Liquid Penetrant Inspection
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1. Introduction
1.1 Non- Destructive Evaluation
Non-destructive testing (NDT) is a wide group of analysis techniques
used to evaluate the properties of a material, component or system
without causing damage. The terms Non-destructive examination (NDE),
Non-destructive inspection (NDI) and Non-destructive evaluation (NDE)
are also commonly used to describe this technology. Flaws/cracks present
a material plays a major role in determination of its fracture strength and
hence life. Hence techniques of crack detection became more popular.
The present report is all about one such non-destructive technique, ie.
Liquid Penetrant Testing.
1.2 Liquid Penetrant Test (LPT)
Indian Standard Institution (IS: 3658-1981) (2)
defines liquid penetrant
test as a non-destructive testing method for detecting discontinuities that
are open to surface.
LPT’s may be effectively used in the inspection of both ferrous and non-
ferrous metals and on non-porous, non-metallic materials, such as
ceramics, plastics and glass. Surface discontinuities such as cracks, cold
shunts are indicated by this method. Flaw detection using the help of
liquid penetrant is being increasingly used in various industries.(2)(3)
and hence it is a relevant NDT process in the nation.
LPT utilizes the natural accumulation of a fluid (penetrant) around the
discontinuity to create a recognizable indication of a crack or other
surface anomaly.
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Capillary action attracts the fluid to the discontinuity in a concentration
heavier than in the surroundings. In order for the fluid concentration to be
recognized, the background (developer) area must be of sufficient
contrast to distinctly reveal the defect on the surface. The complete
penetrant flaw detection system, therefore, consists of the fluid mechanics
on the surface, as well as the recognition system that is used to detect the
indication.
Eg. The testing method can be well explained using the example of a
concrete slab. First the slab is initially wetted with a fluid that flows
readily. Second, the fluid is drawn into the crack by a capillary action.
And, third, the excess fluid is removed from the surface creating a good
contrast between the still moist areas and dried. Finally it is important to
note that the penetrants are useful only for the surface breaking defects.
The crack that did not reach the surface would not be detected.(5)
.
Fig 1.Turbine blade with the application of penetrant
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Fig.2 Turbine blade with the application of developer.
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2. Scientific Principles Involved In LPT
The complete penetrant flaw detection system is based on the fluid
mechanics of the penetrant on the surface. Three major areas of fluid
mechanics that play a significant role in LPT are as follows,
Surface Tension
Capillarity
Viscosity
Surface tension is a fluid property that affects the flow of the fluid and
wettability of the fluid penetrant of the surface to be inspected. Capillarity
is the driving force in the movement of the penetrant the solid surface,
into the crack and out of the crack into the developer. Viscosity is the
property of a fluid, affecting the flow of the fluid on the surface to be
inspected.
2.1 Surface Tension
Surface tension is a property of the surface of a liquid that allows it to
resist external force. The cohesive forces between molecules of a liquid
cause surface tension. An example of the influence of surface tension is
the tendency of free liquid, such as a droplet of water, to contract into a
sphere. In such a droplet, surface tension is counter balanced by the
internal hydrostatic pressure of the liquid.
When the liquid comes into contact with a solid surface, the cohesive
force responsible for surface tension competes with the adhesive force
between the molecules of the liquid and the solid surface. These forces
jointly determine the contact angle, θ, between the liquid and the surface.
Contact angle lesser than 90 degree gives good wetting on the surface,
which is desirable.
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Fig 3.Fluid-solid contact angles (<90, 90,>90) respectively
Fig 4.Surface Tension
The contact angle is a function of the properties of both the fluid and the
material being inspected. The cleanliness of the surface also affects the
contact angle.
The ability of the fluid to enter the surface cavities is directly proportional
to the surface tension, and inversely proportional to the contact angle, the
density and the size of opening.
Table 1: Surface Tension of liquids at 20o c in contact with air
Liquid Surface Tension (m N/m)
Benzene 28.9
Hexane 18.4
Kerosene 26.8
Lube Oil 25-35
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Methanol 22.6
Octane 21.8
Water 72.8
2.2 Capillarity
It is the ability of a liquid to flow in narrow spaces without the assistance
of, and in opposition to external forces like gravity. But recent studies
made by MIT (US) research group on Non Newtanian Fluids shows that
the liquid drops may lose their spherical shape under the influence of
gravity.
There exists a particular length, denoted κ-1
, beyond which gravity
becomes important. It is referred to as the capillary length. It can be
estimated by comparing the Laplace pressure γ/κ-1
to the hydrostatic
pressure ρgκ-1
at a depth κ-1
in a liquid of density ρ submitted to earth's
gravity g=9.8 m/s2. Equating these two pressures defines the capillary
length.(w 1)
(I)
For water, the capillary length = 2.7 mm
It occurs because of inter-molecular attractive forces between the liquid
and solid surrounding surfaces; If the diameter of the tube is sufficiently
small, then the combination of surface tension (which is caused
by cohesion within the liquid) and adhesive forces between the liquid and
container act to lift the liquid as shown in fig 5.
Capillary rise/fall is a function of surface tension, contact angle & density
of the liquid as shown in the equation (I
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(II)
Fig 5.Capillary rise
Even though flaws are not exactly capillary tubes, the phenomenon of
penetration is by capillary action.
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Table 2:Contact angles of Various Liquid-solid Interfaces
2.3Viscosity
Viscosity is a measure of the resistance of a fluid which is being
deformed by either shear or tensile stress. Viscosity describes a fluid's
Internal resistance to flow and may be thought of as a measure of
fluid friction.The viscosity of the liquid is not a factor in the basic
equation of capillary rise. Viscosity is related to the rate at which a liquid
will flow under some applied unbalanced stress; in itself, viscosity has a
negligible effect on penetrating ability. In general, however, very viscous
liquids are unsuitable as penetrants because they do not flow rapidly
enough over the surface of the work piece; consequently, they require
excessively long periods of time to migrate into fine flaws.
Liquid Solid Contact angle
water soda-lime
glass
lead glass
fused quartz
0°
ethanol
diethyl ether
carbon
tetrachloride
glycerol
acetic acid
water paraffin wax 107°
silver 90°
methyl iodide soda-lime
glass
29°
lead glass 30°
fused quartz 33°
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3. History and Evolution Of LPT.
The exact origin of liquid penetrant inspection is not known, but it has
been assumed that the method evolved from the observation that the rust
on a crack in a steel plate in outdoor storage was somewhat heavier than
the rust on the adjacent surfaces as a result of water seeping into the crack
and forcing out the oxide it had helped to produce. The obvious
conclusion was that a liquid purposely introduced into surface cracks and
then brought out again would reveal the locations of those cracks.
The only material that fulfilled the known criteria of low viscosity, good
wettability, and ready availability was kerosene. It was found, however,
that although wider cracks showed up easily, finer ones were sometimes
missed because of the difficulty of detecting, by purely visual means, the
small amounts of kerosene exuding from them. The solution was to
provide a contrasting surface that would reveal smaller seepages. The
properties and availability of whitewash made it the logical choice. This
method, known as the kerosene-and-whiting test, was the standard for
many years. The sensitivity of the kerosene-and-whiting test could be
increased by hitting the object being tested with a hammer during testing.
The resulting vibration brought more of the kerosene out of the cracks
and onto the whitewash.
Although this test was not as sensitive as those derived from it, it was
quick, inexpensive, and reasonably accurate. Thus, it provided a vast
improvement over ordinary visual examination. The first step leading to
the methods now available was the development of the fluorescent
penetrant process by R.C. Switzer. This liquid, used jointly with a powder
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developer, brought penetrant inspection from a relatively crude procedure
to a more scientific operation. With fluorescent penetrant, minute flaws
could be readily detected when exposed to ultraviolet light (commonly
called black light). This development represented a major breakthrough in
the detection of surface flaws.
Switzer's work also included the development of the visible-colour
contrast method, which allowed for inspection under white light
conditions. Although not as sensitive as fluorescent penetrant inspection,
it is widely used in industry for noncritical inspection. Through the
developments described above, liquid penetrant inspection has become a
major non-destructive inspection method.
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4. Terminologies Related To LPT As In
IS: 3658- 1981.(2)
Penetrant:- A liquid possessing the property of entering into small
openings and to remain there, making it suitable for liquid
penetrant test.
Developer:-A substance ,which is usually white in colour and
powdered in the form and having the property of blotting out a
penetrant retained in defects, thus intensifying the indications and
providing a contrasting background for the penetrant.
Dwell Time: - The period of time wherein an inspection penetrant
is in contact with the surface of the part.
Emulsifiable Penetrants:-Penetrants capable of being converted
into a water washable condition by the addition of emulsifier.
Post-emulsifiable Penetrant:-A Penetrant which must be treated
with an emulsifying agent to render it water washable.
Pre-emulsifiable Penetrant:-A penetrant with suitable
emulsifiable agent, added to render it directly water washable.
Self-emulsifiable Penetrant:-The property of a liquid penetrant
to combine satisfactorily with water, thus facilitating its removal
by washing with water.
Vapour Degreasing:-The removal of oils, greases, and organic
oils by the use of suitable vapour.
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Ultraviolet Light:-Monochromatic blue light of approximately
4300 Angstrom wavelength, used to cause certain liquid penetrant
s to fluorescence.
Bleed Out:-The action by which the penetrant comes out of the
discontinuities on to the surface of materials due to
blotting/soaking effect of the developer.
Fluorescent Penetrants:-penetrating liquids containing additives
to fluorescence under black light.
Solvent Remove:-A liquid employed for removal of surface
penetrant from parts or for removal of unwanted background
porosity indications.
5. LPT Process Description
LPT process consists of 5 basic steps.viz
1. Surface Preparation. All surfaces to be inspected, whether
localized or the entire work piece must be thoroughly cleaned and
completely dried before being subjected to penetrant inspection.
Flaws exposed to the surface must be free from oil, water, or other
contaminants if they are to be detected.
2. Penetration. After the work piece has been cleaned, penetrant is
applied in a suitable manner so as to form a film of the penetrant
over the surface. This film should remain on the surface long
enough to allow maximum penetration of the penetrant into any
surface openings that are present.
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3. Removal of Excess Penetrant. Excess penetrant must be removed
from the surface. The removal method is determined by the type of
penetrant used. Some penetrants can be simply washed away with
water; others require the use of emulsifiers (lipophilic or
hydrophilic) or solvent/remover. Uniform removal of excess
surface penetrant is necessary for effective inspection, but over
removal must be avoided.
4. Development. Depending on the form of developing agent to be
used, the work piece is dried either before or directly after
application of the developer. The developer forms a film over the
surface. It acts as a blotter to assist the natural seepage of the
penetrant out of surface openings and to spread it at the edges so as
to enhance the penetrant indication.
5. Inspection. After it is sufficiently developed, the surface is
visually examined for indications of penetrant bleed back from
surface openings. This examination must be performed in a
suitable inspection environment. Visible penetrant inspection is
performed in good white light. When fluorescent penetrant is used,
inspection is performed in a suitably darkened area using black
(ultraviolet) light, which causes the penetrant to fluoresce
brilliantly.
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Fig. 6 LPT Process Description
Surface Preparation
Key Recommendations for surface preparation by ISI code 3658
are as follows,
In general satisfactory results can be obtained when the
surface is in the as welded, as rolled, as cast/forged
condition, but surface preparation by grinding/machining or
other suitable method may be necessary, when surface
irregularities could otherwise mask indications of
unacceptable discontinuities.
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Heavy grinding should be avoided to avoid the masking of
fine defects.
The surface to be examined and all adjacent areas within at
least 25 mm should be also dry and clean.
The method of cleaning depends on the nature of material
and contamination. Cleaning using detergents, organic
solvents, alkali solutions, paint remover, descaling solutions,
vapour degreasing, ultrasonic cleaning, abrasive blasting are
in practice.
Penetration
o Classification Of Penetrants
Type I fluorescent penetrant utilizes penetrants that are usually green in
colour and fluorescent brilliantly under ultraviolet light. The sensitivity of
a fluorescent penetrant depends on its ability to form indications that
appear as small sources of light in an otherwise dark area. Type I
penetrants are available in different sensitivity levels classified as
follows:
· Level: Ultralow
· Level 1: Low
· Level 2: Medium
· Level 3: High
· Level 4: Ultrahigh
Type II visible penetrant employs a penetrant that is usually red in
colour and produces vivid red indications in contrast to the light
background of the applied developer under visible light. The visible
penetrant indications must be viewed under adequate white light. The
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sensitivity of visible penetrants is regarded as Level 1 and adequate for
many applications.
Visible light level-Recommended minimum light intensity at
examination site=100 lux/m2.
(1)
Penetrant selection and use depend on the criticality of the
inspection, the condition of the work piece surface, the
type of processing, and the desired sensitivity.
Fig 7.Application of red liquid penetrant
Penetrant Selection Method
Method A, water-washable penetrants are designed for the removal of
excess surface penetrant by water rinsing directly after a suitable
penetration (dwell) time. The emulsifier is incorporated into the water-
washable penetrant. When this type of penetrant is used, it is extremely
important that the removal of excess surface penetrant be properly
controlled to prevent over washing, which can cause the penetrant to be
washed out of the flaws.
Methods B and D, lipophilic and hydrophilic post emulsifiable
penetrants are insoluble in water and therefore not removable by water
rinsing alone. They are designed to be selectively removed from the
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surface of the work piece by the use of a separate emulsifier. The
emulsifier, properly applied and left for a suitable emulsification time,
combines with the excess surface penetrant to form a water-washable
surface mixture that can be rinsed from the surface of the workpiece. The
penetrant that remains within the flaw is not subject to over washing.
However, proper emulsification time must be established experimentally
and maintained to ensure that over emulsification, which results in the
loss of flaws, does not occur.
Method C, solvent-removable penetrants are removed by wiping with
clean, lint-free material until most traces of the penetrant have been
removed. The remaining traces are removed by wiping with clean, lint-
free material lightly moistened with solvent. This type of penetrant is
primarily used where portability is required and for the inspection of
localized areas. To minimize the possibility of removing the penetrant
from discontinuities, the use of excessive amounts of solvent must be
avoided.
Emulsifiers
Emulsifiers are liquids used to render excess penetrant on the surface of a
work piece water washable. There are two methods used in the post
emulsifiable method: method B, lipophilic, and method D, hydrophilic.
Both can act over a range of durations from a few seconds to several
minutes, depending on the viscosity, concentration, method of
application, and chemical composition of the emulsifier, as well as on the
roughness of the work piece surface.
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The length of time an emulsifier should remain in contact with the
penetrant depends on the type of emulsifier employed and the roughness
of the work piece surface.
Method B, lipophilic emulsifiers are oil based, are used as supplied, and
function by diffusion (Fig. 11). The emulsifier diffuses into the penetrant
film and renders it spontaneously emulsifiable in water. The rate at which
it diffuses into the penetrant establishes the emulsification time.
The emulsifier is fast acting, thus making the emulsification operation
very critical. The emulsifier continues to act as long as it is in contact
with the work piece; therefore, the rinse operation should take place
quickly to avoid over emulsification
Fig8. Functioning of Lipophilic Emulsifier
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Method D, hydrophilic emulsifiers are water based and are usually
supplied as concentrates that are diluted in water to concentrations of 5 to
30% for dip applications and 0.05 to 5% for spray applications.
Hydrophilic emulsifiers function by displacing excess penetrant from the
surface of the part by detergent action. The force of the water spray or the
air agitation of dip tanks provides a scrubbing action. Hydrophilic
emulsifier is slower acting than the lipophilic emulsifier; therefore, it is
easier to control the cleaning action. In addition to the emulsifier
application, method D also requires a pre rinse. Utilizing a coarse water
spray, the pre rinse helps remove the excess penetrant to minimize
contamination of the emulsifier. Of greater significance, only a very thin
and uniform layer of penetrant will remain on the surface, thus allowing
easy removal of the surface layer with minimum opportunity of removing
penetrant from the flaws. This step is required because the penetrant is
not miscible with the hydrophilic emulsifier.
Fig 9.Functioning Of Hydrophilic Emulsifier
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Recommendations (2)
Penetration Time
When high viscosity florescent penetrant is used, the penetration
time is longer than normal penetration time. In such a case, time
shall be subjected to agreement between the manufacturer and the
purchaser.
If the testing is to be done out of the range of 15-500 c, the
penetration time should be suitably increased, and the whole
process and the materials is to be checked using a comparator
block.
If water washable penetrants are used, the penetration time shall be
about 1.5 to 2.0 times of 10 minutes.
Rinsing
Using water washable penetrant rinsing should be done with water
supply nozzle. Temperature of water should not exceed 43oC and
pressure of 3 kg /cm2 should be maintained.
On smooth surfaces, it may be possible sometimes to remove
excess penetrant merely by wiping the surface with dry rag.
Using fluorescent type of penetrant, it is helpful to use a portable
black light source, while rinsing, so as to ensure that rinsing operation
is complete.
Developing
Generally half of the penetration time is considered as developing
time. Developers may either of dry type or wet type. A good
practice is to start observation as soon as developer applied.
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Fig 10 Application Of Developer
Inspection
With visible dye penetrants, surface defects are
indicated by bleeding out of penetrant, normally
deep red in colour.
Crack/small openings show a line, tight crack shows
broken lines. Fine porosity will be indicated by
random dots.(see fig 11)
Depth of surface discontinuity can be correlated to
richness of colour and speed of bleeding.
A minimum light intensity of 1000 lx/ m2
is to be
maintained.
