liquid penetrant testing lecture
DESCRIPTION
This lecture is about fundamentals of liquid penetrant testing. Liquid penetrant testing is one of a nondestructive testing technique.TRANSCRIPT
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وزدني علما
Nondestructive Evaluation of Structures
MEC 4867
Dr. Meftah Hrairi
Lecture 7
Liquid Penetrant Testing Fundamentals
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Outline
Introduction
Principles
Materials
Project discussion
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Introduction
Penetrant testing (PT) is one of the most widely used nondestructive testing methods for the detection of surface discontinuities (such as cracks, laps and folds) in nonporous solid materials: most commonly used surface NDT method today because it can be applied
to virtually any magnetic or nonmagnetic material (complementary to MT).
provides industry with a wide range of sensitivities and techniques that make it especially adaptable to a broad range of sizes and shapes.
useful for examinations that are conducted in remote field locations, since it is extremely portable.
appropriate in a production type environment where many smaller parts can be processed in a relatively short period of time.
builds on the principle of Visual Inspection.
increases the “seeability” of small discontinuities that the human eye might not be able to detect alone.
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How Does PT Work ?
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 is applied to pull trapped penetrant back to the surface.
With good inspection technique, visual
indications of any discontinuities
present become apparent.
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Basic Process of PT
1) Clean & Dry Component
2) Apply Penetrant 4) Apply Developer
3) Remove Excess 5) Visual Inspection
6) Post Clean Component
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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.
Some examples: − Plants and trees draw water up
from the ground to their branches
and leaves to supply their
nourishment.
− The human body has miles of
capillaries that carry life sustaining
blood to our entire body.
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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.
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What can be Inspected via PT ?
Almost any material that
has a relatively smooth,
nonporous surface
on which discontinuities
or defects are suspected.
PT can be used on any
material which has a
non-absorbent surface.
PT is generally found to be more sensitive than X-radiography or ultrasonics for fine surface cracks.
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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 coating
that prevent penetrants
from entering defects.
Defect indications become less distinguishable as the background “noise” level increases.
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Choices of Penetrant Materials
Penetrant Type
I - Fluorescent
II - Visible
Remover Removal Method
A Water Washable
B Post-emulsifiable, Lipophilic
C Solvent Removable
D Post-emulsifiable, Hydrophilic
Developer Form
Dry Powder
Wet, Water Soluble
Wet, Water Suspendable
Wet, Non-Aqueous
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Penetrant Material
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.
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Penetrant Materials
The characteristics of a good penetrant liquid are therefore related to its surface tension, density and wetting properties rather than viscosity.
Penetrants are solutions of colored or fluorescent dyes in oil-based liquids.
Penetrants can be classified as: Fluorescent
visible dye
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Penetration of a Liquid into a Crack
Penetrant materials are often pulled into surface breaking defects by capillary action.
The capillary force F driving the penetrant into the crack is a function of the surface tension of the liquid-gas interface, the contact angle, and the size of the defect opening:
F = 2 π r σLV cosθ
r = radius of the crack opening (2 π r is the line of contact between the liquid and the solid tubular surface.)
σLV = liquid-gas surface tension
θ = contact angle
The capillary force balances
the weight of the liquid so that
h = 2 σLV cosθ / r g ρ
σLV
σSV σSL Solid
Vapor
2r
h
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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 ½ - Ultra Low Sensitivity
− Level 1 - Low Sensitivity
− Level 2 - Medium Sensitivity
− Level 3 - High Sensitivity
− Level 4 - Ultra-High 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.
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Visible vs Fluorescent PT
Inspection can be performed using
visible (or red dye) or fluorescent
penetrant materials.
Visible PT is performed under white light. All are in the level 1 sensitivity range.
Fluorescent PT must be performed using an ultraviolet light in a darkened area. 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.
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Penetrant Removal Method
The excess penetrant can be removed in four different ways
1. by water only
2. by emulsifier which is oil soluble (lipophilic), followed by water.
3. by a liquid solvent
4. by water, followed by a penetrant removal solution (detergent) which is
water soluble (hydrophilic), followed by water.
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Penetrant Removal Method
Penetrants are also classified by the method of removing the excess penetrant. 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.
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.
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 react
with the penetrant on the surface but not the
penetrant trapped in the flaw.
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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.
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Penetrant Inspection System
Penetrant systems can therefore be classified as:
1. water-washable
2. post-emulsifiable (i.e. the excess penetrant is emulsified for removal)
3. solvent-removable
The appropriate process to be used on any specific application is based on: 1 The flaw sensitivity required.
2 The surface finish of the component.
3 The compatibility of the materials with the component.
4 The size, shape and accessibility of the region to be inspected.
5 The ultimate use of the component.
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Penetrant Inspection System
Water-removal processes are more suitable for routine high-volume production inspection.
For maximum sensitivity, post-emulsifier fluorescent penetrants with a hydrophilic solution remover are generally preferred.
Often, the final choice of a penetrant system is made in terms of the most suitable penetrant remover related to surface finish, compatibility, flash point, etc.
Some aerospace companies have compared penetrant materials and systems by producing a series of similarly cracked specimens, such as ex-service turbine blades with leading-edge cracks.
The results of comparisons of different processes on these are valuable at the time of the test, but rapidly become out-of-date as newer penetrant materials are marketed.
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Penetrant Inspection System
Equipment available can be divided into three types, as follows:
1. Portable kits are used for carrying out inspection of small areas, for use
on site; these often contain the materials to be used in aerosol form.
2. Fixed installations are used for testing components on a continuous basis,
with a series of processing stations in sequential order to form a flow line.
Increasingly, these are being automated to provide automatic component
handling and timing.
3. Self-contained processing booths are used for testing large components
which cannot be moved during testing.
Portable Penetrant System Stationary Penetrant System
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Verification of Penetrant System Performance
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
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Penetrant Technique Classification Summary
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Penetrant Technique Classification Summary
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Penetrant Technique Classification Summary
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Project Discussion
Next Topics
Liquid penetrant
Further Readings Handouts:
– Liquid penetrant
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