Ultrasonic Imaging Evaluation With Composite-mode Total Focusing Method

The profile reconstruction is helpful for visualizing defect characteristics and qualitative and quantitative analysis in ultrasonic testing.However,the current research on profile reconstruction mainly focuses on the known defects,and it is difficult to reconstruct the whole profile of priori unknown defects.In order to solve the above problems,the composite-mode total focusing method(CTFM)is proposed by selecting several mode waves for composite superposition,considering the directivity of different mode waves.The method is used to reconstruct the profile of regular cracks,side-drilled holes and irregular cracks.The main researches are listed as follows:(1)Considering the different directivity and limited representable range of mode waves,the views corresponding to multiple mode waves can be superimposed to expand the representable range.At the same time,in order to improve the imaging efficiency,it is necessary to select a number of mode waves with uniform detection range and less overlap for composite superposition.In this paper,a 45° longitudinal-wave wedge was taken as an example to calculate the directivity of all 21 mode waves,and the selection principle of mode wave was determined.Finally,eight mode waves were selected for imaging by CTFM.(2)In order to explore the influence of the number of mode waves on CTFM imaging quality,all 21 mode waves and eight mode waves were used to image regular cracks with different orientations.The results showed that both methods can obtain whole crack profiles and the quantitative errors were less than 10%.It is shown that the reconstruction effect of a small amount of mode wave is the same as that of all mode waves,but the computational amount is about one third of that of the latter.(3)In order to explore the ability of CTFM in profile reconstruction of different types of defect,several groups of adjacent holes with the same orientation angles as regular cracks were designed and imaged by CTFM.Compared with the imaging results of cracks,the characteristics of holes were intuitively and clearly presented in each reconstruction image,indicating that the planar and volumetric defects could be accurately characterized by CTFM.The quantitative errors of hole diameters,orientation angles and central depths were not more than 0.18 mm,3.41° and 0.55 mm with-6 dB method,which verified the ability of CTFM in quantitative detection.(4)The influencing factors of CTFM are analyzed from three aspects: wedge angle,the size of probe aperture and FMC data acquisition position.The wedge angle determines the type and directivity of mode waves in specimen,and affects the representable range of CTFM.The size of probe aperture mainly determines the coverage range of main beam and affects the effect of profile reconstruction,which is mainly related to the thickness of specimen.The integrity of defect signal is related to the FMC data acquisition position.When the FMC data acquisition position is not appropriate,the profile is incomplete reconstructed by CTFM.(5)In order to further expand the application range of CTFM,a preliminary exploration was made on the profile reconstruction of irregular crack.The reconstruction quality was improved by the CTFM with diffusion correction,considering the difference of echo energy due to different propagation paths of mode waves.At the same time,because the echo from defect may be shaded by the part of defect,it was difficult to reconstruct the whole profile of irregular cracks by unilateral imaging.Therefore,the imaging results of both sides of defect are superimposed again to solve the above problems.