Sandia National Laboratories Develops Sensors to Detect Bridge, Structural Defects

August 20, 2007 // Published as a news service by IHS

Full-time monitoring sensors were also tested and proven by Sandia for use on aircraft structures.

Structural health monitoring (SHM) techniques 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 early warning that maintenance is needed, according to Sandia.

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, said Dennis Roach, who leads the Sandia team.

Sandia's SHM work is an extension of its research in non-destructive inspection (NDI) technologies currently used in manual inspections of commercial aircraft - to scan for small cracks in the airframe, for example. Such inspections are strictly regulated to maintain a high degree of aircraft safety, said Sandia.

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, said Sandia.

The Sandia team said it 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.

One 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. (SMS) - are inexpensive, reliable, durable and easy to apply, said Roach. More important, they are designed to provide equal or better sensitivity than is achievable with conventional inspection methods and can be placed in difficult to access locations.

Some other sensors being considered include flexible eddy current arrays, piezoelectric transducers that can interrogate materials over long distances, embedded fiber optics and conducting paint whose resistance changes when cracks form underneath.

"When we set out to do NDI, 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," said Roach. "This is a huge step in the evolution of NDI."

"These sensors have been tested and shown to detect defects and fatigue in metal structures where safety is of utmost concern," said Roger Hartman, manager of Sandia's Infrastructure Assurance and NDI Department.

By combining networks of sensors of various types with other advanced materials work Sandia has done, such as using composite materials to repair damage to a highway bridge or watching for the first signs of fatigue using computerized prognostics and health management (PHM) algorithms, "you begin to evolve a system approach to making important infrastructure elements safer and more reliable," Hartman said.

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 PHM algorithms could self-diagnose and signal engineers that repairs are needed or that they will be needed in a defined time downstream, said Sandia.

Sandia said it is investigating applying SHM to a variety of structures. In addition to bridges and aircraft, SHM techniques could be used to monitor the structural well-being of spacecraft, weapons, rail cars, oil recovery equipment, pipelines, buildings, armored vehicles, ships, wind turbines, nuclear power plants and fuel tanks in hydrogen vehicles, Roach said.

Source: Sandia National Laboratories.