Research On Micro-cracks Damage Detection Technology Of X80 Pipeline Steel Based On Nonlinear Ultrasonic Lamb Wave

The damage evolution process of materials starts from the generation of microcracks such as dislocations,resident slip zones,and micro-cracks,and gradually forms macro-cracks until failure and fracture.The early damage mainly in the form of microholes,micro-cracks and other micro-cracks accounts for 80% to 90% of the service life of the material.The nonlinear ultrasonic testing technology can effectively analyze the early damage of the material.In order to explore the detection mechanism of nonlinear ultrasonic testing to characterize micro-cracks,this paper takes dual-phase(B+F)X80pipeline steel as the research object,and uses a combination of finite element analysis and experiment to explore the interaction between ultrasonic Lamb waves and microcracks mechanism,and the relationship between different amounts of tensile deformation and ultrasonic nonlinear parameters.The main work accomplished in this paper is as follows:1.The propagation process of ultrasonic Lamb wave is studied and analyzed.As the propagation distance increases,the second harmonic has a significant cumulative effect.Under the action of ultrasonic waves,the phenomenon of wave transmission,reflection,distortion,phase delay and strain field change of micro-cracks is an important characteristic of ultrasonic nonlinearity.2.The relationship between the geometric parameters of micro-cracks and ultrasonic nonlinear parameters is studied.In this paper,according to the description of the damage of ductile metal materials by the classical mesoscopic GTN model,a micron-scale three-dimensional ellipsoid cracks is set up for the finite element model,and the effects of the length and width of the micro-cracks on the ultrasonic nonlinear parameters are respectively explored.The simulation results show that with the increase of the length of the micro-cracks,the nonlinear coefficient increases;with the increase of the width of the micro-cracks,the nonlinear parameter decreases exponentially.3.The relationship between the number and direction of random cracks and nonlinear coefficients is investigated.This article uses Python programming combined with Abaqus to generate random cracks of different numbers and directions in the middle of the model.Through the simulation analysis,it is found that the nonlinear coefficient increases with the increase of the number of cracks;when the random cracks and the ultrasonic wave are in the direction of 90°,the ultrasonic nonlinear coefficient is the largest.4.The nonlinear ultrasonic testing experiments were carried out on tensile specimens with different deformations.The test results show that as the specimen enters the plastic deformation stage 6%-21%,the ultrasonic nonlinear coefficient is more sensitive and the quadratic function increases.