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Fig 11 Random dots indicating fine porosity in the material
Fig 12 Crack Inspection using fluorescent penetrant in black light
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Fig 12 Crack Inspection using fluorescent penetrant in black light
Table 3.Crack Detection Limits for Dye Penetrant Test (4)
Type of Inspection Crack length (mm)
Production Parts (lab use) 0.127
Production Parts (inspection) 0.76
Structure (Service Inspection) 1.27
Sensitivity of LPT(5)
The sensitivity of LPT is defined as the ratio of number of detected
cracks per scan of UV laser to the total number of actual craks.It is
expressed in percentage. Higher values of sensitivity refer to higher
efficiency of LPT in the crack detection process.
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6. Process Flow Diagrams of LPT(8)
They are the flow diagrams/charts referring the preferences of LPT and
direct the path of conduction of LPT.
Fig 13 Process flow diagram of postemulsifiable, method B, lipophilic
liquid penetrant system
Fig 14. Processing flow diagram for the water-washable liquid
penetrant system
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Fig 15 Processing flow diagram for the solvent-removable liquid
penetrant system
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7. List of ASTM standards for LPT
ASTM E 165- Standard Practices for Liquid-Penetrant Inspection
Method
ASTM E 270- Standard Definitions of Terms Relating to Liquid-
Penetrant Inspection
ASTM E 1208- Standard Method for Fluorescent Liquid-Penetrant
Examination Using the Lipophilic Post-Emulsification Process
ASTM E 1209- Standard Method for Fluorescent-Penetrant
Examination Using the Water-Washable Process
ASTM E 1210- Standard Method for Fluorescent-Penetrant
Examination Using the Hydrophilic Post-Emulsification Process
ASTM E 1219- Standard Method for Fluorescent-Penetrant
Examination Using the Solvent-Removable Process
ASTM E 1220- Standard Method for Visible-Penetrant
Examination Using the Solvent-Removable Process
ASTM E 1135- Standard Test Method for Comparing the
Brightness of Fluorescent Penetrants
ASTM D 2512- Compatibility of Materials with Liquid Oxygen
(Impact-Sensitivity Threshold Technique)
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8. Safety Considerations In LPT(4,7)
Changes in environmental protection standards have placed
restrictions on many chemicals used as liquid penetrants.
Methyl Chloroform (MCF), a liquid penetrant was banned after
Montreal Protocol (2002) since it was a CFC causing ozone
depletion. Then a Ozone friendly penetrant HCFC-123 replaced
MCF. (7)
Certain chemicals which are used as a fluorescent penetrants were
irritants and even carcinogenic.
Gloves, aprons, masks should be used when working with many
chemicals.
UV rays from an arc lamp may burn skin/eyes if the filters are
broken.
Many of the volatile solvents are highly inflammable. So cleaning
area should be ventilated to avoid fire accidents.
Special care must be taken in the inspection of liquid oxygen
tanks.eg. Selection of penetrants/cleansers that will not cause
explosive reactions with liquid oxygen residues.
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Summary of LPT
Advantages of LPT
1. It is portable, well suited for fieldwork.
2. Relatively inexpensive.
3. Applicable to many non-porous materials of wide
range of size and irregular shapes.
4. Flaw orientation will not pose a problem.
5. Large objects can be checked on spot (in situ).
6. It can be designed for high volume production.
Problems with LPT
Only surface anomalies which can absorb the liquid
can be detected.
Rough and porous (low density powder metallurgical
products) surfaces can’t be used.
Significant surface preparation is required for good
results.
Chemically compatible component and penetrant must
be considered.
Sharp corners and complex edges give false indications.
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9. Conclusion
Of the thousands of parts made for the space shuttle, hundreds are
inspected by penetrant testing. Items such as valves, pipelines, tanks and
structural membranes used in space vehicles’ liquid oxygen systems are
penetrant inspected for flaws. Turbine engine blades are usually inspected
using radiography, eddy current and penetrant testing to ensure there is no
premature failure of the blades. (3)
During maintenance of honeycomb and adhesively bonded aircraft
structures, radiography can detect the presence of corrosion or water, but
it cannot detect the origin of the water. Penetrant testing can find these
areas before water and corrosion happen.
The automotive industry uses penetrant testing to ensure flaw-free parts
such as steering spindles, aluminum suspension parts and aluminum rims.
Fluorescent dye is used in locating leaks in air conditioning systems in
automobiles. The fluorescent dye is added to the system and run for
several minutes; a high-intensity black light locates leaks in the system.
Back in the 1960s it was thought by nondestructive testing engineers that
penetrant testing days were numbered and new technology would replace
it.
Penetrant testing may not be used as much as it was in the past, but it still
has a place in a wide range of industries and applications. Penetrant
testing is going to be around for many years to come.