OMNDT PT L-II NOTES Prepared by MAHESH PANDIT(ASNT L-III)

OM NDT TRAINING & CONSULTANCY
PENETRANT TESTING L-II
Prepared by
MAHESH PANDIT
ASNT NDT L-III

Basic principle of a Liquid Penetrant
• DPI is based upon capillary action, where low surface
tension fluid penetrates into clean and dry surface-breaking
discontinuities. Penetrant may be applied to
the test component by dipping, spraying, or brushing.
After adequate penetration time has been allowed, the
excess penetrant is removed and a developer is
applied. The developer helps to draw penetrant out of
the flaw so that an invisible indication becomes visible
to the inspector. Inspection is performed under
ultraviolet or white light, depending on the type of dye
used - fluorescent or non fluorescent (visible).

Capillary action is the ability of a liquid to
flow in narrow spaces without the
assistance of, and in opposition to,
external forces like gravity. The effect can
be seen in the drawing up of liquids
between the hairs of a paint-brush in a
thin tube, in porous materials such as
paper. It occurs because of intermolecular
forces between the liquid and
surrounding solid surfaces

Intermolecular forces are forces of attraction or
repulsion which act between neighboring particles.
Surface tension is a contractive tendency of the
surface of a liquid that allows it to resist an external
force. The cohesive forces between liquid molecules
are responsible for the phenomenon known as
surface tension. Surface tension is typically measured
in dynes/cm, the force in dynes required to break a
film of length 1 cm. Water at 20°C has a surface
tension of 72.8 dynes/cm . The surface tension of
water decreases significantly with temperature .
Soaps and detergents further lower the surface
tension.

• When a liquid comes into contact with a surface
both cohesive and adhesive forces will act on it.
These forces govern the shape which the liquid
takes on. Due to the effects of adhesive forces,
liquid on a surface can spread out to form a thin,
relatively uniform film over the surface, a process
known as wetting. Alternatively, in the presence
of strong cohesive forces, the liquid can divide
into a number of small, roughly spherical beads
which stand on the surface, maintaining minimal
contact with the surface.

• Cohesive forces are the intermolecular forces
which cause a tendency in liquids to resist
separation. These attractive forces exist
between molecules of the same substance.
For instance, rain falls in droplets, rather than
a fine mist, because water has strong cohesion
which pulls its molecules tightly together,
forming droplets.

• Adhesive forces are the attractive forces between
unlike molecules. In the case of a liquid wetting
agent, adhesion causes the liquid to cling to the
surface on which it rests. When water is poured
on clean glass, it tends to spread, forming a thin,
uniform film over the glasses surface. This is
because the adhesive forces between water and
glass are strong enough to pull the water
molecules out of their spherical formation and
hold them against the surface of the glass, thus
avoiding the repulsion between like molecules

• When the cohesive force of the liquid is
stronger than the adhesive force of the liquid
to the wall, the liquid concaves down in order
to reduce contact with the surface of the wall.
When the adhesive force of the liquid to the
wall is stronger than the cohesive force of the
liquid, the liquid is more attracted to the wall
than its neighbors, causing the upward
concavity.

• The meniscus : is the curve in the upper surface of a liquid
close to the surface of the container or another object,
caused by surface tension. It can be either convex or
concave, depending on the liquid and the surface.
• 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.

The ability of a given liquid to flow over a surface
and enter surface cavities depends principally on
the following:
• Cleanliness of the surface
• Configuration of the cavity
• Cleanliness of the cavity
• Size of surface opening of the cavity
• Surface tension of the liquid
• Ability of the liquid to wet the surface
• Contact angle of the liquid

• If θ is less than 90° (Fig. 1a), the liquid is said
to wet the surface, or to have good wetting
ability;
• if the angle is equal to or greater than 90° (Fig.
1b and c), the wetting ability is considered
poor.
• If θ is greater than 90°, the liquid is depressed
in the tube and does not wet the tube wall,
and the meniscus is convex (Fig. 2c).

