| Popis: |
Magnetic fields can be induced in ferromagnetic components and materials either by use of permanent magnets or by use of electric current. Techniques for the use of permanent magnetism are limited; however, considerable ingenuity has been shown in the use of electric current as a source of magnetic induction. Among the reasons for the need for such ingenuity are the geometry and size of components, bars, sheets and billets of material and the fact that even the simplest shaped components or pieces of material must be magnetized in two directions. The complex shapes of many steel components dictate that two directions of magnetization are not sufficient to ensure that magnetic fields are induced in directions which ensure thorough inspection. The physical form of components may also influence the choice of magnetization method: a light ring, for example, is vulnerable to distortion if it is clamped between the heads of an installation. The possibility of burning surfaces is often a serious problem. Where electric current passes directly from its source to the tested material there is always the danger of poor contact leading to arcing and burning. Some specifications have been written with a requirement for penetrant testing of the contact areas to ensure that the heat produced on arcing has not caused cracks. |
