Penetration Testing

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Nondestructive Testing
The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field that plays a critical role in
assuring that structural components and systems perform their function in a reliable and cost effective fashion.
NDT technicians and engineers define and implement tests that locate and characterize material conditions and
flaws that might otherwise cause planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety
of less visible, but equally troubling events. These tests are performed in a manner that does not affect the future
usefulness of the object or material. In other words, NDT allows parts and materials to be inspected and
measured without damaging them. Because it allows inspection without interfering with a product's final use,
NDT provides an excellent balance between quality control and cost-effectiveness. Generally speaking, NDT
applies to industrial inspections. While technologies are used in NDT that are similar to those used in the
medical industry, typically nonliving objects are the subjects of the inspections.
Nondestructive Evaluation
Nondestructive Evaluation (NDE) is a term that is often used interchangeably with NDT. However, technically,
NDE is used to describe measurements that are more quantitative in nature. For example, a NDE method would
not only locate a defect, but it would also be used to measure something about that defect such as its size, shape,
and orientation. NDE may be used to determine material properties such as fracture toughness, formability, and
other physical characteristics.
Take this link to learn about the background of NDT and NDE
NDT/NDE Methods
The number of NDT methods that can be used to inspect components and make measurements is large and
continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to
develop better NDT methods. However, there are six NDT methods that are used most often. These methods are
visual inspection, penetrant testing, magnetic particle testing, electromagnetic or eddy current testing,
radiography, and ultrasonic testing. These methods and a few others are briefly described below.
Visual and Optical Testing (VT)
Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools
such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area.
Visual examiners follow procedures that range from simple to very complex.

Penetrant Testing (PT)
Test objects are coated with visible or fluorescent dye solution. Excess dye is then removed from the surface, and
a developer is applied. The developer acts as blotter, drawing trapped penetrant out of imperfections open to the
surface. With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to
see. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing
imperfections to be readily seen.

Magnetic Particle Testing (MT)
This NDE method is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the

Liquid penetrant inspection is a method that is used to reveal surface breaking flaws by bleedout
of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to
be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called
the "dwell," excess surface penetrant is removed and a developer applied. This acts as a blotter.
It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require
good white light while fluorescent penetrants need to be used in darkened conditions with an
ultraviolet "black light".
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.
A different (though related) method was introduced in the 1940's. The surface under
examination was coated with a lacquer, and after drying, the sample was caused to vibrate by the
tap of a hammer. The vibration causes the brittle lacquer layer to crack generally around surface
defects. The brittle lacquer (stress coat) has been used primarily to show the distribution of
stresses in a part and not for finding defects.
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.

Why a Penetrant Inspection Improves the Detectability of
Flaws

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.
Additional information on the human eye can be found by following the links below.
Visual Acuity
Contrast Sensitivity
Color Sensitivity

Basic Processing Steps of a Liquid Penetrant Inspection
1. Surface Preparation: One of
critical steps of a liquid
inspection is the surface
preparation. The surface must
oil, grease, water, or other
contaminants that may prevent
from entering flaws. The
may also require etching if

the most
penetrant
be free of
penetrant
sample

mechanical operations such as machining, sanding, or grit blasting have been performed.
These and other mechanical operations can smear metal over the flaw opening and
prevent the penetrant from entering.
2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the
penetrant material is applied by spraying, brushing, or immersing the part in a penetrant
bath.
3. 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 for. Minimum dwell times typically
range from five 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 may be very specific to a particular application.

4. Excess Penetrant Removal: This is
the 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 treating the part with an emulsifier and then rinsing with water.
5. Developer Application: A thin layer of developer is then applied to the sample to draw
penetrant trapped in flaws back to the surface where it will be visible. Developers come
in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying
(wet developers).
6. Indication Development: The developer is allowed to stand on the part surface for a
period of time sufficient to permit the extraction of the trapped penetrant out of any

surface flaws. This development time is usually a minimum of 10 minutes. Significantly
longer times may be necessary for tight cracks.
7. Inspection: Inspection is then performed under appropriate lighting to detect indications
from any flaws which may be present.
8. Clean Surface: The final step in the process is to thoroughly clean the part surface to
remove the developer from the parts that were found to be acceptable.

Common Uses of Liquid Penetrant Inspection
Liquid penetrant inspection (LPI) is one of the most widely used nondestructive evaluation
(NDE) methods. Its popularity can be attributed to two main factors: its relative ease of use and
its flexibility. LPI can be used to inspect almost any material provided that its surface is not
extremely rough or porous. Materials that are commonly inspected using LPI include the
following:


Metals (aluminum, copper, steel, titanium, etc.)



Glass



Many ceramic materials



Rubber



Plastics

LPI offers flexibility in performing inspections because it can be applied in a large variety of
applications ranging from automotive spark plugs to critical aircraft components. Penetrant
materials can be applied with a spray can or a cotton swab to inspect for flaws known to occur in
a specific area or it can be applied by dipping or spraying to quickly inspect large areas. In the
image above, visible dye penetrant is being locally applied to a highly loaded connecting point to
check for fatigue cracking.
Penetrant inspection systems have been developed to inspect some very large components. In the
image shown right, DC-10 banjo fittings are being moved into a penetrant inspection system at
what used to be the Douglas Aircraft Company's Long Beach, California facility. These large
machined aluminum forgings are used to support the number two engine in the tail of a DC-10
aircraft.
Liquid penetrant inspection can only be used to inspect for flaws that break the surface of the
sample. Some of these flaws are listed below:


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

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be
open to the surface. To learn more about the advantages and disadvantages of LPI, proceed to the
next page. dvantages and Disadvantages of Penetrant Testing
Like all nondestructive inspection methods, liquid penetrant inspection has both advantages and
disadvantages. The primary advantages and disadvantages when compared to other NDE
methods are summarized below.
Primary Advantages


The method has high sensitivity to small surface discontinuities.



The method has few material limitations, i.e. metallic and nonmetallic, magnetic and
nonmagnetic, and conductive and nonconductive materials may be inspected.



Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.



Parts with complex geometric shapes are routinely inspected.



Indications are produced directly on the surface of the part and constitute a visual
representation of the flaw.



Aerosol spray cans make penetrant materials very portable.



Penetrant materials and associated equipment are relatively inexpensive.

Primary Disadvantages



Only surface breaking defects can be detected.



Only materials with a relatively nonporous surface can be inspected.



Precleaning is critical since contaminants can mask defects.



Metal smearing from machining, grinding, and grit or vapor blasting must be removed
prior to LPI.



The inspector must have direct access to the surface being inspected.



Surface finish and roughness can affect inspection sensitivity.



Multiple process operations must be performed and controlled.



Post cleaning of acceptable parts or materials is required.



Chemical handling and proper disposal is required.

Penetrant Testing Materials
The penetrant materials used today are much more sophisticated than the kerosene and whiting
first used by railroad inspectors near the turn of the 20th century. Today's penetrants are
carefully formulated to produce the level of sensitivity desired by the inspector. To perform well,
a penetrant must possess a number of important characteristics. A penetrant must:


spread easily over the surface of the material being inspected to provide complete and
even coverage.



be drawn into surface breaking defects by capillary action.



remain in the defect but remove easily from the surface of the part.



remain fluid so it can be drawn back to the surface of the part through the drying and
developing steps.



be highly visible or fluoresce brightly to produce easy to see indications.



not be harmful to the material being tested or the inspector.

All penetrant materials do not perform the same and are not designed to perform the same.
Penetrant manufactures have developed different formulations to address a variety of inspection
applications. Some applications call for the detection of the smallest defects possible and have
smooth surfaces where the penetrant is easy to remove. In other applications, the rejectable

defect size may be larger and a penetrant formulated to find larger flaws can be used. The
penetrants that are used to detect the smallest defect will also produce the largest amount of
irrelevant indications.
Penetrant materials are classified in the various industry and government specifications by their
physical characteristics and their performance. Aerospace Material Specification (AMS) 2644,
Inspection Material, Penetrant, is now the primary specification used in the USA to control
penetrant materials. Historically, Military Standard 25135, Inspection Materials, Penetrants, has
been the primary document for specifying penetrants but this document is slowly being phased
out and replaced by AMS 2644. Other specifications such as ASTM 1417, Standard Practice for
Liquid Penetrant Examinations, may also contain information on the classification of penetrant
materials but they are generally referred back to MIL-I-25135 or AMS 2644.
Penetrant materials come in two basic types. These types are listed below:


