PT Theory From
PT Theory From
PT Theory From
A very early surface inspection technique involved the rubbing of carbon black on glazed
pottery, whereby the carbon black would settle in surface cracks rendering them visible.
Later, it became the practice in railway workshops to examine iron and steel components
by the "oil and whiting" method. In this method, a heavy oil commonly available in
railway workshops was diluted with kerosene in large tanks so that locomotive parts such
as wheels could be submerged. After removal and careful cleaning, the surface was then
coated with a fine suspension of chalk in alcohol so that a white surface layer was formed
once the alcohol had evaporated. The object was then vibrated by being struck with a
hammer, causing the residual oil in any surface cracks to seep out and stain the white
coating. This method was in use from the latter part of the 19th century to approximately
1940, when the magnetic particle method was introduced and found to be more sensitive
for ferromagnetic iron and steels.
Many of these early developments were carried out by Magnaflux in Chicago, IL, USA in
association with Switzer Bros., Cleveland, OH, USA. More effective penetrating oils
containing highly visible (usually red) dyes were developed by Magnaflux to enhance
flaw detection capability. This method, known as the visible or color contrast dye
penetrant method, is still used quite extensively today. In 1942, Magnaflux introduced the
Zyglo system of penetrant inspection where fluorescent dyes were added to the liquid
penetrant. These dyes would then fluoresce when exposed to ultraviolet light (sometimes
referred to as "black light") rendering indications from cracks and other surface flaws
more readily visible to inspectors.
The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual
inspection is that it makes defects easier to see for the inspector. There are basically two
ways that a penetrant inspection process makes flaws more easily seen. First, LPI
produces a flaw indication that is much larger and easier for the eye to detect than the
flaw itself. Many flaws are so small or narrow that they are undetectable by the unaided
eye. Due to the physical features of the eye, there is a
threshold below which objects cannot be resolved. This
threshold of visual acuity is around 0.003 inch for a person
with 20/20 vision.
The second way that LPI improves the detectability of a flaw is that it produces a flaw
indication with a high level of contrast between the indication and the background also
helping to make the indication more easily seen. When a visible dye penetrant inspection
is performed, the penetrant materials are formulated using a bright red dye that provides
for a high level of contrast between the white developer. In other words, the developer
serves as a high contrast background as well as a blotter to pull the trapped penetrant
from the flaw. When a fluorescent penetrant inspection is performed, the penetrant
materials are formulated to glow brightly and to give off light at a wavelength that the
eye is most sensitive to under dim lighting conditions.
2. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow
as much penetrant as possible to be drawn from or to seep into a defect. Penetrant
dwell time is the total time that the penetrant is in contact with the part surface.
Dwell times are usually recommended by the penetrant producers or required by
the specification being followed. The times vary depending on the application,
penetrant materials used, the material, the form of the material being inspected,
and the type of defect being inspected. Minimum dwell times typically range from
5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell
time as long as the penetrant is not allowed to dry. The ideal dwell time is often
determined by experimentation and is often very specific to a particular
application.
1. Excess Penetrant Removal: This is a most delicate
part of the inspection procedure because the excess
penetrant must be removed from the surface of the
sample while removing as little penetrant as possible
from defects. Depending on the penetrant system used,
this step may involve cleaning with a solvent, direct
rinsing with water, or first treated with an emulsifier
Fatigue cracks
Quench cracks
Grinding cracks
Overload and impact fractures
Porosity
Laps
Seams
Pin holes in welds
Lack of fusion or braising along the edge of the bond
line
Primary Advantages
Primary Disadvantages
Reference:
Penetrants
Emulsifiers
1. Method A: Water-Washable
2. Method B: Post Emulsifiable, Lipophilic
3. Method C: Solvent Removable
4. Method D: Post Emulsifiable, Hydrophilic
References:
-- Boisvert, B.W., Hardy, G., Dorgan, J.F., and Selner, R.H.,
The Fluorescent Penetrant Hydrophilic Remover Process,
Materials Evaluation, February 1983, pp. 134-137.
