Module 1
Introduction
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In modern days, with rapidly growing industrial demand, it is necessary that
products should be reliable.
Buyers buy a product with some expectation that the product will perform well and
give hassle free service for a stipulated period of time.
Reliability of a machine is also dependent on the individual reliability of
components.
Reliability comes through improving quality of level individual components.
Non-destructive testing (NDT) plays an important role.
Allows evaluation of defects in various materials/components.
Characterization of material properties.
Leads to confidence in material being used.
Opt for lower factor of safety.
Improve reliability.
Used in aerospace, aircraft, nuclear establishments, power plants, etc.
Synonymous terms are non-destructive evaluation (NDE) and non-destructive
inspection (NDI)
Definition
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NDT may be defined as those testing methods in which the material under
inspection is not destroyed.
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The future usefulness of the material under test is not destroyed.
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Contrary to mechanical testing, where a material is made to fail/fracture.
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In NDT, the material retains all its properties, and the component can be used for
purposes it was intended for.
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Common NDT techniques include:
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Visual inspection
Liquid penetrant inspection
Magnetic particle testing
Eddy current testing
Radiography
Ultrasonic testing
Thermography
Advantages and Disadvantages
Advantages:
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Tests are made directly on the component or object.
Many NDT techniques can be applied to a single part.
So, all desired properties may be evaluated.
In-service testing is possible.
Repeated checks over a period of service is possible on a part.
Very little preparation is sufficient.
Most test methods are rapid.
Disadvantages
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Reliability needs to be verified at times.
Skilled judgement and experience is required to interpret results and indications.
Steps Involved
1.
Preparation of test surface.
2.
Application of testing medium/signal.
3.
Modification of testing medium/signal due to presence of defects.
4.
Conversion of modulated/changed signal into convenient form.
5.
Interpretation of results obtained.
6.
Verification of test results.
Types of Flaws
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Defects which may be introduced du ring the manufacture of raw materials or the
production of castings.
– Stress cracking
– Shrinkage porosity
– Gas porosity
– Slag inclusions
– Segregation
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Defects which may be introduced during the manufacture of components
– Machining faults
– Heat treatment defects
– Welding defects
– Residual stress cracks
Types of Flaws Contd…
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Defects which may be introduced during component assembly
– Missing parts
– Incorrectly assembled parts
– Additional welding defects
– Additional stress cracking
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Defects generated during service life
– Fatigue
– Corrosion
– Corrosion + fatigue
– Stress corrosion
– Wear
– Creep
Factors Influencing Reliability
1. Human Factors: Education, Training, experience, working environment.
2. Testing Methods: Adequacy of method, standardization.
3. Test Object: Complexity of shape, accessibility of part in assembly, surface
condition.
4. Nature of defect: Location, shape, size, volume dispersion.
5. Knowledge base and facilities: Understanding of mechanism of interaction
between test object and information generating tool.
6. Risk factors: Probability of defect detection, statistical data for reliable decision
making.
Main NDT System Features
System
Features
Applicability
Liquid
penetrant
Detection of defects which break
the surface
Can be used for any metal,
many plastics, glass and glazed
ceramics
Magnetic
particle
Detection of defects which break the surface Can only be used for
and sub-surface defects close to the surface
ferromagnetic materials (most
steels and irons)
Electrical
methods
(Eddy
currents)
Detection of surface defects and some sub- Can be used for any metal
surface defects. Can also be used to measure the
thickness of a non-conductive coating, such as
paint, on a metal
Ultrasonic
testing
Detection of internal defects but can also detect Can be used for most materials
surface f1aws
Radiography Detection of internal defects, surface defects Can be used for many materials
and the correctness of part assemblies
but there are !imitations on the
maximum material thickness
Support Stages in NDT
Activity/Stage
Design
and
development
NDT Support
product 1. Statistical evaluation of strength, stiffness and dimensional
features.
2. Detection, location, sizing and volume dispersion of defects.
3. Assessment of variation in material homogeneity, isotropy,
residual stresses.
Manufacturing stage
1. Assessment of repeatability of manufacturing processes.
2. Detection, location, sizing and volume dispersion of defects.
3. Providing acceptance criteria for defects and material
inhomogeneity.
Life cycle management
1. Estimation of variation in material homogeneity, dimensions
due to corrosions, erosion, fatigue, creep, impact.
2. Generating defect-property correlating data and specific
information related to life extension of components.
3. Preparation of specifications/documents/test techniques for
periodic NDT inspection for health monitoring.
4. Determine the stage of damage in a structure till it reaches
critical stage of rejection and replacement.
Visual and Optical Inspection
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One of the oldest and simplest NDT technique.
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Easy to apply, quick in interpretation, low cost.
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Commonly used for first inspection of any object.
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Visual inspection of a component by an experienced inspector can reveal the
following information:
– General condition of the component.
– Presence or absence of any oxide film or corrosive product.
– Presence or absence of any cracks, orientation of any cracks and position of cracks
relative various zones in case of welds.
– Surface porosity, unfilled craters, contour of weld beads, etc.
– Potential source of mechanical weakness such as sharp notches or misalignments.