Type 1 - Fluorescent Penetrants



Type 2 - Visible Penetrants

Fluorescent penetrants contain a dye or several dyes that fluoresce when exposed to ultraviolet
radiation. Visible penetrants contain a red dye that provides high contrast against the white
developer background. Fluorescent penetrant systems are more sensitive than visible penetrant
systems because the eye is drawn to the glow of the fluorescing indication. However, visible
penetrants do not require a darkened area and an ultraviolet light in order to make an inspection.
Visible penetrants are also less vulnerable to contamination from things such as cleaning fluid
that can significantly reduce the strength of a fluorescent indication.
Penetrants are then classified by the method used to remove the excess penetrant from the part.
The four methods are listed below:


Method A - Water Washable



Method B - Post-Emulsifiable, Lipophilic



Method C - Solvent Removable



Method D - Post-Emulsifiable, Hydrophilic

Water washable (Method A) penetrants can be removed from the part by rinsing with water
alone. These penetrants contain an emulsifying agent (detergent) that makes it possible to wash
the penetrant from the part surface with water alone. Water washable penetrants are sometimes
referred to as self-emulsifying systems. Post-emulsifiable penetrants come in two varieties,
lipophilic and hydrophilic. In post-emulsifiers, lipophilic systems (Method B), the penetrant is
oil soluble and interacts with the oil-based emulsifier to make removal possible. Postemulsifiable, hydrophilic systems (Method D), use an emulsifier that is a water soluble detergent

which lifts the excess penetrant from the surface of the part with a water wash. Solvent
removable penetrants require the use of a solvent to remove the penetrant from the part.
Penetrants are then classified based on the strength or detectability of the indication that is
produced for a number of very small and tight fatigue cracks. The five sensitivity levels are
shown below:


Level ½ - Ultra Low Sensitivity



Level 1 - Low Sensitivity



Level 2 - Medium Sensitivity



Level 3 - High Sensitivity



Level 4 - Ultra-High Sensitivity

The major US government and industry specifications currently rely on the US Air Force
Materials Laboratory at Wright-Patterson Air Force Base to classify penetrants into one of the
five sensitivity levels. This procedure uses titanium and Inconel specimens with small surface
cracks produced in low cycle fatigue bending to classify penetrant systems. The brightness of
the indication produced is measured using a photometer. The sensitivity levels and the test
procedure used can be found in Military Specification MIL-I-25135 and Aerospace Material
Specification 2644, Penetrant Inspection Materials.
An interesting note about the sensitivity levels is that only four levels were originally planned.
However, when some penetrants were judged to have sensitivities significantly less than most
others in the level 1 category, the ½ level was created. An excellent historical summary of the
development of test specimens for evaluating the performance of penetrant materials can be
found in the following reference.
Reference:
Flaherty, J. J., History of Penetrants: The First 20 Years, 1941-61, Materials Evaluation, Vol. 44,
No. 12, November 1986, pp. 1371-1374, 1376, 1378, 1380, 1382

Penetrants
The industry and military specifications that control penetrant materials and their use, all
stipulate certain physical properties of the penetrant materials that must be met. Some of these
requirements address the safe use of the materials, such as toxicity, flash point, and
corrosiveness, and other requirements address storage and contamination issues. Still others
delineate properties that are thought to be primarily responsible for the performance or
sensitivity of the penetrants. The properties of penetrant materials that are controlled by AMS

2644 and MIL-I-25135E include flash point, surface wetting capability, viscosity, color,
brightness, ultraviolet stability, thermal stability, water tolerance, and removability.
More information on how some of these properties can affect penetrant testing can be found by
following these links.
When removal of the penetrant from a defect due to over-washing of the part is a concern, a postemulsifiable penetrant system can be used. Post-emulsifiable penetrants require a separate
emulsifier to break the penetrant down and make it water-washable. Most penetrant inspection
specifications classify penetrant systems into four methods of excess penetrant removal. These
are listed below:
1. Method A: Water-Washable
2. Method B: Post-Emulsifiable, Lipophilic
3. Method C: Solvent Removable
4. Method D: Post-Emulsifiable, Hydrophilic
Method C relies on a solvent cleaner to remove the penetrant from the part being inspected.
Method A has emulsifiers built into the penetrant liquid that makes it possible to remove the
excess penetrant with a simple water wash. Method B and D penetrants require an additional
processing step where a separate emulsification agent is applied to make the excess penetrant
more removable with a water wash. Lipophilic emulsification systems are oil-based materials
that are supplied in ready-to-use form. Hydrophilic systems are water-based and supplied as a
concentrate that must be diluted with water prior to use .
Lipophilic emulsifiers (Method B) were introduced in the late 1950's and work with both a
chemical and mechanical action. After the emulsifier has coated the surface of the object,
mechanical action starts to remove some of the excess penetrant as the mixture drains from the
part. During the emulsification time, the emulsifier diffuses into the remaining penetrant and the
resulting mixture is easily removed with a water spray.
Hydrophilic emulsifiers (Method D) also remove the excess penetrant with mechanical and
chemical action but the action is different because no diffusion takes place. Hydrophilic
emulsifiers are basically detergents that contain solvents and surfactants. The hydrophilic
emulsifier breaks up the penetrant into small quantities and prevents these pieces from
recombining or reattaching to the surface of the part. The mechanical action of the rinse water
removes the displaced penetrant from the part and causes fresh remover to contact and lift newly
exposed penetrant from the surface.
The hydrophilic post-emulsifiable method (Method D) was introduced in the mid 1970's. Since
it is more sensitive than the lipophilic post emulsifiable method it has made the later method
virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive
to variation in the contact and removal time. While emulsification time should be controlled as

closely as possible, a variation of one minute or more in the contact time will have little effect on
flaw detectability when a hydrophilic emulsifier is used. However, a variation of as little as 15 to
30 seconds can have a significant effect when a lipophilic system is used.
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.
-- Sherwin, A. G., Overremoval Propensities of the Prewash Hydrophilic Emulsifier Fluorescent
Penetrant Process, Materials Evaluation, March 1993, pp. 294-299.

Developers
The role of the developer is to pull the trapped penetrant material out of defects and spread it out
on the surface of the part so it can be seen by an inspector. The fine developer particles both
reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant,
causing more efficient fluorescence. The developer also allows more light to be emitted through
the same mechanism. This is why indications are brighter than the penetrant itself under UV
light. Another function that some developers perform is to create a white background so there is
a greater degree of contrast between the indication and the surrounding background.
Developer Forms
The AMS 2644 and Mil-I-25135 classify developers into six standard forms. These forms are
listed below:
1. Form a - Dry Powder
2. Form b - Water Soluble
3. Form c - Water Suspendable
4. Form d - Nonaqueous Type 1 Fluorescent (Solvent Based)
5. Form e - Nonaqueous Type 2 Visible Dye (Solvent Based)
6. Form f - Special Applications
The developer classifications are based on the method that the developer is applied. The
developer can be applied as a dry powder, or dissolved or suspended in a liquid carrier. Each of
the developer forms has advantages and disadvantages.
Dry Powder

Dry powder developer is generally considered to be the least sensitive but it is inexpensive to use
and easy to apply. Dry developers are white, fluffy powders that can be applied to a thoroughly
dry surface in a number of ways. The developer can be applied by dipping parts in a container of
developer, or by using a puffer to dust parts with the developer. Parts can also be placed in a dust
cabinet where the developer is blown around and allowed to settle on the part. Electrostatic
powder spray guns are also available to apply the developer. The goal is to allow the developer to
come in contact with the whole inspection area.
Unless the part is electrostatically charged, the powder will only adhere to areas where trapped
penetrant has wet the surface of the part. The penetrant will try to wet the surface of the penetrant
particle and fill the voids between the particles, which brings more penetrant to the surface of the
part where it can be seen. Since dry powder developers only stick to the area where penetrant is
present, the dry developer does not provide a uniform white background as the other forms of
developers do. Having a uniform light background is very important for a visible inspection to be
effective and since dry developers do not provide one, they are seldom used for visible
inspections. When a dry developer is used, indications tend to stay bright and sharp since the
penetrant has a limited amount of room to spread.
Water Soluble
As the name implies, water soluble developers consist of a group of chemicals that are dissolved
in water and form a developer layer when the water is evaporated away. The best method for
applying water soluble developers is by spraying it on the part. The part can be wet or dry.
Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods
are less desirable. Aqueous developers contain wetting agents that cause the solution to function
much like dilute hydrophilic emulsifier and can lead to additional removal of entrapped
penetrant. Drying is achieved by placing the wet but well drained part in a recirculating, warm
air dryer with the temperature held between 70 and 75°F. If the parts are not dried quickly, the
indications will will be blurred and indistinct. Properly developed parts will have an even, pale
white coating over the entire surface.
Water Suspendable
Water suspendable developers consist of insoluble developer particles suspended in water. Water
suspendable developers require frequent stirring or agitation to keep the particles from settling
out of suspension. Water suspendable developers are applied to parts in the same manner as
water soluble developers. Parts coated with a water suspendable developer must be forced dried
just as parts coated with a water soluble developer are forced dried. The surface of a part coated
with a water suspendable developer will have a slightly translucent white coating.
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
Plastic or lacquer developers are special developers that are primarily used when a permanent
record of the inspection is required.

