1.
Introduction
Liquid Penetrant Testing (LPT) is one of the oldest Non-Destructive Testing
(NDT) techniques.
It is used to detect surface defects in a variety of materials.
Only applicable to non-porous materials like:Metals and alloys,Non-metals
(plastics, polymers, glassware)
LPT relies on capillary action to locate surface discontinuities.
2. Capillary Action
It is the ability of a liquid to flow into narrow spaces (like cracks) without
external force assistance.
The process involves cohesive and adhesive forces acting on the liquid.
3. Steps of Penetrant Testing
3.1 Pre-Cleaning
3.2 Penetrant Application
3.3 Penetrant Dwell Time
3.4 Excess Penetrant Removal
3.5 Developer Application
3.6 Development Time
3.7 Interpretation
3.8 Evaluation
3.9 Post-Cleaning
4. Dye Penetrant Testing Process
4.1 Clean the contaminated test piece.
4.2 Apply penetrant and allow dwell time.
4.3 Perform intermediate cleaning (spraying, brushing, or dipping).
4.4 Apply developer and allow developer time.
4.5 Conduct inspection and documentation.
5. Important Points About LPT
It's having high sensitivity; we can find the defects up to 0.01 inch.
We can find those defects which are open to the surface with LPT.
Removal of paint coating is necessary.
LPT can be applied only on Non porous material.
We can apply LPT on wide range of material.
6. Penetrant Properties
Wetting Ability: The ability to uniformly spread across the surface.
Contact Angle: Measures liquid interaction with a solid surface (should be
<5° for better wetting).
Viscosity: The resistance of a fluid to motion.
Volatility: The liquid’s tendency to evaporate.
Flash Point: The lowest temperature at which a flammable liquid produces
vapors to ignite.
Chemical Compatibility: Penetrant must be compatible with the test object
(max 1% chlorine & sulfur).
Temperature Stability: Ability to maintain properties under varying
temperatures.
7. Advantages of LPT
High sensitivity (detects small defects).
Works on diverse materials (metallic/non-metallic, magnetic/non-
magnetic).
Fast inspection for large areas and volumes.
Suitable for complex shapes.
Provides a visual representation of flaws.
Portable (available in aerosol cans).
Cost-effective (affordable materials and equipment).
Step 1 - Pre-cleaning
This is the most important step in the PT process.
Parts must be free of dirt, rust, scale, oil, grease, etc., for a reliable
inspection.
The cleaning process must remove contaminants from the part's surfaces
and defects without masking any defects.
Some machining and surface finishing can cause metal smearing, covering
the defect.Etching may be required before inspection.
Step 2 - Penetrant Application
Several application methods are available:
Brushing
Spraying
Dipping/Immersing
Flowing
Step 3 - Penetrant Dwell Time
Dwell time is how long the penetrant remains in contact with the part.
It is usually recommended by the penetrant producer or required by
specific guidelines.
Factors affecting dwell time:
Application method
Penetrant material
Material form being inspected
Type of discontinuity
Typical dwell times range from 5 to 60 minutes.
Penetrant Materials
A well-performing penetrant must:
Spread easily over the surface.
Be drawn into surface-breaking defects by capillary action.
Stay in the defect but be easily removable from the surface.
Be highly visible or fluoresce brightly.
Be safe for the inspector and tested materials.
step 4 - Excess Penetrant removal.
. This is the most delicate part of the inspection procedure because the
excess penetrant must be removed from the surface of the sample while
removing as little penetrant as possible from defects.
· The removal technique depends upon the type of penetrant used-
· Water Washable Penetrant = water washing
· Solvent Removable Penetrant = Solvent Cleaning
· Post Emulsifiable Penetrant = Post Emulsification
Penetrant Removal Process
Method A: Water-Washable
Method B: Post-Emulsifiable (Lipophilic)
Method C: Solvent-Removable
Method D: Post-Emulsifiable (Hydrophilic)
Penetrant Systems According to Sensitivity Criteria
Water Washable
Post-Emulsifiable
Solvent Removable
Sensitivity Levels
Penetrants come in various sensitivity levels:
Level 4: Ultra-High Sensitivity
Level 3: High Sensitivity
Level 2: Medium Sensitivity
Level 1: Low Sensitivity
Level ½: Ultra-Low Sensitivity
Higher sensitivity levels increase the likelihood of detecting small defects but
also produce more non-relevant indications.
Step 5 - Developer Application
A thin layer of developer is applied to draw trapped penetrant back to the
surface for visibility.
The method depends on the developer type:
Dry
Wet
Aqueous Wet
Non-Aqueous Wet
Step 6 - Development Time
Theoretical development time: 7 to 30 minutes
Optimal development time: 10 minutes
Correct development time provides sharp and distinct indications.
Excessive development time causes wider and diffused indications.
Step 7 - Interpretation
Measurement of indications: Evaluate the cause and nature of detected
discontinuities.
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.
a water spray is used to remove the excess penetrant.
pressure (typically not more than 40 psi)
water temperature(typically 50-100°F)
(Image with Process Flow)
Apply Penetrant 4 to 52°C
Spraying Dipping Brushing Flooding
Dwell Time
10 Mins 20 Mins [<10°C]
Wash Rinse 50 - 100°F 25 - 40 PSI *Control Overwashing
Non Aqueous Developer Dry < 160°F
Solvent Removable Method-C
Solvent Removable Penetrant Are oily Penetrants that, doesn't have an
emulsifying agent and are removed only by solvents which specially
designed for that purpose.
Excess surface penetrant is removed by wiping
First The part is wiped with a clean dry lint-free cloth to remove the bulk of
the excess penetrant.
Again use other clean dry lint-free cloth slightly moistened with solvent
cleaner/remover to remove the remaining excess penetrant.
It is not recommended that excess surface penetrant be removed by
flooding the surface with solvent cleaner/remover or direct action,
Because the solvent will dissolve the penetrant within the defect and
indications will not be produced.
Dwell time chart :
Dwell time is the time allowed for penetrant to enter discontinuities.
Tight discontinuities require longer dwell time.
Suggested timing for some common product forms :
Typical Dwell Time Chart
Aluminu Magnesi Ste
Form Type of Flaw m um el
Casting Porosity, Cold Shut 5 5 10
Forgings Laps 10 10 10
Weldmen Lack of Fusion,
ts Porosity 5 10 20
All Forms Cracks 10 10 20
* At a temperature range of 16 to 52℃
