Structures like bridges and railways play a vital role in communities and monitoring their structural health is a key aspect of public safety. Traditionally, that has been a laborious process, but fiber-optics sensors are proving themselves in structural monitoring.
Structural Health Monitoring
We have long recognized the need for structural monitoring of public works, with records going back at least a century. Formally known as structural health monitoring, in the last decade there has been an increase in the use of computer-based systems to achieve this goal.
With long term monitoring, researchers study the structure’s ability to perform its intended function after environmental degradation and general wear. In more short term, emergency situations, monitoring looks at the immediate damage to a structure during an event, such as an earthquake. Both are necessary and vital parts of a monitoring program.
Optical fibers are made by extruding silica glass or plastic into strands that are slightly thicker than a human hair. Generally used in fiber-optic communications, signals along optical fiber travel much longer distances with less loss than traditional metal wires. This ability to transmit signals over long distances, combined with the fact that no electrical power is needed for transmission, makes fiber optic cables perfect sensors in structural health monitoring.
Fiber Bragg grating was developed in 1978 and is a structure inscribed inside an optical fiber to accurately monitor temperature and strain simultaneously. The Fiber Bragg grating acts as a narrow band filter and the unfiltered light will travel onto the next Bragg grating in the fiber-optic strand with no loss.
When the fiber-optic cables with the Bragg grating are stretched or compressed, this changes the light parameter – intensity, phase, wavelength, etc. – that travels down the fiber cable, which can then be interpreted on the receiving end. A change in the silica refraction index can be used to measure temperature changes along the sensor. Using multiplexing, or the combination of multiple signals into one signal, many other parameters, like displacement, can be measured with fiber-optic sensors.
Developed jointly by Victoria Public Sector Innovation Fund (PSIF) in Australia, the Palo Alto Research Center (PARC) and University of Melbourne (UoM), the monitoring system, Fibridge, uses fiber-optics sensors. Embedded sensors measure the “structural strain, thermal response, bending moments, shear/impact loads, and corrosion of bridges”. This combined information provides a broad view of the health of the bridge.
The system uses the internet of things, detection technology and algorithms to “enable effective real-time monitoring, performance management, better reliability, improved safety, and optimized bridge design”.
The smart monitoring system has been tested on VicRoads highway bridge in Melbourne, Australia. Researchers considered the study on that bridge to be successful. It shows the value in using fiber-optic sensors in structural health monitoring. The program will now expand to include multiple other bridges as part of an extended trial in Victoria, Australia. If all goes well, researchers expect to expand the technology to other structures that have complex and expensive monitoring, like buildings.