Preparation of Part
One of the most critical steps in the penetrant inspection process is preparing the part for
inspection. All coatings, such as paints, varnishes, plating, and heavy oxides must be removed to
ensure that defects are open to the surface of the part. If the parts have been machined, sanded, or
blasted prior to the penetrant inspection, it is possible that a thin layer of metal may have
smeared across the surface and closed off defects. It is even possible for metal smearing to occur
as a result of cleaning operations such as grit or vapor blasting. This layer of metal smearing
must be removed before inspection.
Contaminants
Coatings, such as paint, are much more elastic than metal and will not fracture even though a
large defect may be present just below the coating. The part must be thoroughly cleaned as
surface contaminates can prevent the penetrant from entering a defect. Surface contaminants can
also lead to a higher level of background noise since the excess penetrant may be more difficult
to remove.
Common coatings and contaminates that must be removed include: paint, dirt, flux, scale,
varnish, oil, etchant, smut, plating, grease, oxide, wax, decals, machining fluid, rust, and residue
from previous penetrant inspections.
Some of these contaminants would obviously prevent penetrant from entering defects, so it is
clear they must be removed. However, the impact of other contaminants such as the residue from
previous penetrant inspections is less clear, but they can have a disastrous effect on the
inspection. Take the link below to review some of the research that has been done to evaluate the
effects of contaminants on LPI sensitivity.
Click here to learn more about possible problems with Cleaning Practices.
A good cleaning procedure will remove all contamination from the part and not leave any residue
that may interfere with the inspection process. It has been found that some alkaline cleaners can
be detrimental to the penetrant inspection process if they have silicates in concentrations above
0.5 percent. Sodium metasilicate, sodium silicate, and related compounds can adhere to the

surface of parts and form a coating that prevents penetrant entry into cracks. Researchers in
Russia have also found that some domestic soaps and commercial detergents can clog flaw
cavities and reduce the wettability of the metal surface, thus reducing the sensitivity of the
penetrant. Conrad and Caudill found that media from plastic media blasting was partially
responsible for loss of LPI indication strength. Microphotographs of cracks after plastic media
blasting showed media entrapment in addition to metal smearing.
It is very important that the material being inspected has not been smeared across its own surface
during machining or cleaning operations. It is well recognized that machining, honing, lapping,
hand sanding, hand scraping, grit blasting, tumble deburring, and peening operations can cause
some materials to smear. It is perhaps less recognized that some cleaning operations, such as
steam cleaning, can also cause metal smearing in the softer materials. Take the link below to
learn more about metal smearing and its affects on LPI
Click here to learn more about metal smearing.
References:
Robinson, Sam J., Here Today, Gone Tomorrow! Replacing Methyl Chloroform in the Penetrant
Process, Materials Evaluation, Vol. 50, No. 8, August 1992, pp. 936-946.
Rummel, W., Cautions on the Use of Commercial Aqueous Precleaners for Penetrant Inspection,
Materials Evaluation, Vol. 16, No. 5, August 1998, pp. 950-952.
Glazkov, Y.A., Some Technological Mistakes in the Application of Capillary Inspection to
Repairs of Gas Turbin Engines, translation from Defektoskopiya - The Soviet Journal of
Nondestructive Testing, Vol. 26, No. 3, New York, NY Plenum/Consultants Bureau, January
1990, pp. 361-367.
Glazkov, Yu . A., Bruevich, E.P., and Samokhin, N.L, Special Features of Application of
Aqueous Solutions of Commercial Detergents in Capillary Flaw Inspection, Defektoskopiya The Soviet Journal of Nondestructive Testing, Vol. 19, No. 8, August 1982, pp. 83-87.

Selection of a Penetrant Technique
The selection of a liquid penetrant system is not a straightforward task. There are a variety of
penetrant systems and developer types that are available for use, and one set of penetrant
materials will not work for all applications. Many factors must be considered when selecting the
penetrant materials for a particular application. These factors include the sensitivity required,
materials cost, number of parts, size of area requiring inspection, and portability.

When sensitivity is the primary consideration for choosing a penetrant system, the first decision
that must be made is whether to use fluorescent penetrant or visible dye penetrant. Fluorescent
penetrants are generally more capable of producing a detectable indication from a small defect.
Also, the human eye is more sensitive to a light indication on a dark background and the eye is
naturally drawn to a fluorescent indication.
The graph below presents a series of curves that show the contrast ratio required for a spot of a
certain diameter to be seen. The ordinate is the spot diameter, which was viewed from one foot.
The abscissa is the contrast ratio between the spot brightness and the background brightness. To
the left of the contrast ratio of one, the spot is darker than the background (representative of
visible dye penetrant testing); and to the right of one, the spot is brighter than the background
(representative of fluorescent penetrant inspection). Each of the three curves right or left of the
contrast ratio of one are for different background brightness (in foot-Lamberts), but simply
consider the general trend of each group of curves right or left of the contrast ratio of one. The
curves show that for indication larger than 0.076 mm (0.003 inch) in diameter, it does not really
matter if it is a dark spot on a light background or a light spot on a dark background. However,
when a dark indication on a light background is further reduced in size, it is no longer detectable
even though contrast is increased. Furthermore, with a light indication on a dark background,
indications down to 0.003 mm (0.0001 inch) were detectable when the contrast between the flaw
and the background was high.
From this data, it can be seen why a fluorescent penetrant offers an advantage over a visible
penetrant for finding very small defects. Data presented by De Graaf and De Rijk supports this
statement. They inspected "identical" fatigue cracked specimens using a red dye penetrant and a
fluorescent dye penetrant. The fluorescent penetrant found 60 defects while the visible dye was
only able to find 39 of the defects.
Ref: De Graaf, E. and De Rijk, P., Comparison Between Reliability, Sensitivity, and Accuracy of
Nondestructive Inspection Methods, 13th Symposium on Nondestructive Evaluation
Proceedings, San Antonio, TX, published by NTIAC, Southwest Research Institute, San Antonio,
TX, April 1981, pp. 311-322.

Ref: Thomas, W.E., An Analytic Approach to Penetrant Performance, 1963 Lester Honor
Lecture, Nondestructive Testing, Vol. 21, No. 6, Nov.-Dec. 1963, pp. 354-368.
Under certain conditions, the visible penetrant may be a better choice. When fairly large defects
are the subject of the inspection, a high sensitivity system may not be warranted and may result
in a large number of irrelevant indications. Visible dye penetrants have also been found to give
better results when surface roughness is high or when flaws are located in areas such as
weldments.
Since visible dye penetrants do not require a darkened area for the use of an ultraviolet light,
visible systems are more easy to use in the field. Solvent removable penetrants, when properly
applied, can have the highest sensitivity and are very convenient to use. However, they are
usually not practical for large area inspection or in high-volume production settings.
Another consideration in the selection of a penetrant system is whether water washable, postemulsifiable or solvent removable penetrants will be used. Post-emulsifiable systems are
designed to reduce the possibility of over-washing, which is one of the factors known to reduce
sensitivity. However, these systems add another step, and thus cost, to the inspection process.
Penetrants are evaluated by the US Air Force according to the requirements in MIL-I-25135 and
each penetrant system is classified into one of five sensitivity levels. This procedure uses
titanium and Inconel specimens with small surface cracks produced in low cycle fatigue bending
to classify penetrant systems. The brightness of the indications produced after processing a set of
specimens with a particular penetrant system is measured using a photometer. A procedure for
producing and evaluating the penetrant qualification specimens was reported on by Moore and
Larson at the 1997 ASNT Fall Conference. Most commercially available penetrant materials are

listed in the Qualified Products List of MIL-I-25135 according to their type, method and
sensitivity level. Visible dye and dual-purpose penetrants are not classified into sensitivity levels
as fluorescent penetrants are. The sensitivity of a visible dye penetrant is regarded as level 1 and
largely dependent on obtaining good contrast between the indication and the background.

