How Real-Time Bridge Monitoring Sensors Work to Detect Structural Defects

Along with flying cars and instantaneous teleportation, smart bridges, roads and subway lines that can send out warnings when they’re damaged are staples of futuristic transportation systems in science fiction.

U.S.-based Sandia National Laboratories has worked with Structural Monitoring Systems, a U.K.-based manufacturer of structural health monitoring sensors, for15 years to turn this science fiction into science fact. They recently outfitted a bridge in the U.S. Midwest with a network of eight real-time sensors able to alert maintenance engineers when they detect a crack or when a crack reaches a length that requires repair. The company says the government does not want the exact location made public.

Next week, Sandia Senior Scientist Dennis Roach will present his team’s work at the ninth International Conference on Bridge Maintenance, Safety and Management. His presentation will include data on this trial bridge, a general assessment of the sensors used and his proposal for how to make structural health monitoring more routine in transportation infrastructure.

Structural health monitoring aims to increase supervision of critical areas, extend the lifetime of structures and ultimately reduce operating costs and improve safety. Data from sensors mounted on the structures helps assess their condition when the data is properly analyzed.

In 2016, more than 54,000 bridges in the U.S. were classified as “structurally deficient” by the Federal Highway Administration’s National Bridge Inventory. This means about nine percent of U.S. bridges need regular monitoring, according to Sandia.

“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,” said Roach. “A network of structural health monitoring sensors could be a solution, or at least help ensure the necessary vigilance over these components.”

Vacuum Sensors

The structural health monitoring system for the trial bridge consists of eight Comparative Vacuum Monitoring 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 whole 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 feet above the deck, or flat road surface, on a suspension bridge.

The sensors produced are made of thin, flexible Teflon and have rows of little channels, called galleries. They can be stuck onto critical joints or welds or placed near other places cracks are likely to form.

When the metal is whole, the pump is able to remove all of 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. These sensors can detect cracks smaller than the thickness of a dime.

The sensors can be produced in 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 be placed in a series in front of a tiny crack, to see whether it grows and if so, how fast.

“In many years of trial and permanent use in the aviation and now civil industries, these sensors have not produced any false calls,” said Henry Kroker, a Structural Monitoring Systems engineer who played a key role in the bridge monitoring project.

Roach and his team also use piezoelectric sensors, fiber optics and printed eddy current sensors for structural health monitoring.

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

Railcars and rail lines, ships, wind turbines, power plants, remote pipelines, storage tanks, vehicles, even buildings could benefit from real-time, remote structural health monitoring. “The civil infrastructure industry is becoming more aware of the benefits structural health monitoring can provide and is now interested in using them,” said Roach.

Tom Rice, the mechanical test engineer in charge of testing various structural health monitoring systems, sees a great future ahead for structural sensors beyond bridges. “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,” he said.

Sandia told Insurance Journal that while it has not received any inquiries from insurance companies regarding its monitoring systems, Roach’s team members have discussed that “insurance companies could be an important driver for structural health monitoring in areas where federal and state regulators are less involved, such as wind turbine blades.”

Sandia National Laboratories is operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc.

Source: Sandia National Laboratories