– Results of visual inspection may be of great assistance to other tests
Visual and Optical Inspection Contd…
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Cleaning of the object surface is necessary and very important.
Sandblasting or shot blasting may be used.
Inspection area should be properly illuminated.
Some optical aids are:
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Mirrors
Magnifying lenses
Microscopes
Periscopes
Telescopes
Endoscopes
Flexible fibre optics boroscopes
Applications include:
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Inspection of plant components for any leakage.
Misalignment of parts.
Corrosion, erosion, cracks, fracture, etc.
Defects in weldments.
Visual and Optical Inspection Contd…
Porosities in Ni-B-W coating
Liquid Penetrant Inspection
Liquid Penetrant Inspection
Introduction:
• Liquid penetrant inspection is a technique which can be used to detect
defects in a wide range of components, provided that the defect breaks the
surface of the material.
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The principle of the technique is that a liquid is drawn by capillary
attraction into the defect and, after subsequent development, any surfacebreaking defects may be rendered visible to the human eye.
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In order to achieve good defect visibility, the penetrating liquid will either
be colored with a bright and persistent dye or else contain a fluorescent
compound.
Liquid Penetrant Inspection
Introduction contd…:
• In the former type the dye is generally red and the developed surface can be
viewed in natural or artificial light. In the latter case the component must be
viewed under ultra-violet light if indications of defects are to be seen.
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It can be used to indicate the presence of defects such as cracks,
laminations, laps and zones of surface porosity in a wide variety of
components.
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The method is applicable to almost any component, whether it be large or
small, of simple or complex configuration, and it is employed for the
inspection of wrought and cast products in both ferrous and non-ferrous
metals and alloys, glassware and some polymer components.
Components
Almost any material that has a
relatively smooth, non-porous
surface on which discontinuities
or defects are suspected.
SURFACE BREAKING DEFECTS
Components
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.
Components difficult to detect
• Components with rough surfaces, such as
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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.
Principle
• 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
applied to pull trapped penetrant back the surface.
• Developer provides a contrasting
background for visual indications
of any discontinuities present
become apparent.
developer (blotter) is
Stages of LPI
Principle / Stages:
a) Surface preparation.
Material surface clean
and grease-free
Penetrant absorbed
into defect
b) Application of
penetrant.
c)
Removal of excess
penetrant.
Excess penetrant removed,
but liquid remains in defect
Developer applied
to surface
d) Development.
e)
Observation and
inspection.
Penetrant
absorbed
into developer giving
indication of defect
Working of LPI
• Surface Tension : An elastic force that acts tangential to the fluid surface to reduce
the area is called surface tension
• 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
Working of LPI
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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.
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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.
Wetting of different fluids. A shows a
fluid with very little wetting, while C
shows a fluid with more wetting.
Working of LPI
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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°
low wettability
θ = 180°
perfectly
non-wetting
•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
Working of LPI
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°
Working of LPI
• 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
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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 :
• Cohesive force< Adhesive force :
Convex surface -> liq. fall below
Concave surface -> liq. rise up
Finer Tube, greater liquid rise
finer defect (hairline) -> greater indication
Penetrability
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For cylindrical volume capillary pressure P = 2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.
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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 penetration
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
• Viscosity, 
– Not significantly affect penetrant ability
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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
Distance travelled
by a liquid in a
uniform capillary
cross-sec
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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 during the dwell
cycle
Liquid Penetrant Inspection
Surface preparation
• All surfaces of a component must be thoroughly cleaned and completely dried
before it is subjected to inspection.
• It is important that any surfaces to be examined for defects must be free from oil,
water, grease or other contaminants if successful indication of defects is to be
achieved.
Application of penetrant
• After surface preparation, liquid penetrant is applied in a suitable manner, so as to
form a film of penetrant over the component surface.
• The liquid film should remain on the surface for a period sufficient to allow for full
penetration into surface defects.
Liquid Penetrant Inspection
Removal of excess penetrant
• It is now necessary to remove excess penetrant from the surface of the component.
• Some penetrants can be washed off the surface with water, while others require the
use of specific solvents.
• Uniform removal of excess penetrant is necessary far effective inspection.
Liquid Penetrant Inspection
Development
• The development stage is necessary to reveal c1early the presence of any defect.
• The developer is usually a very fine chalk powder.
• This may be applied dry, but more commonly is applied by spraying the surface
with chalk dust suspended in a volatile carrier fluid.
• A thin uniform layer of chalk is deposited on the surface of the component.
• Penetrant liquid present within defects will be slowly drawn by capillary action into
the pores of the chalk.
• There will be some spread of penetrant within the developer and this will magnify
the apparent width of a defect.
• When a dye penetrant is used the dye color must be in sharp contrast to the uniform
white of the chalk-covered surface.
• The development stage may sometimes be omitted when a fluorescent penetrant is
used.
Liquid Penetrant Inspection
Observation and inspection
• After an optimum developing time has been allowed, the component surface is
inspected for indications of penetrant 'bleedback' into the developer.
• Dye penetrant inspection is carried out in strong lighting conditions, while
fluorescent penetrant inspection is performed in a suitable screened area using ultraviolet light.
