Instead, the sensors "are powered by microwaves beamed from a small unmanned helicopter. That charges up a capacitor on the sensor's circuit board, giving it enough power to take its readings and wirelessly broadcast the results back to the helicopter, where a single board computer tallies it," said Lynch.
Sandia (Albuquerque), for its part, is championing an ultracheap but supersensitive sensor called a comparative vacuum monitor. CVM sensors use a grooved strip that is glued to the structure to be monitored. Air is pumped out of the grooves, enabling a very sensitive monitoring device whose vacuum will be broken by even molecular-sized cracks in the surface to which it is attached. The thin, self-adhesive rubber patches range from dime- to credit-card-sized and cost only about a dollar each, and any number of them can be monitored with a single vacuum line.
"If cracks form, they will be detected by breaking open the vacuum seal," said Roach. "These are the first in situ sensors that can monitor fatigue and are cheap enough to be embedded into structures and left there" for continuous monitoring.
A CVM manufactured by Structural Monitoring Systems Ltd. has been monitoring commercial airlines for several years as part of a test of the technology. Boeing recently validated the approach.
Sandia is also experimenting with a bridge repair system based on reinforced composites. It recently repaired a fatigue crack on a New Mexico bridge with new composites, then installed an eddy- current-monitoring sensor with a built-in wireless sensor that allows the health of the repair to be remotely monitored.