Developers
Developer Forms
Dry Powder
Water Soluble
Nonaqueous
Nonaqueous
developers suspend the
developer in a volatile
solvent and are
typically applied with
a spray gun.
Nonaqueous
developers are
commonly distributed
in aerosol spray cans for portability. The solvent tends to pull
penetrant from the indications by solvent action. Since the
solvent is highly volatile, forced drying is not required. A
nonaqueous developer should be applied to a thoroughly dried
part to form a slightly translucent white coating.
Special Applications
Preparation of Part
Contaminants
References:
Oils
Klein showed that when a test specimen was contaminated
with cutting oil, there was a reduction in sensitivity even
when the specimen's was vapor degreased before inspection.
The specimens used for this study were quenched cracked
2024 aluminum blocks. The reduction in sensitivity was
believed to be the result of incomplete removal of the cutting
oil from the defects.
Etchants
Kleint warns that acid entrapment from a prepenetrant etch
can have disastrous effects on the penetrant inspection. The
article states that the sodium hydroxide caustic often used to
etch aluminum parts does not affect penetrants but that acids
used to etch parts of other materials do have an effect. Experts
in the penetrant field warn that caustics can in fact reduce
penetrant brightness. Careful cleaning of both acid and caustic
etches before penetrant inspection is highly recommended.
Cleaning Chemicals
References:
1. Method A: Water-Washable
2. Method B: Post Emulsifiable, Lipophilic
3. Method C: Solvent Removable
4. Method D: Post Emulsifiable, Hydrophilic
-- Senda, T., Maeda, N., Kato, M., Ebata, M., Ooka, K., and
Miyoshi, S., Factors Involved in Formation of Penetrant
Testing Indications, NDE in the Nuclear Industry:
Proceedings of the 6th International Conference, Zurich,
Switzerland, November - December 1984, pp. 807-810.
Develope
Advantages Disadvantages
r
Ease of coating
entire part
Indications are
bright and sharp
Indications weaken
Suspendib
White coating for and become
le
good contrast can diffused after time
be produced which
work well for both
visible and
fluorescent systems
Indications show-
up rapidly and are
well defined
Provides highest
sensitivity
References:
Vaerman evaluated the effect that rinse time had on one high
sensitivity water-washable penetrant and two post-
emulsifiable penetrants (one medium and one high
sensitivity). The evaluation was conducted using TESCO
panels numerous cracks ranging in depth from 5 to 100
microns deep. A 38 percent decrease in sensitivity for the
water-washable penetrant was seen when the rinse time was
increased from 25 to 60 seconds. When the rinse times of two
post-emulsifiable penetrants were increased from 20 to 60
seconds, a loss in sensitivity was seen in both cases but it was
much reduced from the loss seen with the water-washable
system. The relative sensitivity loss over the range of crack
depths was 13 percent for the penetrant with medium
sensitivity and roughly percent for the high sensitivity
penetrant.
-- Vaerman, J., Fluorescent Penetrant Inspection, Quantified
Evolution of the Sensitivity Versus Process Deviations,
Proceedings of the 4th European Conference on Non-
Destructive Testing, Pergamon Press, Maxwell House,
Fairview Park, Elmsford, New York, Volume 4, September
1987, pp. 2814-2823.
Solvent Suspendible
Development Time
Part should be allowed to develop for a minimum of 10
minutes and no more than 2 hours before inspecting.
Light Measurement
References:
Rummel, W.D. and Matzkanin, G. A., Nondestructive
Evaluation (NDE) Capabilities Data Book, Published by the
Nondestructive Testing Information Analysis Center
(NTIAC), NTIAC #DB-95-02, May 1996.
Clark, R., Dover, W.D., and Bond, L.J., The Effect of Crack
Closure on the Reliability of NDT Predictions of Crack Size,
NDT International, Vol. 20, No. 5, Guildford, United
Kingdom, Butterworth Scientific Limited, October 1987, pp.
269-275.
Chemical Safety