Machinery's Handbook, 31st Edition
1628 Nondestructive Testing transmitted to the film or digital sensors. As shown in Fig. 2, this produces relatively darker regions in the resulting imagery, as compared to thicker and/or denser areas.
Source
Film
Fig. 2. Radiographic Absorption Effect on a Steel Step Wedge RT can detect voids, such as porosity, slag, and tungsten inclusions, that indicate weld discontinuities. Critical flaws, including cracks and incomplete fusion, are more difficult to see unless the radiation source is oriented at a suitable direction with respect to the flaw. A key advantage of using radiographic film or digital media is that a permanent record of the testing can be stored. A disadvantage of RT (unlike UT) is that it requires access to both sides of the material: one for the transmission source and the other for the film. Another issue is the danger posed to humans by exposure to radiation, particularly when using portable X-ray equip- ment involving isotopes such as iridium-192 and cobalt-60. Training in radiation safety is mandatory, along with safeguards to protect personnel in the test vicinity. Magnetic Particle Testing (MT): Used to detect surface and near-surface discontinuities that are primarily linear in nature, magnetic particle testing is relatively easy to use, is low in cost, and provides immediate results. However, it is only effective on ferromagnetic materials. Magnetization is achieved by passing an electric current through the material, or by using an external magnetizing coil; permanent magnets also may be used. Finely divided magnetic particles—either as a dry powder or suspended in liquid carrier—are applied to and removed from the part during magnetization. An example of longitudinal magnetiza- tion is shown below in Fig. 3a . If there are no discontinuities interrupting the flow of the current, the magnetic particles are easily removed. However, if there is an interruption in the flow of current, as shown in Fig. 3b , flux leakage occurs, creating new north and south poles, and the particles remain fixed at the discontinuity.
Discontinuity
Fig. 3a. Magnetic Flux Lines Fig. 3b. Interruption of Flow Evaluation of test results is arguably the most critical stage of MT and requires distin- guishing among false, nonrelevant, and relevant indications. Experience and familiar- ity with codes and specifications, application of proper techniques, stringent equipment performance checks, and adherence to the detailed and qualified procedure are essential.
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