History of PT
• 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
by Magna flux in (Chicago)

History of PT
• In this method, a heavy oil 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

Why a Penetrant Inspection Improves
the Detectability of Flaws?
• 1) It produces a flaw indication that is much
larger and easier for the eye to detect than
the flaw itself.
• 2) it produces a flaw indication with a high
level of contrast between the indication and
the background
• 3) The developer serves as a high contrast
background as well as a blotter to pull the
trapped penetrant from the flaw.

Visual Acuity of the Human 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 (0.076mm) inch for a person with 20/20
vision.
• 20/20 vision, it means that when you stand 20 feet away from
the chart you can see what the "normal" human being can see.
• The human eye is more sensitive to a light indication on a dark
background and the eye is naturally drawn to a fluorescent
indication.
• With a light indication on a dark background, indications down
to 0.003 mm (0.0001 inch) may be seen when the contrast
between the flaw and the background was high.
• But dark indication on a lighter background can’t.

The eye has a visual acuity threshold below which an
object will go undetected. This threshold varies from
person to person, but as an example, the case of a
person with normal 20/20 vision can be considered.
As light enters the eye through the pupil, it passes
through the lens and is projected on the retina at the
back of the eye.
Muscles called extra ocular muscles, move the
eyeball in the orbits and allow the image to be
focused on the central retinal or fovea.

The retina is a mosaic of two basic types of
photoreceptors: rods, and cones. Rods are
sensitive to blue-green light with peak
sensitivity at a wavelength of 498 nm, and
are used for vision under dark or dim
conditions. There are three types of cones
that give us our basic color vision: L-cones
(red) with a peak sensitivity of 564 nm, M-cones
(green) with a peak sensitivity of 533
nm, and S-cones (blue) with a peak
sensitivity of 437 nm.

• The standard definition of normal visual acuity (20/20
vision) is the ability to resolve a spatial pattern separated
by a visual angle of one minute of arc. Since one degree
contains sixty minutes, a visual angle of one minute of arc is
1/60 of a degree.
• For the case of normal visual acuity the angle Theta is 1/60
of a degree. By bisecting this angle we have a right triangle
with angle Theta/2 that is 1/120 of a degree. Using this
right triangle it is easy to calculate the distance X/2 for a
given distance d.
• X/2 = d (tan Theta/2)
• under normal lighting conditions, the eye is most sensitive
to a yellowish-green color.

• When the light levels drop to near total darkness, the
response of the eye changes significantly by the scotopic
response curve .
• At this level of light, the rods are most active and the
human eye is more sensitive to the light present, and less
sensitive to the range of color.
• At this very low light level, sensitivity to blue, violet, and
ultraviolet is increased, but sensitivity to yellow and red is
reduced.
• Fluorescent penetrant inspection materials are designed to
fluoresce at around 550 nanometers to produce optimal
sensitivity under dim lighting conditions.

Basic Processing Steps of a Liquid
Penetrant Inspection
• 1) Surface preparation: The surface must be
free of oil, grease, water, or other
contaminants that may prevent penetrant
from entering flaws.
• 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.

System performance checks
• System performance checks involve processing a test
specimen with known defects to determine if the
process will reveal discontinuities of the size required.
• 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.

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. 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.
4) Excess Penetrant Removal:
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.

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.
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.

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.
• 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

Pre-cleaning
• Regardless of the penetrant chosen, adequate pre-cleaning
of work pieces prior to penetrant inspection is
absolutely necessary for accurate results. Without
adequate removal of surface contamination, relevant
indications may be missed because:
• The penetrant does not enter the flaw
• The penetrant loses its ability to identify the flaw
because it reacts with something already in it
• The surface immediately surrounding the flaw retains
enough penetrant to mask the true appearance of the
flaw

Cleaning
• 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.
• 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.

Cleaning methods
Selection of a cleaning method depends upon
the type of contaminant to be removed and the
type of alloy being cleaned.
This cleaning methods are generally classified as
• Chemical,
• Mechanical,
• Solvent,
• or any combination of these.

