Research On Ultrasonic Lamb Wave Detection For Lapped Laser Welded Joint Of Stainless Steel Sheet

Aiming at the quality control requirements of Semi-penetration type lap laser welding joint in stainless steel urban railway carriage bodies,this paper has studied the basic theory of ultrasonic Lamb wave detection and obtained the frequency dispersion characteristics of SUS301 L austenitic stainless steel which commonly used in railway carriage bodies.In this paper,it was determined that A1 and S0 mode Lamb waves can be used as the information carriers for ultrasonic nondestructive detection of such thin plate lap welded joints,and its detection principle was explained theoretically.On this basis,a corresponding ultrasonic Lamb wave nondestructive testing method for Laser welded joint was developed.According to the established testing method,this paper designed an ultrasonic Lamb wave testing system for laser welded joints with a transmitter and receiver.The software system included functional modules such as ultrasound parameter setting,data display,data storage and reproduction.Finally,the main detection functions such as ultrasonic probe scanning and positioning,excitation and reception of ultrasonic Lamb waves,and real-time display and storage of ultrasonic detection signals for the semipenetration type lap laser welding joint of stainless steel sheets were realized.In order to deeply understand the influence of the internal connection state of a lap laser welded joint on the propagation process of ultrasonic Lamb waves,and determine the signal characteristic values that can reflect the internal fusion width of the joint,this paper used the finite element method to numerically simulate the acoustic field of ultrasonic Lamb waves inside the laser welded joint,and studied the propagation characteristics of S0 and A1 mode ultrasonic Lamb waves inside the laser welded joint.The performance ability of ultrasonic Lamb mode and acoustic characteristics on the internal fusion width of the joint were analyzed,and A1 mode was determined as the best mode for ultrasonic Lamb wave detection under current laser welded joint conditions;On this basis,the quantitative correspondence between fusion width and acoustic characteristics of A1 mode ultrasonic Lamb wave were further studied.The results showed that the maximum acoustic pressure amplitude of the A1 mode Lamb wave was negatively correlated with the internal fusion width of the laser welded joint.In order to establish a computational model that can quantitatively evaluate the internal fusion width of semi-penetration lap laser welded joints,this paper conducted time domain,frequency domain,and HHT time-frequency analysis of ultrasonic Lamb wave detection signals,and studied the relationship between the acoustic characteristics of ultrasonic Lamb wave signals and the internal fusion width of joints from different perspectives.The results showed that the maximum normalized amplitude of the time domain signal,the first and second peak values of the frequency domain signal,the maximum peak values of the time-frequency decomposition IMF2 component signal,the first peak values of the IMF3 component signal,the maximum instantaneous amplitude peak values of the IMF2 component,and the maximum marginal spectrum peak values of the IMF2 component all exhibited significant correlations with the weld width.Using the above characteristic values as input information,this paper established a laser welded joint internal weld width evaluation model based on BP neural network algorithm,which can effectively distinguish three types of welding quality: incomplete fusion,insufficient weld width,and qualified weld width.The evaluation accuracy rate can reach 94.7%,which can meet the current production requirements for nondestructive testing and quality control of laser welding quality of stainless steel urban rail carriage bodies.