Research On Theory And Application Of Automatic Nondestructive Testing For Cylindrical Components Based On Phased Array Ultrasonic Imaging

Cylindrical components such as pipe,axle,bar,etc.,are widely used in the field of national economy,national defense construction and high technology,and the operating performance and service life of the instruments are directly determined by the quality of cylindrical components,so the demand for nondestructive testing(NDT)technology is very strong.Although there are various NDT methods,the ultrasonic NDT has competitive technology advantage over other NDT methods,especially,the phased array ultrasonic technology which has the capability for flexible beam synthesis and adaptive control,can easily meet different application requirement and effectively reduce the dead zone and blind area.Therefore,the phased array ultrasonic technology has become one of the research focuses in the field of NDT,and is moving towards the direction of automatic ultrasonic imaging testing driven by the trend of nondestructive evaluation,thus,it is of great development potential for the quantitative NDT of cylindrical components.However,the automatic NDT prefers to be carried out in the immersion environment,and the immersion testing can be regarded as testing of a layered object in which the propagation characteristics of ultrasonic wave is quite complex.As a result,the ultrasonic signal is affected by the interface acoustic effect in addition to the excitation pulse and the medium,besides,transmitting and receiving the ultrasonic signal during the scanning movement of the transducer will aggravate the diffraction effect.Due to these complicated resolution affecting factors,the existing phased array ultrasonic imaging technology has a lot of shortcomings in the imaging resolution and realtime performance,and it is difficult to satisfy the requirement of automatic quantitative NDT of cylindrical components.According to the background above,the research on theory and application of automatic nondestructive testing for cylindrical components based on phased array ultrasonic imaging is carried out in this doctoral dissertation which is supported by National Natural Science Foundation of China(No.51175465).On the basis of analyzing the research status and development tendency of ultrasonic NDT and phased array ultrasonic imaging,and understanding the testing mechanism of phased array ultrasonic imaging of cylindrical components,beam-domain adaptive forming of phased array ultrasonic beam,phased array ultrasonic sparse inversion imaging and full-matrix frequency-domain imaging in layered structure are mainly researched.Meanwhile,by combining the morphological preprocessing and image segmentation method with analysis of imaging results,the objective to quantitative NDT of cylindrical components is achieved.Furthermore,a phased array ultrasonic imaging inspection system is developed for the automatic NDT requirement of seamless steel pipe.The specific research contents and innovative points of this dissertation are presented as below:In chapter one,the great impact of the cylindrical components on the development of national economy and the construction of modern national defense,and the signification of research on the automatic phased array ultrasonic imaging inspection technology are elaborated.By analyzing the research status and development tendency of ultrasonic NDT and phased array ultrasonic imaging,the problems on relevant theory and technology of phased array ultrasonic imaging inspection are clarified,indicating the research direction in this dissertation.Meanwhile,detailed contents of this dissertation are also listed.In chapter two,the theoretical basis of phased array ultrasonic imaging inspection for cylindrical components are researched.Based on the wave equation and its general solution in cylindrical coordinates,the propagation mechanism of acoustic wave in a cylindrical medium and the factors that affect the imaging resolution are clearly defined,and the radiation field of a phased array ultrasonic transducer is calculated and analyzed by spatial impulse response acoustic model.Meanwhile,by considering the layered cylindrical structure as application object,the control method of phased array ultrasonic incident beam is researched,implementing beam deflection and focus.These contents lay a theoretical foundation for the following study.In chapter three,a beam-domain adaptive forming technique of phased array ultrasonic beam is proposed,and the realtime performance of beamforming is effectively improved.After the echo signals received by the phased array elements are transformed to beam domain,then in beam domain,the objective signal subspace is approximated by the high order power multiplication of the inverse of the signal covariance matrix,the weight vector for beamforming is obtained by the minimum variance method and is projected to the signal subspace,the optimal weight vector in beam domain is determined,resulting in the realization of adaptive beamforming.The simulated and experimental results indicate that the proposed technique can guarantee the lateral and contrast resolution of phased array ultrasonic imaging and substantially reduce the computation quantity,in favor of practical engineering applications.In chapter four,a phased array ultrasonic sparse inversion imaging in layered structure is proposed,and the phased array ultrasonic imaging based on beamforming is implemented with high resolution and efficiency in layered structures.By establishing the linear model of phased array ultrasonic imaging which includes the acoustic diffraction and electrical effect using the impulse response method,the image reconstruction is transformed to a regularized inverse problem based on the sparse distribution of defects,an optimization objective function which contains l2-norm as well as l1-norm the optimal solution is obtained by a separable approximation algorithm,then the high resolution imaging result is quickly achieved.The simulated and experimental results indicate that the proposed technique can obviously enhance the lateral and vertical resolution of phased array ultrasonic imaging with high realtime performance.In chapter five,a phased array ultrasonic full-matrix frequency-domain imaging in layered structure is proposed,and the full-matrix imaging technique based on virtual focus is extended to the application of layered structures.The frequency-domain expression of ultrasonic array receiving signal model in full-matrix capture mode is established by Fourier transform,and the spectrum of receiving field at any depth in the layered structure is reconstructed recursively.Simultaneously,the ultrasonic array transmitting field is reconstructed as well and correlated with the receiving field,then the imaging condition is applied to realize virtual focus at the imaging point,in result the aim of high resolution full-matrix imaging in layered structures is achieved.The simulated and experimental results indicate that the proposed technique is suitable for phased array ultrasonic full-matrix imaging in layered structures with high imaging resolution and realtime performance.In chapter six,phased array ultrasonic image processing technique is researched and the quantitative ultrasonic NDT of defects is realized.On the basis of adaptive wavelet threshold denoising,bilinear interpolation and morphological opening and closing reconstruction for phased array ultrasonic image,the defect image region is determined by automatic imagesegmentation and boundary tracking method,and the shape and size of the defect is obtained by local normalization and twice image segmentation of the defect region,then the automatic quantitative inspection of defects is implemented.In chapter seven,according to research results from the inspection theory and key techniques of phased array ultrasonic imaging in above chapters,the automatic NDT for cylindrical components based on phased array ultrasonic imaging is researched.After completing the overall design,the multi-channel phased array ultrasonic testing instrument is developed based on the PXI bus virtual instrument architecture system.Then combining with mechatronics technology,an automatic phased array ultrasonic imaging system for cylindrical components is developed.Meanwhile,the practical experiment of seamless steel pipe is performed on this testing platform and automatic phased array ultrasonic imaging inspection of pipe is implemented to verify the feasibility and effectiveness of relevant theory and techniques proposed in this dissertation.In chapter eight,the research achievements and the innovative points of this dissertation are summarized,and the future research works are also forecasted.