1. NDT involves noninvasive testing methods to inspect components without impairing their future use, while PT specifically uses penetrant liquids to detect surface or near-surface discontinuities. 2. Key steps in PT include cleaning parts, applying penetrant, removing excess surface penetrant, applying developer to extract penetrant from flaws, and inspecting for indications. 3. An ideal penetrant wets surfaces well, penetrates cracks capillarity, and is visible; the developer enhances penetrant visibility against a contrasting background.

1. Introduction to NDT and Penetrant Testing (PT)
G.Jothinathan
Scientific Officer Gr.I
Department of Metallurgical and
Materials Engineering
I I T M, Chennai 600 036
Email – gjndt@yahoo.com

2. Introduction To NDT
Non Destructive Testing (NDT) is a
noninvasive method and the testing is carried
out without impairing further usefulness of
the material, component and structure
The component can be put into use after the
Testing. The signals that are employed do not
alter the properties permanently .
Method Signal Remarks
UT US waves - elastic waves Elastic deformation
RT X-rays and gamma rays No damage during
the course of testing
Radiation damage!!!
MT Magnetic field Remnent field-
Demagnetisation
- interesting and highly paying field
- interdisciplinary of metallurgy, physics and
chemistry
NDT accounts for 1/3 of the cost of an aircraft
(without raw material and fabrication cost !!)

3. Definition of terms
NDT
NDI Used interchangeably- detection and
reporting
The scope is broader –serviceability is
NDE ascertained
Also includes material characterisation
Material anomalies-interruption ,break,disruption
Discontinuity- can meet the service requirements
Defect/flaw - renders unsuitable for service
Anamoly whether Discontinuity or defect/flaw
1.Nature of the imperfection –linear-volumetric
2.Material –structural or high strength
3.Criticality of use – storage vessel or boiler
boiler in thermal power plant
boiler in nuclear power plant
Linear -crack – length > 3 times width
Volumetric - porosity and inclusion

4. Applications of NDT
The main application of NDT is
flaw detection and evaluation.
It is also used for
geometric dimension measurement.
material characterization,
bond integrity testing ,
condition monitoring etc
Material characterisation:
2. E and μ determination
3. Grain size evaluation
4. Proportion of microstructural phases
5. Extent of deformation
6. Nodularity of nodular cast iron etc
7. Fracture toughness determination

5. Purpose of NDT
Fracture mechanics approach – damage
tolerance approach (propagates the idea of
“live with discontinuities”)
A material property, fracture toughness (KIc)
is defined and this characterizes the material
behaviour in presence of discontinuities
which the earlier strength of materials design
approach has not taken into account.
KIc can be experimentally determined.
The stress intensity factor (KI) the stress
distribution around a discontinuity
is given by KI = G σ √πa where G is
geometric constant, σ is the working stress
and ’ a’ is flaw size.
Failure occurs when KI is equal to or greater
than KIc
To determine KI,, flaw size information
needed and is provided by NDT personnel.
Life extension and life predication methods

6. APPLIED YIELD TENSILE
STRESS STRENGTH
(a) The Strength of materials approach
APPLIED
STRESS
FLAW FRACTURE
SIZE TOUGHNESS
(b) The Fracture Mechanics approach
Comparison of classical design philosophy with fracture mechanics
approach

7. NDT methods
1.Visual testing (VT)
2.Liquid Penetrant Testing (PT)
3.Magnetic particle Testing (MT)
Surface NDT
4.Radiographic Testing (RT)
5.Ultrasonic Testing (UT) Surface &internal
6.Eddy Current Testing (ET) - highly sensitive
7.Acoustic Emission Testing (AET) monitors activity
8.Thermal Infrared imaging (TIFR)
9.Leak testing (LT)

8. Visual Testing (VT)
This ancient and original method of
examination by human eyes is still widely
employed to find gross discontinuities,
surface irregularities, roughness and
corrosion products on the surface.
Many gadgets like lenses, cameras are used.
For surfaces inaccessible to the human eyes
as in the case of inside surfaces of pipes and
boilers, boroscopes and flexible fibroscopes
are employed. These are called endoscopes
(internal vision). They are nothing but lens
arrangement to transfer the image or the
optical fibers arrangement for transferring the
image.

