Photonic crystal-based strain visualization film is promising for detecting the age-related deterioration of large man-made structures and public infrastructures. However, as the number of target structures increases, monitoring them all will become a major problem. We propose two solutions: (1) a portable solar-battery-powered automated monitoring station to monitor the color of photonic coatings, and (2) the application of real-time image analysis using mobile phones to record color changes. Both solutions make use of the power of small computers, while the former assists us with efficient data collection, and the latter helps non-experts to inspect structures without using expensive spectroscopes.
The portable monitoring station consists of a micro-computer connected to a 3G mobile network, a USB camera, and a solar battery system installed in a waterproof box. Photographs of the strain visualization film are taken once every hour and, at all other times, the computer disconnects the camera to save electricity. We placed four monitoring stations in the shade of a bridge or a tree and ran them continuously for more than a year.
The application displays a real-time image in which only the strain-free area of the film is extracted. As a result, the region under strain and the background appear in white. This software runs on many mobile computers with built-in cameras and the OSs including Android, iOS, Windows, and Linux. This is possible due to the versatility of the computer vision library we used, namely OpenCV, which is widely used in robotics and automatic car-driving.
We have been developing smart photonic coating for structural materials to visualize strain mapping on steel or aluminum and to detect cracks on concrete. In the technology, strain imaging sheet made of colloidal photonic crystal film coated on a polyethylene terephthalate sheet. The strain imaging sheets change structural color by mechanical deformation. Now we have been testing smart photonic coating for real concreate bridges One of the key issues is the durability of the sheet for long-term use at the outfield site. In outdoor exposure test and in laboratory accelerated exposure test, polystyrene particles in the colloidal photonic crystal film were damaged and lost the structural color. To protect the deteriorating, top coat layer containing ultraviolet absorber was effective to reduce the damage of the colloidal photonic crystal film.
Here we will propose the conceptual new idea of the inspection of concrete bridge using smart materials and mobile IoT system. We apply opal photonic crystal film to detect cracks on concrete infrastructures. High quality opal photonic crystal films were coated on black color PET sheet over 1000 cm2 area. The opal film sheet was cut and adhered to concrete or mortar test pieces by epoxy resin. In the tensile test, the structural color of the opal sheet was changed when the crack was formed. As a demonstration, we have installated the opal film sheet on the wall of the concrete bridge. Our final purpose is the color change will be recorded by portable CCD devices, and send to expert via IoT network.
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