Monitoring Strategic Hydraulic Infrastructures by Brillouin Distributed Fiber Optic Sensors

We present a case study of a Structural Health Monitoring (SHM) hybrid system based on Brillouin Distributed Fiber Optic Sensors (D-FOS), Vibrating Wire (VW) extensometers and temperature probes for an existing historical water penstock bridge positioned in a mountain valley in Valle d’Aosta Region, Northwestern Italy. We assessed Brillouin D-FOS performances for this kind of infrastructure, characterized by a complex structural layout and located in a harsh environment. A comparison with the more traditional strain monitoring technology offered by VW strain gauges was performed. The D-FOS strain cable has been bonded to the concrete members using a polyurethane-base adhesive, ensuring a rigid strain transfer. The raw data from all sensors are interpolated on a unique general timestamp with hourly resolution. Strain data from D-FOS and VW strain gauges are then corrected from temperature effects and compared. Considering the inherent differences between the two monitoring technologies, results show a good overall matching between strain time series collected by D-FOS and VW sensors. Brillouin D-FOS proves to be a good solution in terms of performance and economic investment for SHM systems on complex infrastructures such as hydropower plants, which involve extensive geometry combined with the need for detailed and continuous strain monitoring.

Strain Monitoring of an Aluminium Joint with an Optical System

The stresses distribution can be easily determined in the cross-section of the elements but in a joint, the distribution of stresses is more complicated. Its complexity is also increased if stiffeners are added to the joint and if the connecting bolts are not positioned in a regular configuration. An aluminium cantilever with a two bolts connection is experimentally tested to determine the real capacity. Stiffeners reduce the stress intensity in the connection, but they are prone to instability problems if they are subjected to compression. In order to determine the real stress development in the stiffeners, the Digital Image Correlation (DIC) technique was used. This technique determines the strains in the loaded parts which then can be equivalated to the stress distribution. The paper presents the stress development in the compressed stiffeners of an aluminium joint considering also improvement solution for increasing the bending capacity by reducing the instability parameters.