Penetrant Application and Dwell Time
The penetrant material can be applied in a number of different ways, including spraying,
brushing, or immersing the parts in a penetrant bath. The method of penetrant application has
little effect on the inspection sensitivity but an electrostatic spraying method is reported to
produce slightly better results than other methods. Once the part is covered in penetrant it must
be allowed to dwell so the penetrant has time to enter any defect present.
There are basically two dwell mode options, immersion-dwell (keeping the part immersed in the
penetrant during the dwell period) and drain-dwell (letting the part drain during the dwell
period). Prior to a study by Sherwin, the immersion-dwell mode was generally considered to be
more sensitive but recognized to be less economical because more penetrant was washed away
and emulsifiers were contaminated more rapidly. The reasoning for thinking this method was
more sensitive was that the penetrant was more migratory and more likely to fill flaws when kept
completely fluid and not allowed to lose volatile constituents by evaporation. However, Sherwin
showed that if the specimens are allowed to drain-dwell, the sensitivity is higher because the
evaporation increases the dyestuff concentration of the penetrant on the specimen. As pointed-out
in the section on penetrant materials, sensitivity increases as the dyestuff concentration increases.
Sherwin also cautions that the samples being inspected should be placed outside the penetrant
tank wall so that vapors from the tank do not accumulate and dilute the dyestuff concentration of
the penetrant on the specimen.
-- Vaerman, J., Fluorescent Penetrant Inspection, Quantified Evolution of the Sensitivity Versus
Process Deviations, Proceedings of the 4th European Conference on Nondestructive Testing,
Pergamon Press, Maxwell House, Fairview Park, Elmsford, New York, Volume 4, September
1987, pp. 2814-2823.
-- Sherwin, A.G., Establishing Liquid Penetrant Dwell Modes, Materials Evaluation, Vol. 32, No.
3, March 1974, pp. 63-67.
Penetrant Dwell Time
Penetrant dwell time is the total time that the penetrant is in contact with the part surface. The
dwell time is important because it allows the penetrant the time necessary to seep or be drawn
into a defect. Dwell times are usually recommended by the penetrant producers or required by
the specification being followed. The time required to fill a flaw depends on a number of
variables which include the following:


The surface tension of the penetrant.



The contact angle of the penetrant.



The dynamic shear viscosity of the penetrant, which can vary with the diameter of the
capillary. The viscosity of a penetrant in microcapillary flaws is higher than its viscosity
in bulk, which slows the infiltration of the tight flaws.



The atmospheric pressure at the flaw opening.



The capillary pressure at the flaw opening.



The pressure of the gas trapped in the flaw by the penetrant.



The radius of the flaw or the distance between the flaw walls.



The density or specific gravity of the penetrant.



Microstructural properties of the penetrant.

The ideal dwell time is often determined by experimentation and is often very specific to a
particular application. For example, AMS 2647A requires that the dwell time for all aircraft and
engine parts be at least 20 minutes, while ASTM E1209 only requires a five minute dwell time
for parts made of titanium and other heat resistant alloys. Generally, there is no harm in using a
longer penetrant dwell time as long as the penetrant is not allowed to dry.
The following tables summarize the dwell time requirements of several commonly used
specifications. The information provided below is intended for general reference and no
guarantee is made about its correctness. Please consult the specifications for the actual dwell
time requirements.

Some Research Results on Dwell Time
An interesting point that Deutsch makes about dwell time is that if the elliptical flaw has a length
to width ratio of 100, it will take the penetrant nearly ten times longer to fill than it will a
cylindrical flaw with the same volume.
-- Deutsch, S. A, Preliminary Study of the Fluid Mechanics of Liquid Penetrant Testing, Journal
of Research of the National Bureau of Standards, Vol. 84, No. 4, July-August 1979, pp. 287-291.
Lord and Holloway looked for the optimum penetrant dwell time required for detecting several
types of defects in titanium. Both a level 2 post-emulsifiable fluorescent penetrant (Magnaflux
ZL-2A penetrant and ZE-3 emulsifier) and a level 2 water washable penetrant (Tracer-Tech P133A penetrant) were included in the study. The effect of the developer was a variable in the
study and nonaqueous wet, aqueous wet, and dry developers were included. Specimens were also
processed using no developer. The specimen defects included stress corrosion cracks, fatigue
cracks and porosity. As expected, the researchers found that the optimal dwell time varied with
the type of defect and developer used. The following table summarizes some of the findings.

-- Lord, R. J. and Holloway, J. A., Choice of Penetrant Parameters for Inspecting Titanium,
Materials Evaluation, October 1975, pp. 249-256.
C

Penetrant Removal Process
The penetrant removal procedure must effectively remove the penetrant from the surface of the
part without removing an appreciable amount of entrapped penetrant from the defect. If the
removal process extracts penetrant from the flaw, the flaw indication will be reduced by a
proportional amount. If the penetrant is not effectively removed from the part surface, the
contrast between the indication and the background will be reduced. As discussed in the Contrast
Sensitivity Section, as the contrast increases, so does visibility of the indication.
Removal Method
Penetrant systems are classified into four methods of excess penetrant removal. These include
the following:
1. Method A: Water-Washable
2. Method B: Post-Emulsifiable, Lipophilic
3. Method C: Solvent Removable
4. Method D: Post-Emulsifiable, Hydrophilic
Method C, Solvent Removable, is used primarily for inspecting small localized areas. This
method requires hand wiping the surface with a cloth moistened with the solvent remover, and is,
therefore, too labor intensive for most production situations. Of the three production penetrant
inspection methods, Method A, Water-Washable, is the most economical to apply. Waterwashable or self-emulsifiable penetrants contain an emulsifier as an integral part of the
formulation. The excess penetrant may be removed from the object surface with a simple water
rinse. These materials have the property of forming relatively viscous gels upon contact with
water, which results in the formation of gel-like plugs in surface openings. While they are
completely soluble in water, given enough contact time, the plugs offer a brief period of
protection against rapid wash removal. Thus, water-washable penetrant systems provide ease of
use and a high level of sensitivity.
When removal of the penetrant from the defect due to over-washing of the part is a concern, a
post-emulsifiable penetrant system can be used. Post-emulsifiable penetrants require a separate
emulsifier to breakdown the penetrant and make it water washable. The part is usually immersed
in the emulsifier but hydrophilic emulsifiers may also be sprayed on the object. Spray
application is not recommended for lipophilic emulsifiers because it can result in non-uniform
emulsification if not properly applied. Brushing the emulsifier on to the part is not recommended
either because the bristles of the brush may force emulsifier into discontinuities, causing the

entrapped penetrant to be removed. The emulsifier is allowed sufficient time to react with the
penetrant on the surface of the part but not given time to make its way into defects to react with
the trapped penetrant. The penetrant that has reacted with the emulsifier is easily cleaned away.
Controlling the reaction time is of essential importance when using a post-emulsifiable system. If
the emulsification time is too short, an excessive amount of penetrant will be left on the surface,
leading to high background levels. If the emulsification time is too long, the emulsifier will react
with the penetrant entrapped in discontinuities, making it possible to deplete the amount needed
to form an indication.
The hydrophilic post-emulsifiable method (Method D) is more sensitive than the lipophilic postemulsifiable method (Method B). Since these methods are generally only used when very high
sensitivity is needed, the hydrophilic method renders the lipophilic method virtually obsolete.
The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the
contact and removal time. While emulsification time should be controlled as closely as possible,
a variation of one minute or more in the contact time will have little effect on flaw detectability
when a hydrophilic emulsifier is used. On the contrary, a variation of as little as 15 to 30 seconds
can have a significant effect when a lipophilic system is used. Using an emulsifier involves
adding a couple of steps to the penetrant process, slightly increases the cost of an inspection.
When using an emulsifier, the penetrant process includes the following steps (extra steps in
bold): 1. pre-clean part, 2. apply penetrant and allow to dwell, 3. pre-rinse to remove first layer
of penetrant, 4. apply hydrophilic emulsifier and allow contact for specified time, 5. rinse to
remove excess penetrant, 6. dry part, 7. apply developer and allow part to develop, and 8.
inspect.
Rinse Method and Time for Water-Washable Penetrants
The method used to rinse the excess penetrant from the object surface and the time of the rinse
should be controlled so as to prevent over-washing. It is generally recommended that a coarse
spray rinse or an air-agitated, immersion wash tank be used. When a spray is being used, it
should be directed at a 45° angle to the part surface so as to not force water directly into any
discontinuities that may be present. The spray or immersion time should be kept to a minimum
through frequent inspections of the remaining background level.
Hand Wiping of Solvent Removable Penetrants
When a solvent removable penetrant is used, care must also be taken to carefully remove the
penetrant from the part surface while removing as little as possible from the flaw. The first step
in this cleaning procedure is to dry wipe the surface of the part in one direction using a white,
lint-free, cotton rag. One dry pass in one direction is all that should be used to remove as much
penetrant as possible. Next, the surface should be wiped with one pass in one direction with a rag
moistened with cleaner. One dry pass followed by one damp pass is all that is recommended.
Additional wiping may sometimes be necessary; but keep in mind that with every additional
wipe, some of the entrapped penetrant will be removed and inspection sensitivity will be
reduced.