9. VT of welds(contd)
• Mainly for noncritical welds
• Before, during and after
welding
• Before -Surface roughness and
cleanliness (oil)
• During - Electrode size and
welders performance
• After -Dimensional accuracy of
weldments Conformity of
welds to size and contour
requirements

10. VT of welds (contd)
• Acceptability of weld appearance
with regard to:
• Presence of surface discontinuities
such as:
• surface roughness weld spatter
• cleanliness underfill pores
• Undercuts overlaps
• Cracks spatter
• Establishing definite procedure to
ensure uniformity and accuracy

11. Liquid Penetrant Testing (PT)
-is applicable to discontinuities that are
open to the surface or surface connected.
-is extension of visual testing
-an indication is obtained whose width
is very much larger than the actual
width of the crack so as to be seen by
the unaided eye.
Crack indication

12. Sensitivity and applications
-Sensitivity – equal or better than MT
– better than RT for surface
discontinuities
1 μm x 10 μm x 50 μm
can be detected
Applications – on all materials
– metals(ferrous and nonferrous)
nonmetals(rubber,plastic etc)
– all type of defects (open)
– leak testing
Normally not applicable to porous materials
(unfired ceramics and powder metallurgical
parts)
Presently Filtered particle penetrant for porous
materials
Rough surfaces pose problem- > 125 μm-
background poses problem

13. Principle of Penetrant Testing PT
Highly coloured (visible or fluorescent)
organic dye liquid which is also surface
active in nature (called penetrants) is applied
on to the clean surface of the component and
allowed sufficient time for penetration into
discontinuities. The excess surface penetrant
on the component is removed. This leaves a
clean surface of the component with pentrant
residing in the discontinuities. At this point
Of time, developer, which is highly absorptive
in nature, is applied. The developer brings
back or bleeds out the penetrant thereby
providing an indication in a contrasting
background of white colour of developer.
1.Preparation of part
2. Application of penetrant
3. Removal of excess surface penetrant
4. Development
5. Inspection and Evaluation
6. Post Cleaning

14. Clean surface of Component
Penetrant application
After removal of excess surface penetrant and
Removal of excess surface penetrant before application developer
Inspection and Interpretation
Application of developer
Fig.1. Principle of Penetrant Testing

15. Principle Of PT

16. Properties of penetrant
The entire penetrant testing is based on the
ability of the liquid to penetrate into
discontinuities and later ability to come out.
The required properties are 1. Wettability
2. Capillarity
The ability of the liquid to wet the solid surface
or spread over the solid surface is determined by
the surface energies of the liquid-gas interface,
the solid-liquid interface and the solid-gas
interface.
Mathematically expressed
SSL = γSG - (γLg + γSL)
Where: SSL is the wetting ability of liquid on a clean solid.
γLg is the surface energy of the liquid-gas interface.
γSL is the surface energy of the solid-liquid interface.
γSG is the surface energy of the solid-gas interface.
The liquid to spread over the solid surface, should replace the previously
existing solid –gas interface. This can happen when the energy
difference γSG -γSL) is positive or if γSG> > γSL). Or in other words the
surface energy of the solid gas interface should exceed, the surface
energy of the solid liquid interface. The difference in these energies is
responsible for the liquid to spread over the solid surface.

