Research On Thickness Measurement Of Aluminum Alloy Plate Based On Electromagnetic Acoustic Resonance

With the rapid development of China’s defense industry and infrastructure construction,aluminum alloy materials have been widely used.During the service process of the aluminum alloy plate,severe working conditions will adversely affect its service state,which may lead to a series of serious consequences.Therefore,it is necessary to accurately detect the service state of the aluminum alloy plate.The thickness change of the metal plate during the service process needs to be paid attention to since the thickness reduction area may cause macroscopic damage such as cracks and perforations over time,thus affecting the normal operation of the equipment.Electromagnetic ultrasonic testing is to utilize electromagnetic acoustic transducer to excite ultrasonic waves inside the tested metal,and then capture the received ultrasonic signals to analyze the internal structural characteristics of the specimen,such as thickness and defects.This detection method has the characteristics of couplant-free,high detection efficiency,and suitable for complex working conditions,and has gradually developed into a relatively popular metal performance detection scheme around the world.Electromagnetic acoustic resonance detection is based on electromagnetic ultrasound,using long-period excitation to superimpose ultrasonic signals inside the specimen,and performing high-precision detection on the thickness of the specimen through the resonant spectrum signal obtained by frequency sweep,thus has higher signal-to-noise ratio and detection accuracy than traditional electromagnetic ultrasonic detection.This dissertation first proposes a Halbach array electromagnetic acoustic longitudinal wave transducer.In order to improve its efficiency as well as increase the excitation signal and the amplitude of the received signal,orthogonal test and uniform test schemes are used to optimize the structural parameters of the transducer.The model of Halbach array electromagnetic acoustic longitudinal wave transducer was established by finite element simulation software.Several coil structure parameters were designed and the echo signals obtained were analyzed and compared.The optimal parameters were selected and the influences of different lift distance and excitation current frequency on received signals were analyzed.Finally,the Halbach transducer was made successfully using the optimized structural parameters.Then,the traditional electromagnetic acoustic resonance needs long-period excitation and frequency sweep to obtain the resonant spectrum,this dissertation proposes a broadband excitation electromagnetic acoustic resonance detection scheme.The characteristics of ultrasonic excitation and propagation under broadband excitation are analyzed by simulation,and the Fast Fourier Transform of the received signal is directly applied to verify the feasibility of electromagnetic acoustic resonance detection using broadband excitation.Secondly,a broadband excitation electromagnetic ultrasonic testing platform was built,and the optimized Halbach array electromagnetic acoustic longitudinal wave transducer was used to measure the thickness of 7075 aluminum alloy specimens with different thickness by using the broadband excitation electromagnetic acoustic resonance detection scheme.The model of probability of detection was introduced to quantitatively analyze the accuracy level of the detection scheme.Finally,aiming at the thickness measurement issue of 7075 aluminum alloy with nonparallel plate with an inclined angle,a rotating scanning detection scheme based on electromagnetic acoustic resonance was proposed.The finite element simulation software was used to establish the sloped specimen model with different thickness at both ends,and the simulation results were analyzed to verify the theoretical feasibility of the scheme,and multiplication of n-times compression was used to process the spectral signals.Then,the electromagnetic acoustic resonance detection platform was set up,and the thickness of the fixed point on the surface of the sloped specimen was detected by rotating scanning scheme.The corresponding relationship between the measured thickness of the specimen at different point positions and the rotation angles of the transducer was obtained,and the best rotation angle range was obtained by comparing with the real thickness of the position.Finally,the power spectral density was used to analyze the signals by frequency sweep at different rotation angles,so as to judge the inclined direction of the sloped specimen.