Figure 7-60.Liquid penetrant inspection.
materials and to make sure that the surface is entirely
dry before using the liquid penetrant.
Maintain the temperature of the inspection piece
and the liquid penetrant in the range of 50°F to 100°F.
Do not attempt to use the liquid penetrant when this
temperature range cannot be maintained. Do not use an
open flame to increase the temperature because some of
the liquid penetrant materials are flammable.
After thoroughly cleaning and drying the surface,
coat the surface with the liquid penetrant. Spray or brush
on the penetrant or dip the entire piece into the penetrant.
To allow time for the penetrant to soak into all the cracks,
crevices, or other defects that are open to the surface,
keep the surface of the piece wet with the penetrant for
a minimum of 15 or 30 minutes, depending upon the
penetrant being used.
After keeping the surface wet with the penetrant for
the required length of time, remove any excess penetrant
from the surface with a clean, dry cloth, or absorbent
paper towel. Then dampen a clean, lint-free material
with penetrant remover and wipe the remaining excess
penetrant from the test surface. Next, allow the test
surface to dry by normal evaporation or wipe it dry with
a clean, lint-free absorbent material. In drying the sur-
face, avoid contaminating it with oil, lint, dust, or other
materials that would interfere with the inspection.
After the surface has dried, apply another substance,
called a developer. Allow the developer (powder or
liquid) to stay on the surface for a minimum of 7 minutes
before starting the inspection. Leave it on no longer than
30 minutes, thus allowing a total of 23 minutes to
evaluate the results.
The following actions take place when using dye
penetrants. First, the penetrant that is applied to the
surface of the material will seep into any passageway
open to the surface, as shown in figure 7-60, view A.
The penetrant is normally red in color, and like penetrat-
ing oil, it seeps into any crack or crevice that is open to
the surface. Next, the excess penetrant is removed from
the surface of the metal with the penetrant remover and
a lint-free absorbent material. Only the penetrant on top
of the metal surface is removed (fig. 7-60, view B),
leaving the penetrant that has seeped into the defect.
Finally, the white developer is applied to the surface
of the metal, as shown in figure 7-60, view C. The
developer is an absorbing material that actually draws
the penetrant from the defect. Therefore, the red pene-
trant indications in the white developer represent the
defective areas. The amount of red penetrant drawn
from the defective areas indicates the size and some-
times the type of defect. When you use dye penetrants,
the lighting in the test area must be bright enough to
enable you to see any indications of defects on the test
The indications you see during a liquid penetrant
inspection must be carefully interpreted and evaluated.
In almost every inspection, some insignificant indica-
tions are present. Most of these are the result of the
failure to remove all the excess penetrant from the
surface. At least 10 percent of all indications must be
removed from the surface to determine whether defects
are actually present or whether the indications are the
result of excess penetrant. When a second inspection
does not reveal indications in the same locations, it is
usually safe to assume that the first indications were