T.O. 33B-1-1
2-30
SECTION IV
MECHANISM, PROPERTIES AND APPLICATION OF PENETRANT
2.4
MECHANISM, PROPERTIES AND APPLICATION OF PENETRANT.
2.4.1
Summary.
This section provides basic, operating, and advanced level information on the penetrant portion of the inspection
process. It explains the basic theory of penetrating action on the mechanism of penetration, describes the physical and
chemical properties of penetrants and discusses their effects on the inspection process, describes methods and provides
instructions on applying penetrants, and presents information and guidance on the penetrant dwell process.
2.4.2
Requirements of A Penetrant.
There are a number of characteristics desired in a material if it is to function as a penetrant. The four primary
requirements are:
a. It must be capable of entering and filling surface openings even though they may be very small.
b. Penetrant in a discontinuity must resist removal during removal of the excess penetrant material on the
surface of the part.
c. It must exit from the discontinuity after the surface penetrant has been removed.
d. It must present a readily visible or noticeable indication of the discontinuity.
The primary requirements listed do not include the factors of being economical, safe, and practical to use. The primary
requirements, combined with the additional factors, complicate the formulation of a penetrant material. The behavior
of a penetrant is controlled by a number of physical and chemical properties, many of which are conflicting. As a
result, commercial penetrants are a complex mixture of chemicals that are formulated for specific performance
characteristics. Unfortunately, there is no simple rule for formulating a penetrant material, nor is there a set of
characteristics which, if provided, will ensure a final material that is completely satisfactory for all applications.
2.4.3
Mechanism of Penetration.
2.4.3.1
Physical Principles.
The penetrant inspection process depends on a liquid that can flow over the surface. The ability of a liquid to cover the
surface of a part and enter any surface opening depends on surface tension, wetting ability and capillary action.
2.4.3.1.1
Surface Tension.
The surface of a liquid exhibits certain features resembling the properties of a stretched elastic membrane. These
features are due to the cohesive forces holding the surface molecules together, hence the term “surface tension”. As an
example, one may lay a needle or safety razor blade upon the surface of water, and it will lie at rest in a shallow
depression caused by its weight, much as if it were on a rubber air cushion. The forces drawing surface molecules
together can be very strong. These forces, or surface tension, cause a droplet of liquid to have a spherical shape. A
sphere has the smallest surface for a given volume of liquid. This has a direct effect upon the ability of a penetrant to
wet a surface. Surface tension usually decreases with a decrease in temperature.
2.4.3.1.2
Wetting Ability.
When a liquid comes into contact with a solid surface, the cohesive force responsible for surface tension competes with
or is countered by the adhesive force between the liquid molecules and the solid surface. These forces determine the
contact angle that the liquid forms with the surface. Figure 2-8 illustrates three examples of contact angle. Contact
angle is designated by the Greek letter q (theta). If the contact angle is less than 90 degrees, the liquid spreads over the
surface and is said to “wet the surface”, or to have good wetting ability.
