MME 101 Materials Engineering Fundamentals

Lecture 36
A. K. M. Bazlur Rashid
Professor, Dept. of Materials and Metallurgical Eng.
Part E: Processing and Applications
Bangladesh Univ. of Eng. and Tech., Dhaka-1000 7 – Non-destructive inspection

Lecture outcome (LO)

At the end of this lecture, students should be able to

1. explain importance of non-destructive testing and inspection,
2. describe the process, advantages and applications of different non-
destructive inspections.

Reference:
SH Avner. Introduction to Physical Metallurgy, 2nd Ed., pp.45-60.

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 1. Introduction

❑ Non-destructive inspection (NDI) or testing (NDT) 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.

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 ❑ There are FIVE basic elements in any nondestructive inspection:

 SOURCE - provides a probing medium to inspect the item under test

 MODIFICATION - the probing medium must change or be modified
due to 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

❑ Five most common nondestructive inspection methods:

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

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 2. 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.

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.
❑ The film darkness (density) will vary with the amount
of radiation reaching the film through the test object.
X-ray film

less exposure

Top view of developed film more exposure

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 Some radiographic images

❑ 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.

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 Advantages Disadvantages
• Technique is not limited by material type or density. • Many safety precautions needed for the use
of high intensity radiation.
• 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.

• 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.

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3. 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.

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 How does it work?
 A ferromagnetic test specimen is magnetized FLUX LEAKAGE
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.
MAGNETIC FIELD LINES
MAGNETIC PARTICLES

 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).

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Some examples

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 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 specimen surface.
❑ Considered low cost compared to many other NDI methods.
❑ Availability of portable inspection method especially when used with battery powered
equipment.

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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 15 mm (under ideal conditions).
❑ Post cleaning, and post demagnetization is often necessary.
❑ Alignment between magnetic flux and defect is important.

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 3. 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.

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How does it work?

Applying
penetrant
❑ 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 Washing
discontinuities via capillary action and other mechanisms. of excess
penetrant
❑ Excess penetrant is removed from the surface.
❑ A developer (powder) is applied to pull the trapped penetrant Applying
out of the defect and spread it on the surface where it can be developer
seen.
❑ With good inspection technique (under UV light), visual
indications of any discontinuities present become apparent. Inspection

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 What CAN be tested using PT? What CANNOT be tested using PT?

❑ All defects that are open to the surface. ❑ Components with rough surfaces (e.g. sand
• Rolled products – cracks, seams, laminations. castings), that trap and hold penetrant.
• Castings – cold shuts, hot tears, porosity, ❑ Porous ceramics
blow holes, shrinkage.
• Forgings – cracks, laps, external bursts. ❑ Wood and other fibrous materials.
• Welds – cracks, porosity, overlap, lack of fusion,
❑ Plastic parts that absorb or react with the
lack of penetration
penetrant materials.
❑ Components with coatings that prevent
penetrants from entering defects.

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Advantages Disadvantages
❑ Relative ease of use. ❑ Only detects surface breaking defects.
❑ Can be used on a wide range of material types. ❑ Requires relatively smooth nonporous material.
❑ Large areas or large volumes of parts/materials ❑ Precleaning is critical. Contaminants can mask
can be inspected rapidly and at low cost. defects.
❑ Parts with complex geometries are routinely ❑ Requires multiple operations under controlled
inspected. conditions.
❑ Indications are produced directly on surface of the ❑ Chemical handling precautions necessary
part providing a visual image of the discontinuity. (toxicity, fire, waste).
❑ Initial equipment investment is low. ❑ Metal smearing from machining, grinding and
other operations inhibits detection. Materials
❑ Aerosol spray cans can make equipment very
may need to be etched prior to inspection.
portable.
❑ Post cleaning is necessary to remove chemicals.

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 4. Ultrasonic Inspection
❑ Ultrasonic testing uses high frequency What can be tested?
sound energy to conduct examinations and
make measurements. ❑ A wide variety of material forms including castings,
forgings, welds, and composites can be tested.
❑ Sound is produced by a vibrating body and
travels in the form of a wave. Sound waves ❑ Information obtained by this test include:
travel through materials by vibrating the • Flaw detection (cracks, inclusions, porosity, etc.)
particles that make up the material. • Erosion and corrosion thickness gauging
• Assessment of bond integrity in adhesively joined and
❑ The pitch of the sound is determined by the brazed components
frequency of the wave. Ultrasound is sound • Estimation of void content in composites and plastics
with a pitch too high (1-5 million Hz) to be • Measurement of case hardening depth in steels; part or
detected by the human ear. coating thickness
• Estimation of grain size in metals

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How does it work? Testing techniques

❑ Ultrasonic waves are introduced into a material ❑ Ultrasonic testing is a very versatile
by a transducer where they travel in a straight inspection method, and inspections can be
line and at a constant speed until they accomplished in either of the following ways:
encounter a surface.
 Pulse-echo
❑ At surface interfaces some of the wave energy One transducer is used in one side of
is reflected and some is transmitted. the sample, both as transmitter and
receiver
❑ The amount of reflected or transmitted energy
can be detected and provides information  Through Transmission
about the size of the reflector. Two transducers are used in both sides
of the sample, one as transmitter and
❑ The travel time of the sound can be measured, the other as receiver
and this provides information on the distance
that the sound has traveled.

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 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

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

2
T R

2
0 2 4 6 8 10

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 Advantages Disadvantages
• Sensitive to both surface and subsurface discontinuities. • Surface must be accessible to transmit ultrasound.
• Depth of penetration for flaw detection or measurement • Skill and training is more extensive than with some other
is superior to other methods. methods.
• Only single-sided access is needed when pulse-echo • Normally requires a coupling medium to promote transfer of
technique is used. sound energy into test specimen.
• High accuracy in determining reflector position and • Materials that are rough, irregular in shape, very small,
estimating size and shape. exceptionally thin or not homogeneous are difficult to inspect.
• Minimal part preparation required. • Cast iron and other coarse-grained materials are difficult to
inspect due to low sound transmission and high signal noise.
• Electronic equipment provides instantaneous results.
• Linear defects oriented parallel to the sound beam may go
• Detailed images can be produced with automated systems.
undetected.
• Other uses include thickness measurements,
• Reference standards are required for both equipment
in addition to flaw detection.
calibration, and characterization of flaws.

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5. Eddy-Current Inspection

How does it work?

❑ Eddy current testing uses
electromagnetic induction to detect flaws. Coil Coil's
magnetic field
❑ A varying magnetic field is produced if a
source of alternating current is connected
to a coil. Eddy current's
Eddy currents magnetic field
❑ When this field is placed near a test
specimen capable of conducting an conductive
electric current, eddy currents will be material
induced in the specimen.

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 ❑ 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 to detect a crack

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Advantages Disadvantages

❑ Detection of very small surface and sub- ❑ Only conductive materials can be tested.
surface cracks and other irregularities.
❑ Surface of the material must be accessible.
❑ Minimal preparation of surface.
❑ Depth of penetration is limited by materials’
❑ Samples with complex geometry can be conductivity
investigated.
❑ Flaws lie parallel to the probe cannot be
❑ Variations in composition and heat treatment detected.
conditions.
❑ Bad surface finish can cause bad reading.
❑ Measurement of electrical conductivity and
coating thickness.

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 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.

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@ 01747 219894
and make an appointment
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