To study the effects of the wiping process, Japanese researchers manufactured a test specimen
out of acrylic plates that allowed them to view the movement of the penetrant in a narrow cavity.
The sample consisted of two pieces of acrylic with two thin sheets of vinyl clamped between as
spaces. The plates were clamped in the corners and all but one of the edges sealed. The unsealed
edge acted as the flaw. The clearance between the plates varied from 15 microns (0.00059055
inch) at the clamping points to 30 microns (0.0011811 inch) at the midpoint between the clamps.
The distance between the clamping points was believed to be 30 mm (1.18 inch).
Although the size of the flaw represented by this specimen is large, an interesting observation
was made. They found that when the surface of the specimen was wiped with a dry cloth,
penetrant was blotted and removed from the flaw at the corner areas where the clearance
between the plate was the least. When the penetrant at the side areas was removed, penetrant
moved horizontally from the center area to the ends of the simulated crack where capillary forces
are stronger. Therefore, across the crack length, the penetrant surface has a parabola-like shape
where the liquid is at the surface in the corners but depressed in the center. This shows that each
time the cleaning cloth touches the edge of a crack, penetrant is lost from the defect. This also
explains why the bleedout of an indication is often largest at the corners of cracks.
-- 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.

Use and Selection of a Developer
The use of developer is almost always recommended. One study reported that the output from a
fluorescent penetrant could be multiplied by up to seven times when a suitable powder developer
was used. Another study showed that the use of developer can have a dramatic effect on the
probability of detection (POD) of an inspection. When a Haynes Alloy 188, flat panel specimen
with a low-cycle fatigue crack was inspected without a developer, a 90 % POD was never
reached with crack lengths as long as 19 mm (0.75 inch). The operator detected only 86 of 284
cracks and had 70 false-calls. When a developer was used, a 90 % POD was reached at 2 mm
(0.077 inch), with the inspector identifying 277 of 311 cracks with no false-calls. However, some
authors have reported that in special situations, the use of a developer may actually reduce
sensitivity. These situations primarily occur when large, well defined defects are being inspected
on a surface that contains many nonrelevant indications that cause excessive bleedout.
Type of Developer Used and Method of Application
Nonaqueous developers are generally recognized as the most sensitive when properly applied.
There is less agreement on the performance of dry and aqueous wet developers, but the aqueous
developers are usually considered more sensitive. Aqueous wet developers form a finer matrix of
particles that is more in contact with the part surface. However, if the thickness of the coating
becomes too great, defects can be masked. Also, aqueous wet developers can cause leaching and
blurring of indications when used with water-washable penetrants. The relative sensitivities of
developers and application techniques as ranked in Volume II of the Nondestructive Testing
Handbook are shown in the table below. There is general industry agreement with this table, but

some industry experts feel that water suspendable developers are more sensitive than watersoluble developers.
Sensitivity ranking of developers per the Nondestructive Testing Handbook.
Sensitivity Ranking (highest to lowest) Developer Form Application Technique.
Ranking
1
2
3
4
5
6
7
8
9
10

Developer Form
Nonaqueous, Wet Solvent
Plastic Film
Water-Soluble
Water-Suspendable
Water-Soluble
Water-Suspendable
Dry
Dry
Dry
Dry

Method of Application
Spray
Spray
Spray
Spray
Immersion
Immersion
Dust Cloud (Electrostatic)
Fluidized Bed
Dust Cloud (Air Agitation)
Immersion (Dip)

The following table lists the main advantages and disadvantages of the various developer types.
Developer

Advantages

Dry

Does not form contrast
Indications tend to remain
background so cannot be used
brighter and more distinct over
with visible systems
time
Easily to apply

Ease of coating entire part
Soluble

Disadvantages

Difficult to assure entire part
surface has been coated

Coating is translucent and
provides poor contrast (not
recommended for visual
systems)

White coating for good
contrast can be produced
which work well for both
Indications for water washable
visible and fluorescent systems
systems are dim and blurred

Ease of coating entire part
Indications are bright and
sharp
Suspendable
White coating for good
contrast can be produced
which work well for both
visible and fluorescent systems

Indications weaken and
become diffused after time

Very portable
Easy to apply to readily
accessible surfaces

Nonaqueous

Difficult to apply evenly to all
White coating for good
surfaces
contrast can be produced
which work well for both
More difficult to clean part
visible and fluorescent systems
after inspection
Indications show-up rapidly
and are well defined
Provides highest sensitivity

To review a summary of some of the research that has been done on developer usage and
performance, take this link.
Research on Developer Use
References:
- Brittain, P. I., The Amplifying Action of Developer Powders, QUALTEST 3 Conference,
Cincinnati OH, Oct 1984.
- Rummel, W. D., Probability of Detection as a Quantitative Measure of Nondestructive Testing
End-To-End Process Capabilities, Materials Evaluation, January 1998, pp. 35.
- Nondestructive Testing Handbook, Vol. 2, Liquid Penetrant Tests, Robert McMaster, et al.,
American Society for Nondestructive Testing, 1982, pp. 283-319.

Process Control of Temperature

The temperature of the penetrant materials and the part being inspected can have an effect on the
results. Temperatures from 27 to 49oC (80 to 120oF) are reported in the literature to produce
optimal results. Many specifications allow testing in the range of 4 to 52oC (40 to 125oF). A tip to
remember is that surfaces that can be touched for an extended period of time without burning the
skin are generally below 52oC (125oF).
Since the surface tension of most materials decrease as the temperature increases, raising the
temperature of the penetrant will increase the wetting of the surface and the capillary forces. Of
course, the converse is also true, so lowering the temperature will have a negative effect on the
flow characteristics. Raising the temperature will also raise the speed of evaporation of
penetrants, which can have a positive or negative effect on sensitivity. The impact will be
positive if the evaporation serves to increase the dye concentration of the penetrant trapped in a
flaw up to the concentration quenching point and not beyond. Higher temperatures and more
rapid evaporation will have a negative effect if the dye concentration exceeds the concentration
quenching point, or the flow characteristics are changed to the point where the penetrant does not
readily flow.
The method of processing a hot part was once commonly employed. Parts were either heated or
processed hot off the production line. In its day, this served to increase inspection sensitivity by
increasing the viscosity of the penetrant. However, the penetrant materials used today have 1/2 to
1/3 the viscosity of the penetrants on the market in the 1960's and 1970's. Heating the part prior
to inspection is no longer necessary and no longer recommended.

Quality Control of Penetrant
The quality of a penetrant inspection is highly dependent on the quality of the penetrant materials
used. Only products meeting the requirements of an industry specification, such as AMS 2644,
should be used. Deterioration of new penetrants primarily results from aging and contamination.
Virtually all organic dyes deteriorate over time, resulting in a loss of color or fluorescent
response, but deterioration can be slowed with proper storage. When possible, keep the materials
in a closed container and protect from freezing and exposure to high heat. Freezing can cause
separation to occur and exposure to high temperature for a long period of time can affect the
brightness of the dyes.
Contamination can occur during storage and use. Of course, open tank systems are much more
susceptible to contamination than are spray systems. Contamination by another liquid will
change the surface tension and contact angle of the solution. Water is the most common
contaminant. Water-washable penetrants have a definite tolerance limit for water, and above this
limit they do not function properly. Cloudiness and viscosity both increase with increasing water
content. In self-emulsifiable penetrants, water contamination can produce a gel break or
emulsion inversion when the water concentration becomes high enough. The formation of the gel

is an important feature during the washing processes, but must be avoided until that stage in the
process. Data indicates that the water contamination must be significant (greater than 10%) for
gel formation to occur. Most specifications limit water contamination to around 5% to be
conservative. Water does not readily mix with the oily solution of lipophilic post-emulsifiable
systems and it generally settles to the bottom of the tank. However, the inspection of parts that
travel to the bottom of the tank and encounter the water could be adversely affected.
Most other common contaminates, such as cleaning solvents, oils, acids, caustics and chromates,
must be present in significant quantities to affect the performance of the penetrant. Organic
contaminants can dilute the dye and absorb the ultraviolet radiation before it reaches the dye, and
also change the viscosity. Acids, caustics, and chromates cause the loss of fluorescence in watersoluble penetrants.
Regular checks must be performed to ensure that the material performance has not degraded.
When the penetrant is first received from the manufacturer, a sample of the fresh solution should
be collected and stored as a standard for future comparison. The standard specimen should be
stored in a sealed, opaque glass or metal container. Penetrants that are in-use should be
compared regularly to the standard specimen to detect changes in color, odor and consistency.
When using fluorescent penetrants, a brightness comparison per the requirements of ASTM E
1417 is also often required. This check involves placing a drop of the standard and the in-use
penetrants on a piece of Whatman #4 filter paper and making a side by side comparison of the
brightness of the two spots under UV light.
Additionally, the water content of water washable penetrants must be checked regularly. Waterbased, water washable penetrants are checked with a refractometer. The rejection criteria is
different for different penetrants, so the requirements of the qualifying specification or the
manufacturer's instructions must be consulted. Non-water-based, water washable penetrants are
checked using the procedure specified in ASTM D95 or ASTM E 1417.
Application of the Penetrant
The application of the penetrant is the step of the process that requires the least amount of
control. As long as the surface being inspected receives a generous coating of penetrant, it really
doesn't matter how the penetrant is applied. Generally, the application method is an economic or
convenience decision.
It is important that the part be thoroughly cleaned and dried. Any contaminates or moisture on
the surface of the part or within a flaw can prevent the penetrant material from entering the
defect. The part should also be cool to the touch. The recommended range of temperature is 4 to
52oC (39 to 125oF).

