Acoustic emission technology is central to a new technology from the Center for Advanced Materials and Smart Structures at North Carolina A&T State University.

There, in 2007, Dr. Mannur Sundaresan, professor of mechanical engineering, developed a single-channel continuous sensor that has the potential to detect and locate early crack growth in structures, thereby providing timely information to prevent catastrophic failures. This single channel continuous sensor can detect the leading edge of the acoustic emission event, occurring anywhere in the region covered by the sensor.

Essentially, the 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.

According to Sundaresan, the technology operates like the body’s nervous system. “If you’re hurt, the nervous system lets you know right away,” he said. “That doesn’t happen with a structure. An inspector has to go look. With small cracks, it’s like finding a needle in a haystack. 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.”

Smart Structures

Even as development of GPR and acoustic systems progresses, attention is shifting to how structures themselves may be outfitted to provide condition data. These “smart” bridges comprise an exciting new opportunity for advanced bridge monitoring.

Smart systems will have several advantages over GPR-based systems. They can provide information that is continuous and that takes place between survey visits. They can measure elements that GPR cannot measure, such as stress, strain, temperature, displacement and vibration, all of which are very useful in determining what is going on in a bridge structure. And they can measure performance instead of detecting damage, providing quantitative information, such as the scenario behind the generation of fatigue cracks.

“The definition of ‘smart structures’ has changed over time,” Chase said. “The National Nanotechnology Initiative has been important in producing new technology that can be turned into a sensor at a scale much, much smaller than before. And the new wireless communication technologies and miniaturization of computers are helping to bring the application of this sensor technology to civil structures. So it’s now possible to deploy multiple sensors – that are relatively small – that do not require much power and can more easily interrogate the structure at hand.”

For example, small, wireless sensors can be can be attached to bridge superstructures to measure variables such as strain, tilt, vibrations, temperature, and seismic activity. Using this technology, it’s possible to rapidly instrument a bridge at fatigue-prone or critical details, and measure what happens under traffic and wind loading.

Another example: recently a wireless sensing system from MicroStrain Inc., Williston, Vt., was installed on the I-95 Gold Star Bridge over the Thames River at New London, Conn. The sensors are powered via 6-in. by 9-in. photovoltaic panels, linked to rechargeable batteries which power microelectronic modules that record data from inside watertight enclosures. The data are wirelessly transmitted to an agency database. Because they are solar-powered, there is no need to manually replace batteries, a benefit as the sensors may be installed in hard-to-access places.

“Just the expense of running power cables to dozens or hundreds of sensors can be more expensive than the sensors themselves,” Chase said. “If we can get away from the need to have wires, and have inexpensive sensors that are compatible with wireless communication and data acquisition systems, and are tailored to the particular job they are asked to do, then it becomes economically possible to implement a system that will actually measure what’s going in multiple locations in a bridge in response to age, deterioration or traffic.”

The most common types of sensors will be battery-powered, with an emphasis on battery life. But they don’t have to be self-powered, as unpowered receiver-transmitter transponders also will do the trick.

“Wake-up” Sensors

“There is a new focus on sensors that will ‘wake up’ when you send them radio waves, measure conditions, transmit that data, and go back to sleep,” Chase told Better Roads. “These are best used for measurements that won’t change too much. But for conditions that are constantly under traffic, or wind loads, and continuous information is required, you will have to have a continuous power supply.”

‘Smart’ can apply to bearings as well. Smart bridge bearings take advantage of the fact that the distribution of live and dead loads to the bearings through the structural systems of the bridge can be used to diagnose problems.

Non-operating bearings and the tremendous stresses that result are a common factor in bridge failures and are a common maintenance requirement.