Metallurgy

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Nondestructive Inspection
 It is an examination of an object in a manner which will not
impair the future usefulness of the object.
 Does not provide a direct measurement of mechanical
properties of the object.

 Very valuable in locating material defects that could impair the
performance of the object when placed in service.
 Common reasons for performing nondestructive inspections (NDI):






To detect faulty material before it is formed or machined into component parts
To detect faulty component before assembly
To discover defects that may have developed during service
For routine examination in service, permitting their removal before failure occurs
To improve and control manufacturing process to make products more reliable,
safe and economical.

 Five basic elements in any nondestructive inspection:
 SOURCE
provides a probing medium that can be used to inspect the item under test
 MODIFICATION
the probing medium must change or be modified as a result of the variations
or discontinuities within the object being tested
 DETECTION
a detector capable of determining the changes in the probing medium
 INDICATION
a means of indicating or recording the signals from the detector

 INTERPRETATION
a method of interpreting these indications

Lec 32, Page 2/18

Classification of
Nondestructive Inspection
 Five most common nondestructive inspection methods:






Radiographic inspection
Magnetic particle inspection
Die penetrant inspection
Ultrasonic inspection
Eddy current inspection

Radiographic Inspection
 Radiography uses penetrating radiation
that is directed towards a component.

 The component stops some of the radiation.
The amount that is stopped or absorbed is affected by
material density and thickness differences.

 These differences in “absorption” are recorded
on film, or electronically.

Lec 32, Page 3/18

General principle
 The part is placed between
the radiation source and the
radiographic film. The part
will stop some of the radiation.
Thicker and more dense area
will stop more of the radiation.

X-ray film

 The film darkness (density)
will vary with the amount of
radiation reaching the film
through the test object.

less exposure
Top view of developed film

more exposure

Some radiographic images

Lec 32, Page 4/18

 The radiation used in
radiography testing is either
X-ray or gamma-ray, a
higher energy (shorter
wavelength) version of the
electromagnetic waves
capable of penetrating
relatively large thickness of
metal.
 Industrial radiography is often subdivided into “X-ray
Radiography” or “Gamma-ray Radiography”, depending on the
source of radiation used.

Advantages
 Technique is not limited by material type or density.
 Can inspect assembled components.
 Minimum surface preparation required.
 Sensitive to changes in thickness, corrosion, voids, cracks,
and material density changes.
 Detects both surface and subsurface defects.
 Provides a permanent record of the inspection.

Lec 32, Page 5/18

Disadvantages
 Many safety precautions for the use of high intensity radiation.

 Many hours of technician training prior to use.
 Orientation of equipment and flaw can be critical.
 Determining flaw depth is impossible without additional
angled exposures.
 Expensive initial equipment cost.

Magnetic Particle Inspection
 Magnetic particle inspection can detect both production
discontinuities (inclusions, seams, laps, tears, grinding
cracks and quenching cracks) and in-service damage
(fatigue and overload cracks) in ferromagnetic materials
such as iron and steel.
 Can detect surface discontinuities too fine to be detected
by the naked eye, and will also detect discontinuities
which lie slightly below the surface.

Lec 32, Page 6/18

How does it work?
 A ferromagnetic test specimen is magnetized with a strong
magnetic field created by a magnet or special equipment.
If the specimen has a discontinuity, the discontinuity will
interrupt the magnetic field flowing through the specimen
and a leakage field will occur.

 Finely milled iron particles coated with a dye pigment are applied
to the test specimen. These particles are attracted to leakage fields
and will cluster to form an approximate shape of the surface
projection of the discontinuity. This indication can be visually
detected under proper lighting conditions (e.g., ultraviolet light).

Lec 32, Page 7/18

Some examples

Advantages
 Can detect both surface and near sub-surface defects.
 Can inspect parts with irregular shapes easily.
 Pre-cleaning of components is not as critical as it is for some
other inspection methods. Most contaminants within a flaw
will not hinder flaw detectability.
 Method of inspection is fast and indications are visible
directly on the specimen surface.
 Considered low cost compared to many other NDI methods.

 A very portable inspection method especially when used with
battery powered equipment.

Lec 32, Page 8/18

Disadvantages
 Cannot inspect non-ferrous materials such as aluminum,
magnesium or most stainless steels.
 Inspection of large parts may require use of equipment
with special power requirements.
 Some parts may require removal of coating or plating to
achieve desired inspection sensitivity.
 Limited subsurface discontinuity detection capabilities.
Maximum depth sensitivity is approximately 0.6” (under
ideal conditions).
 Post cleaning, and post demagnetization is often necessary.
 Alignment between magnetic flux and defect is important.

Die Penetrant Inspection
 Penetrant Testing, or PT, is a nondestructive testing method
that builds on the principle of Visual Inspection.
 PT increases the “seeability” of small discontinuities that
the human eye might not be able to detect alone.
 It is a very sensitive inspection method of detecting minute
discontinuities such as cracks, shrinkage, and porosity that
are open to the surface.

Lec 32, Page 9/18

How does it 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.
 A developer (powder) is applied to pull
the trapped penetrant out of the defect
and spread it on the surface where it can
be seen.

Applying
penetrant

Washing
of excess
penetrant

Applying
developer

 With good inspection technique (under Inspection
UV light), visual indications of any
discontinuities present become apparent.

What CAN be tested using 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, overlap, lack of fusion, lack of penetration

What CANNOT be tested using 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 coatings that prevent penetrants from entering defects.

Lec 32, Page 10/18

Advantages
 Relative ease of use.
 Can be used on a wide range of material types.
 Large areas or large volumes of parts/materials can be
inspected rapidly and at low cost.
 Parts with complex geometries are routinely inspected.
 Indications are produced directly on surface of the part
providing a visual image of the discontinuity.
 Initial equipment investment is low.
 Aerosol spray cans can make equipment very portable.