Quality Control of Wash Temperature and Pressure
The wash temperature, pressure and time are three parameters that are typically controlled in
penetrant inspection process specification. A coarse spray or an immersion wash tank with air
agitation is often used. When the spray method is used, the water pressure is usually limited to
276 kN/m2 (40 psi). The temperature range of the water is usually specified as a wide range
(e.g.. 10 to 38oC (50 to 100oF) in AMS 2647A.) A low-pressure, coarse water spray will force
less water into flaws to dilute and/or remove trapped penetrant and weaken the indication. The
temperature will have an effect on the surface tension of the water and warmer water will have
more wetting action than cold water. Warmer water temperatures may also make emulsifiers and
detergent more effective. The wash time should only be as long as necessary to decrease the
background to an acceptable level. Frequent visual checks of the part should be made to
determine when the part has be adequately rinsed.
Summary of Research on Wash Method Variables
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 with numerous cracks ranging in depth from five
to 100 microns deep. A 38% 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 postemulsifiable penetrants were increased from 20 to 60 seconds, a loss in sensitivity was seen in
both cases, although much reduced from the loss seen with the water-washable system. The
relative sensitivity loss over the range of crack depths was 13% for the penetrant with medium
sensitivity.
-- 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.
In a 1972 paper by N.H. Hyam, the effects of the rinse time on the sensitivity of two level 4
water-washable penetrants were examined. It was reported that sensitivity decreased as sprayrinse time increased and that one of the penetrants was more affected by rinse time than the
others. Alburger, points out that some conventional fluorescent dyes are slightly soluble in water
and can be leached out during the washing processes.
-- Hyam, N. H., Quantitative Evaluation of Factors Affecting the Sensitivity of Penetrant
Systems, Materials Evaluation, Vol. 30, No. 2, February 1972, pp. 31-38.

Brittian evaluated the effect of wash time on a water-washable, level 4 penetrant (Ardrox
970P25) and found that indication brightness decreases rapidly in the first minute of wash and
then slows. The brightness value dropped from a relative value of 1100 to approximately 500 in
the first minute and then continued to decrease nearly linearly to a value of 200 after five
minutes of wash. Brittian concluded that wash time for water-washable systems should be kept to
a minimum.
-- 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.
Robinson and Schmidt used a Turner fluorometer to evaluate the variability that some of the
processing steps can produce in the brightness of indications. To find out how much effect the
wash procedure had on sensitivity, Tesco cracked, chrome-plated panels, were processed a
number of times using the same materials but three different wash methods. The washing
methods included spraying the specimens with a handheld nozzle, holding the specimens under a
running tap, and using a washing machine that controlled the water pressure, temperature, spray
pattern and wash time. The variation in indication brightness readings between five trials was
reported. The variation was 16% for the running tap water, 14% for the handheld spray nozzle
and 4.5% for the machine wash.
-- Robinson, S. J. and Schmidt, J. T., Fluorescent Penetrant Sensitivity and Removability - What
the Eye Can See, a Fluorometer Can Measure, Materials Evaluation, Vol. 42, No. 8, July 1984,
pp. 1029-1034.

Quality Control of Drying Process
The temperature used to dry parts after the application of an aqueous wet developer or prior to
the application of a dry powder or a nonaqueous wet developer, must be controlled to prevent
"cooking" of the penetrant in the defect. High drying temperature can affect penetrants in a
couple of ways. First, some penetrants can fade at high temperatures due to dye vaporization or
sublimation. Second, high temperatures can cause the penetrant to dry in the the flaw, preventing
it from migrating to the surface to produce an indication. To prevent harming the penetrant
material, drying temperature should be kept to under 71oC.
The drying should be limited to the minimum length of time necessary to thoroughly dry the
component being inspected.

Quality Control of Developer

The function of the developer is very important in a penetrant inspection. It must draw out of the
discontinuity a sufficient amount of penetrant to form an indication, and it must spread the
penetrant out on the surface to produce a visible indication. In a fluorescent penetrant inspection,
the amount of penetrant brought to the surface must exceed the dye's thin film threshold of
fluorescence, or the indication will not fluoresce. Additionally, the developer makes fluorescent
indications appear brighter than indications produced with the same amount of dye but without
the developer.
In order to accomplish these functions, a developer must adhere to the part surface and result in a
uniform, highly porous layer with many paths for the penetrant to be moved due to capillary
action. Developers are either applied wet or dry, but the desired end result is always a uniform,
highly porous, surface layer. Since the quality control requirements for each of the developer
types is slightly different, they will be covered individually.
Dry Powder Developer
A dry powder developer should be checked daily to ensure that it is fluffy and not caked. It
should be similar to fresh powdered sugar and not granulated like powdered soap. It should also
be relatively free from specks of fluorescent penetrant material from previous inspection. This
check is performed by spreading a sample of the developer out and examining it under UV light.
If there are ten or more fluorescent specks in a 10 cm diameter area, the batch should be
discarded.
Apply a light coat of the developer by immersing the test component or dusting the surface. After
the development time, excessive powder can be removed by gently blowing on the surface with
air not exceeding 35 kPa or 5 psi.
Wet Soluble/Suspendable Developer
Wet soluble developer must be completely dissolved in the water and wet suspendable developer
must be thoroughly mixed prior to application. The concentration of powder in the carrier
solution must be controlled in these developers. The concentration should be checked at least
weekly using a hydrometer to make sure it meets the manufacturer's specification. To check for
contamination, the solution should be examined weekly using both white light and UV light. If a
scum is present or the solution fluoresces, it should be replaced. Some specifications require that
a clean aluminum panel be dipped in the developer, dried, and examined for indications of
contamination by fluorescent penetrant materials.
These developers are applied immediately after the final wash. A uniform coating should be
applied by spraying, flowing or immersing the component. They should never be applied with a
brush. Care should be taken to avoid a heavy accumulation of the developer solution in crevices

and recesses. Prolonged contact of the component with the developer solution should be avoided
in order to minimize dilution or removal of the penetrant from discontinuities.
Solvent Suspendable (AKA Nonaqueous Wet)
Solvent suspendable developers are typically supplied in an sealed aerosol spray can. Since the
developer solution is in a sealed vessel, direct check of the solution is not possible. However, the
way that the developer is dispensed must be monitored. The spray developer should produce a
fine, even coating on the surface of the part. Make sure the can is well shaken and apply a thin
coating to a test article. If the spray produces spatters or an uneven coating, the can should be
discarded.
When applying a solvent suspendable developer, it is up to the inspector to control the thickness
of the coating. with a visible penetrant system, the developer coating must be thick enough to
provide a white contrasting background but not heavy enough to mask indications. When using a
fluorescent penetrant system, a very light coating should be used. The developer should be
applied under white light and should appear evenly transparent.
Development Time
Parts should be allowed to develop for a minimum of 10 minutes and no more than 2 hours
before inspecting.

Quality Control of Lighting
After a component has been properly processed, it is ready for inspection. While automated
vision inspection systems are sometimes used, the focus here will be on inspections performed
visually by a human inspector, as this is the dominant method. Proper lighting is of great
importance when visually inspecting a surface for a penetrant indication. Obviously, the lighting
requirements are different for an inspection conducted using a visible dye penetrant than they are
for an inspection conducted using a fluorescent dye penetrant. The lighting requirements for each
of these techniques, as well as how light measurements are made, are discussed below.
Lighting for Visible Dye Penetrant Inspections
When using a visible penetrant, the intensity of the white light is of principal importance.
Inspections can be conducted using natural lighting or artificial lighting. When using natural
lighting, it is important to keep in mind that daylight varies from hour to hour, so inspectors must
stay constantly aware of the lighting conditions and make adjustments when needed. To improve
uniformity in lighting from one inspection to the next, the use of artificial lighting is
recommended. Artificial lighting should be white whenever possible and white flood or halogen

