BMFB 4283
NDT & FAILURE ANALYSIS
Lectures for Week 2
Prof. Qumrul Ahsan, PhD
Department of Engineering Materials
Faculty of Manufacturing Engineering
Issues to address
2.0 Liquid Penetrant
2.1 Introduction
2.2 Fundamentals
2.3 Techniques
2.4 Applications
Introduction
• This lecture is intended to provide an
introduction to the NDT method of penetrant
testing.
• Penetrant Testing, or PT, is a nondestructive
testing method that builds on the principle
of Visual Inspection.
• PT increases the
“seeability” of small
discontinuities that
the human eye might
not be able to detect
alone.
PENETRANT TESTING (PT)
oPT effectively requires:
 Discontinuities open to the surface of the part
(subsurface discontinuities or surface discontinuities
not open to the surface aren’t detected)
 Special cleaning of parts
 Good eyesight
What Can Be Inspected Via PT?
Almost any material
that has a relatively
smooth, non-porous
surface on which
discontinuities or
defects are suspected.
SURFACE BREAKING DEFECTS
What Types of Discontinuities
Can Be Detected Via PT?
All defects that are open to
the surface.
– Rolled products-- cracks,
seams, laminations.
– Castings--cold shuts, hot tears,
porosity, blow holes,
shrinkage.
– Forgings– cracks, laps, external
bursts.
– Welds– cracks, porosity,
undercut, overlap, lack of
fusion, lack of penetration.
Principles of Penetrant Testing
•In penetrant testing, a liquid with high surface wetting
characteristics is applied to the surface of a
component under test.
•The penetrant “penetrates” into surface breaking
discontinuities via capillary action and other
mechanisms.
•Excess penetrant is removed from the surface and a
developer (blotter) is applied to
pull trapped penetrant back
the surface.
•Developer provides a contrasting
background for visual indications
of any discontinuities present
become apparent.
Basic Process of PT
1) Clean & Dry Component
4) Apply Developer
2) Apply Penetrant
5) Visual Inspection
3) Remove Excess
6) Post Clean Component
What Makes PT Work?
• Surface Tension : An elastic force that acts tangential to the fluid surface to reduce
the area is called surface tension
The surface tension for a droplet of liquid is  = ½ (po - pi ) r, where (po - pi ) is the difference
between the bubble’s exterior and interior pressure and r is the radius of curvature
• The surface tension at interface of two mediums can be expressed as lg
• Wetting is the ability of a liquid to maintain contact with a solid surface, resulting
from intermolecular interactions when the two are brought together.
• The degree of wetting (wettability) is determined by a force balance between
adhesive and cohesive forces.
• Adhesive forces between a liquid and solid cause a liquid drop to spread
across the surface.
• Cohesive forces within the liquid cause the drop to ball up and avoid contact
with the surface
At the liquid-solid surface interface, if the molecules of the
liquid have a stronger attraction to the molecules of the solid
surface than to each other (the adhesive forces are stronger
than the cohesive forces), wetting of the surface occurs.
Alternately, if the liquid molecules are more strongly
attracted to each other than the molecules of the solid
surface (the cohesive forces are stronger than the adhesive
forces), the liquid beads-up and does not wet the surface of
the part.
Figure 2: Wetting of different fluids. A
shows a fluid with very little wetting,
while C shows a fluid with more
wetting.
What Makes PT Work?
•The contact angle is the angle formed by the solid/liquid interface
and the liquid/vapor interface measured from the side of the liquid
Contact
angle
Degree of
wetting
θ=0
Perfect
wetting
0 < θ < 90°
high
wettability
90° ≤ θ <
180°
θ = 180°
Strength of:
Sol./Liq.
interactions
Liq./Liq.
interactions
strong
weak
strong
strong
weak
weak
low
wettability
weak
strong
perfectly
non-wetting
weak
strong
•For a penetrant material to be effective, the contact
angle should be as small as possible
•Typical penetrant materials have contact angles on
the order of 10o
Contact angle of a liquid
droplet wetted to a rigid solid
surface
What Makes PT Work?
