| 摘要 |
An electromagnetic acoustic transducer for exciting ultrasonic vibration within a test object comprises at least one magnet (10, Fig 5), a fluxguide (8, Fig 5) shaped to direct the magnetic field towards a test face (11, Fig 5) due to repulsion between magnetic fields, an electrical coil 12, at least an active portion of which covers the test face, and within the active portion conductors of the coil are parallel, straight and carrying current in the same direction. The coil may be a butterfly coil comprising two adjacent spiral coils wound in opposite senses, the medial portion of the coil being the active portion. The magnetic field may pass from magnet to fluxguide in directions that are non-normal to the test face. The repulsion of the magnetic field lines may increase the flux density within the fluxguide. The EMAT may comprise a housing with a low magnetic permeability. The ratio of the radius of a circle containing a projection of the test face, to the radius of a circle containing the at least one magnet ranges from 0.2 to 0.8. Driving electric current through the coil excites a substantially mode pure and uni-directionally polarised shear wave in the test object. |
