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Japan's Daelim Group Co., Ltd. and Lazoc Co., Ltd. have jointly developed a "fiber AE sensor." The product can be used for long-term observation of structural changes in tunnels and other environments where the humidity is relatively high or there is a risk of gas ejection. By using optical fibers in the sensors and signal paths, it is possible to master the damage caused by the bedrock and the lining without the need for electricity.
An example of an optical fiber AE sensor is set in a mountain tunnel.
An optical fiber AE sensor is constructed by connecting a sensor portion mounted on a structure and a detection device provided outside the site using a cable as a signal path. The working principle is to use the sensor to detect the tiny elastic fluctuations (acoustic emission, AE) generated when the structure is deformed, thereby grasping the state of the structure.
The fiber expands and contracts due to external forces, and its internal light changes its frequency due to the Doppler effect. By detecting this amount of frequency variation, acoustic emission of the bedrock and concrete with the sensor installed can be measured.
Even with long-distance transmission, the loss of light is very small, and therefore accurate detection can be performed even if the distance between the sensor and the detection device is long. In the absence of an amplifying device, detection can be carried out at a distance of up to 4 km.
The sensor is an optical fiber rolled into an oval, and then made of 26 mm high by 15.5 mm wide by 15 mm long coil shape. Since the center portion of the optical fiber is made of quartz glass, it is not only heat-resistant but also not easily corroded by water and chemicals.
Example of a metal body set in a bedrock layer. The sensor is built into the body.
Fiber AE sensor shape and size.
5 to 10 years will be cheaper than electrical
The previous AE detection technology used electrical methods. Therefore, in order to use it for a long time in a place with severe environmental conditions, sensors and cables must be specially treated. For example, in tunnels where there is a risk of flammable gas emissions, the sensor must be of explosion-proof construction.
The fiber-optic AE sensor does not require electricity because it does not energize the sensor and the cable. And will not be affected by electromagnetic waves and thunder.
Compared with conventional electrical AE sensors, fiber-optic AE sensors can reduce operating costs despite higher initial costs. The Daelim Group stated that after 5 to 10 years, the initial and operational costs would be lower than the electrical AE sensor.
The Daelim Group applied fiber-optic AE sensors to the detection of the bedrock firmness five years ago in the project of the "Po-National Oil and Gas Reserve Base" contracted by Japan's Oil and Gas Mineral Resources Agency. So far no transmission failure has occurred and the sensor has been in normal operation.
In the future, in addition to the maintenance and management of infrastructure such as tunnels and bridges, the Daelim Group also plans to propose proposals for long-term detection of radioactive wastes, such as geological disposal. (Reporter of "Nikkei Civil Engineering": Morishita Shinichi)
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