Research On Defect Detection And Imaging Algorithm Of Shell Weld Based On Post Beamforming

Ultrasonic imaging testing technology is widely used in the detection of shell weld defects to obtain the size,position and other information of defects to provide a visual basis for defect detection and evaluation.Beamforming,which can be divided into prebeamforming and postbeamforming,is the key to achieve high quality imaging in the detection area in ultrasonic imaging.Prebeamforming is to form wavefronts in different directions or realize beam focusing by adjusting the excitation of time sequence.This method has high requirements on hardware circuits,and the increase of circuit complexity and cost restrict the popularization and application of this technology.Then postbeamforming is to process the received signal to achieve the purpose of later beamforming.Although the postbeamforming reduces the dependence on hardware,it still has some problems such as high complexity of operation and large amount of data processing.Therefore,a postbeamforming method combined spatial sampling with coherence factor is proposed in this thesis which reduce the amount of processing data and the computational complexity at the same time.The validity of the proposed method was verified through different defects detection and imaging tests.Based on the analysis of the development status of postbeamforming,the relationship between decimation factor and array element number is deduced.Then,spatial smoothing and diagonal loading were performed on the sampled data,and the weighted values were calculated based on the principle of minmum variance.After coherence factor was applied,the output of the beamformer was obtained,and the total focus imaging under the condition of postbeamforming was realized.The main research work and conclusions of this thesis are as follows:(1)A postbeamforming algorithm based on spatial sampling and coherence factor fusion was proposed.Firstly,by analyzing the beam mode of complete echo data and sampling data,the relationship between the decimation factor and the array element number was deduced,and the spatial sampling of data was realized.Secondly,after preprocessing the sampled data,the weighted value of the sampled data was calculated based on the principle of minmum variance,and weighted the sampled data.When calculating the weighted value,the sampled data covariance matrix was constructed as Toeplitz matrix structure,which reduced the complexity of matrix operation.The coherence factor was introduced to weight the sampled data.And the proportion of coherent energy in the sampled data was enhanced to improve the imaging quality finally.This method can effectively reduce the computational complexity of the algorithm from three dimensions to two dimensions,and the imaging time can be reduced by more than 85% compared with the conventional minimum variance beamforming method.Under the same spatial sampling condition,the imaging effect of this method is better than that of other algorithms.(2)A total focus algorithm for double-layer media based on postbeamforming was proposed.The data processed by postbeamforming was used for total focus imaging.The propagation characteristics of ultrasound and the propagation mode of acoustic beam in double-layer medium were analyzed firstly.According to Fermat's principle and Snell's principle,the incident and reflection points of the acoustic beam at the interface were calculated secondly.The time index value was calculated finally,and the amplitude of the corresponding imaging point can be obtained according to the time index.This method can improve the imaging resolution of cracks effectively.(3)A test platform for defect detection of shell weld was built.The test objects are processed standard test blocks with cracks and horzontial holes and embedded defect shell weld test blocks.Through ultrasonic array defect detection and postbeamforming imaging,the imaging resolutions of several defects were compared and analyzed by array performance indicator.The results show that the method combined postbeamforming technology with total focusing algorithm can improve the imaging resolution effectively,which provides a basis for quantitative evaluation of defects.