24 Nov 1998

2
 Definition of NDT (NDE)
The use of nondestructive testing techniques to
determine the integrity of a material, component or structure
or quantitatively measure some characteristic of an object.
i.e. Inspect or measure without doing harm.

Nondestructive Evaluation (NDE) is a term that is often

used interchangeably with NDT. For example, a NDE method
would not only locate a defect, but it would also be used to
measure something about that defect such as its size, shape,
and orientation.
 What are Some Uses
of NDE Methods?

Flaw Detection and Evaluation

Leak Detection
Location Determination
Dimensional Measurements Fluorescent penetrant indication

Estimation of Mechanical and Physical Properties

Stress (Strain) and Dynamic Response Measurements
 Test piece too precious to be destroyed
Test piece should be useable after
inspection
Test piece is in service
For quality control purpose.
When are NDE Methods Used?
To assist in product development
To screen or sort incoming materials
To monitor, improve manufacturing processes
To verify proper processing such as heat
treating
To verify proper assembly
To inspect for in-service damage
Defects in Solids
0D, Point defects: atoms missing or in irregular places
in the lattice (vacancies, interstitials, impurities)
1D, Linear defects: groups of atoms in irregular
positions (e.g. screw and edge dislocations).
2D, Planar defects: the interfaces between
homogeneous regions of the material (grain boundaries,
external surfaces)
3D, Volume defects: extended defects
(pores, cracks)

7
8
 A Composite material is a material system composed
of two or more macro constituents that differ in shape
and chemical composition and which are insoluble in
each other.
Applications:
Aerospace industry
Sporting Goods Industry
Automotive Industry
Home Appliance Industry

9
Composite flaws can occurs
1.In the matrix or fibre matrix bond
2.During production
3.During service
4.Fibre and ply misalignment
5.Fibre wrinkling or waviness

10
 Detection of surface flaws
Visual
Magnetic Particle Inspection
Fluorescent Dye Penetrant Inspection
Detection of internal flaws
Radiography
Ultrasonic Testing
Eddy current Testing
24 Nov 1998
12
 The emission of energy as electromagnetic
waves or as moving subatomic particles.
RADIATION:
-Heat from the sun warming your face
-Heat from a light bulb
-Heat from a fire
Also there is ultraviolet, infrared, gamma,
microwave, X, alpha, beta, visible light

13
 Radiation is a way in which energy moves from
one place to another. Thus, the energy released
when a stone is dropped into water radiates
away in circular waves.
Sound energy radiates from a speaker's mouth
to a listener's ear;
light and heat energy radiate from the sun to the
earth.
Electrons, radiating from a hot wire, provide the
energy that forms the picture in a television set.
In the first four examples the radiation consists
of waves--water waves, sound waves, light
waves, heat waves.

14
 Gamma radiation and X-rays are
electromagnetic radiation like visible light,
radio waves, and ultraviolet light. These
electromagnetic radiations differ only in the
amount of energy they have. Gamma rays and
X-rays are the most energetic of these.
Gamma radiation is able to travel many meters
in air and many centimeters in human tissue. It
readily penetrates most materials and is
sometimes called "penetrating radiation.

15
 producing an image on a radiosensitive surface
by radiation other than visible light.
Radiography involves the use of penetrating
gamma- or X-radiation to examine material's
and product's defects and internal features
Radiation is directed through a part and on to
film or other media. The resulting shadowgraph
shows the internal features and soundness of
the part. Material thickness and density
changes are indicated as lighter or darker
areas on the film. The darker areas in the
radiograph below represent internal voids in
the component.

16
 x-ray source
X-rays are electromagnetic radiation with very short
wavelength ( 10-8 -10-12 m)
The energy of the x-ray can be calculated with the equation

E = h = hc/

Production of X-rays
X-rays are produced whenever
high-speed electrons collide
with a metal target.

17
18
Two of the most commonly used sources of
radiation in industrial radiography are x-ray
generators and gamma ray sources. Industrial
radiography is often subdivided into X-ray
Radiography or Gamma Radiography,
depending on the source of radiation used.

