Research On The High Frequency Response Mechanism Of Metal Plate Crack Defect Using Electromagnetic Excitation

The development of high-quality and novel nondestructive testing methods has practical significance for improving defect detection efficiency of the products and keeping the stable operation of the machinery.Among the numerous nondestructive testing(NDT)methods,electromagnetically induced stress wave(EMSW)testing technique is widely used due to independent of couplant,extreme environmental adaptability and high detection efficiency.However,the crucial part of electromagnetically induced acoustic and nonlinear electromagnetic acoustic has been rarely researched,which limits the application of this technology.In this dissertation,various EMSW methods are carried out for multi-scale crack detection,laying a theoretical foundation for practical application of EMSW.The main researches and conclusions of this dissertation are as follows:(1)Research on the macro crack detection using electromagnetic ultrasonic method.Based on governing equations of electromagnetic ultrasonic transducer working in the Lorentz force principle,the model of transduction process from electromagnetism to structure is established by combining the finite element and the analytical method based on discrete point source.Then the relationship between transduction efficiency and structure parameters of electromagnetic acoustic transducer is studied.Next,the detectability of inner defect by transverse wave and surface opening crack by Rayleigh wave is analyzed and estimated.The results show that the characteristic of crack dimension,position and number can be detected effectively using electromagnetic ultrasonic method.(2)The theoretical analysis for electromagnetically induced acoustic emission(EMAE).Based on the electrical activated dislocation dynamics and free electron theory,the formulation for EMAE energy radiated is established.The effect of EMAE signal parameters on electromagnetical excitation is explored.In addition,the signal characteristics under different exciting conditions are analyzed by Hilbert-Huang transform.The results show that EMAE is generated by the energy exchanged between drift electrons and dislocations.(3)An electromagnetic ultrasonic/EMAE composite detection method is proposed.The composite detection principles of this system have been researched,and the composite coil,transduction magnetic field,impedance matching are designed.Through the analysis of the composite detection efficiency,the two detection techniques have strong complementarities.The results show that this composite method realized effective carrier of acoustic emission characteristics for long distance.(4)Research on the nonlinear electromagnetic ultrasonic testing method.Aiming at solving the difficult detection of the micro crack and plastic deformation by traditional linear ultrasonic techniques,combine nonlinear ultrasonic technique and contactless electromagnetic ultrasonic method.The angle incidence technique and mode-conversion at the contact interfaces as well as the normal and tangential interface stiffness have been taken into account.The behaviors of nonlinear longitudinal wave in plastic specimen and Rayleigh wave in soild-soild contact interfaces from a numerical perspective and experiments are investigated.The results show that the early damage detection can be measured using the nonlinear electromagnetic ultrasonic method.