Disadvantages
 Only detects surface breaking defects.
 Requires relatively smooth nonporous material.
 Precleaning is critical. Contaminants can mask defects.
 Requires multiple operations under controlled conditions.
 Chemical handling precautions necessary (toxicity, fire, waste).
 Metal smearing from machining, grinding and other operations
inhibits detection. Materials may need to be etched prior to
inspection.
 Post cleaning is necessary to remove chemicals.

Lec 32, Page 11/18

Ultrasonic Inspection
 Ultrasonic testing uses high frequency sound energy to
conduct examinations and make measurements.
 Sound is produced by a vibrating body and travels in the form
of a wave. Sound waves travel through materials by vibrating
the particles that make up the material.
 The pitch of the sound is determined by the frequency of the
wave. Ultrasound is sound with a pitch too high (1-5 million
Hz) to be detected by the human ear.

How does it work?
 Ultrasonic waves are introduced into a material by a transducer
where they travel in a straight line and at a constant speed until
they encounter a surface.
 At surface interfaces some of the wave energy is reflected and
some is transmitted.
 The amount of reflected or transmitted energy can be detected
and provides information about the size of the reflector.
 The travel time of the sound can be measured and this provides
information on the distance that the sound has traveled.

Lec 32, Page 12/18

Testing techniques
 Ultrasonic testing is a very versatile inspection method,
and inspections can be accomplished in either of the
following ways:
 Pulse-echo
One transducer is used in one side of the sample, both as
transmitter and receiver

 Through Transmission
Two transducers are used in both sides of the sample, one as
transmitter and the other as receiver

Pulse-echo system
 A transducer sends out a pulse of energy and the same transducer listens for reflected
energy (an echo) from the discontinuities (if any) and the surfaces of the test article.
 The amount of reflected sound energy is displayed versus time, which provides the
inspector information about the size and the location of features that reflect the sound.

initial
pulse

back surface
echo

crack
echo

0

2

4

6

8

10

crack
oscilloscope,or flaw detector screen

sample plate

Lec 32, Page 13/18

Through-transmission system
 Two transducers located on opposing sides of the
test specimen are used. One transducer acts as
a transmitter, the other as a receiver.

11
T

 Discontinuities in the sound path will result in a
partial or total loss of sound being transmitted and
be indicated by a decrease in the received signal
amplitude.

R

R

T

2

 Through transmission is useful in detecting
discontinuities that are not good reflectors, and
when signal strength is weak. It does not provide
depth information.

11

2

0

2

4

6

8

10

What can be tested?
 Ultrasonic examinations can be conducted on a wide variety
of material forms including castings, forgings, welds, and
composites.
 A considerable amount of information about the part being
examined can be collected some of which are mentioned below:
 Flaw detection (cracks, inclusions, porosity, etc.)
 Erosion and corrosion thickness gauging
 Assessment of bond integrity in adhesively joined and brazed components
 Estimation of void content in composites and plastics

 Measurement of case hardening depth in steels; part or coating thickness
 Estimation of grain size in metals

Lec 32, Page 14/18

Advantages
 Sensitive to both surface and subsurface discontinuities.
 Depth of penetration for flaw detection or measurement is superior to
other methods.
 Only single-sided access is needed when pulse-echo technique is used.
 High accuracy in determining reflector position and estimating size and
shape.
 Minimal part preparation required.
 Electronic equipment provides instantaneous results.
 Detailed images can be produced with automated systems.
 Has other uses such as thickness measurements, in addition to flaw
detection.

Disadvantages
 Surface must be accessible to transmit ultrasound.
 Skill and training is more extensive than with some other methods.
 Normally requires a coupling medium to promote transfer of sound
energy into test specimen.
 Materials that are rough, irregular in shape, very small,
exceptionally thin or not homogeneous are difficult to inspect.
 Cast iron and other coarse grained materials are difficult to inspect
due to low sound transmission and high signal noise.
 Linear defects oriented parallel to the sound beam may go
undetected.

 Reference standards are required for both equipment calibration,
and characterization of flaws.

Lec 32, Page 15/18

Eddy-Current Inspection
How does it work?
Eddy current testing uses electromagnetic
induction to detect flaws.
A varying magnetic field
is produced if a source of
alternating current is
connected to a coil.
When this field is
placed near a test
specimen capable of
conducting an electric
current, eddy currents
will be induced in the
specimen.

Coil

Eddy currents

Coil's
magnetic field

Eddy current's
magnetic field
conductive
material

The detection unit will measure this new magnetic field and convert the signal
into a voltage that can read on a meter or cathode-ray tube.
Variations in the electrical conductivity or magnetic permeability of the test object
or the presence of any flaws will cause a change in eddy current and a
corresponding change in the phase and amplitude of the measured current.

A small surface probe is
scanned over the part
surface in an attempt to
detect a crack

Lec 32, Page 16/18

Advantages
 Detection of very small surface and sub-surface cracks
and other irregularities.
 Minimal preparation of surface.
 Samples with complex geometry can be investigated
 Variations in composition and heat treatment conditions.

 Measurement of electrical conductivity and coating thickness.

Disadvantages
 Only conductive materials can be tested.
 Surface of the material must be accessible.
 Depth of penetration is limited by materials’ conductivity
 Flaws lie parallel to the probe cannot be detected.
 Bad surface finish can cause bad reading.

Nondestructive Inspection:
A Summary
Table 1.8: Major nondestructive methods
Indicating when to use, where to use, advantages, and
limitations of each nondestructive inspection method
described earlier.

Lec 32, Page 17/18

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