Ultrasonic Post-process Imaging For Irregular Layered Media Based On Virtual Source

Ultrasonic imaging detection is widely used in aerospace,petroleum,ship,railway and other fields because of its advantages such as visualization of defect display,easy qualitative and quantitative analysis,high detection efficiency and so on.Many pivotal structural parts in the above-mentioned industrial fields have complex curved surface structures.The stress concentration often occurs in the process of use,which is easy to produce various defects,causing hidden danger and harm to the safety of the structure.In the process of ultrasonic testing,in order to ensure the effective incidence of acoustic waves into curved surface components,water immersion or filling wedge is usually used for coupling.This coupling mode enables sound waves to propagate in a doublelayer medium consisting of coupling medium and tested components.The traditional synthetic aperture focusing technique(SAFT)based on the assumption of constant acoustic velocity medium is difficult to apply.Although SAFT can be combined with Fermat's law or Ray Tracing to detect surface components,the above methods need to know the surface function of components in advance,and the algorithm is complex,lead to a slow imaging speed.In order to solve the problem of low detection efficiency of irregular layered media by traditional synthetic aperture focusing imaging method,a synthetic aperture focusing technique based on virtual source(Virtual Source Synthetic aperture focusing technique,VS-SAFT)is developed in this paper.Firstly,the principle of Delay And Sum imaging in single medium and double medium is introduced.On the basis of analyzing the principle of virtual source construction based on pulse-echo method,a imaging algorithm of virtual source-synthetic aperture focusing technique is derived.Then,the imaging detection of convex and concave components is carried out by numerical simulation and experiments.The conclusions are as follows:(1)The B-scan and VS-SAFT post-processing images of convex curved component with radius of curvature 60 mm and concave component with radius of curvature 200 mm are obtained by water immersion test and numerical simulation.The results show that the VS-SAFT post-processing image based on the water immersion experiment and numerical simulation can accurately characterize the correct location of the defect and can measure the surface structure of the material through the virtual source coordinates.Simulation results are in good agreement with the experiment.(2)VS-SAFT replaces the excitation source-refraction point-image point propagation path with the excitation source-virtual source-image point by establishing a virtual source on the surface of the component.The post-process focusing image of components with an unknow surface is obtained while the refraction point calculation is avoided.The parameters used in the imaging implementation include only the pitch of the array,the sound velocity of the medium,the sampling frequency and other usual parameters,no need additional hardware facilities.(3)When the virtual source is constructed by VS-SAFT,the parallel assumption that the sound wave is excited by adjacent elements at the interface causes the error of the virtual source coordinates,and the error value increases with the decrease of the radius of curvature on the surface of the inspected component.The simulation results show that when the thickness of the water layer is 10 mm and the element pitch is 1mm,VS-SAFT can effectively represent the internal defect position of the convex component with the curvature radius of 15 mm and the concave component with the radius of curvature of 20 mm.(4)The error between the virtual source and the interface increases with the increase of the thickness of the water layer when the surface component is immersed in water.For convex components,the thickness of water layer decreases the resolution of defects in VS-SAFT images.For concave components,when the thickness of the water layer is greater than or equal to the radius of curvature,the sound wave propagates in the form of focus or rotation after reflection on the concave interface,meanwhile,the propagation of acoustic wave in the concave component becomes complex,which affects the imaging quality of VS-SAFT.Theoretically,when the thickness of the water layer is greater than the radius of curvature,VS-SAFT is no longer applicable.