Non-Destructive Testing
2. LIQUID PENETRANT TESTING
Introduction
Liquid Penetrant Testing is one of the oldest and simplest Non-Destructive Testing methods.
This method is used to reveal surface discontinuities by bleedout of a coloured or fluorescent
dye from the flaw. It is used in inspection of all non-porous materials
Basic Principle
The Liquid Penetrant Testing works on the principle of Capillary action i.e. rise and fall of
liquid. The settling down of penetrant into the discontinuities is called Capillary fall and
rising of penetrant from the discontinuities by the application of developer is known as
Capillary rise.
Fig.2.1 Principle of LPT
Penetrants
Penetrants are carefully formulated to produce the level of sensitivity desired by the
inspector.
Characteristics of Penetrants
Penetrants spread easily over the surface of the material being inspected to provide
complete and even coverage.
They must draw into the surface breaking defects by capillary action.
The penetrant must remain in the defect but should be easily removed easily from the
surface of the material.
They should not be harmful to the material being tested or to the inspector
Basic types of Penetrants
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Fluorescent Penetrants
Visible Penetrants
Methods used for excess removal of Penetrants
Water washable
Solvent removable
Post-Emulsifiable
Lipophilic
Hydrophilic
Fig. 2.2 Penetrant application and removal process
Application of Penetrants
By spraying
By brushing
By pouring the penetrant on material(when large work material is used
By dipping the material into the penetrant
Developers
The role of the developer is to pull the trapped penetrant material out of defects and spread it
out on the surface of the part so it can be seen by the inspector.
Developers used with Visible penetrants create a white background so there is a great
degree of contrast between the indication and the surrounding background
Developers used with Fluorescent penetrants both reflect and refract the incident UV
light, allowing more of it to interact with the penetrant , causing more efficient
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fluorescence. Under UV rays , the defects are visible as green-yellow colour whereas
the developer is visible as black-blue colour.
Characteristics of Developers
The developers should provide a contrast background for indication and should be
visible.
They should form a uniform coating over surface
They should have high absorption
Developers viscosity should be less
Developers should be non-toxic.
Basic types of Developers
Dry developers
Water soluble developers
Water suspendable developers
Non-aqueous developer
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Fig. 2.3 Developer application and obtaining of indication
Basic steps involved in Liquid Penetrant Testing
1. Pre- Cleaning
It is the most critical step in Liquid Penetrant Testing
All coatings such as paints, varnishes, heavy oxides must be removed to
ensure that defects are open to surface of the part
Processes like machining, sand blasting, steam cleaning can cause metal
smearing. This layer of metal smearing must removed before inspection.
Various methods of cleaning are:
i. Water washing
ii. Solvent or Detergent cleaning
iii. Vapour degreasing
iv. Steam cleaning
v. Ultra-sonic cleaning
vi. Sand blasting
vii. Shot or Abrasive blasting
2. Penetrant application
The penetrant material can be applied in a number of ways, such as spraying,
brushing or immersing the part in a penetrant bath
The penetrant must entirely cover the part of the material to be tested
3. Penetrant Dwell Time
It is defined as the time taken by the penetrant to settle inside the cracks of the
material to be tested
Penetrants require a dwell time of 5-10 minutes
The dwell time depends upon:
i. The contact angle of the penetrant
ii. The capillary pressure at the flaw opening
iii. The specific gravity of the penetrant
4. Excess Penetrant Removal
The penetrant removal procedure must effectively remove the penetrant from
the surface of the part without removing an appreciable amount of entrapped
penetrant from the discontinuity
Excess penetrant removal methods
i. Water washable
ii. Penetrant removable
iii. Post-Emulsifiable
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a. Lipophilic: The emulsifier is oil-based and interacts with the oil
soluble penetrant to make removable possible
b. Hydrophilic: The emulsifier is water soluble detergent which
lifts excess penetrant from surface of part with a water wash
5. Developer Application
The main function of the developer is to provide a contrast background for
indications and the developer must be bright
Commonly a white contrast developer is used
Types of developers are:
i. Dry developers
ii. Water soluble developers
iii. Water suspendable developers
iv. Non-aqueous developers
Non-aqueous developers are generally recognized as the most sensitive when
properly applied
6. Developing Time
It is defined as the time taken by developer to pull the penetrant out from the
defect to make an indication
Developing time is around 2-5 minutes
7. Result and Interpretation
Results:
i. False Indications: It can be caused by improper cleaning. It should be
eliminated by using proper lint-free clothes and air covers
ii. Non-Relevant Indications: These are caused due to geometrical
changes in the part
iii. Relevant Indications: These are produced by actual discontinuities in
the part
a. Acceptable: If the indications matches the codes and standards
b. Reject-able: If the indications does not matches the codes and
standards
Interpretations(Weld Discontinuities):
i. Tungsten Inclusions: This is caused when the filler metal comes in
contact with the tip of tungsten electrode. This discontinuity is located
in sub-surface of material
ii. Offset or Mismatch: This is caused where two pieces being welded
together are not properly aligned. This discontinuity is located on
surface of material
iii. Internal Undercut: This is caused by over-sized weld pool. This
discontinuity is located in sub-surface of material
iv. Lack of Fusion: This is caused due to the failure of filler metal to join
with base metal. This discontinuity is located in sub-surface of material
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v. Lack of Penetration: It is caused due to the failure of penetration of
weld joint root by weld metal. This discontinuity is located on surface
and in sub-surface of material
vi. Slag Inclusions: It is caused due to improper cleaning of previous weld
parts and mixing of oxides on base metal surfaces into the weld pool.
