Research On Early Metal Damage Evaluation Based On Electromagnetic Ultrasonic

Electromagnetic ultrasonic testing technology is a non-destructive testing method with the advantages of non-contact,no coupling,high temperature resistance,etc,and has a wide range of applications in the field of metal performance and structure testing.Among them,linear electromagnetic ultrasound is mainly aimed at macroscopic defect damage,and nonlinear electromagnetic ultrasound is more inclined to early damage.The early plastic damage and fatigue damage of metal components during service affect the safe operation of the equipment,and their detection is the guarantee of safe operation and maintenance.In view of the difficulties in the detection of early damage,the changes in material properties caused by early damage,changes in sample thickness,and changes in the internal crystal structure of the material can be used for detection and analysis to realize the characterization of early damage.The development of early fatigue damage of metal is divided into three stages: plastic deformation,crack initiation and crack propagation.In the process of metal plastic deformation,the relative permeability,Young’s modulus,electrical conductivity,and magnetostriction curve of the material properties change.Therefore,this paper proposes the use of electromagnetic ultrasonic testing technology to detect and analyze them.Based on the analysis of the magnetizing force,Lorentz force and magnetostrictive force and the receiving principle of the electromagnetic ultrasonic transducer,a finite element simulation model including three transduction principles is established.The influence of various material properties on the electromagnetic ultrasonic conversion efficiency was analyzed by using the simulation model,and the influence of plastic deformation on electromagnetic ultrasonic testing in fatigue damage was explored by preparing wheel specimens with different plastic deformations.The simulation and test results show that the electromagnetic ultrasonic detection technology can realize the detection of metal plastic deformation.For crack initiation and propagation,the Murnaghan model and the closed crack equivalent spring model are used to simulate plastic deformation and closed cracks.Firstly,based on the force distribution of the closed crack and the principle of nonlinear effect,it is found that the displacement of each point of the interface is different when the closed crack is stressed.The closed crack is simulated by changing the spring coefficient and damping coefficient in the equivalent spring model.By building a closed crack equivalent spring simulation model,the effects of damping coefficient,spring coefficient and the number of equivalent springs on ultrasonic nonlinearity are analyzed.Secondly,based on the Murnaghan model and the closed crack equivalent spring model,the nonlinear electromagnetic ultrasonic surface wave and longitudinal wave simulation models are built respectively.By changing the value of Murnaghan’s third-order elastic constant and the number and size of closed cracks,the relationship between different degrees of fatigue damage and relative nonlinear coefficients is obtained.The results show that the results of the two detection tests are consistent,and the relative nonlinear coefficient can effectively characterize the early fatigue damage of metals.Finally,7075 aluminum alloy specimens with different fatigue cycles were prepared and tested by nonlinear ultrasonic and nonlinear electromagnetic ultrasonic longitudinal waves.The Duffing chaotic system has good noise immunity and high sensitivity to weak signals.In order to improve the detection efficiency,signal processing is performed on the detection results of the nonlinear electromagnetic ultrasonic test.By calculating the phase trajectory,Lyapunov exponent,correlation dimension,Kolmogorov entropy characteristic quantity of the chaotic system,and comparing with the relative nonlinear coefficient.The results show that the sensitivity of the Lyapunov exponent is two to three times that of the relative nonlinear coefficient,and the chaotic system can effectively improve the detection performance.