Research On Electromagnetic Ultrasonic Nondestructive Testing Technology Based On Lorentz Force Mechanism

Aluminum plate is one of the most important raw materials in the industry,and its quality has a crucial impact on many products in downstream industries.As a necessary means to control the quality of plates,electromagnetic ultrasonic nondestructive testing technology has been widely concerned because of its many advantages,such as no coupling agent,no direct contact with the surface,and the ability to inspect in harsh environments.However,as the core component of electromagnetic ultrasonic nondestructive testing technology,the electromagnetic acoustic transducer(EMAT)has the problems of low transduction efficiency,wide radiation pattern,poor directivity of the sound field,and unfocused energy.For the EMAT applied to aluminum plate defect detection,a two-dimensional finite element model of bilateral line focusing shear vertical wave EMAT(BLFSV-EMAT)is established by finite element simulation software.The model covers all the physical fields involved in the EMAT transducer process,which effectively solves the problems of the existing model in terms of accuracy and completeness and lays the foundation for studying the optimization design technology of the line focus transducer.For the BLFSV-EMAT has two-way radiation and poor sound field directivity problems,a unilateral line-focusing SV wave EMAT(ULFSV-EMAT)is proposed,which uses a double-coil excitation to control the main energy direction of the acoustic wave by changing the relative position and delay time parameters of the double-coil.Then an optimization method based on orthogonal test design is proposed to determine the optimal combination of transducer parameters.The simulation results show that the optimized EMAT in-plane displacement amplitude,out-of-plane displacement amplitude,and main flap peak are increased by 10.12,6.9,and 9.13 times,respectively,compared with the original signal,and the acoustic beam width is reduced from the original 7.2 mm to 5.1mm.For the problem of weak signal strength at the focal line position of ULFSV-EMAT,three new EMATs based on the principle of Halbach array permanent magnets are proposed,and the performances of the three transducers are compared using finite element simulation software,and finally it is determined that the rectangular Halbach array transducer has higher signal strength at the focal line position.On this basis,the structural parameters of the selected transducer’s permanent magnet are optimized,and the optimized rectangular Halbach array transducer has 1.73 and 2.12 times higher inplane displacement and out-of-plane displacement amplitude at the focal line position compared with the conventional ULFSV-EMAT.Finally,in order to verify the detection capability of the designed ULFSV-EMAT for defects in aluminum plates,three finite element models of internal rectangular defects,equilateral triangular defects,and through-hole defects were established,and the effects of different types of defect locations and sizes on the echo signals were analyzed,thus laying the foundation for the subsequent defect detection experiments.