This discontinuity is located in sub-surface of material
vii. Cluster Porosity: This is caused when vapourised constituents in
molten weld metal are entrapped during solidification. This
discontinuity is located on surface and in sub-surface of material
viii. Porosity: This is caused when vapourised constituents in molten weld
metal are entrapped during solidification. This discontinuity is located
on surface and in sub-surface of material
ix. Suck Back: this is caused when weld metal has contracted to its root
and has been drawn up into the root of metal. This discontinuity is
located on surface and in sub-surface of material
Fig. 2.4 Various discontinuities obtained on a weld part
8. Post Cleaning
It is the method of cleaning the material after inspection
9. Final Report
The final report is obtained by providing the photo copy of the procedure or by
drawing
Emulsifier
The function of the emulsifier is to remove the excess of penetrant from the
surface of the material
Emulsifier is give enough time to react with penetrant, but not enough time to
diffuse into the penetrant trapped in the defect
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When there is excess penetrant, Post-Emulsification method is used
In this method, emulsifier is applied on the part and given some time for
dwelling
Functions of Emulsifiers
Penetrant inside the defect should not be over-washed
Excess penetrant should be removed easily
Emulsifier should have good sensitivity
Types of Emulsifiers
Lipophilic Emulsifiers
Hydrophilic Emulsifiers
Basic Steps involved in Post-Emulsification Method
1. Pre-Cleaning
2. Penetrant Application
3. Penetrant Dwell Time
4. Application of Emulsifier
5. Emulsifier Dwell Time
6. Excess Penetrant Removal
7. Developer Application
8. Developing Time
9. Result and Interpretation
10. Post Cleaning
11. Final Report
Safety Precautions
Two types of problems may cause to the inspector
Skin Irritation
Precaution: Wearing gloves, hand crings
Breathing problem/Air pollution
Precaution: Wearing masks and install exhaust fans
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Fig. 2.5 A Inspector carrying out LPT process with wearing safety equipments
Chemical Activity
Penetrant should be non-corrosive towards the material which is being tested and the
containers in which they are stored
Penetrant with Chlorine, Sulphur and Fluorine are usually restricted with Titanium
and high Nickel alloys
Wetting Ability and Contact Angle
The ability of the liquid to wet the surface of material is called as wetting ability
Liquids having good wetting ability have very low contact angle
Contact angle is represented by ’’
Liquids having contact angle less than 90, have a good wetting ability
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Fig. 2.6 Wetting ability and contact angle diagram
Advantages of Liquid Penetrant Testing
High sensitivity i.e., small discontinuities can be detected
Rapid inspection of large areas and volumes
Suitable for parts with complex shapes
Indications are produced directly on the surface of the part and constitute avisual
representation of the flaw
Low cost i.e., the materials and related equipment are relatively inexpensive
Dis-advantages of Liquid Penetrant Testing
Only surface defects can be detected
Pre-cleaning is critical since contaminants can mask defects
Only materials with a relatively non-porous surface can be inspected
Surface finish and roughness can affect inspection sensitivity
Applications of Liquid Penetrant Testing
This method is used in the inspection of grinding, casting, welding defects
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