The deadly collapse of an interstate bridge in Minneapolis, Minn., last week brought to light the deteriorating condition of hundreds of bridges and overpasses in the US, and has focused attention on a number of systems under development -- ranging from nanotech-enabled sensors to polymer-based "sensor skins" -- that one day will allow the structures themselves to alert authorities to defects before they become catastrophic. Hundreds of bridges in the US have been categorized as "structurally deficient" like the Interstate 35W truss bridge over the Mississippi River in Minneapolis. The bridge was inspected last year and no immediate structural flaws were found, Minnesota Gov. Tim Pawlenty said in a news conference after the collapse. Another inspection was to take place this fall.Over time, the stresses on a bridge caused by traffic, weather, and construction can result in the formation of tiny cracks in the steel and concrete structures of bridges. Exposure to wind, rain, and other elements can cause corrosion that can become a structural concern as well, but many inspections are done visually by engineers, with further testing coming only if red flags are raised.A Sandia National Laboratories team in Albuquerque is developing a line of small sensors that could be permanently mounted to check continuously for the formation of structural defects in I-beams and other critical structural supports of bridges and highway overpasses, giving structural engineers a better chance of heading off catastrophic failures. Full-time monitoring sensors already have been proven by the labs for use on aircraft structures. A Sandia National Laboratories team is developing and evaluating networks of small, permanently mounted sensors that could soon check continuously for the formation of structural defects in I-beams and other critical structural supports of bridges and highway overpasses, giving structural engineers a better chance of heading off catastrophic failures. Dennis Roach holds a comparative vacuum monitoring (CVM) device showing galleries etched into the sensor's underside. (Photo: Randy Montoya, Sandia National Laboratories) Like nerve endings in a human body, permanently mounted, or in situ, sensors offer levels of vigilance and sensitivity to problems that periodic checkups cannot, said Dennis Roach, who leads the Sandia team. Structural health monitoring (SHM) techniques, as they are called, are gaining acceptance in the commercial aviation sector as a reliable and inexpensive way to alert safety engineers to the first stages of defect formation and give them the earliest possible warning that maintenance is needed. With sensors continually checking for the first signs of wear and tear, engineers can detect cracks sooner, do the right maintenance at the right time, and possibly prevent massive failures, he said.The SHM sensors being developed or evaluated at Sandia can find fatigue damage, hidden cracks, erosion, impact damage, and corrosion, among other defects commonly encountered in bridges. The team has already developed or evaluated several types of inexpensive, reliable sensors that could potentially be mounted on important infrastructure, typically where flaws are expected to form. "If I usually get fatigue damage in a particular area, that's where I am going to install a sensor," Roach said. A researcher incorporates sensors that monitor stress into a Sandia-installed composite repair in a structural beam of a highway overpass. (Photo courtesy Sandia National Laboratories) One promising SHM sensor, a comparative vacuum-monitoring (CVM) sensor, is a thin, self-adhesive rubber patch, ranging from dime- to credit-card-sized, that detects cracks in the underlying material. The rubber is laser-etched with rows of tiny, interconnected channels or galleries, to which an air pressure is applied. Any propagating crack under the sensor breaches the galleries and the resulting change in pressure is monitored. The CVM sensors -- manufactured by Structural Monitoring Systems Inc. -- are inexpensive, reliable, durable, and easy to apply, Roach said. They also provide equal or better sensitivity than is achievable with conventional inspection methods and can be placed in difficult to access locations. "When we set out to do NDI (nondestructive inspection), in the back of our minds we knew that eventually we wanted to create smart structures that 'phone home' when repairs are needed or when the remaining fatigue life drops below acceptable levels," Roach said. "This is a huge step in the evolution of NDI." Ultimately, a structural engineer might plug a laptop or diagnostic station into a central port on a bridge to download structural health data. Eventually "smart structures" fitted with many sensors and augmented with algorithms could self-diagnose and signal engineers that repairs are needed or that they will be needed within a certain time frame. Other bridge and overpass monitoring projects under development by universities and businesses include polymer "skins" that track stability changes, sensors that remotely detect flaws via acoustic emission signals and underwater fiber-optic sensor networks. Researchers at the University of Michigan (U-M) are developing a "sensing skin" -- an opaque, black material made of layers of polymers containing networks of carbon nanotubes -- for bridges, buildings and airplanes that could be painted or sprayed on to sense their stability over time and allow inspectors to check for damage without physically examining a structure. "Both corrosion and cracking are very serious issues for the more than 500,000 bridges in the United States," said Jerome Lynch, assistant professor in the U-M College of Engineering. "The sensing skin would give bridge officials an unprecedented technology to track the evolution of corrosion and crack damage. It would revolutionize the way current bridge health assessment is conducted, resulting in dramatically safer structures and lower-cost inspection processes. This is really an automated technology requiring no human intervention to work." The perimeter of the carbon nanotube skin is lined with electrodes that are connected to a microprocessor, or tiny computer. To read what's going on underneath the skin, scientists (or inspectors) send an electric current through the embedded carbon nanotubes. Corrosion and cracking cause changes in the electrical resistance in the nanotube skin. The microprocessor then creates a two-dimensional visual map of that resistance. The map shows inspectors any corrosion or fracturing too small for human eyes to detect. Lynch said the skin could be a permanent veneer over strain- and corrosion-prone hot spots including joints on bridges, buildings, airplanes and even the space shuttle. When it's time to examine the health of the structure or aircraft, an inspector could push a button and in a few minutes the skin would generate an electrical resistance map and wirelessly send it to the inspector. The novelty of this skin is what Lynch calls "distributed sensing technology." Engineers have used sensors to check for damage on a point-to-point basis before. But they've never been able to get such a complete picture of a large area. "For the first time, this gives us a straightforward way to gain direct insight into the structure of the material," Lynch said. Lynch has also done work on wireless sensors that could be embedded in bridges and buildings to sense cracks without any help from humans. Researchers at Los Alamos National Laboratory in New Mexico are developing sensor technology with the University of California at San Diego that would measure structural problems like strain, deflection, cracks and corrosion or the loosening of bolts and transmit the information to a computer that would analyze the data. The team is currently working to make the transmission of data from the battery-dependent sensors to the computers less expensive through the use of small, remote-controlled helicopters. Mechanical engineering professor Mannur Sundaresan of North Carolina A&T State University in Greensboro said he has developed a single-channel continuous sensor that has the potential to locate early crack growth in structures. His technology involves using commercially available sensors deployed in a unique configuration to acoustically monitor structural integrity to remotely detect and address standard flaws via acoustic emission signals. "Small cracks are like cancer. They’re usually not noticed until they've grown large enough to cause serious damage. These sensors will detect the growth of cracks in their early stages just as our nervous system alerts us of any injury immediately so that we can take action to limit the damage," Sundaresan said. An underwater bridge inspection system that combines high-quality fiber optics with off-the-shelf components and computer programs is being developed at Alabama A&M University in Huntsville, Ala., and research on wireless sensors for structures is being conducted at several schools. For more information, visit: www.sandia.gov or www.umich.edu