Research On Ultrasonic Detection Method For Concrete Defects Based On Φ-OTDR

The distributed acoustic wave sensing system,which based on Φ-OTDR technology,realizes long-distance and high-density distributed vibration sensing by detecting and demodulating the phase information carried by Rayleigh scattering optical signals,which is widely used in oil exploration,pipeline security and other fields.The study of this article is Establishing an ultrasonic testing system to detect and extract ultrasonic signals in concrete based on the Φ-OTDR technology,and achieve the detection and positioning of defects in concrete.The main tasks are as follows:In order to achieve the acquisition and processing of ultrasound signals,a high-speed upper computer acquisition software was designed and developed.After testing,the upper computer software received and processed UDP data packets at a rate of nearly 3Gbps,with a continuous and stable storage rate of 2Gbps.It can store ultrasound signals collected by the DAS system in real-time at a sampling rate of 500 k,greatly improving the speed of DAS system acquisition and processing of ultrasound signals.Aiming at the problem that the signal noise ratio of the original ultrasonic testing signal is low,and the signal components reflecting the characteristics of concrete structure are difficult to extract,the wavelet de-noising simulation verification shows that the denoising effect is the best when the sym8 wavelet function is used for 4-layer wavelet decomposition,the signal-to-noise ratio is increased by 10.17 d B,and the root mean square error is 0.1056;Attempting to zero the fourth approximation component of the original detection signal before performing wavelet threshold denoising results in a higher signal-to-noise ratio and reduced distortion compared to traditional IIR bandpass filtering.Aiming at the problem of how DAS system can effectively detect the internal defects of concrete,the propagation characteristics of ultrasonic waves in concrete specimens were simulated using finite difference software,and two ways for the DAS system to detect concrete defects were confirmed:(1)extracting the reflected echo signal of the defect interface;(2)Detect direct wave signals with delayed arrival.Set up a Ultrasonic testing system,and successfully extract the ultrasonic signal in the 20～160k Hz frequency band from the concrete;Finally,the experiment verifies that the DAS system is an effective way to use Ultrasonic testing to detect concrete defects.The results show that the internal defects of concrete specimens can be effectively detected by detecting the ultrasonic direct wave signal in concrete specimens through the fiber ring array.To address the issue of how the DAS system can effectively locate internal defects in concrete,a mathematical model for detecting concrete defects using penetration method is established.The problem of deriving the coordinate position of the defect is transformed into solving the problem of ultrasonic propagation time in each concrete unit.The coordinate position of internal defects in concrete is accurately derived,which is basically consistent with the actual coordinate position of the defect.