• In liquid penetrant testing, two conditions must be met.
– First, the surface energy of the solid-gas interface must be greater than
the combined surface energies of the liquid-gas and the solid-liquid
interfaces.
– Second, the surface energy of the solid-gas interface must exceed the
surface energy of the solid-liquid interface.
• Capillary action : If a tube is sufficiently narrow
and the liquid adhesion to its walls is sufficiently
strong, surface tension can draw liquid up the
tube. The height the column is lifted to is given
by:
where
h is the height the liquid is lifted,
 is the liquid-air surface tension,
 is the density of the liquid,
r is the radius of the capillary,
g is the acceleration due to gravity,
θ is the angle of contact described
above.
Illustration of capillary
rise and fall.
Red=contact angle less
than 90°;
blue=contact angle
greater than 90°
What Makes PT Work?
•Every step of the penetrant process is done to
promote capillary action.
•This is the phenomenon of a liquid rising or
climbing when confined to small openings
due to surface wetting properties of the
liquid.
•Surface tension of liquid vs tube surface
wetting
• Surface tension – cohesive force
• Tube surface wetting – adhesive force
•Cohesive force> Adhesive force : Convex surface -> liq. fall below
•Cohesive force< Adhesive force : Concave surface -> liq. rise up
Finer Tube, greater liquid rise
finer defect (hairline) -> greater indication
Penetrability
• For cylindrical volume capillary pressure P = 2gl cosθ/r = 2Scosθ/r
– Where S is the surface tension, r is the radius of the crack and θ is the
contact angle.
– Influenced by variables: surface condition, and type of test object, type of
penetrant, temperature of test object and contamination.
• Fluid penetration into a real crack will generally be different from
the above estimation
– Crack width is not a constant ( a crack typically narrows with depth)
– Portions of the crack may be closed
– Trapped gas or contaminants within the crack limits fluid penetrationion
A liquid penetrant will continue to fill the void until an opposing force
balances the capillary pressure. This force is usually the pressure of trapped
gas in a void, as most flaws are open only at the surface of the part. Since the
gas originally in a flaw volume cannot escape through the layer of penetrant,
the gas is compressed near the closed end of a void.
Penetrability
• High P  High S (High S is not necessarily a
good penetrant
High Surface Tension
Low Wetting Ability
– e.g. Water has high surface tension, yet poor
penetrant
• Static Penetration Parameter (SPP) = Scosθ
– Smaller contact angle, θ  higher P
– Good penetrant 5o
• Width of discontinuity; D(2r)
– Narrower  higher P  Longer time to rise
– Finer defect  longer dwell time
Low Surface Tension
High Wetting Ability
• Viscosity, 
Penetrability
– Not significantly affect penetrant ability
High Surface Tension
Low Wetting Ability
into discontinuity
– Strongly affected with temperature
 penetrant inspection
– Kinetic Penetration Parameter, KPP = Scosθ/ 
•
•
Highly viscous penetrant: longer time to enter
into defect  longer dwell time
Drain more slowly and cause excessive loss of
penetrant due to drag
Low Surface Tension
High Wetting Ability
• Density
– has a slight to negligible effect on the performance of a penetrant.
– Increasing the specific gravity by decreasing the percent of solvent (by
volume) in the solution will increase the penetration speed.
