3-1. General. This chapter describes the basic principles of penetrant inspection methods, the reason for penetrant
inspection, and the capabilities of penetrant inspection.
3-2. Purpose. Penetrant inspection is a quick and reliable nondestructive test method used for detecting various types
of discontinuities which are opened to the surface of an object or part.
3-3. Basic Principles of Penetrant Inspection. The basic principle of penetrant inspection is to increase the visible
contrast between the discontinuity and its background. This is done by treating the whole object with an appropriate
searching liquid of high mobility and penetrating power and then encouraging the liquid to emerge from the discontinuity
to reveal the flaw pattern to the inspecting personnel.
3-4. Limitations. Penetrant inspections only detect defects that are on the surface on the item. Defects below the
surface, such as casting voids, can not be detected by penetrant inspection. Also, penetrant inspection does not work
well on absorbent items, such as some plastics, rubber and synthetic parts.
3-5. Importance of Skilled Operators. Since correct evaluation of a defect depends on accurate interpretation, the
operator is the key man in the inspection process. The success and reliability of penetrant Inspection depends upon the
thoroughness with which the operator prepares the part from the pre-cleaning process all the way through to the final
interpretation of the indications. Penetrant inspection is not a method by which a part is processed through a machine
which separates the good parts from the bad. The operator must carefully process the part, search out indications, and
then decide the seriousness of defects found, to determine the disposition of parts according to the severity of the flaw
indications. Remember poor processing can be worse than no inspection; because, if improper processing yields no
indications for the operator to detect, the part is considered satisfactory whether it is or not.
3-6. General Procedures for Penetrant Inspections. The following general procedures are for penetrant inspections:
The appropriate inspection process shall be determined by the testing facilities available, the type and amount of
parts to be tested, and the results anticipated and desired.
If the part to be tested could be affected by oil, sulphur or chlorine, tests shall be performed to ensure that the
parts are not damaged when placed under penetrant inspection method tests
The part to be inspected shall be precleaned in order to achieve reliable penetrant inspection.
Parts which have been precleaned shall be dried to achieve reliable penetrant inspection
Penetrant shall be applied to a part under test In a manner appropriate to the type of part or facilities available.
Sufficient dwell time shall be allowed for optimum penetration
Penetrant shall be removed from the surface of the part under test in the manner dictated by the type of
Developer shall be applied to the part under test as appropriate to the process being used and the configuration
of the part under test. Sufficient dwell time shall be allowed for optimum results.
The part shall be inspected and the discontinuity interpreted.
The developer shall be removed after inspection interpretation and prior to returning the part to service.
3-7. Precautions In Penetrant Inspections. Precautions to be taken during penetrant inspections are described in the
Pre-operative Precautions. Precautions to be taken prior to performing penetrant inspections are as follows: