Dye-Penetrant Examination Applications and Limitations

PT can be used to detect cracks that have occurred due to stress-corrosion (environmental) cracking, creep, mechanical or thermal fatigue, and hydrogen attack. However, it must be realized that PT will only detect flaws on the surface of the vessel shell. Surface cracks are often visually apparent upon close scrutiny after the PT has been performed, but this does not imply that the cracks would have been detected by VT alone if they were not previously located by PT.

PT does not give any indication of the depth of a surface crack, which is the primary characteristic of a crack that can affect the integrity and reliability of a vessel. The intensity of the dye stain revealed by the developer is an indication of the quantity of liquid penetrant that reemerges, but a deep crack that is tightly closed may exhibit a less intense stain than a shallow crack with a relatively wide opening. Corrosion scales that fill a deep crack would also reduce the intensity of the dye stain. In addition, the spread of the dye stain in the developer can exaggerate the apparent surface length of a crack, especially for relatively short cracks. Therefore, it is usually necessary to determine the size of cracks detected by PT by other NDE procedures, such as ultrasonic examination (UT), to evaluate their significance, unless the PT indications are removed by grinding.

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