– The gravitational force acting on the penetrant liquid can be working either
with or against the capillary force depending on the orientation of the flaw
Flaw Entrapment Efficiency
•Ability of penetrant to form an indication large
enough to be detected
•Factors influenced efficiency
– Volume of defect
– Length of defect
– Contaminants
–Penetrant dye
–Processing
Flaw Entrapment Efficiency
•Volume of defect
– Size of indication reflects
the volume of defect it entered
– Larger discontinuity: depth or width
More penetrant it holds
More penetrant is present to form
indication
• Length of Defect
– Affect volume of penetrant
– Strongly affect the visibility
Very fine defect has insufficient width
Reduce the ability of human eye to detect
indication visually
Can only be located when the defect has
sufficient length
Flaw Entrapment Efficiency
•Contaminants
–Fine and clean discontinuity vs wide and
contaminated dicontinuity
 Affect the penetration of penetrant
– Inservice inspection encounter defects
contaminated with oil, water and corrosion
products
Reduce the volume available for penetration
Water adversely influence contact angle
Acidic or alkaline contaminants also fading the
dye visibility
Heat and prolonged exposure under UV light
cause penetrant lose their sensitivity
Flaw Entrapment Efficiency
•Penetrant dye
–Type of dye
Affect the sensitivity of penetrant visibility
in terms of brilliance and intensity of the
dye color
Fluorescent dye is more sensitive than
color contrast dye
–
Concentration of dye
Different concentration within a
classification affect the sensitivity level
Altering dye concentration affect the
ability of penetrant penetration
Flaw Entrapment Efficiency
•Processing
– Method of processing
Dip and drain allows more volatile constituents of penetrant to
evaporate during dwell time
Increases the concentration of dye within remaining penetrant
– Degree of penetrant removal
• No background coloration (cleaned component) high degree of
•
•
•
contrast penetrant indication
Should be interpreted carefully: over-emulsification or over
washing  removes penetrant from defects
Small degree of background coloration : over-washing not occur
Brightness of indication must exceed background brightness
What Can NOT be Inspected Via PT?
•Components with rough
surfaces, such as sand castings,
that trap and hold penetrant.
•Porous ceramics
•Wood and other fibrous
materials.
•Plastic parts that absorb or
react with the penetrant
materials.
•Components with coatings that
prevent penetrants from
entering defects.
Defect indications
become less
distinguishable as
the background
“noise” level
increases.
Choices of Penetrant Materials
Penetrant
Type
I Fluorescent
II Visible
Method
A Water Washable
B Postemulsifiable - Lipophilic
C Solvent Removable
D Postemulsifiable - Hydrophilic
Developer
Form
Dry Powder
Wet, Water Soluble
Wet, Water Suspendable
Wet, Non-Aqueous
PREPARATION AND PRE-CLEANING
WATER WASHABLE
POST-EMULSIFIABLE
SOLVENT REMOVABLE
CHECK PENETRANT REMOVAL
POWDER
DEVELOPER
WATER SOLUBLE
DEVELOPER
WATER SUSPENDABLE
DEVELOPER
INSPECT
RECORD
CLEAN AND PROTECT
SOLVENT BASED
DEVELOPER
Penetrant Materials
Penetrants are formulated to possess a number of
important characteristics. To perform well, a
penetrant must:
– Spread easily over the surface being inspected.
– Be drawn into surface breaking defects by capillary action
or other mechanisms.
– Remain in the defect but remove easily from the surface of
the part.
– Remain fluid through the drying and developing steps so it
can be drawn back to the surface.
– Be highly visible or fluoresce brightly to produce easy to see
indications.
– Not be harmful to the inspector or to the material being
tested.
