Area Type Defect Imaging Method Based On Multi-mode Synthetic Aperture Focusing Technology

Area type defect is a kind of highly harmful defects,and it is of great significance to carry out nondestructive testing.Ultrasonic testing technology has been widely used in the detection of this defect.With the development of industry and the requirement of high performance manufacturing,the requirement of NDE is not only to detect defects,but also to accurately represent the shape and size of defects.The existing ultrasonic detection technology can detect the defect and visualize the image of this defect,but it is still very difficult to characterize the whole shape of this defect.In order to solve the above problem,this paper based on synthetic aperture focusing technique(SAFT)proposed a multi-modes SAFT method,which effectively receive the reflected echo after wave conversion occurred in the test block and achieve a complete characterization of the defect area.This paper discusses the principle of multi-modes SAFT,established a steel model with area type defects,the use of simulation for the detection and signal acquisition model of simulation software,the signal processing algorithm is written,reconstruction of the detection with B scan images,the method demonstrates the resolution,the ability to distinguish between the qualitative defect detection ability of the different orientation of defects,the specific contents are as follows:(1)Based on the principle of synthetic aperture focusing technique and the method of image reconstruction,the classification of the multi-modes SAFT is studied,discussed the beam propagation path of direct mode,half-skip mode and full-skip mode,according to the delay and sum principle,three kinds post processing algorithms of multi-modes SAFT are written.The relationship between the frontier and the defects of probe 8 half span mode and 4 mode full span horizontal distance and wedge angle,specimen thickness and defect depth is discussed through the geometry obtained at the shear block mode is advanced into the defects and the minimum horizontal distance of probe.Based on the geometric relationship,the relationship between the horizontal distance of the probe tip and the defect and the wedge angle,the thickness of the test block and the depth of the defect is discussed,got a conclusion of the horizontal distance between the probe edge and the defect is the lowest when the transverse wave is shear wave.(2)The model of carbon steel with deep 30.0 mm and long 5.0 mm longitudinal crack was established,and the quantitative error of the multi-mode SAFT was less than PAUT and the traditional SAFT technique,and it could be used to characterize the overall defect morphology.Compared with the results of multi-modes SAFT and traditional SAFT,it could be found that the multi-modes SAFT can clearly distinguish the two defects.Based on the detection of longitudinal cracks with different lengths,the multi-modes SAFT technique can be used to characterize the overall shape of the 1.0 mm defects,which has high detection resolution and qualitative discrimination.(3)The model of carbon steel with 5.0 mm length,0°-75° orientation is established,the overall shape of 0°-20° cracks can be achieved by the TTT mode,the overall shape of 0°-10° cracks can be achieved by the TLL mode,the overall shape of 0°-20° cracks can be achieved by the TTT mode,the overall shape of 45°-60° cracks can be achieved by the TTTT mode.The quantitative error of the three modes is 0.28 mm,0.29 mm and 0.68 mm,respectively.