Cleaning methods
• Chemical cleaning methods include alkaline or acid
cleaning, pickling or chemical etching.
• Mechanical cleaning methods include tumbling, wet
blasting, dry abrasive blasting, wire brushing, and high
pressure water or steam cleaning.
• Mechanical cleaning methods should be used with care
because they often mask flaws by smearing adjacent metal
over them.
• Solvent cleaning methods include vapor degreasing,
solvent spraying, solvent wiping, and ultrasonic immersion
using solvents.
• Probably the most common method is vapor degreasing.
However, ultrasonic immersion is by far the most effective
means of ensuring clean parts, but it can be a very
expensive capital equipment investment.

Mechanical methods
• Abrasive tumbling : Removing light scale, burrs,
welding flux, braze stop-off, rust, casting mold, and
core material;
• Wire brushing removing light deposits of scale, flux,
and stopoff.
• Stop-Off is a brazing aid commonly used in silver and
aluminum brazing. It is used to prevent the flow of flux
and metal to unwanted areas during brazing.
• High-pressure water and steam used with an alkaline
cleaner or detergent; removing typical machine shop
soils such as cutting oils, polishing compounds,
grease, chips etc.
• Ultrasonic cleaning used with detergent and water or
with a solvent; removing adherent shop soil from
large quantities of small parts

Chemical methods
• Alkaline cleaning Removing braze stopoff,
rust, scale, oils, greases, polishing material,
and carbon deposits; ordinarily used on large
articles where hand methods are too
laborious;
• Acid cleaning Strong solutions for removing
heavy scale; mild solutions for light scale;
weak (etching) solutions for removing lightly
smeared metal

Solvent methods
• Vapor degreasing removing typical shop soil,
oil, and grease; usually employs chlorinated
solvents; not suitable for titanium
• Solvent wiping Same as for vapor degreasing
except a hand operation; may employ non-chlorinated
solvents; used for localized low-volume
cleaning

Common Uses of Liquid Penetrant Inspection
• LPI can be used to inspect almost any material
provided that its surface is not extremely rough
or porous. It include the following:
• Metals (aluminum, copper, steel, titanium, etc.)
• Glass
• Many ceramic materials
• Rubber
• Plastics

It can only be used to inspect for flaws that break
the surface of the sample. Some of these flaws
are listed below:
1. Fatigue cracks
2. Quench cracks
3. Grinding cracks
4. Overload and impact fractures
5. Porosity
6. Laps
7. Seams
8. Pin holes in welds
9. Lack of fusion along the edge of the bond line

Advantages of Penetrant Testing
• High sensitivity to small surface
discontinuities.
• Large areas and large volumes of
parts/materials can be inspected rapidly and
at low cost.
• Parts with complex geometric shapes are
routinely inspected
• Aerosol spray cans make penetrant materials
very portable.

Disadvantages of Penetrant Testing
• Only surface breaking defects can be
detected.
• Only materials with a relatively nonporous
surface can be inspected.
• Pre-cleaning is critical since contaminants can
mask defects.
• Metal smearing from machining, grinding, and
grit or vapor blasting must be removed prior
to LPI.

Disadvantages of Penetrant Testing
• The inspector must have direct access to the
surface being inspected.
• Surface finish and roughness can affect
inspection sensitivity.
• Post cleaning of acceptable parts or materials
is required.
• Chemical handling and proper disposal is
required.

TYPES OF PENETRANT MATERIALS
 Type 1 - Fluorescent Penetrants: High sensitive,
comes usually green in color and fluoresce
brilliantly under ultraviolet light.
 Type 2 - Visible Penetrants : Less sensitive,
usually red in color, viewed under adequate
white light. less vulnerable to
 Type 3 – Dual mode penetrants : Viewed under
black light or white light.

The Type- I , Penetrant have five sensitivity
levels:-
 Level ½ - Ultra Low Sensitivity
 Level 1 - Low Sensitivity
 Level 2 - Medium Sensitivity
 Level 3 - High Sensitivity
 Level 4 - Ultra-High Sensitivity

Before selection of a type of penetrant
method, we must have a knowledge of
• Surface condition of the work piece being
inspected
• Characteristics of the flaws to be detected
• Time and place of inspection
• Size of the work piece
• Sensitivity required
• Materials cost, number of parts, size of area
requiring inspection, and portability.