Penetrant Properties
•
•
•
•
•
•
•
Wetting ability
Specific gravity
Volatility
Chemical activity
Solubility
Solvent ability
Health hazard
• Tolerance to
contaminants
• Flammability / flash
point
• Electrical conductivity
• Availability and cost
Penetrant Properties
• Wetting ability:
– Affect penetrability and bleed-back characteristics
– Contact angle and surface tension of penetrant control
wetting ability
• Specific gravity:
– Ratio of density of penetrant with density of distilled
water at 40C
– Not a problem with oil base penetrant
– Penetrant used in tank system must have specific
gravity less than 1
 to ensure water will not float on top of penetrant
 prevent penetrant from covering the test object
Penetrant Properties
• Flash point:
– Temperature at which enough vapor is given off to form
combustible mixture
– Typical min 93C
– Should not be flammable
• Volatility:
– Characterize by vapor pressure or boiling point
– Good penetrant usually evaporate too quickly
– Low volatility is desirable  so the penetrant dry from the
surface, leave stained and from any discontinuity, leave
precipitated dye
Penetrant Properties
• Chemically inert:
– Must be inert, non-corrosive as possible  chemically compatible
with the material being tested
– Penetrant is contaminant (contain sulphur, sodium, halogen) 
potential reactions must be considered
– To avoid possibility of embrittlement or cracking over years
• Viscosity:
– Affect thickness of penetration due to molecular/internal friction
– Low viscosity penetrant
• Solubility:
– Penetrant contain dye in liquid solution
– Must hold sufficient dye at ambient or high temperature
– Must not come out from solution if temperature drop
Penetrant Properties
• Solvent ability:
– Solvent must be able to remove surplus penetrant from
test specimen
– To ensure clean, clear background
– Must not dissolve the penetrant in defect
• Tolerance to contaminants:
– Penetrant will be contaminated after a period of time,
even if a great care is taken
– Must be periodically check to ensure all is well, no residue
left
Penetrant Properties
• Health hazard:
– Must comply with or exceed the most stringent
HSE requirements
• Toxicity, odour, skin contact
– To prevent allergies or contaminants
• Availability and cost:
– Dye materials are easily obtained
– Low cost
Penetrant Properties
• Electrical conductivity:
– Electrostatic spraying becomes popular
uniform coverage with complicated shapes
Reduces over spraying
Requires less penetrant over all
– Spray gun applies –ve charge to penetrant
– Test object ground potential
– Electrostatic attraction cause penetrant be strongly
attracted to the part
 low viscosity and easily attracted to the part
Must readily accept and hold the electrical charge
Sensitivity Levels
• Penetrants are also formulated to produce a variety of
•
sensitivity levels. The higher the sensitivity level, the
smaller the defect that the penetrant system is capable of
detecting.
The five sensitivity levels are:
–
–
–
–
–
Level 4 - Ultra-High Sensitivity
Level 3 - High Sensitivity
Level 2 - Medium Sensitivity
Level 1 - Low Sensitivity
Level 1/2 – Ultra-Low Sensitivity
• As the sensitivity level increases, so does the number of
nonrelevent indications. Therefore, a penetrant needs to
be selected that will find the defects of interest but not
produce too many nonrelevent indications.
Why is Visible Penetrant Red and Fluorescent
Penetrant Green?
• Visible penetrant is
usually red because
red stands out and
provides a high level of
contrast against a light
background
• Fluorescent penetrant
is green because the
eye is most sensitive to
the color green due to
the number and
arrangement of the
cones (the color
receptors) in the eye.
Visible Vs Fluorescent PT
• Inspection can be performed
using visible (or red dye) or
fluorescent penetrant
materials.
• Visible Pt is performed under
white light while fluorescent
PT must be performed using
an ultraviolet light in a
darkened area. All are all in
the level 1 sensitivity range.
• Fluorescent PT is more
sensitive than visible PT
because the eye is more
sensitive to a bright
indication on a dark
background. Sensitivity
ranges from 1 to 4.
Photo Courtesy of Contesco
Type of UV light
• Mercury vapour arc lamp
– Street lamp that has filter to reduce the visible
light to minimum but allow UV-A to transmit
• GE or Westinghouse lamp
– Has separate filter, Hg arc is drawn between
electrodes in quartz tube
• 400 W Hg vapour flood lamp
– Used for very large component
Quality of fluorescent dye
• Depends on how efficient dye absorb UV
light and convert into visible light
• Influenced by:
– The intensity of UV-A light at the surface
– The ability of dye to absorb UV-A
– The concentration of dye
– The ability of dye to produce visible light
– Film thickness
Penetrant Removal Method
Penetrants are also classified by the method of
removing the excess penetrant.
•Solvent Removable penetrants are removed by
wiping with a cloth dampened with solvent. They are
supplied in aerosol cans for portability and are primarily
used for spot checks.
•Water Washable penetrants are removed with a
course spray of water. They are the easiest to employ
and most cost effective when inspecting large areas.
•Post-Emulsifiable penetrants are water-washable
only after they have reacted with an emulsifier solution.
A post-emulsifiable system is used when washing the
penetrant out of the defect is a concern. The emulsifier
is given time to reacts with the penetrant on the surface
but not the penetrant trapped in the flaw.