17. The ability of the liquid to spread or wet the solid
surface is related to the contact angle θ, which
quantifies the resultant adhesive and cohesive forces,
The contact angle is defined as the angle between the
solid surface and the tangent drawn to the liquid at
the point of contact. It can be seen that spreading
ability and contact angle are inversely related. The
Figs. Show the contact angle and wetting ability.
a. low contact angle contact angle 90 deg. High contact angle
Normally, the penetrants need to have a very low
contact angle and the commercial penetrants
have contact angles between 0 –5.. Contact angle
depends on the solid surface to be wetted. Water-
glass has a contact angle of 0 deg. compared to
water-silver which is 90 deg
Once the liquid wets the surface, the ability of the
liquid to rise in the capillary or enter into the
openings is determined by surface tension (T)
Hence the main properties of penetrant are T and θ
However, the speed of penetration is determined by
viscosity η

18. Other penetrant required properties
1.Visibility
2.Nontoxicity (noncorroding etc)
Visibility is next most important property of the
penetrant
Colour contrast ratio of visible dye is ~1:10
(The light reflected by the white background to bright
red of the dye)
Colour contrast ratio of the fluorescent dye is ~1:100
(light emitted by the indication to the light emitted by
the dark background)
Because of this colour contrast ratio, the indication is
better seen in the case of fluorescent indication . The
human eye brings an effect called halation effect, the
ability to magnify the indication

19. Classification penetrants
There are mainly three types of penetrants namely
1. Visible dye or colour contrast penetrant (Type II )
2. Fluorescent or brightness contrast penetrant (Type I )
3. Dual mode (visible and fluorescent) (Type III)
Other unclassified type is . filtered particle penetrant
(Type I, II, or III is based based on the type of dye
that is incorporated -visible or fluorescent or both )
Each of these are further classified as methods
1. Water washable WW ( Method A)
2. Post emulsifiable PE lippophilic (Method B)
3. Solvent removable SR (Method C )
4. Post emulsifiable Hydrophilic ( Method D)
-classification is based on the method by which
the excess penetrant is removed in the excess
penetrant removal step.
Simple water washing – water washable
Solvent wiping – solvent removable
Emulsifying &removing – post emulsifiable
lippo & hydro-
philic

20. Composition of penetrants
1. Oil base
2. Dye material (visible or fluorescent)
3. Solvents and stabilising agents.
The composition of solvent removable and post
emulsifiable penetrant are essentially same.
Solvent removable – by solvent action
Post emulsifiable – by dispersing the penetrant
into fine particles by the
applied emulsifier,
making it water removable
Water washable - Simple water washing
Simple water washing cannot remove the oil base
penetrant. Water washable penetrant has one more
constituent namely built in emulsifier The moment
water is applied, penetrant is dispersed by the in-
built emulsifier making it amenable for water
washing

21. Sensitivity of penetrants
All fluorescent methods are more sensitive
than visible dye penetrant
Penetrant Type Visisble dye Fluorescent
Properties dye
Indication Bright red Yellowish
colour green(on
excitation)
Background White colour Dark (max. 2
of the ft.candles)
developer
Colour contrast ratio ~1:10 ~1:100
Visibility(seeability) Medium Very high
Halation Normal High
effect(ability of the
eye to magnify the
indication)
Sensitivity Medium High
In weld inspection, lower sensitivity methods,
namely water Washable and solvent removable are
employed due to surface roughness of the weld.
Hence, use of high sensitivity penetrant namely
fluorescent dye penetrant may be preferred.

22. Preparation of parts
3.1.2 The possible contaminants on the components
are
1.Oil and grease
2. Rust or scales (Oxidation products)
3. Paints and conversion coatings
4. Carbon, Varnish etc
5. water
3.1.3 Sources
1. Fabrication processes and subsequent treatment
2. Surface protection against corrosion
3. Surface treatment for improvement of properties
3.1.4 Interference by contaminants
1.physical blocking of the discontinuity (eg
Rust or scale paint or conversion coatings)
2.disturbing the balanced composition of the
penetrant (eg. Oil, water etc)
3. Entering and occupying the discontinuities. Oil,
water etc)