Penetrants are classified on the basis
of penetrant type
• Type I: Fluorescent
• Type II: Visible
Method A: Water washable
Method B: Post emulsifiable-lipophilic
Method C: Solvent removable
Method D: Post emulsifiable-hydrophilic

Water-washable penetrant (method A)
• Designed so that the penetrant is directly
water washable from the surface of the work
piece.
• It is a self emulsifying penetrant.
• It is susceptible to over washing.

Post-emulsifiable penetrants
• Emulsifiers are liquids used to render excess penetrant on
the surface of a work piece water washable.
• Method B, lipophilic emulsifiers: oil based, are used as
supplied in ready-to-use form, and function by diffusion.
• work with both a chemical and mechanical action.
• mechanical action remove excess penetrant as the mixture
drains from the part
• In Chemical action, the emulsifier diffuses into the
remaining penetrant and the resulting mixture is easily
removed with a water spray.
• Water content (method B, lipophilic) Monthly Not to
exceed 5%

Emulsifiers
• 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.
• Hydrophilic emulsifier is slower acting than the
lipophilic emulsifier Concentration (method D,
hydrophilic) Weekly Not greater than 3% above
initial concentration

Hydrophilic emulsifiers
• The major advantage of hydrophilic emulsifiers is
that they are less sensitive to variation in the
contact and removal time.
• It is more sensitive than the lipophilic post
emulsifiable.
• No diffusion takes place
• Work with both a chemical and mechanical
action.
• Emulsification Time: ranges from approximately
30 s to 3 min.

Pre-rinse
• When using method D (hydrophilic), a coarse water
spray pre-rinse is needed to assist in penetrant removal
and to reduce contamination of the emulsifier.
• A coarse water spray is recommended, using a pressure
of 275 to 345 kPa (40 to 50 psi).
• The pre-rinse water temperature should be 10 to 40 °C
(50 to 100 °F).
• The pre-rinse time should be kept to a minimum (that
is, 30 to 90 s) because the purpose is to remove excess
penetrant so that the emulsifier does not become
contaminated quickly.
• Rinse time should be determined experimentally for
specific workpieces; it usually varies from 10 s to 2 min.

Drying
• Drying is best done in a recirculating hot-air drier
that is thermostatically controlled.
• The temperature in the drier is normally between
65 and 95 °C (150 and 200 °F).
• The temperature of the work pieces should not
be permitted to exceed 70 °C (160 °F).
• Excessive drying at high temperatures can impair
the sensitivity of the inspection.
• Because drying time will vary, the exact time
should be determined experimentally for each
type of work piece.

Post-emulsifiable penetrants
(methods B and D)
• Designed to ensure the detection of minute
flaws in some materials.
• Separate emulsification is required to remove
the penetrant.
• The danger of over washing the penetrant out
of the flaws is reduced.
• These methods are the most reliable for
detecting minute flaws.

Methods B and D
1. pre-clean part, 2. apply penetrant and allow to dwell,
3. pre-rinse to remove first layer of penetrant, 4. apply
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
Processing steps

Solvent-removable penetrant
(method C)
• Used to inspect only a localized area of a work
piece
• Inspect a work piece at the site rather than on a
production inspection basis.
• Normally, the same type of solvent is used for pre
cleaning and for removing excess penetrant.
• This method is labor intensive.
• When properly conducted and when used in the
appropriate applications, the solvent-removable
method can be one of the most sensitive
penetrant methods available.

Solvent-removable penetrant
(method C)
• The use of excessive amounts of solvent must
be avoided.

Solvent Cleaner/Removers
• Remove excess surface penetrant through direct
solvent action.
• There are two basic types of solvent removers:
• flammable and nonflammable.
• Flammable cleaners are essentially free of
halogens but are potential fire hazards.
• Nonflammable cleaners are widely used.
However, they do contain halogenated solvents,
which may render them unsuitable for some
applications.