Developers
• The role of the developer is to pull trapped
penetrant out of defects and to spread it
out on the surface so that it can be seen.
Also provides a light background to
increase contrast when visible penetrant is used.
• Developer materials are available in several different forms
– Dry Powder is a mix of light fluffy powder that clumps together where penetrant
bleeds back to the surface to produces very defined indications.
– Wet, Water Suspendable is a powder that is suspended in a water that covers the
surface with a relatively uniform layer of developer when the water is evaporated. The
solution is somewhat difficult to maintain as the powder settles out over time.
– Wet, Water Soluble is a crystalline powder that forms a clear solution when mixed
with water. The solution recrystallizes on the surface when the water is driven off.
Indications sometimes lack definition and look milky. Not recommended for use with
water-washable penetrants.
– Wet, Non-Aqueous - is supplied in a spray can and is the most sensitive developer
for inspecting small areas. It is too costly and difficult to apply to large areas.
Basic mechanism of Developer
• Developer works due to :
– Capillarity
– Light scattering
– Solvent action
Capillarity
• Capillary attraction of developer overcomes the
opposing attraction of the discontinuity
– Increase the surface area of indication
– Spread the penetrant laterally on surface  widening
indication
– Expands the bulk dye into many thin films  enhance
brightness
• Too large developer particle size will result low
capillary pressure
• Too small will cause block any orifice
Light scattering
• Very important when involves fluorescent
penetrants
– Brightness of indication is amplified per unit area
– Each particle provide scattering multiple reflector
 both UV-A and fluorescent radiation
– This improve contrast in dark condition 
improved sensitivity of penetrant system
Solvent action
• Applies to non-aqueous method
– Have no capability for drawing penetrant out of
discontinuity
– The developer damp the test surface
– The remaining solvent will bridge the gap
between the developer particles and the
penetrant in the discontinuity
– Important for fine defect
Developer properties
• Good developer:
– Material must be absorptive to perform blotting action
– Must have fine texture
– Must mask out background contours and colors
– Must be easily and evenly applicable
– Must form light and even coat
– Must be no fluorescing of developer when fluorescent
penetrant is used
– Penetrant bleeding must easily wet the material
– Must be high color contrast, white is the best
– Must be readily removable after test
– Must be in-toxic and non-irritant
6 Steps of Penetrant Testing
1.
2.
3.
4.
5.
6.
Pre-Clean
Penetrant Application
Excess Penetrant Removal
Developer Application
Inspect/Evaluate
Post-clean
Pre-cleaning – Step 1
• Parts must be free of dirt,
rust, scale, oil, grease, etc.
to perform a reliable
inspection.
• The cleaning process must
remove contaminants from
the surfaces of the part
and defects, and must not
plug any of the defects.
Pre-cleaning is the most
important step in the PT
process!!!
Why Pre-cleaning important?
• Penetrant unable to wet the surface of the test
object
– due to oils, water/hydrates left after evaporation or
polishing and buffing lubricants
• Penetrant is unable to enter a discontinuity
(blockage)
– Peening or smearing of discontinuity, carbon, scale,
paint/coatings, penetrant residues
• Penetrant bleed out from discontinuity is restricted
– Carbon, scale, rust, anodising
Cleaning methods
• Mechanical methods:
– Brushing
– Blasting
• Chemical methods:
–
–
–
–
–
–
–
Hot solvent degreasing
Vapor degreasing
Cold solvent degreasing
Alkaline degreasing
Acid pickling
Steam cleaning
Paint strippers
Physical Cleaning
•Grinding
•Abrasive Blasting
•Wire brushing
Defect
Peened or Closed
After abrasive
cleaning
Before
Cleaning
Light Acid Etching
Light Acid applied
Thin layer of the
surface dissolved
Light Acid Etching
The defect opened again to
the surface
After Acid Etching
Chemical Methods
Hot Solvent Degreasing
Solvent
Components
Heating
Element
Vapour Degreasing
Components
Condensor
vapour
Drip Tray
Solvent
Heating
Element
The most effective method for degreasing
Steam Cleaning
• For large objects
Chemical Methods
Other methods
• Cold solvent
Degreasing
• Solvent materials with
Emulsifiers
• Acid / Alkaline
Cleaning
• Paint Removal
• Ultrasonic Cleaning
Ultrasonic Cleaning
Solvent/
water
Components
Ultrasonic
Crystal
Caution About Metal Smearing
Some machining, surface finishing and cleaning
operations can cause a thin layer of metal to
smear on the surface and prevent penetrant from
entering any flaws that may be present.
