Experimental And Numerical Studies Of The Oblique Defects In The Pipes Using Chaotic Oscillators Based On Ultrasonic Guided Waves

Based on the current problems and challenges of ultrasonic guided wave detection of the oblique defects in pipes, a new defect detection method using chaotic oscillators is proposed in this dissertation. The research significance is introduced firstly; and then the main methods of pipe defect detections are classified; finally, the recent advance in the study of pipe defect detections using ultrasonic guided wave is summarized. Effect of defects on the pipage was analyzed based on the corresponding experimental and numerical studies. Considering the angle and length of defects, wave attenuation, and weak wave signal caused by propagation distance, an improvement on defect detection efficiency using the chaotic oscillator detection technology was developed.The longitudinal ultrasonic guided waves in oblique defects pipes were investigated experimentally and numerically in the dissertation. The experimental devices, experimental specimen and experimental method are introduced. The group velocity curves are calculated by using the program Disperse. Experimental results show that the longitudinal ultrasonic guided wave is sensitive to the defects in pipes; the velocities of experimental signals are close to that of L(0, 2) mode for intact pipe calculated by DISPERSE; the reflection coefficient of the reflected signal from defects decreases with the increase of the angle; the 45-90°defects were not recognized by band-pass filter and FFT Smoothing. Based on the experiment, the corresponding simulations were conducted by using finite element software LS-DYNA. Numerical results show that the velocities of both experimental and numerical signals are close to that of L(0, 2) mode for intact pipe calculated by DISPERSE; the longitudinal ultrasonic guided wave could detect to the defects in pipes; the echoes from defects were weaker with the increase of the angle of defect; the reflection coefficient of the reflected signal from defects decreases with the increase of the angle of defect; longitudinal ultrasonic guided wave is not sensitive to the longitudinal defects in pipes. Numerical result is agreed with the experimental result.Simulations of the straight through oblique defect in pipes were conducted by using finite element software ANSYS, where stress method is used to solve the stress intensity factor. The performance of resisting the internal pressure of the pipeline with oblique defect is analyzed. The stress at the oblique defect tip is used for calculation stress strength factor. Meanwhile, the influences of defect angle, defect length, defect aspect ratio, pipe radius, and pipeline internal pressure are further studied. Numerical simulation results show that oblique defects are harmful to the pipeline; tensile stress perpendicular to oblique defect is the main factors causing pipes damaged; Keq is positively correlated with KⅠ; stress strength factor increases with the increase of the angle of defect; KⅠ increases with the increase of defect angle, defect length, defect aspect ratio, pipe radius, and pipeline internal pressure, respectively.A novel method of the defect detection for pipes is proposed based on the Duffing-Holmes chaotic oscillator. Research shows that the chaos system is immune to noise under a suitable noise level; the proposed method could be used to inspect numerical guided waves; the proposed method could be used to inspect weaker experimental guided waves. The tested records of the oblique defects in the pipes and the phase trace depending on the selected signal are not correlativity. The method of Lyapunov exponents based on Duffing-Holmes chaotic oscillator is effective recognition depending on signal of the 45°oblique defects pipes.Another method of the defect detection for pipes is proposed based on the Van der Pol chaotic oscillator in this dissertation. The defect location of two kinds of Van der pol system depending on the guided wave is introduced. Van der pol system and cubic Van der Pol system are proposed and compared. Researches of immuned to noise of two kinds of Van der pol system are discussed. A method of determination of Lyapunov exponents based on the two kinds of Van der pol system is introduced. Research shows that the chaos system is immune to noise under a suitable noise level; the proposed method could be used to inspect numerical and experimental guided waves; The Dif of cubic Van der Pol system is 1.5-1.73 times to the value of Van der Pol system; the method can get the defect location depending on the guided wave; the method of defect location is effective recognition depending on signal of the 0-45°oblique defects pipes. These results are worthy of engineering application in both extending the detection range of ultrasonic guided waves and increasing the sensitivity of detecting micro defects.