Solvent Cleaner/Removers
• Excess surface penetrant is removed by
wiping, using lint-free cloths slightly
moistened with solvent cleaner/remover.
• It is not recommended that excess surface
penetrant be removed by flooding the surface
with solvent cleaner/remover,
• Because the solvent will dissolve the
penetrant within the defect and indications
will not be produced

Penetrant Application
• Penetrants can be applied by :-
• Immersing
• Spraying
• Brushing
• The emulsifier on to the part is not
recommended by brushing either because the
bristles of the brush may force emulsifier into
discontinuities, causing the entrapped
penetrant to be removed

Penetrant Application and Dwell Time
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.

Penetrant Application and Dwell Time
• 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.
• 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.

Dwell Time
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
• The atmospheric pressure at the flaw opening.
• 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.

Dwell Time
• 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.
• 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

Physical and Chemical Characteristics
• Chemical stability and uniform physical consistency
• A flash point not lower than 95 °C (200 °F);
• penetrants that have lower flash points constitute a
potential fire hazard.
• A high degree of wettability
• Low viscosity to permit better coverage and minimum
drag out
• Ability to penetrate discontinuities quickly and
completely
• Sufficient brightness and permanence of color

Physical and Chemical Characteristics
• Chemical inertness with materials being
inspected and with containers
• Low toxicity to protect personnel
• Slow drying characteristics
• Ease of removal
• Inoffensive odor
• Low cost
• Resistance to ultraviolet light and heat fade

Chemical stability
• Tendency of a material to resist change or
decomposition due to internal reaction, or
due to the action of air, heat, light, pressure,
etc.
• 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.

ultraviolet & Thermal stability
Excessive heat:
1. evaporates the more volatile constituents which
increases viscosity and adversely affects the rate
of penetration.
2. alters wash characteristics.
3. "boils off" chemicals that prevent separation
and gelling of water soluble penetrants.
4. kills the fluorescence of tracer dyes.
2. Generally, thermal damage occurs when
fluorescent penetrant materials are heated
above 71oC

Temperature
• The temperature of the penetrant materials and the part
being inspected should be from 10 to 49oC (80 to 120oF) .
• 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.
• 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.

Flash point
• The evaporation of the volatile constituents of
penetrants can alter their chemical and
performance characteristics,
• Resulting in changes in inherent brightness,
removability, and sensitivity.
• Liquid penetrant materials qualified to MIL-I-
25135D (and subsequent revisions) have a
flash point requirement of a minimum of 95 °C

Properties of a penetrant
• 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.
• Dilution of the penetrant liquid will affect the
concentration of the dye and reduce the
dimensional threshold of fluorescence.

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.

Function of developers
• Increase the brightness intensity of
fluorescent indications and the visible contrast
of visible-penetrant indications.
• The developer also provides a blotting action,
which serves to draw penetrant from within
the flaw to the surface, spreading the
penetrant and enlarging the appearance of
the flaw.
• Decreases inspection time by hastening the
appearance of indications.

Developer properties
• The developer must be adsorptive to maximize
blotting
• It must have fine grain size and a particle shape
that will disperse and expose the penetrant at a
flaw to produce strong and sharply defined
indications of flaws
• It must be capable of providing a contrast
background for indications when color-contrast
penetrants are used
• It must be easy to apply
• It must form a thin, uniform coating over a
surface

Developer properties
• It must be non fluorescent if used with
fluorescent penetrants
• It must be easy to remove after inspection
• It must not contain ingredients harmful to
parts being inspected or to equipment used in
the inspection
• It must not contain ingredients harmful or
toxic to the operator

Developer Forms
• Form A, dry powder
• Form B, water soluble
• Form C, water sus-pendible
• Form d , Non-aqueous Type 1 Fluorescent
(Solvent Based)
• Form e ,Non-aqueous Type 2 Visible Dye
(Solvent Based)

Form A, dry powder
• Dry developer does not provide a uniform
white background as the other forms of
developers do
• Least sensitive but it is inexpensive to use and
easy to apply.
• Excessive powder can be removed by gently
blowing on the surface with air not exceeding
35 kPa or 5 psi.