Etching of the surface prior to inspection is
sometimes required.
Before Sanding
After Sanding
After Etching
Penetrant Application – Step 2
Many methods
of application
are possible
such as:
– Brushing
– Spraying
– Dipping/
Immersing
– Flow-on
– And more
Dwell Time
•The penetrant solution must
be allowed to “dwell” on the
surface of the part to allow
the penetrant time to fill any
defects present.
•The dwell time vary
according to penetrant type,
temperature, material type
and surface finish.
Excess Penetrant Removal – Step 3
The removal technique depends upon the
type of penetrant used, as stated earlier…
– Solvent Removable
– Water Washable
– Post Emulsifiable
Excess Penetrant Removal – Step 3 (cont.)
Water Washable
• A coarse water spray is
used to remove the excess
penetrant.
• The procedure used as a
guideline for the
inspection will specify
water temperature
(typically 50-100°F) and
pressure (typically not
more than 40 psi), etc.
Excess Penetrant Removal – Step 3 (cont.)
Solvent Removable
•The part is wiped with a
clean dry cloth to remove
the bulk of the excess
penetrant.
•Then, a cloth lightly
dampened with solvent is
used to remove any
remaining penetrant on
the surface.
Excess Penetrant Removal – Step 3 (cont.)
Solvent Removable (cont.)
Any time a solvent is
used in the penetrant inspection
process, a
suitable flash time is
required to allow excess solvent to
evaporate.
Excess Penetrant Removal – Step 3 (cont.)
Post Emulsifiable
•When there is concern about removing much of the penetrant
a post emulsifiable system is used.
•This involves an additional step
in which an emulsifier is applied
to the surface of the part after
the penetrant dwell time.
•The emulsifier is given just
enough time to react with
the penetrant on the surface
to render it water washable
but not enough time to diffuse
into the penetrant trapped in
the defects.
Developerr Application –Step 4
• Bleed the penetrant back to the surface by the
reverse capillary action
• Spread the penetrant into larger area : easier
to be seen
• Improve background contrast
Developer Application – Step 4
The method of developer application is is
dependent on the type of developer used. The
primary methods for the following main developer
types will be covered in the following slides.
– Dry
– Wet
– Nonaqueous Wet
Dry powder developer
Developer Application – Step 4 (cont.)
Dry Powder Developer
•Prior to applying a dry
powder developer, the
component must be
thoroughly dried. Drying is
usually accomplished in a hot
air circulating oven.
•The developer is then applied
by immersing the part in the
powder or by dusting of the
part with the powder.
•The part can also be placed in
a developer dust cloud
chamber.
Dry powder developer
ADVANTAGES
• Easy to handle
• No hazardous vapours
• Easy to remove
DISADVANTAGES
• Difficult to ensure if it
has been properly
applied
• Fine powders can be
hazardous
• Do not offer a high
degree of colour contrast
Aqueous Developer
TWO TYPES
SOLUTION
SUSPENSION
The particles
settled down if
not agitated
Developer Application – Step 4 (cont.)
Wet Developer (water- suspended
and water- soluble)
•Wet developers are applied by
immersing or spraying the part
while it is still wet from the
penetrant removal process.
•The part is completely coated and
the excess liquid allowed to drain
to prevent pooling
•The part is then dried in a hot air
circulating oven.
Aqueous Developer
ADVANTAGES
• No vapours or dust
• Cheaper than nonaqueous
DISADVANTAGES
• Difficult to apply
evenly
• Requires drying after
application
• Proper mixing
required
Too little developer particle : Very weak indications
Too much :
Developer layer will crack when dry
Developer Application – Step 4 (cont.)
