Research On Monitoring Optical Fiber Acoustic Emission Sensor Of Crack Initiation For Towery Steel Structures

Modern acoustic emission technology has been developing since1950s. Acoustic emission technology has been extended from laboratory to engineering applications. Testing frequency has been increased from audio frequency to tens of MHz. Detection instruments have been improved to detection and analysis system of multichannel, digital form and high computerization. However, the piezoelectric acoustic emission sensor, which the electricity preamplifier should be closed with, is currently widely used. Both of the sensor and the preamplifier are sensitive to mechanical and electrical noise. Besides, the sensor's work band is narrow and the sensor must come into contact with the specimen to be tested. While the fiber optic acoustic emission sensor is not sensitive to electromagnetic interference, its work band is wide and it does not have to contact with the specimen. What's more, the sensor is small in size, and can be placed far apart with the relevant electrical equipments. Optical fiber acoustic emission sensor has become a new research direction in the field of acoustic emission detection technology.In the paper, based on optic fiber Fabry-Perot sensor and acoustic emission detection technology, by research on the sensor's response to the acoustic emission signal excitated in crack initiation and expansion, it was demonstrated that the designed optic fiber Fabry-Perot sensors could be used for online monitoring the crack initiation of towery steel structure. The main work of the article is:(1) Acoustic emission testing technology in the field of metal material was analyzed, optical fiber sensing technology in the field of structural damage detection was summarized, and optical fiber Fabry-Perot sensors for crack damage were designed.(2) According to the working principle of optical fiber Fabry-Perot sensor and propagation characteristics of the acoustic emission signals in metal material, two kinds of optical fiber F-P acoustic emission sensors were designed and fabricated, one with diaphragm, and the other with capillary glass tube.(3) Performance of the two kinds of fabricated sensors was tested, including the basic optical properties and temperature characteristics of the sensors. From the experimental results, the sensors were with good optical performance; and the sensor with capillary glass tube was with good temperature stability and reproducibility:in the process of temperature rising from-20℃to80℃, one single level peak drift of interference stripes less than7nm; while the peak drift of the sensor with diaphragm the changed linearly.(4) Acoustic emission signals, generated when loads were applied on the crack of metal specimen, was detected by the optical fiber acoustic emission sensors, and weak acoustic emission signals, generated when pencil leads were broken on the surface of the metal specimen, could be also detected by the optical fiber F-P sensor with diaphragm. Therefore, it was demonstrated that the designed optic fiber F-P sensors could be used for detection of the crack of steel structure.(5) Based on the summary of the experimental study, measures to improve the sensors for detection of the steel structure crack were raised, and detection system based on the sensors was looked forward to.