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breaks the surface for its entire length or a partially filled seam will also produce an intermittent linear indication as
shown in Figure 2-32c.
2.7.6.2.1.3 Round or Dot Indications.
Rounded indications are characterized as having a length and width of approximately equal dimensions. Porosity or
relatively small areas of unsoundness in metal components usually form rounded indications. However, the actual
surface opening may be irregular in shape. Deep discontinuities, such as weld crater cracks, may appear rounded due
to the large volume of entrapped penetrant. Figure 2-32d illustrates the appearance of large and small rounded
indications.
Figure 2-32. Typical Penetrant Indications.
2.7.6.2.2
Manufacturing Discontinuities.
Many discontinuities result from the manufacturing and repair processes. These will probably be detected each time
the part is reinspected. The NDI inspector must, therefore, be familiar with their appearance and cause, in order to
make valid interpretations of inspection results. Some of the common types of manufacturing discontinuities are
described and illustrated in the following paragraphs.
2.7.6.2.2.1 Porosity.
Porosity is common to all cast parts, particularly aluminum and magnesium. Porosity occurs when gases are entrapped
in the molten metal during pouring and solidification and may also occur during welding. It does not always break the
surface, and internal porosity is not detected by penetrant inspection. Porosity can be very small and distributed
throughout the material, in which case it is called microporosity. Microporosity may or may not cause a penetrant
indication. When it does, it produces an overall background. The larger pores are called macroporosity. In castings,
porosity is not usually considered a defect, unless it is extensive enough to cause a structural weakness or allow the
leakage of a fluid intended to be contained by the casting.