Nonaqueous Developer (AKA
Solvent-Suspended)
• Nonaqueous developer is
applied by a aerosol spray to a
thoroughly dried and cooled
part.
• A thin even coating should be
applied. The coating should be
white but still slightly
transparent when performing a
visible dye penetrant
inspection, and even thinner
when performing a fluorescent
penetrant inspection.
Non-Aqueous Developer
ADVANTAGES
• Most sensitive
• Useable with
fluorescent or colour
contrast
DISADVANTAGES
• Hazardous solvents
• Higher cost
• Need to be correctly
applied
Inspection/Evaluation – Step 5
In this step the inspector
evaluates the penetrant
indications against specified
accept/reject criteria and
attempts to determine the
origin of the indication.
The indications are judged
to be either relevant, nonrelevant or false.
Non-relevant weld geometry indications
Relevant crack indications from an
abusive drilling process
Inspection/Evaluation – Step 5
• Should be inspect immediately after developer is
applied
– Development time between 0 to 30 min
– Inspection should be carried out throughout the
development time
• Inspector is the most critical element
– Acceptance and rejection based on inspector judgment
– Must have good near vision acuity & capable of colour
discrimination
Inspection/Evaluation – Step 5
• For colour contrast penetrants:
– Should be viewed in bright white light colour
– Min 500 lux is recommended = bright daylight
Inspection/Evaluation – Step 5
• For fluorescent method inspection
– The room must be darkened below 20 lux visible
light
– Min level of UV-A is 1.0mW/cm2
– Inspector must be in dark room for 10 min
• To adapt for low light level
– Wear photochromatic spectacles
• To reduce eyestrain
– Should not view continuously more than 30 mins
Inspection/Evaluation – Step 5
A very important step of
evaluation is to
document findings on
an inspection report
form or other record
keeping form.
This may be supported
with sketch drawings or
photos of indications,
adhesive tapes, video,
etc
Indications
• True or Relevant
indications
Indications caused by
defects
• Non Relevant indications
Indications caused by
assembly or geometry of
the component
• False indications
Indications caused by
operator faults
PRESS FITTED
SPLINES
HOLES
POROUS NATURE
CRACKS
RIVETS
POROSITY
LAP
IMPROPER
ROUGH
PROFILE
THREAD
WASHING
Post Clean – Step 6
The final step in the penetrant
inspection process is to
thoroughly clean the part that
has been tested to remove all
penetrant processing
materials.
Penetrant chemicals residues are required to
be removed because
They may be harmful to the component
Penetrant and emulsifier are alkaline:
may cause surface pitting, especially on
aluminium.
Developer will entrapped moisture: may
cause corrosion
or
They may impair subsequent processing
Penetrant Inspection Systems
Penetrant systems can be highly portable or
stationary.
Image courtesy of Nebraska Army National Guard
Portable Penetrant System
Stationary Penetrant System
CHOICE OF PENETRANT SYSTEM
• Factors influenced:
– Size and type of defect
– Geometry and intricacy
– Surface condition
– Other factors such as: component material, size
and position of item, equipment and expertise
available, cost, number of component to be tested
Size and type of defect
• Wide shallow defect are most likely to be
detected using post-emulsifiable method
• Fine defect is best located with fluorescent
method due to high sensitivity of eye to
fluorescent than colour contrast indication
Geometry and intricacy
• High intricate components have large number
of section changes
• Threaded component causes problem during
excess penetrant removal
• Results in excessive background colouration
– Reduce the detection of defect
Surface condition
• Rough surface is difficult to fully clean
• Fluorescent mehod is less suited to test
rough component
– Difficulty in adequately monitoring penetrant
removal
Other factors
• Component material:
– Solvent removable methods may lead to surface damage
due to incompatibility between penetrant and material
under test
• Size and position of the item
– On-site welds are unlikely tested by fluorescent method
due to test method requirement (dark room)
• Equipment and expertise available
– Fluorescent method involves flow lines  more suited for
factory use than onsite test
Other factors
• Cost
– Water washable method is much cheaper than solvent
removable method due to availability of cleaning fluid
• Number of components to be tested
– Fluorescent method is recommended for batch inspection
due to high sensitivity of human eye to fluorescent
indication compare to visible colour contrast
 reduce fatigues during perform the test
 inspector able to perform at higher level for longer
period
Quality and Process Control
Since penetrant testing
involves multiple processing
steps, the performance of the
materials and the processes
should be routinely checked
using performance verification
tools, which include:
– TAM Panels
– Crack Sensitivity Panels
– Run Check Panels
Equipment checks
• Overall system performance
• Control check:
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water wash temperature and pressure
Colour intensity
Penetrant remover check
Developer check
UV lamp output check
Water removable penetrant, water tolerance check
• Maintenance check:
– Equipment cleanliness
– Airline cleanliness
– Processing unit
Advantages of Penetrant Testing
• Relative ease of use.
