Using Fiber Optics to Pinpoint Structural Problems Early

A new optical system can detect problems in structures such as natural-gas pipes and concrete columns more precisely and potentially earlier than before. University of Ottawa physicists presented the invention this week at the Optical Fiber Communication/National Fiber Optical Engineer Conference (OFC/NFOEC) in Anaheim, Calif.

Already being considered for commercial production, the new system can catch much earlier signs of costly and dangerous structural failures than previously possible.

Called the Distributed Brillouin Sensor (DBS), the system uses fiber optics to detect deformation, cracks, and bending in two structures under real-world conditions.

In one demonstration, conducted with Edmonton-based engineering firm C-FER Technologies and TransCanada PipeLines Ltd., the Ottawa researchers (Xiaoyi Bao, xbao@uottawa.ca) deployed the DBS system on a section of steel pipe designed to transport natural gas. Laying 10 lines of optical fiber along the axis of the pipe, they created one pulse of laser light and one continuous light wave, each traveling in opposite directions in the fiber.

When the researchers applied tension and compression to the pipe to mimic real-world disturbances, it produced vibrations (sound waves). Through a phenomenon called the Brillouin effect, these sound waves then slightly changed the speed of light in the affected part of the fiber and consequently altered the frequency difference between the two propagating light waves, providing precise information on the mechanical strains that were applied to the pipe.

Unlike present structural health analysis, which is done on a spot-by-spot basis, DBS can detect problems over all points in the entire structure and pinpoint the location of a structural deformation to within 5 centimeters, while measuring mechanical strains as low as 20 microstrains (20 millionths of a strain, a dimensionless property that generally reflects the structure's change in length over its original length). This exceeds the 1-meter resolution and 50 microstrains that the construction industry has wanted and expected.

In another demonstration, conducted with civil engineers at the University of Ottawa, the researchers tested the DBS system on a concrete column encased with fiber-reinforced rods and sheets. Subjecting the column to simulated seismic forces such as those that would occur in an earthquake or tsunami, the researchers could detect signs of debonding (in which the concrete detached from the fiber casing) and the cracks (crushing) of concrete as a result of compression forces.

Unlike competing techniques, the system could readily tell the difference between debonding and crushing. The Ottawa researchers say that DBS can prevent potentially life-threatening and environmentally damaging accidents and multimillion-dollar repairs. In addition, the technique can improve the testing of structures and materials by providing valuable information during the testing process.

Paper OTuL7 at OFC/NFOEC meeting