• The latter technique causes the penetrant to emit visible light, and defects are
brilliantly outlined.
Liquid Penetrant Inspection
The following penetrant characteristics are desired:
• The penetrant must have the ability to enter extremely fine surface defects or other
openings in the component under test.
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The penetrant must have a good surface wetting ability and be able to maintain a
surface film on the component, and hence, continue to feed into a defect over a
considerable period of time.
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The penetrant should have the ability to drain away from the component well but
with a minimum amount of dragging-out of penetrant from within defects.
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If necessary, the penetrant should be capable of dissolving a path into contaminated
defects, through a wide range of contaminants.
Liquid Penetrant Inspection
The following penetrant characteristics are desired:
• The liquid penetrant should be stable over a wide range of temperature and
humidity and should not form a scum or lose its volatile constituents while it is kept
in open tanks or when stored in drums.
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It must be possible to remove excess penetrants from component surfaces easily
without affecting the penetrant within any defects.
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A penetrant must resist drying out, and complete bleed out, during hot-air drying of
the component after the wash operation has been completed.
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The dye used in dye penetrants should be such that a good, deep colour can be
given to the penetrant by a comparatively small amount of dye.
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Red is the most commonly used colour in dye penetrants as this colour is the most
readily seen by the human eye.
Liquid Penetrant Inspection
The following developer characteristics are desired:
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Absorption A developer must be easily wetted by the penetrant at the flaw, and
highly absorptive to draw the maximum amount of penetrant from the defect.
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Application It must be easy to apply and capable of forming a thin uniform surface
coating. In addition, it must be easy to remove after inspection.
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Background masking It must be capable of effectively masking out interference
from background colours, and capable of providing a contrasting background for
indications, especially when coloured penetrants are used.
Liquid Penetrant Inspection
The following developer characteristics are desired:
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Physical characteristics It must have a grain size and a particle shape that will
disperse the penetrant at the flaw, so that a clearly defined indication is attained,
without excessive spread.
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In addition, the material must be neither hygroscopic nor excessively dusty.
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Chemical characteristics It must not contain ingredients which may be harmful to
either the parts being inspected or to the operator.
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Dry powders, aqueous powder suspension, solvent suspendible and water soluble.
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Applied by spray, immersion, passing through a developer dust cloud chamber.
Liquid Penetrant Inspection
The following cleaner characteristics are desired:
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Acetone or carbon tetrachloride based fluid may be used.
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For bigger components cleaning may be done by dipping in a series of tanks
containing hot water, soap solution, detergents, cold water and chemical solution.
Liquid Penetrant Inspection
Water-washable system:
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This system (using a fluorescent or visible dye penetrant) is designed so that the
penetrant can be directly removed from the component surface by washing with
water.
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It is extremely important, however, to maintain a controlled washing operation,
especially where the removal of excess penetrant is by means of water sprays.
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Optimisation of the processing conditions:
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Water pressure and temperature
Duration of rinse cyde,
Surface condition of the workpiece
The inherent removal characteristies of the penetrant.
Even so, it is possible that penetrant may be washed away from small defects.
Liquid Penetrant Inspection
Post-emulsification system:
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When it is necessary to detect minute defects, high-sensitivity penetrants that are
not water washable are usually employed.
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Such penetrants have an oil base and require an additional processing step.
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An emulsifier is applied after the penetrant has had sufficient time to be absorbed
into defects.
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The major advantage of this system is that the emulsifier renders the excess
penetrant soluble in water, and hence, capable of being rinsed away.
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The system is more expensive, because of the penetrant-emulsifier costs and the
additional time required for the operations.
Liquid Penetrant Inspection
Solvent-removable system:
• It is often necessary, to inspect only a small area of a component, or to inspect a
component in situ.
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Solvent-removable penetrants are widely used for such situations.
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Normally, the same type of solvent is used both for pre-cleaning and for the
removal of excess penetrant.
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There are two basic solvent types: flammable and non-flammable.
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The flammable cleaners are potential fire hazards but are free from halogens, while
the non-flammable cleaners are halogenated solvents, but have high toxicity.
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The excess surface penetrant is usually removed by wiping the component with a
lint-free cloth moistened with solvent.
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The costs are relatively high, because of high material expense and the fact that it is
a more labor-intensive process.
Liquid Penetrant Inspection
Advantages of LPI:
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The liquid penetrant process is comparatively simple.
The equipment necessary is cheaper than that required for other non-destructive
testing systems.
Portable kit (aerosol spray cans).
High sensitivity to small surface defects.
Large areas can be inspected at low cost.
Parts with complex shape can be routinely investigated.
Liquid Penetrant Inspection
Disadvantages of LPI:
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Only surface flaws can be detected.
Only non-porous surfaces can be inspected.
False indications, since each pore will register as a potential defect.
Pre-cleaning is critical.
Wide shallow cracks are difficult to detect.
Homework
Range of Applications of LPI and examples.
Cleaning Methods
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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.
Cleaning Methods
• 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:
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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
Before
Cleaningcleaning
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
Solvent
Drip Tray
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
Thank You