Research On Theory And Application Of On-line Ultrasound Imaging For Cylindrical Components Based On Synthetic Aperture

Cylindrical components such as pipe, bar, axle, etc., have been widely used in the national economy and the national defense, which gives rise to the strong requirement for the nondestruc-tive testing (NDT). Because of the high reliability, high safety, high flexibility, richness in char-acteristic parameter, convenience in combining with information technology, the ultrasonic NDT has become one of the most widely used and the most promising technique among the NDT meth-ods However, with the large scale of modern industrial production, as well as the relevant facilities becoming bigger, running faster, being used in extreme environment, the type, structure and size of cylindrical components are more various, their usage amount, working temperature and pres-sure are further improved. All of these put forward higher requirements on the on-line ultrasonic NDT technique. The on-line ultrasonic NDT perfers to be carried out with an exterior cylindrical transducer in the immersion environment. However, if the liquid coupling is taken as a prat of the inspected object, the immersion testing can be viewed as testing of a layered object, in which the propagation feature of sound waves is quite complicated. In addition, transmitting and receiving the ultrasonic signals during the scanning movement of transducer will make the diffraction ef-fect even more apparent. How to guaranteeing the resolution of the on-line ultrasound imaging for cylindrical components, there are still many theoretical and technical challenges. Meanwhile, transducers used in the practical application are classified as flat transducers and focused transduc-ers. Due to the differences between the wave pathes of flat and focused transducers, the imaging techniques that were developed based on the assumption of flat transducer are no long suitable for the application of the focused transducer.According to the background above, the research on theory and application of on-line ultra-sound imaging for cylindrical component based on synthetic aperture is carried out in this disser-tation, which is supported by National Natural Science Foundation of China (No.51175465). By analyzing the present research status and the development trend of ultrasound imaging technology, understanding the principles of wave propagation in cylinder and synthetic aperture imaging, the frequency-domain synthetic aperture imaging techniques for the cases of immersion testing using a focused transducer, non-uniform coupling condition and exterior cylindrical scan are mainly re-searched. Thus a frequency-domain synthetic aperture imaging technique developed for on-line quantitative NDT of cylindrical components is proposed, which has the capability of imaging with high efficiency and high resolution. Moreover, an on-line ultrasonic imaging system for cylindri- cal components is developed. The practical on-line experiment for a pipe is also carried out. The detailed contents and innovative points of this dissertation are presented as below:In chapter one, the influence of the cylindrical component on the development of national economy and the construction of national defense, the importance of research on the on-line ultra-sound imaging technology for the cylindrical component are introduced. By analyzing the present research status and the development trend of ultrasound imaging and synthetic aperture technol-ogy, the problems and bottlenecks on relevant theory and technique of synthetic aperture imaging are clarified. Meanwhile, the research direction is pointed out and the detailed contents of this dissertation are also listed.In chapter two, the wave propagation principle in cylinder and the basic theory of synthetic aperture imaging are researched. After establishing the wave equation in cylindrical coordinates, the general solution is obtained by the separation of variables in cylindrical coordinates to un-derstand the wave propagation principle in cylinder. Then combining with the calculation of transducer radiation sound field, the main factor that causes the decline in lateral resolution with increasing depth is clearly defined. Meanwhile, based on the basic principle of time-domain syn-thetic aperture imaging, the frequency-domain implementations are also studied. Since the imag-ing algorithm is operated in frequency domain, the frequency-domain synthetic aperture imaging has the capability of imaging with high efficiency and high resolution. These lay a foundation for the following study.In chapter three, a frequency-domain synthetic aperture imaging technique for immersion testing using focused transducer is proposed. By establishing the equivalent relationship between the conventional phase shift migration imaging method and the time-domain synthetic aperture imaging method, the difference of acoustic pathes between flat and focused transducers is con-verted into phase compensation. Then a recursive wave field extrapolation formula for immersion testing using focused transducer is derived. Finally, a synthetic aperture imaging technique based on phase shift migration is achieved to solve the overcorrection problem and improve the imaging resolution and efficiency.In chapter four, a frequency-domain synthetic aperture imaging technique for the non-uniform coupling condition is proposed. By calculating the root-mean-square velocity of the non-uniform coupling media and building a nonstationary filter, the spectrum resampling in tranditional method is transformed to filtering process in mixed domain. Then a non-recursive synthetic aperture imag-ing in frequency domain is developed for the application of non-uniform coupling condition, where the imaging technique based on phase shift migration is limited. These lay a necessary technical foundation for on-line ultrasound imaging.In chapter five, a frequency-domain synthetic aperture imaging technique for exterior cylin-drical scan in homogeneous medium is proposed. According to the general solution of wave equation in cylindrical coordinates, the frequency-domain expression of ultrasonic signal model in exterior cylindrical scan is established by Fourier transform, and the formula used for wave field reconstruction in homogeneous cylinder is derived. Then the spectrum of sound pressure on the cylndrical surface of interest is reconstructed. Finially, the focused image of homogeneous cylinder is obtained by extracting and stacking the image data at t=0 from the inverse Fourier transform result of sound pressure reconstructed. In addition, simulation and experiment are car-ried out to confirm the feasibility and effectiveness of the method proposed in this chapter.In chapter six, a frequency-domain synthetic aperture imaging technique for multilayer cylin-drical object using an exterior rotating transducer is proposed. Combining the formula used for wave field reconstruction in homogeneous cylinder and the acoustic effect of the interface between adjacent cylindrical layers, the spectrum of sound pressure on the cylndrical surface of interest is reconstructed recursively. Finially, the focused image of multilayer cylindrical object is obtained by extracting and stacking the image data at t=0 from the inverse Fourier transform result of sound pressure reconstructed. In addition, simulation and experiment are carried out to confirm the feasibility and effectiveness of the method proposed in this chapter.In chapter seven, an on-line ultrasonic imaging technique for cylindrical components is pro-posed on the basis of frequency-domain synthetic aperture imaging techniques which were pro-posed in the previous chapters. After completing the overall design, the multi-channel ultrasonic inspection device is developed in the architecture of virtual instrument. Then combining with mechatronics technology, an on-line ultrasonic imaging system for cylindrical components is de-veloped. Finally, the practical experiment is performed on this platform to verify the feasibility and effectiveness of this methodology proposed in this dissertation.In chapter eight, the research achievements and the innovative points of this dissertation are summarized, and the future works are also forecasted.