Non Destructive Testing ?

Flourescent Penentrant Testing

Flourescent Penentrant Testing applications. Exposure to intense ultraviolet light and elevated temperatures can have a negative effect on fluorescent penetrant indications. Fluorescent materials can loose their brightness after a period of exposure to high intensity UV light. One study measured the intensity of fluorescent penetranttesting indications of a sample that was subjected to multiple UV exposure cycles. Each cycle consisted of 15 minutes of 800 microwatts/cm² UV light and 2.5 minutes of 1500 microwatts/cm² UV light. Two fluorescent 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 fluorescent penetrants faded with increased UV exposure. After eight exposure cycles the brightness of the indications was less than one half their original values.

At an elevated temperature, fluorescent 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 fluorescent penetration.
2. alters wash characteristics, affect fluorescent penetration testing
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 fluorescent 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 a reduced fluorescent effect.

This explanation is only valid when the part and the fluorescent penetrant are at an elevated temperature. When the materials cool, the fluorescence penentrant 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. These are the principal benefits of Flourescent Penentrant Testing