and Thermal Stability of Penetrant Indications
Exposure to intense ultraviolet light and elevated temperatures
can have a negative effect on fluorescent penetrant indications.
Fluorescent materials can lose their brightness after a period
of exposure to high intensity UV light. One study measured the
intensity of fluorescent penetrant indications on a sample that
was subjected to multiple UV exposure cycles. Each cycle consisted
of 15 minutes of 800 microwatt/cm² UV light and 2.5 minutes
of 1500 microwatt/cm² UV light. Two penetrants were tested
in the study, water washable, level 3 and a post emulsifiable,
level 4. The results from the study showed that the indications
from both penetrants faded with increased UV exposure. After eight
exposure cycles, the brightnesses of the indications were less than
one half their original values.
At an elevated temperature, penetrants can experience heat degradation
or "heat fade." Excessive heat:
1. evaporates the more volatile constituents which increases
viscosity and adversely affects the rate of penetration.
2. alters wash characteristics.
3. "boils off" chemicals that prevent separation and
gelling of water soluble penetrants.
4. kills the fluorescence of tracer dyes.
This fourth degradation mechanism involves the molecules of
the penetrant materials. The phenomenon of fluorescence involves
electrons that are delocalized in a molecule. These electrons
are not specifically associated with a given bond between two
atoms. When a molecule takes up sufficient energy for the excitation
source, the delocalized bonding electrons rise to a higher electronic
state. After excitation, the electrons will normally lose energy
and return to the lowest energy state. This loss of energy can
involve a "radiative" process such as fluorescence or
"non-radiative" processes. Non-radiative processes include
relaxation by molecular collisions, thermal relaxation, and chemical
reaction. Heat causes the number of molecular collisions to increase,
which results in more collision relaxation and less fluorescence.
This explanation is only valid when the part and the penetrant
are at an elevated temperature. When the materials cool, the fluorescence
will return. However, while exposed to elevated temperatures,
penetrant solutions dry faster. As the molecules become more closely
packed in the dehydrated solution, collision relaxation increases
and fluorescence decreases. This effect has been called "concentration
quenching" and experimental data shows that as the dye concentration
is increased, fluorescent brightness initially increases but reaches
a peak and then begins to decrease. Airflow over the surface on
the part will also speed evaporation of the liquid carrier, so
it should be kept to a minimum to prevent a loss of brightness.
Generally, thermal damage occurs when fluorescent penetrant materials
are heated above 71oC (160oF). It should be noted that the sensitivity
of an FPI inspection can be improved if a part is heated prior
to applying the penetrant material, but the temperature
should be kept below 71oC (160oF). Some high temperature penetrants
in use today are formulated with dyes with high melting points,
greatly reducing heat related problems. The penetrants also
have high boiling points and the heat related problems are greatly
reduced. However, a loss of brightness can still take place when
the penetrant is exposed to elevated temperatures over an extended
period of time. When one heat resistant formulation was tested,
a 20 % reduction was measured after the material was subjected
to 163oC (325oF) for 273 hours. The various types of fluorescent
dyes commonly employed in today's penetrant materials begin decomposition
at 71oC (160oF). When the temperature approaches 94oC (200oF),
there is almost total attenuation of fluorescent brightness of
the composition and sublimation of the fluorescent dyestuffs.
Brittain, P.I., Assessment of Penetrant Systems by Fluorescent
Intensity, Proceedings of the 4th European Conference on Nondestructive
Testing, Vol. 4, Published by Perganon Press, 1988, pp. 2814-2823.
Muller, F.A. and Fantozzi, F.F., Advances in Improving the Heat-Fade
Resistance of Fluorescent Penetrants, Materials Evaluation, July
1987, pp. 848-850.
Sherwin, A. G. and Holden, W. O., Heat Assisted Fluorescent
Penetrant Inspection, Materials Evaluation, Sept. 1979, pp. 52-56,
Robertson, A.J., Heat Stable Fluorescent Penetrants, Paper S2,
4th Pan Pacific Conference on Nondestructive Testing, Vol. 1,
Parkville, Victoria, Australia, Australian Institute for Non-Destructive
Testing, November 1983.
Lovejoy, D.J., The Importance of the Physical Nature of Fluorescence
in Penetrant Testing, Reliability in Non-Destructive Testing:
Proceedings of the 27th Annual British Conference on Non-Destructive
Testing, London, UK, Pergamon Press, 1989, pp. 483-491.