| Aluminum magnesium alloy structural parts have been widely used in infrastructure construction and aviation industry in China due to their advantages of low density and high strength.In the face of real complex working conditions,alloy structural members will encounter material corrosion,external force impact and other factors during service,which will cause irreversible plastic damage to their interior,and eventually lead to major accidents.As a new non-contact testing method,electromagnetic ultrasonic transducer(EMAT),the core component of electromagnetic ultrasonic testing technology,generates ultrasonic signals inside the material by exciting high-frequency signals,and evaluates the defects on the surface or inside of the material according to the characteristics of the echo signals.Compared with piezoelectric ultrasound,EMAT has many advantages,such as no need to pretreat the surface of the tested part,rich excitation waveform,and can be applied to complex field environment,and is widely used in the field of nondestructive testing of metal structural parts.Taking the internal damage detection and plate thickness identification of aluminum magnesium alloy structural parts as the research object,this paper has carried out the research on the sector scanning defect detection and imaging technology based on electromagnetic ultrasonic volume wave and the characterization and imaging of thickness step surface.Aiming at the problem of oblique incidence horizontal polarized shear wave(SV)wave transducer with different coil design mechanisms to detect the internal defects of the specimen with a circular surface at the bottom,a two-dimensional finite element simulation model of oblique incidence SV wave EMAT with fixed coil spacing mode is established,and the influence of different coil turns and coil excitation signal frequency on the radiated sound field and detection energy of oblique incidence SV is analyzed,and the acoustic pointing performance is verified through experiments.Then,the simulation modeling of the line focusing oblique incidence EMAT with different coil spacing modes is carried out.By improving its coil excitation mode,the signal strength of the interference signal on the nonfocusing side of the coil is weakened,the focus of the sound field on the single side of the coil is realized,and its focusing performance is improved.Then through the design of the magnet arrangement structure,the horizontal bias magnetic field strength in the coil area is greatly enhanced,and the energy conversion efficiency of EMAT is improved.Finally,its pointing performance is verified by experiments.At the same time,based on the electromagnetic ultrasonic SV detection technology,the finite element simulation model of the unidirectional linear focusing EMAT through-hole defect is established,the acoustic beam data coordinate conversion process of the timedomain A-scan signal of the defect echo at different incidence angles under the sector scanning mode is studied,and the principle of the nearest neighbor interpolation,bilinear interpolation and bicubic interpolation methods and the interpolation effect of different image interpolation methods on the converted image are analyzed.Then,through the expansion,corrosion,open and close operations in the morphological filling method,the image visualization effect of different structural elements and different expansion and corrosion operations on the image after morphological filling is analyzed.Finally,the fan-scan imaging effect of oblique incidence SV wave EMAT with different modes is verified by testing the specimen with through-hole defect inside.Finally,aiming at the problem that the thickness step surface cannot be effectively identified in the electromagnetic ultrasonic testing method,a moving scanning identification and characterization scheme of the thickness step surface based on the electromagnetic acoustic resonance technology is proposed.In the frequency domain,the finite element simulation is used to scan the internal thickness step surface of the test piece horizontally to obtain the resonance signal of each part,and the frequency-frequency energy density separation method is used to process the signal to verify the theoretical feasibility of the scheme.Then set up the electromagnetic acoustic resonance test and detection platform.Firstly,the cylindrical specimen with fan-shaped opening angle is scanned in a large range to roughly locate the thickness step surface.Then,the proposed method is used to move and scan the thickness step surface to obtain the energy density ratio under different moving distances,and then realize the visual identification of the thickness step surface. |