lamps are most commonly used. The light intensity is required to be 100 foot-candles at the
surface being inspected. It is advisable to choose a white light wattage that will provide sufficient
light, but avoid excessive reflected light that could distract from the inspection.
Lighting for Fluorescent Penetrant Inspections
When a fluorescent penetrant is being employed, the ultraviolet (UV) illumination and the visible
light inside the inspection booth is important. Penetrant dyes are excited by UV light of 365nm
wavelength and emit visible light somewhere in the green-yellow range between 520 and 580nm.
The source of ultraviolet light is often a mercury arc lamp with a filter. The lamps emit many
wavelengths and a filter is used to remove all but the UV and a small amount of visible light
between 310 and 410nm. Visible light of wavelengths above 410nm interferes with contrast, and
UV emissions below 310nm include some hazardous wavelengths.
Standards and procedures require verification of lens condition and light intensity. Black lights
should never be used with a cracked filter as output of white light and harmful black light will be
increased. The cleanliness of the filter should also be checked as a coating of solvent carrier, oils,
or other foreign materials can reduce the intensity by up to as much as 50%. The filter should be
checked visually and cleaned as necessary before warm-up of the light.
Since fluorescent brightness is linear with respect to ultraviolet excitation, a change in the
intensity of the light (from age or damage) and a change in the distance of the light source from
the surface being inspected will have a direct impact on the inspection. For UV lights used in
component evaluations, the normally accepted intensity is 1000 microwatt per square centimeter
when measured at 15 inches from the filter face (requirements can vary from 800 to 1200
µW/cm2). The required check should be performed when a new bulb is installed, at startup of the
inspection cycle, if a change in intensity is noticed, or every eight hours of continuous use.
Regularly checking the intensity of UV lights is very important because bulbs lose intensity over
time. In fact, a bulb that is near the end of its operating life will often have an intensity of only
25% of its original output.
Black light intensity will also be affected by voltage variations. A bulb that produces acceptable
intensity at 120 volts will produce significantly less at 110 volts. For this reason it is important to
provide constant voltage to the light. Also, most UV light must be warmed up prior to use and
should be on for at least 15 minutes before beginning an inspection.
When performing a fluorescent penetrant inspection, it is important to keep white light to a
minimum as it will significantly reduce the inspectors ability to detect fluorescent indications.
Light levels of less than 2 fc are required by most procedures with some procedures requiring
less than 0.5 fc at the inspection surface. Procedures require a check and documentation of
ambient white light in the inspection area. When checking black light intensity at 15 inches a

reading of the white light produced by the black light may be required to verify white light is
being removed by the filter.
Light Measurement
Light intensity measurements are made using a radiometer. A radiometer is an instrument that
translate light energy into an electrical current. Light striking a silicon photodiode detector
causes a charge to build up between internal layers. When an external circuit is
connected to the cell, an electrical current is produced. This current is linear with respect to
incident light. Some radiometers have the ability to measure both black and white light, while
others require a separate sensor for each measurement. Whichever type is used, the sensing area
should be clean and free of any materials that could reduce or obstruct light reaching the sensor.
Radiometers are relatively unstable instruments and readings often change considerable over
time. Therefore, they should be calibrated at least every six months.
Ultraviolet light measurements should be taken using a fixture to maintain a minimum distance
of 15 inches from the filter face to the sensor. The sensor should be centered in the light field to
obtain and record the highest reading. UV spot lights are often focused, so intensity readings will
vary considerable over a small area. White lights are seldom focused and depending on the
wattage, will often produce in excess of the 100 fc at 15 inches. Many specifications do not
require the white light intensity check to be conducted at a specific distance.

ystem Performance Check
System performance checks involve processing a test specimen with known defects to determine
if the process will reveal discontinuities of the size required. The specimen must be processed
following the same procedure used to process production parts. A system performance check is
typically required daily, at the reactivation of a system after maintenance or repairs, or any time
the system is suspected of being out of control. As with penetrant inspections in general, results
are directly dependent on the skill of the operator and, therefore, each operator should process a
panel.
The ideal specimen is a production item that has natural defects of the minimum acceptable size.
Some specification delineate the type and size of the defects that must be present in the specimen
and detected. Surface finish is will affect washability so the check specimen should have the
same surface finish as the production parts being processed. If penetrant systems with different
sensitivity levels are being used, there should be a separate specimen for each system.
There are some universal test specimens that can be used if a standard part is not available. The
most commonly used test specimen is the TAM or PSM panel. These panel are usually made of
stainless steel that has been chrome plated on one half and surfaced finished on the other half to

produced the desired roughness. The chrome plated section is impacted from the back side to
produce a starburst set of cracks in the chrome. There are five impacted areas to produce range of
crack sizes. Each panel has a characteristic “signature” and variances in that signature are
indications of process variance. Panel patterns as well as brightness are indicators of process
consistency or variance.
Care of system performance check specimens is critical. Specimens should be handled carefully
to avoid damage. They should be cleaned thoroughly between uses and storage in a solvent is
generally recommended. Before processing a specimen, it should be inspected under UV light to
make sure that it is clean and not already producing an indication.

Nature of the Defect
The nature of the defect can have a large affect on sensitivity of a liquid penetrant inspection.
Sensitivity is defined as the smallest defect that can be detected with a high degree of reliability.
Typically, the crack length at the sample surface is used to define size of the defect. A survey of
any probability-of-detection curve for penetrant inspection will quickly lead one to the
conclusion that crack length has a definite affect on sensitivity. However, the crack length alone
does not determine whether a flaw will be seen or go undetected. The volume of the defect is
likely to be the more important feature. The flaw must be of sufficient volume so that enough
penetrant will bleed back out to a size that is detectable by the eye or that will satisfy the
dimensional thresholds of fluorescence.

Above is an example of fluorescent penetrant inspection probability of detection (POD) curve
from the Nondestructive Evaluation (NDE) Capabilities Data Book. Please note that this curve is
specific to one set of inspection conditions and should not be interpreted to apply to other
inspection situations.
In general, penetrant inspections are more effective at finding


small round defects than small linear defects. Small round defects are generally easier
to detect for several reasons. First, they are typically volumetric defects that can trap
significant amounts of penetrant. Second, round defects fill with penetrant faster than
linear defects. One research effort found that elliptical flaw with length to width ratio of
100, will take the penetrant nearly 10 times longer to fill than a cylindrical flaw with the
same volume.



deeper flaws than shallow flaws. Deeper flaws will trap more penetrant than shallow
flaws, and they are less prone to over washing.



flaws with a narrow opening at the surface than wide open flaws. Flaws with narrow
surface openings are less prone to over washing.



flaws on smooth surfaces than on rough surfaces. The surface roughness of the part
primarily affects the removability of a penetrant. Rough surfaces tend to trap more
penetrant in the various tool marks, scratches, and pits that make up the surface.
Removing the penetrant from the surface of the part is more difficult and a higher level of
background fluorescence or over washing may occur.



flaws with rough fracture surfaces than smooth fracture surfaces. The surface
roughness that the fracture faces is a factor in the speed at which a penetrant enters a
defect. In general, the penetrant spreads faster over a surface as the surface roughness
increases. It should be noted that a particular penetrant may spread slower than others on
a smooth surface but faster than the rest on a rougher surface.



flaws under tensile or no loading than flaws under compression loading. In a 1987
study at the University College London, the effect of crack closure on detectability was
evaluated. Researchers used a four-point bend fixture to place tension and compression
loads on specimens that were fabricated to contain fatigue cracks. All cracks were
detected with no load and with tensile loads placed on the parts. However, as
compressive loads were placed on the parts, the crack length steadily decreased as load
increased until a load was reached when the crack was no longer detectable.

References:
Rummel, W.D. and Matzkanin, G. A., Nondestructive Evaluation (NDE) Capabilities Data Book,
Published by the Nondestructive Testing Information Analysis Center (NTIAC), NTIAC #DB95-02, May 1996.
Alburger, J.R., Dimensional Transition Effects in Visible Color and Fluorescent Dye Liquids,
Proceedings, 23rd Annual Conference, Instrument Society of America, Vol. 23, Part I, Paper No.
564.
Deutsch, S. A, Preliminary Study of the Fluid Mechanics of Liquid Penetrant Testing, Journal of
Research of the National Bureau of Standards, Vol. 84, No. 4, July-August 1979, pp. 287-291.
Kauppinen, P. and Sillanpaa, J., Reliability of Surface Inspection Methods, Proceedings of the
12th World Conference on Nondestructive Testing, Amsterdam, Netherlands, Vol. 2, Elsevier
Science Publishing, Amsterdam, 1989, pp. 1723-1728.
Vaerman, J. F., Fluorescent Penetrant Inspection Process, Automatic Method for Sensitivity
Quantification, Proceedings of 11th World Conference on Nondestructive Testing, Volume III,
Las Vegas, NV, November 1985, pp. 1920-1927.
Thomas, W.E., An Analytic Approach to Penetrant Performance, 1963 Lester Honor Lecture,
Nondestructive Testing, Vol. 21, No. 6, Nov.-Dec. 1963, pp. 354-368.
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.

