| The bias magnetic field plays an important role in the guided wave inspection based onmagnetostrictive effects, which manifests in selecting the guided wave mode, reducing thedouble-frequency effects, improving the conversion efficiency between magnetic energy andacoustic energy and so on. Therefore, it’s very important to research the influence of the biasmagnetic field on the guided wave inspection based on magnetostrictive effects. In this thesis,the experimental method is used to study how the bias magnetic field affects transmitting andreceiving the longitudinal guided wave based on magnetostrictive effects in the free state or inthe tension state. The research result could provide support for the magnetizer design.Firstly, considered a steel pipe as experimental subject, the functions of bias magneticfield to longitudinal guided wave testing based on magnetostrictive effects in free state arestudied. The experiment is performed to study the relationship between the effects of the biasmagnetic field and the magneto-elastic coupling coefficient based on the action mechanismsof bias magnetic field. The experimental result shows that the changing rule in the position ofthe transmitting sensor is corresponding to the receiving sensor, but the bias magnetic fieldstrength which makes the conversion efficiency maximal is different. Meanwhile, theinfluence of the bias magnetic on the magnetostrictive liner workspace is studied. Theexperimental result shows that the better the linearity of workspace is, the higher theconversion efficiency is.Secondly, considered the a high strength galvanized steel wire as experimental subject,the functions of bias magnetic field to longitudinal guided wave testing based onmagnetostrictive effects in the tension state are studied. The experiment is performed to studythe influence of the tensile stress on the magnetizing effects of bias magnetic field in theposition of transmitting and receiving independently. Taking the conversion efficiency asreference, the best bias magnetization area is defined. The experimental result shows that thetensile stress could expand the best bias magnetization area when the tensile stress varies from100MPa to500MPa. Then, the changing rule of the magnetostrictive conversion efficiencyversus the tensile stress is studied. The experimental result shows that the tensile stress couldimprove the conversion efficiency when the tensile stress varies from50MPa to500MPa.At last, against the problem that the effects of the bias magnetic field produced by thepermanent magnetizer are difficult to be estimated, an assessment method is proposed to solvethe problem based on the equivalent relationship of the permanent magnetizer and the DC magnetizing coil magnetizer. The permanent magnetizer is chosen to provide the biasmagnetic field after comparing the permanent magnetizer to the DC magnetizing coilmagnetizer. Then, the equivalent relation is obtained on the basis of the symmetry that theconversion efficiency is only dependent on the strength and not on the direction of the biasmagnetic field. Through that, the magnetizing effect is estimated, which could providereference for magnetizer design. |
PDF Full Text Request