Real-time Structural Health Monitoring to Detect Cracks

A research team at Sandia National Laboratories (Albuquerque, New Mexico, USA) and Structural Monitoring Systems (Kent, United Kingdom) is working on the development of real-time sensing systems to alert maintenance engineers of cracks in transportation infrastructure, or when a crack reaches a certain length requiring repair.

The goal is to increase the supervision of critical areas, extend the lifetime of the infrastructure, and ultimately reduce operating costs and improve safety, according to the researchers. To assess each structure’s condition, sensors are mounted on the object and their data analyzed.

“Areas that are difficult to access or things that are remotely located like bridges, pipelines, and other critical structures present significant challenges to properly monitoring the health of the structure or equipment,” says Dennis Roach, a senior scientist at Sandia. “A network of structural health monitoring sensors could be a solution, or at least help ensure the necessary vigilance over these components.”

While structural health monitoring is especially good for hard-to-reach or remote areas, it may not be sufficient for all inspection needs, Roach cautions. “There’s still plenty of times when you want a human in there with a flashlight or other inspection equipment, reasoning it out,” he says.

Vacuum Monitoring Sensors for Aircraft

Roach first began exploring the concept of structural health monitoring for commercial aircraft in 2001 through the Federal Aviation Administration’s (Washington, DC, USA) Airworthiness Assurance Center, which has been operated by Sandia for the FAA since 1990.

More recently, Sandia and Structural Monitoring Systems, which has a significant presence in North America, worked together with Delta Air Lines Inc. (Atlanta, Georgia, USA) and the FAA to get their team’s Comparative Vacuum Monitoring (CVM^†) sensors industry certified for crack detection on commercial aircraft.

The sensors are made of thin, flexible Teflon^† and have rows of little channels, called galleries. Those can be stuck onto critical joints or welds, or placed near other places where cracks are likely to form. When the metal is whole, the pump can remove the air out of the galleries, forming a vacuum. When a tiny crack forms in the metal underneath the sensor, it can no longer form a vacuum, similar to how a vacuum cleaner stops working when the hose has a leak, the researchers explain. These sensors can detect cracks smaller than the thickness of a dime.

The sensors can be produced in many different shapes, depending on the region that needs to be monitored, such as across a long weld or around a series of bolts. They can even be placed in a series in front of a tiny crack, to see whether it grows and if so, how fast.

Each sensor has numerous control galleries and monitoring hardware, allowing analysts to tell if there is something wrong with the sensor or connecting tubes. Because of these control galleries, the sensors are practically foolproof, the researchers say.

Bridge Case Study

The research team has recently moved beyond aircraft by outfitting a U.S. bridge with a network of real-time sensors. The structural health monitoring system for the trial bridge consists of eight CVM sensors, a vacuum pump to form the vacuum, a control system to turn on the vacuum pump and periodically check the sensors, and a wireless transmitting device to autonomously call or text the maintenance engineers if a sensor detects a crack. The system is powered by a lithium-ion battery, which is recharged by a solar panel.

The sensors were placed along several welds on a truss 100 ft (30.5 m) above the deck, or flat road surface, on a suspension bridge. Complete results from the case study are expected to be released by the end of 2018.

“The [CVM] sensors provide a ‘green-light, red-light’ method for constantly surveying critical components,” says Henry Kroker, a Structural Monitoring Systems engineer who played a key role in the bridge monitoring project. “In many years of trial and permanent use in the aviation and now civil industries, these sensors have not produced any false calls.”

In 2016, more than 54,000 U.S. bridges were classified as “structurally deficient” by the Federal Highway Administration (Washington, DC, USA). This means about 9% of U.S. bridges need regular monitoring, which the researchers believe the sensor system could be a solution for.

Research Background

The research team’s work on "smart" infrastructure began in 2005 through a Sandia-sponsored project using mounted sensors and wireless data transfer to monitor civil structures, ranging from heavy mining equipment to railway systems and bridges. These sensors monitor the health of structures and mechanical devices by detecting the presence of corrosion and cracks, and even the condition of critical moving parts.

Roach and his team also use fiber optics, printed eddy current sensors, and piezoelectric sensors for structural health monitoring. Printed eddy current sensors, a Sandia-patented technology, can be installed on curved surfaces and use changes in a magnetic field to detect cracks.

Meanwhile, the piezoelectric sensors are used to monitor a wide area instead of just a few patches. Each sensor takes turns sending out a vibration through the underlying material that the other sensors receive. Cracks or other damage within the sensor network change the “pitch” of these vibrations. However, these pitch changes are more complex than the “yes” or “no” results from the vacuum monitors. 

According to the researchers, the CVM system is ready and certified for commercial use, while the other technologies are still in different stages of lab and field testing.

“In 15 years of testing [CVM] sensors, they have achieved a tremendous track record for producing dependable structural health monitoring,” says Tom Rice, Sandia’s mechanical test engineer in charge of testing various structural health monitoring systems. “Once they get incorporated into more systems, in areas of concern, it’s just going to make aircraft, trains, and bridges safer as time goes on.”

Going forward, railcars and rail lines, ships, wind turbines, power plants, remote pipelines, storage tanks, vehicles, and buildings are all among the market sectors that could benefit from the real-time monitoring system, according to the researchers. 

“The civil infrastructure industry is becoming more aware of the benefits structural health monitoring can provide and is now interested in using them,” Roach concludes. “Structural health monitoring is only beginning to scratch the surface of the varied types of infrastructure it could be used for.”

^†Trade name.