23. Simple wiping
Vapour degreasing
1. Solvent cleaning Oil and grease
Ultrasonic cleaning
Acid cleaning
Rust, scale
2. Chemical cleaning Detergent cleaning oxide etc
Alkaline cleaning
Wire brushing, sand
blasting etc
3.Mechanical cleaning Rust, scale.
Oxide etc
Machining, grinding etc

24. Application of penetrant
Penetrant can be applied by
immersion, dipping ,spraying, swabbing and pouring.
The only requirement is that a thin layer of
penetrant should be present for the specified
time (dwell time) on the surface to be inspected.
Dwell time : The total time the penetrant is
contact with the test surface
including the time required for application and
for drain .
Dwell time = application time + drain time
Normally 5-30 mts. Depends on the size and
nature of discontinuity and the material and
surface condition of the material etc.

25. Removal of excess penetrant
The excess surface penetrant on the surface is
carefully removed without affecting the penetrant
that is residing in the discontinuity.
This is an important step as the unremoved excess
surface penetrant will affect subsequently by
affecting the contrast of the indication (excessive
background) and if any penetrant in the
discontinuity is disturbed the volume of the
penetrant indication will get reduced (The volume
of penetrant is already very small)
In any NDT method the Signal to Noise ratio is
important and this should be as high as possible
S- Volume of penetrant that has entered into the discontinuity
N- Unremoved excess surface penetrant in the discontinuity
It is obvious that in PT to achieve a high S/N ratio,
the excess penetrant on the surface should be
removed as completely as possible and the penetrant
is the discontinuity should not be lost by overwashing

26. Penetrant removal methods
Dissolve and remove (SR)
2. Disperse (emulsify) and remove by water PE
3. Simply water wash if penetrant contains built in
Emulsifier (WW)
In each case, care need be exercised so as to
completely remove the excess penetrant on the
surface completely and at the same time the
penetrant in the discontinuity is not affected.
Wash – don’t over wash .
In the case, welds, when need be inspected
without flushing the crown, the surface
roughness poses problems giving background
colouration thereby affecting the visibility of the
Indication. Hence Solvent removable and water
washable variations are normally employed.
Here overwashing tendency of these methods, is
taken to advantage, meaning, the surface is
much free from the background colouration.
Of course, the sensitivity suffers. Sensitivity
depends on the amount of retained penetrant in
the discontinuity

27. Developing an indication
3.5.0 DEVELOPING
After the removal of the excess surface penetrant ,
in developing step, the penetrant from the
discontinuity is brought to the surface so as to
form an visible indication.
3.6.1 Developer functions
The developer functions are
1.To assist the natural seepage of the penetrant in
the discontinuity and extract or blot out the
penetrant so as to form an visual indication
(mechanism of visual indication)
2. To provide a contrasting base which enhances
the detection an indication. Blue-black to
yellowish green: white to red
3. To spread the penetrant so as to increase the
apparent size of the indication
4. To mask some confusing indications

28. Mechanism of development
3.6.3 Mechanism of developing action :
Natural seepage is assisted and the absoptive
developers blot out more penetrant from the
discontinuity. The thickness of the penetrant layer
is increased to the levels above the threshold
visibility .( Some fluorescent penetrant indications
can be seen without developing with high
intensity black light -3000 microwatt/sq.cm)
Particles
Developer Absorbed
No
Developer Penetrant
Width
is
more

29. Type of developers and the developer
used in weld inspection
Developer has two variations: Dry and Wet. In
the case of wet, aqueous and non aqueous.
As the sensitivity of wet developers are better
than, dry developers and since in weld
inspection, less sensitive methods namely water
washable and solvent removable are employed ,
it is better to use wet developers (mostly
nonaqueous) in weld inspection in spite surface
roughness. Normally for high surface roughness,
dry powder is better suited .

