Experimental Research On Steel Plate Testing Based On Electromagnetic Acoustic Transducer Methods

Electromagnetic acoustic transducer(EMAT) technology is a new ultrasonic testingmethod. Compared with the traditional piezoelectric ultrasonic, EMATs have manyadvantages, such as contactlessness, need-no-couplant. These features make EMATsparticularly suitable for the nondestructive inspection under high temperature and other severeenvironment. The EMATs inspection method for steel plate is researched with experiments indetail in this thesis.Firstly, the method of transit time calculation used to thickness measurement is speciallyresearched. According to the deficiency of existing methods of propagation time measurement,a method of transit time measurement is presented based on spline interpolation andcross-correlation time delay estimation. The experimental results show that this method gets1ns time resolution under100MHz hardware sampling rate. A3mm thick steel plate isrepeatedly measured with this method, and the relative error less than0.02%.Secondly, the magnetostrictive-based shear horizontal(SH) wave detection method ofsteel plates is researched. Because of the flexible frequency control, the horizontalmagnetization and a meanderline coil are chosen to constitute the transducer structure. Thesignal of conventional EMAT for the SH wave is weak. An improved transducer structure ispresented, and finite element calculation results of magnetic field show that this structureenhances the intensity of magnetic field, so as to increase the signal amplitude. Theexperimental results show that this structure increases the signal amplitude and therebyimproving the detectability. In the2mm thick steel plate, a through hole located in thedistance of100mm from the receiving transducer is detected, the diameter of which is2mm.Finally, according to the phenomenon that the SH wave is driven weakly in thick steelplates and get multi-modes signals, which interfere the detection results, the surface wavedetection method is researched. The vertical magnetization and a meanderline coil are chosento constitute the transducer structure for its higher magnetized efficiency. The influence ofseveral key parameters is studied. The experimental results show that this transducer structurecan be used to generate and receive surface wave, and thereby detecting defects. In the6mmthick steel plate, a flat bottom hole located in the distance of60mm from the receivingtransducer is detected, the diameter of which is6mm and depth is20%of the plate thickness. In the8mm thick steel plate, a spherical blind hole located in the distance of300mm from thereceiving transducer is obvious detected, the diameter of which is22mm and depth is80%ofthe plate thickness.