Health and Safety Precautions in Liquid Penetrant Inspection
When proper health and safety precautions are followed, liquid penetrant inspection operations
can be completed without harm to inspection personnel. However, there are a number of health
and safety related issues that must be addressed. Since each inspection operation will have its
own unique set of health and safety concerns that must be addressed, only a few of the most
common concerns will be discussed here.
Chemical Safety
Whenever chemicals must be handled, certain precautions must be taken as directed by the
material safety data sheets (MSDS) for the chemicals. Before working with a chemical of any
kind, it is highly recommended that the MSDS be reviewed so that proper chemical safety and
hygiene practices can be followed. Some of the penetrant materials are flammable and, therefore,
should be used and stored in small quantities. They should only be used in a well ventilated area
and ignition sources avoided. Eye protection should always be worn to prevent contact of the
chemicals with the eyes. Many of the chemicals used contain detergents and solvents that can
dermatitis. Gloves and other protective clothing should be worn to limit contact with the
chemicals.
Ultraviolet Light Safety
Ultraviolet (UV) light or "black light" as it is sometimes called, has wavelengths ranging from
180 to 400 nanometers. These wavelengths place UV light in the invisible part of the
electromagnetic spectrum between visible light and X-rays. The most familiar source of UV
radiation is the the sun and is necessary in small doses for certain chemical processes to occur in
the body. However, too much exposure can be harmful to the skin and eyes. Excessive UV light
exposure can cause painful sunburn, accelerate wrinkling and increase the risk of skin cancer.
UV light can cause eye inflammation, cataracts, and retinal damage.
Because of their close proximity, laboratory devices, like UV lamps, deliver UV light at a much
higher intensity than the sun and, therefore, can cause injury much more quickly. The greatest
threat with UV light exposure is that the individual is generally unaware that the damage is
occurring. There is usually no pain associated with the injury until several hours after the
exposure. Skin and eye damage occurs at wavelengths around 320 nm and shorter which is well
below the 365 nm wavelength, where penetrants are designed to fluoresce. Therefore, UV lamps
sold for use in LPI application are almost always filtered to remove the harmful UV
wavelengths. The lamps produce radiation at the harmful wavelengths so it is essential that they
be used with the proper filter in place and in good condition.

Health and Safety Precautions in Liquid Penetrant Inspection

When proper health and safety precautions are followed, liquid penetrant inspection operations
can be completed without harm to inspection personnel. However, there are a number of health
and safety related issues that must be addressed. Since each inspection operation will have its
own unique set of health and safety concerns that must be addressed, only a few of the most
common concerns will be discussed here.
Chemical Safety
Whenever chemicals must be handled, certain precautions must be taken as directed by the
material safety data sheets (MSDS) for the chemicals. Before working with a chemical of any
kind, it is highly recommended that the MSDS be reviewed so that proper chemical safety and
hygiene practices can be followed. Some of the penetrant materials are flammable and, therefore,
should be used and stored in small quantities. They should only be used in a well ventilated area
and ignition sources avoided. Eye protection should always be worn to prevent contact of the
chemicals with the eyes. Many of the chemicals used contain detergents and solvents that can
dermatitis. Gloves and other protective clothing should be worn to limit contact with the
chemicals.
Ultraviolet Light Safety
Ultraviolet (UV) light or "black light" as it is sometimes called, has wavelengths ranging from
180 to 400 nanometers. These wavelengths place UV light in the invisible part of the
electromagnetic spectrum between visible light and X-rays. The most familiar source of UV
radiation is the the sun and is necessary in small doses for certain chemical processes to occur in
the body. However, too much exposure can be harmful to the skin and eyes. Excessive UV light
exposure can cause painful sunburn, accelerate wrinkling and increase the risk of skin cancer.
UV light can cause eye inflammation, cataracts, and retinal damage.
Because of their close proximity, laboratory devices, like UV lamps, deliver UV light at a much
higher intensity than the sun and, therefore, can cause injury much more quickly. The greatest
threat with UV light exposure is that the individual is generally unaware that the damage is
occurring. There is usually no pain associated with the injury until several hours after the
exposure. Skin and eye damage occurs at wavelengths around 320 nm and shorter which is well
below the 365 nm wavelength, where penetrants are designed to fluoresce. Therefore, UV lamps
sold for use in LPI application are almost always filtered to remove the harmful UV
wavelengths. The lamps produce radiation at the harmful wavelengths so it is essential that they
be used with the proper filter in place and in good condition.

References and Resources
Cartz, Louis, Nondestructive Testing, ASM Intl, Metals Park, OH, 1995, ISBN: 0871705176

Introduction to Capillary Testing Theory, Borovikov, A.S. (Ed.), Minsk, Nauka i Tekhnika
Publishing, 1988
Liquid Penetrant Testing, Nondestructive Testing Handbook, Volume 2, Tracy, Noel (Tech. Ed.),
Moore, Patrick (Ed.) American Society for Nondestructive Testing, Columbus, OH, 1999, ISBN
1-57117-028-6
Larson, B.F., Study of the Factors Affecting the Sensitivity of Liquid Penetrant Inspections:
Review of Literature Published from 1970 to 1998, FAA Technical Report Number
DOT/FAA/AR-01/95, Office of Aviation Research, Washington, DC, Jan 2002 (pdf 1.0 meg)

Penetrant Inspection Quizzes
These quizzes draw from the same database of questions and differ only in the number of
questions presented. Each time a quiz is opened, a new set of random questions will be produced
from the database. The Collaboration for NDE Education does not record the names of
individuals taking a quiz or the results of a quiz.

Liquid Penetrant Inspection 20 Question Quiz

~ First name ~

1

~ Last name ~

The advantage that liquid penetrant testing has over an unaided
visual inspection is that:
The actual size of the discontinuity can be measured
The depth of the defect can be measured
The cause of the impact can be seen
It makes defects easier to see for the inspector

2

When fluorescent penetrant inspection is performed, the penetrant
materials are formulated to glow brightly and to give off light at a
wavelength:
Close to infrared light
Close to the wavelength of x-rays
That the eye is most sensitive to under dim lighting conditions
In the red spectrum

3

The threshold of visual acuity for a person with 20/20 vision is
about:
0.003 inches
0.03 inches
0.03 mm
0.3cm

4

When performing a liquid penetrant test, the surface of the part
under inspection should be:
Slightly damp
Clean and smooth to the touch
Free of oil, grease, water and other contaminants
All of the above

5

Once the surface of the part has been cleaned properly, penetrant can
be applied by:

Spraying
Brushing
Dipping
All of the above

6

If the surface of the part has been machined, sanded or grit blasted:
The part may also require etching
It can be immersed in penetrant for its entire dwell time
It will require a shorter dwell time
It will need to be heated in order to open any cracks that have
been peened over

7

The total time that the penetrant is in contact with the part surface is
called the:
Soak time
Baking time
Dwell time
Immersion time

8

Developer times are usually in the range of:
10 minutes
10 seconds
20-30 minutes

5-60 minutes

9

LPI can be used to test most materials provided the surface of the
part is:
Heated to a temperature above 100o F
Is not extremely rough or porous
Smooth and uniform
Cleaned with number 005 grit

10

Minimum penetrant dwell times are usually:
1-5 minutes
1-30 minutes
5-60 minutes
60-100 minutes

11

Developers come in a variety of forms and can be applied by:
Dusting
Dipping
Spraying
All of the above

12

Generally, there is no harm in using a longer penetrant dwell time as
long as the penetrant:

Is not allowed to dry
Stays viscous
Does not form clumps on the surface of the part
Is mixed with emulsifier

13

Which of the following is an advantage to LPI?
Large areas can be inspected
Parts with complex shapes can be inspected
It is portable
All of the above is an advantage

14

Penetrants are designed to:
Perform equally
Perform the same no matter who manufacturers them
Shift in grade and value when the temperature changes
Remain fluid so it can be drawn back to the surface of the part

15

Which of the following is a disadvantage of LPI?
Only surface breaking flaws can be detected
Surface finish and roughness can affect inspection sensitivity
Post cleaning is required
All of the above

16

A penetrant must:
Change viscosity in order to spread over the surface of the part
Spread easily over the surface of the material
Have a low flash point
Be able to change color in order to fluoresce

17

Which type of penetrant is a visible penetrant?
Type I
Type II
Type III
Type IV

18

The pentrants that are used to detect the smallest defects:
Should only be used on aerospace parts
Will also produce the largest amount of irrelevant indications
Can only be used on small parts less than 10 inches in surface
area
Should not be used in the field

19

Which type of penetrant is a fluorescent penetrant?
Type I
Type II

Type III
Type IV

20

Which type of penetrant is most sensitive?
Type I
Type II
Type III
Type IV

Copyright � The Collaboration for NDT Education..