30. Inspection, interpretation and evaluation
Developing action at the end of developer time
reveals an indication. In this step the indications
are interpreted as to the origin of the indication
namely whether True or false indications and if
true indication whether relevant or nonrelevant
and if relevant as to the nature of
discontinuities linear or volumetric.
Inspection environement depends on the type of
dye - Visible or fluorescent dye
white light for visible and black light
for fluoresecent
True indication Relevant
Non relevant
False indication Improper processing
Standard test specimen in PT
3. Aluminum comparator blocks – penetrant quality
2. Chrome plated steel specimen – sensitivity

31. 2.8.1 Inspection environment
Visible dye penetrant indications are inspected under white light
and fluorescent dye penetrant indications are viewed under
blach light (ultraviolet light?)
Electromagnetic radiation spectrum
Visible and ultraviolet spectrum
Visible light spectrum - 380 nm to 770 nm or ( V to R ) or
3800 Ao to 7000 Ao
Ultraviolet spectrum - 10 nm to 380 nm or 100 -3800 Ao
Black light spectrum - 320 nm to 380 nm or (with peak at
365 nm)
Hg arc vapour lamp produces ultraviolet light in addition to
visible light. The filters are used to remove the harmful portion
of UV (below 320 nm) and also visible light which affects the
fluorescence of the fluorescent dye. The filters that are
commonly used give black light with maximum peak intensity at
365 nm..

32. Colour and fluorescent colours
Colour – part of the white light that is reflected by the surface
that is flooded with the white light
Fluorescent colour – Emission of visible light (any colour) on the
impingement of high energy radiation on the surface.
In PT & MT, the impingement of black light (UV) leads to
emission of yellowish green light
Black light
Yellowish
green light
Fluorescent dye

33. Weld defects that can be detected by PT
The preponderance of weld joints are
fusion welded. The following
discontinuities can be detected.
4. Porosity
5. Lack of root penetration (on single side)
6. Lack of fusion (at toe of welds)
7. Cracks in heat affected zone
8. Crater and other cracks
Weldments at the site is inspected with
visible dye penetrant using solvent removable
method

34. Some hints on penetrant testing of welds
• Due to surface roughness of unflushed
welds, it is better to use, water washable and
solvent removable methods to avoid
undesirable background.
• In the case of Solvent removable pentrants,
usual precaution of wiping with solvent
moistened lint free cloth may not yield the
desired results because of excessive
background. Sometimes, the solvent is
directly applied to the weld to overcome this
difficulty. Of course fine cracks may be
missed
• In the case of TIG and MIG where better
surface is obtained, it is better to go solvent
wiping method
• Inspection with multiple pass welds with
penetrants requires that each weld bead is
inspected prior to the next pass to provide the
next the best reliability. Cleaning after each
testing

35. Hints for weld inspection (contd)
5.Water washable fluorescent penetrants
can be used on large pressure vessels or other
large structures by washing with a hose and air
drying.
Developent with solvent suspended developer
can provide good sensitivity
Water washable penetrants can be washed more
readily from most weld
6. It is important to remove all the of the slag from
welds before penetrant testing.
Grinding off the ripples on rough welds is
desirable. Grinding should not smear the metal
7. Brazed joints can discontinuities similar to weld
beads that can be detected by PT.
A braze that does not wet the surface is
indication of a poor joint.

36. Selection guide for NDT
Method Application Advantages Disadvantage
Visual Surface Economical test Limited to visual
discontinuities acuity of the
inspector
Liquid Surface cracks Relatively Cleaning is needed
penetrant and porosity inexpensive & before and after
Reasonably fast inspection .surface
films hide
discontinuities
Magnetic Surface and Relatively Applicable only
particlei subsurface economical and ferromagnetic
discontinuities expedient materials
Radiographic Volume Provides Plane
discontinuities permanent record discontinuities
Surface & must be favourably
subsurface aligned with
radiation bean
Ultrasonic Most Sensitive to Small thickness
discontinuities planar plate may be
discontinuities. difficult to inspect.
High penetration Requires skilled
capability operator
Eddy current Surface and Painted or coated Many variables
testing subsurface can surface signal affect the test
be inspected High speed