As previously mentioned, one of the important characteristics
of a liquid penetrant material is its ability to freely wet the
surface of the object being inspected. At the liquid-solid surface
interface, if the molecules of the liquid have a stronger attraction
to the molecules of the solid surface than to each other (the
adhesive forces are stronger than the cohesive forces), wetting
of the surface occurs. Alternately, if the liquid molecules are
more strongly attracted to each other than the molecules of
the solid surface (the cohesive forces are stronger than the adhesive
forces), the liquid beads-up and does not wet the surface
of the part.
One way to quantify a liquid's surface wetting characteristics
is to measure the contact angle of a drop of liquid placed on
the surface of an object. The contact angle is the angle
formed by the solid/liquid interface and the liquid/vapor interface
measured from the side of the liquid. (See the figure below.) Liquids
wet surfaces when the contact angle is less than 90 degrees. For
a penetrant material to be effective, the contact angle should
be as small as possible. In fact, the contact angle for most liquid
penetrants is very close to zero degrees.
Wetting ability of a liquid is a function of the
surface energies of the solid-gas interface, the liquid-gas interface,
and the solid-liquid interface. The surface energy across an interface
or the surface tension at the interface is a measure of the energy
required to form a unit area of new surface at the interface.
The intermolecular bonds or cohesive forces between the molecules
of a liquid cause surface tension. When the liquid encounters
another substance, there is usually an attraction between the
two materials. The adhesive forces between the liquid and the
second substance will compete against the cohesive forces of the
liquid. Liquids with weak cohesive bonds and a strong attraction
to another material (or the desire to create adhesive bonds) will
tend to spread over the material. Liquids with strong cohesive
bonds and weaker adhesive forces will tend to bead-up or form
a droplet when in contact with another material.
In liquid penetrant testing, there are usually three surface
interfaces involved, the solid-gas interface, the liquid-gas interface,
and the solid-liquid interface. For a liquid to spread over the
surface of a part, two conditions must be met. First, the surface
energy of the solid-gas interface must be greater than the combined
surface energies of the liquid-gas and the solid-liquid interfaces.
Second, the surface energy of the solid-gas interface must exceed
the surface energy of the solid-liquid interface.
A penetrant's wetting characteristics are also largely responsible
for its ability to fill a void. Penetrant materials are often
pulled into surface breaking defects by capillary action. The
capillary force driving the penetrant into the crack is a function
of the surface tension of the liquid-gas interface, the contact
angle, and the size of the defect opening. The driving force for
the capillary action can be expressed as the following formula:
Force = 2 prs
LG cosq
Where:
r = radius of the crack opening (2pr is the line of contact
between the liquid and the solid tubular surface.)
s LG = liquid-gas surface
tension
q = contact angle
Since pressure is the force over a given area, it can be written
that the pressure developed, called the capillary pressure, is
Capillary Pressure = (2 s
LG cosq)/
r
The above equations are for a cylindrical defect but the relationships
of the variables are the same for a flaw with a noncircular cross
section. Capillary pressure equations only apply when there is
simultaneous contact of the penetrant along the entire length
of the crack opening and a liquid front forms that is an equidistant
from the surface. A liquid penetrant surface could take-on a complex
shape as a consequence of the various deviations from flat parallel
walls that an actual crack could have. In this case, the expression
for pressure is
Capillary Pressure = 2(s
SG - ss
SL)/r = 2S /r
Where:
s SG = the surface
energy at the solid-gas interface.
s SL = the surface energy
at the solid-liquid interface.
r = the radius of the opening.
S = the adhesion tension (sSG
- s SL).
Therefore, at times, it is the adhesion tension that is primarily
responsible for a penetrant's movement into a flaw and not the
surface energy of the liquid-gas interface. Adhesion tension is
the force acting on a unit length of the wetting line from the
direction of the solid. The wetting performance of the penetrant
is degraded when adhesion tension is the primary driving force.
It can be seen from the equations in this section that the surface
wetting characteristics (defined by the surface energies) are
important in order for a penetrant to fill a void. A
liquid penetrant will continue to fill the void until an opposing
force balances the capillary pressure. This force is usually the
pressure of trapped gas in a void, as most flaws are open only
at the surface of the part. Since the gas originally in a flaw
volume cannot escape through the layer of penetrant, the gas is
compressed near the closed end of a void.
Since the contact angle for penetrants is very close to zero,
other methods have been devised to make relative comparisons of
the wetting characteristics of these liquids. One method is to
measure the height that a liquid reaches in a capillary tube.
However, the solid interface in this method is usually glass and
may not accurately represent the surface that the penetrant inspection
will be performed on. Another method of comparative evaluation
is to measure the radius, the diameter,
or the area of a spot formed when a drop of penetrant is placed
on the test surface and allowed to stand undisturbed for a specific amount of time. However, using this method, other factors
are also acting in the comparison. These methods include the density,
viscosity, and volatility of the liquid, which do not enter into
the capillarity equations, but may have an effect on the inspection
as discussed in the related pages.
References:
Cartz, L., Nondestructive Testing, ASM International, Materials
Park, OH, 1995, pp. 135-136.
Tugrul, A. B., Capillarity Effect Analysis for Alternative Liquid
Penetrant Chemicals, NDT & E International, Volume 30 Number
1, Published by Elsevier Science Ltd., Oxford England, February
1997, pp. 19-23.