Form A, dry powder
• Widely used with fluorescent penetrants, but should not be
used with visible dye penetrants because they do not
produce a satisfactory contrast coating on the surface of
the work piece.
• It should be light and fluffy to allow for ease of application
and should cling to dry surfaces in a fine film.
• powders should not be hygroscopic, and they should
remain dry.
• If they pick up moisture when stored in areas of high
humidity, they will lose their ability to flow and dust easily,
and they may agglomerate, pack, or lump up in containers
or in developer chambers.
• Dry-developer form inspected daily Must be fluffy, not
caked.

Safety requirement
• Handled with care because it can dry the skin
and irritate the lining of the air passages,
causing irritation.
• Rubber gloves and respirators may be
desirable if an operator works continuously
with this.

Water-soluble developers (form B)
• It can be used for both type I or type II
penetrants.
• It is not recommended for use with water-washable
penetrants, because of the potential to
wash the penetrant from within the flaw if the
developer is not very carefully controlled.
• Supplied as a dry powder concentrate
• Dispersed in water from 0.12 to 0.24 kg/L
• The bath concentration is monitored for specific
gravity with hydrometer.
• They should never be applied with a brush.

Water-suspendible developers (form C)
• It can be used with either fluorescent (type I) or
visible (type II) penetrants.
• With fluorescent penetrant, the dried coating of
developer must not fluoresce, nor may it absorb
or filter out the black light used for inspection.
• supplied as a dry powder concentrate, which is
then dispersed in water in recommended
proportions, usually from 0.04 to 0.12 .
• Specific gravity checks should be conducted
routinely, using a hydrometer to check the bath
concentration.

Water-suspendible developers (form C)
• It contains dispersing agents to help retard
settling and caking as well as inhibitors to
prevent or retard corrosion of work pieces
• It contains biocides to extend the working life
of the aqueous solutions.
• It contains wetting agents to ensure even
coverage of surfaces and ease of removal after
inspection.
• They should never be applied with a brush.

Drying
• 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 be blurred and indistinct.
• Properly developed parts in water soluble
developer will have an even, pale white coating
over the entire surface.
• The surface of a part coated with a water
suspendable developer will have a slightly
translucent white coating.

Advantages
• Not require any agitation in water soluble but
water suspendable developers require frequent
stirring or agitation to keep the particles from
settling out of suspension.
• Applied prior to drying, thus decreasing the
development time
• The dried developer film on the work piece is
completely water soluble and is thus easily and
completely removed by simple water rinsing.

Non-aqueous solvent-suspendible
developers (form D)
• used for both the fluorescent and the visible
penetrant process.
• This coating yields the maximum white color
contrast with the red visible penetrant indication
and extremely brilliant fluorescent indication.
• Supplied in the ready-to-use condition and
contain particles of developer suspended in a
mixture of volatile solvents.
• It also contain surfactants in a dispersant whose
functions are to coat the particles and reduce
their tendency to clump or agglomerate.

developers
• Most sensitive form of developer used with type I
because the solvent action contributes to the
absorption and adsorption mechanisms.
• It enters the flaw and dissolves into the
penetrant. This action increases the volume and
reduces the viscosity of the penetrant.
• There are two types of solvent-base developers:
• nonflammable (chlorinated solvents) and
flammable (non-chlorinated solvents). Both types
are widely used.

developers
• Since the solvent is highly volatile, forced
drying is not required.
• A non-aqueous developer should be applied
to a thoroughly dried part to form a slightly
translucent white coating.
• If the spray produces spatters or an uneven
coating, the can should be discarded.

Stationary Inspection Equipment
The type of equipment most frequently used in
fixed installations consists of a series of modular
subunits.
• Drain and/or dwell stations
• Penetrant and emulsifier stations
• Pre- and post-wash stations
• Drying station
• Developer station
• Inspection station
• Cleaning stations

Developer
• Developer Station. The type and location of the
developer station depend on whether dry or wet
developer is to be used.
• For dry developer, the developer station is
downstream from the drier, but for wet
developer it immediately precedes the drier,
following the rinse station.
• For wet, there should also be a rack or conveyor
on which parts can rest after dipping. This will
permit excess developer to run back into the
tank.

