Research On Ultrasonic Testing Of Welds Based On Synthetic Aperture Focusing Theory

The welding seam is a very important link in the mechanical connection structure.The traditional ultrasonic testing(UT)of the welding seam is based on the reflected signal of the defect to determine the size,shape and nature of the defects.There is deviations in defect ultrasound imaging,missed detection or misjudgment of defects.The synthetic aperture focusing technology(SAFT)is a post-processing technology for the data processing in the ultrasonic flaw detection.A small aperture is used to synthesize a large aperture to form a point-by-point focus signal,which improves the resolution of scanning image and can effectively solve the above problems.In the thesis,the research works are carried out including the following aspects:1.The definition on the nondestructive testing(NDT)and the working principle of the common detection technologies are discussed.The current domestic and international research status about the synthetic aperture imaging technologies and theories about the ultrasonic testing of welds are described.The principles about the ultrasonic phased array and the time of flight diffraction(TOFD)detection are introduced.The corresponding experimental research are carried out.The synthetic aperture-based ultrasonic weld imaging algorithm is proposed.The research on the application of the finite element numerical simulation in the ultrasonic testing of welds is done.2.The synthetic aperture-based ultrasound weld imaging algorithm is proposed.The mechanism about the formation of the delayed superimposed beams is studied.The calculation of the focus deflection delays is deduced.The research about the beam simulation is done.It is shown that the lateral resolution of ultrasound imaging is determined by the width of the main lobe.The contrast of the ultrasound imaging is determined by the amplitude level of the side lobe.At the same time,the effects of variables such as the array element spacing,the number of array elements and the number of sub-aperture array elements on the beam simulation results are analyzed.3.Finite element simulation is performed using ANSYS software to establish the sound field simulation model.The theoretical value about the time of the reflected wave of the defect that the receiving probe obtains is used as the reference.The errors between the simulation values is 3% and the errors between the experimental values is 8%.The accuracy in the simulation results is much higher.The numerical simulation results by the TOFD method are compared with the experimental signals.It is shown that the error range of the experimental values is 2%-11% and the error range of the simulation values is 3%-8%.The error range is much lower than the allowable range in the engineering applications.The simulation accuracy is 44% higher than the experimental accuracy.The simulated diffraction wave error is 2%-5% smaller than the experimental error.The simulation method matches the theoretical situation.The feasibility of the simulation-assisted experimental determination method is demonstrated.4.The method about the ultrasonic detection of welds based on synthetic aperture focusing theory is proposed.The Multi2000 ultrasonic experimental platform is used to scan the defect points on the weld specimen to obtain the ultrasonic image and the original B-scan data.The synthetic aperture focusing algorithm is used to process the ultrasonic image and the data.The echo image resolution before and after processing is estimated according to the-6d B method.The resolution of the B-scan image processed by the synthetic aperture focusing algorithm is at least 25% higher than the resolution of the original image.