Defect Interaction and Anomalous Source Models in Remote Field Eddy Current and Through Transmission Testing.
An experimental investigation of electromagnetic defect interactions was undertaken using a through transmission system. The responses from axial slits in ferromagnetic and non-ferromagnetic pipes were examined using two different exciter coil geometries. A long, internal solenoidal exciter was used to produce strong circumferential eddy currents to examine the eddy current responses to the defects. A special internal ferrite cored exciter coil was used to produce a strong circumferential ac magnetic field to examine the magnetic responses to the defects. Anomalous source models were developed to describe the responses of the defects. In non-ferromagnetic materials, the defect response may be modelled by using anomalous eddy current sources. Two types of anomalous eddy current source distributions were developed to deal with fully and partially penetrating slits. In ferromagnetic material, the magnetic interaction may be modelled by an anomalous magnetization source. The orientation of the defect with respect to the excitation field determines whether the eddy current or magnetic interaction will dominate in ferromagnetic materials. In their present state, the anomalous source models provide qualitative predictions of defect responses for different excitation fields. These predictions can be used to optimize the excitation field or the detector configuration for a particular electromagnetic system. The anomalous fields produced by the sources may be added to the background or defect-free distributions to obtain the total field distribution in the presence of a defect.
- Pub Date:
- November 1995
- Physics: Electricity and Magnetism