Developer
• Suspendible developer baths settle out when not in use;
therefore, a paddle for stirring should be provided.
Continuous agitation is essential because the settling rate is
rapid.
• Pumps are sometimes incorporated into the developer
station for flowing the developer over large work pieces
through a hose and nozzle and for keeping the developer
agitated.
• In automatic units, special methods of applying developer
are required. Flow-on methods are frequently used.
• This technique requires a nozzle arrangement that permits
the work pieces to be covered thoroughly and quickly.

Inspection Station
• Inspection station is simply a worktable on which work
pieces can be handled under proper lighting.
• For fluorescent methods, the table is usually
surrounded by a curtain or hood to exclude most of the
white light from the area.
• For visible-dry penetrants, a hood is not necessary.
• Generally, black (ultraviolet) lights (100 W or greater)
are mounted on brackets from which they can be lifted
and moved about by hand.
• Because of the heat given off by black lights, good air
circulation is essential in black light booths.

Black light Intensity
• UV ranging from 180 to 400 nanometers.
• Recommended black light intensity is 1000 to 1600
W/cm2.
• The intensity of the black light should be verified at
regular intervals by the use of a suitable black light
meter such as a digital radiometer.
• Warm up prior to use--generally for about 10 min.
• UV light must be warmed up prior to use and should be
on for at least 15 minutes before beginning an
inspection.
• The inspector should allow time for adapting to
darkness; a 1-min period is usually adequate.
• White light intensity should not exceed 20 lx (2 ftc) to
ensure the best inspection environment.

Black light Intensity
• 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.
• UV emissions below 310nm include some
hazardous wavelengths.
• 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.

Effect of UV light
• 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
• 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.
• UV lamps sold for use in LPI application are
almost always filtered to remove the harmful UV
wavelengths.

visible light intensity
• visible light intensity should be adequate to
ensure proper inspection; 320 to 540 lx (30 to 50
ftc) is recommended.
• Lighting intensity should be verified at regular
intervals by the use of a suitable white light
meter such as a digital radiometer & it 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

Dimensional Threshold of
Fluorescence
• The performance of penetrants based on the
physical constraints of the dyes can be predicted
using Beer's Law equation. This law states that
the absorption of light by a solution changes
exponentially with the concentration of the
solution.
• This equation does not hold true when very thin
layers are involved but works well to establish
general relationships between variables.
• I = Io x e-lCt

Dimensional Threshold of
Fluorescence
Where:
I = Transmitted light intensity
Io = Incident light intensity
e = Base of natural log (2.71828)
l = Absorption coefficient per unit of concentration
C = Dye concentration
t = Thickness of the absorbing layer trolled to a
certain degree by the concentration of the
fluorescent tracer dye in the penetrant

Post cleaning
• Some residue will remain on work pieces after
penetrant inspection is completed.
• Residues can result in the formation of voids
during subsequent welding or unwanted
stopoff in brazing,
• In the contamination of surfaces (which can
cause trouble in heat treating), or in
unfavorable reactions in chemical processing
operations.

Post cleaning
• ultrasonic cleaning may be the only satisfactory
way of cleaning deep crevices or small holes.
However, solvents or detergent-aided steam or
water is almost always sufficient.
• The use of steam with detergent is probably the
most effective of all methods.
• It has a scrubbing action that removes
developers, the heat and detergent remove
penetrants, it leaves a work piece hot enough to
promote rapid, even drying, and it is harmless to
nearly all materials.

Post cleaning
• Vapor degreasing is very effective for
removing penetrants, but it is practically
worthless for removing developers.
• It is frequently used in combination with
steam cleaning.
• If this combination is used, the steam cleaning
should always be done first because vapor
degreasing bakes on developer films.

Probability of detection
In general, penetrant inspections are more
effective at finding
• small round defects than small linear defects
• deeper flaws than shallow flaws
• flaws with a narrow opening at the surface
than wide open flaws
• flaws on smooth surfaces than on rough
surfaces

OMNDT PT L-II NOTES Prepared by MAHESH PANDIT(ASNT L-III)

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OMNDT PT L-II NOTES Prepared by MAHESH PANDIT(ASNT L-III)