• Can be used on a wide range of material types.
• Large areas or large volumes of parts/materials can be
inspected rapidly and at low cost.
• Parts with complex geometries are routinely
inspected.
• Indications are produced directly on surface of the
part providing a visual image of the discontinuity.
• Initial equipment investment is low.
• Aerosol spray cans can make equipment very
portable.
Limitations of Penetrant Testing
• Only detects surface breaking defects.
• Requires relatively smooth nonporous material.
• Precleaning is critical. Contaminants can mask
defects.
• Requires multiple operations under controlled
conditions.
• Chemical handling precautions necessary (toxicity,
fire, waste).
• Metal smearing from machining, grinding and other
operations inhibits detection. Materials may need to
be etched prior to inspection.
• Post cleaning is necessary to remove chemicals.
Summary
• Penetrant testing (PT) is one of the most widely
used nondestructive testing methods.
• Its popularity can be attributed to two main factors,
which are its relative ease of use and its flexibility.
• However, PT involves a number of processing steps
that must be closely control to achieve optimal
sensitivity.
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Glossary of Terms
Capillary Action - the tendency of certain liquids to travel or climb when
exposed to small openings.
Contrast - the relative amount of light emitted or reflected between and
indication and its background.
Developer - a finely divided material applied over the surface of a part to help
promote reverse capillary action and thus bring out a penetrant indication.
Dwell Time - the period of time that a penetrant or developer must remain in
contact with the surface of a part under test.
Emulsification Time - the time allowed for the emulsifier to render the
penetrant water washable and thus allow the part to be washed.
Emulsifier - a material applied over a film of penetrant that renders it water
washable.
Evaluation - the process of deciding as to the severity of the condition after
an indication has been interpreted.
False Indication - an indication caused by improper processing; not caused by
a relevant or non-relevant condition.
Flash Time - the time required for the solvent to evaporate from the surface
of a part when used to preclean or remove excess penetrant.
Fluorescent Dye - a dye which becomes fluorescent (gives off light) when
exposed to short wave radiation such as ultraviolet light.
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Glossary of Terms
Indication - the visible evidence or penetrant bleed-out on the surface of the
specimen
Interpretation - the process of evaluating an indication in an attempt to
determine the cause and nature of the discontinuity.
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Non-Aqueous Developer - a developer in which developing powder is applied as
a suspension in a quick drying solvent
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Penetrant - a liquid used in fluorescent or visible dye penetrant inspection to
penetrate into the surface openings of parts inspected via these methods
Relevant Indication - an indication that has been determined not to be false or
non-relevant - and actual discontinuity
Seeability - the characteristic of an indication that enables it to be seen against
the adverse conditions of background, outside light, etc.
Sensitivity - the ability of a penetrant to detect surface openings. Higher
sensitivity indicates smaller discontinuities can be detected
Ultraviolet Light (or Black Light) - light energy just below the visible range of
violet light (356 nanometers).
Viscosity - the resistance of a fluid to the motion of its particles
Washability - the property of a penetrant which permits it to be cleaned from the
surface of a part by washing with water
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