Two of the more common industrial gamma-ray

sources for industrial radiography are iridium-192
and cobalt-60.
Cobalt-60 will emit a 1.33 and a 1.17 MeV gamma
ray, and iridium-192 will emit 0.31, 0.47, and 0.60
MeV gamma rays
 Gamma rays are
produced by a
radioisotope.
A radioisotope has an
unstable nuclei that does
not have enough binding
energy to hold the
nucleus together.
The spontaneous
breakdown of an atomic
nucleus resulting in the
release of energy and
matter is known as
radioactive decay.
 Most of the radioactive
material used in
industrial radiography
is artificially
produced.
This is done by
subjecting stable
material to a source of
neutrons in a special
nuclear reactor.
This process is called
activation.
Unlike X-rays, which are
produced by a machine,
gamma rays cannot be turned
off. Radioisotopes used for
gamma radiography are
encapsulated to prevent
leakage of the material.

The radioactive capsule is

attached to a cable to form
what is often called a pigtail.
The pigtail has a special
connector at the other end
that attaches to a drive cable.
A device called a camera is used to store,
transport and expose the pigtail containing the
radioactive material. The camera contains
shielding material which reduces the
radiographers exposure to radiation during use.
A hose-like device
called a guide tube is
connected to a
threaded hole called
an exit port in the
camera.

The radioactive
material will leave
and return to the
camera through this
opening when
performing an
exposure!
A drive cable is connected
to the other end of the
camera. This cable,
controlled by the
radiographer, is used to force
the radioactive material out
into the guide tube where the
gamma rays will pass
through the specimen and
expose the recording device.
26
Unlike gamma rays, x-rays are produced by an X-
ray generator system. These systems typically
include an X-ray tube head, a high voltage
generator, and a control console.
 X-rays are produced by establishing a very high
voltage between two electrodes, called the anode
and cathode.
To prevent arcing, the anode and cathode are
located inside a vacuum tube, which is protected
by a metal housing.
 The cathode contains a small High Electrical Potential
filament much the same as in a
light bulb. Electrons
Current is passed through the + -
filament which heats it. The
heat causes electrons to be X-ray Generator
stripped off. or Radioactive
The high voltage causes these Source Creates
Radiation
free electrons to be pulled
toward a target material
(usually made of tungsten)
located in the anode. Radiation
The electrons impact against Penetrate
the Sample
the target. This impact causes
an energy exchange which Exposure Recording Device
causes x-rays to be created.
 A spectrum of x-ray is
produced as a result of the I
k
interaction between the characteristic
incoming electrons and the radiation continuous
inner shell electrons of the radiation
target element. k
Two components of the
spectrum can be identified,
namely, the continuous
spectrum and the
characteristic spectrum.

SWL - short-wavelength limit
 Fast moving e- will then be deflected or
decelerated and EM radiation will be
emitted.
The energy of the radiation depends on
the severity of the deceleration, which is
more or less random, and thus has a
continuous distribution.
These radiation is called white
radiation or bremsstrahlung (German
word for braking radiation).

If an incoming electron has sufficient

kinetic energy for knocking out an
electron of the K shell (the inner-most
shell), it may excite the atom to an high-
energy state (K state).
One of the outer electron falls into the
K-shell vacancy, emitting the excess
energy as a x-ray photon -- K-shell
emission Radiation.
 All x-rays are absorbed to some extent in passing
through matter due to electron ejection or scattering.
The absorption follows the equation

I0 , I

x

I I 0 e x I 0 e

where I is the transmitted intensity;

x is the thickness of the matter; x
is the linear absorption coefficient (element dependent);
is the density of the matter;
(/) is the mass absorption coefficient (cm2/gm).
Several different imaging methods are
available to display the final image in
industrial radiography:
Film Radiography
Digital Radiography (DR)
Real Time Radiography
Computed Tomography (CT)
Computed Radiography (CR)
 One of the most widely used
and oldest imaging
mediums in industrial
radiography is radiographic
film.
Film contains microscopic
material called silver bromide.
Once exposed to radiation and
developed in a darkroom,
silver bromide turns to black
metallic silver which forms the
image.
 Film must be protected from visible light. Light,
just like x-rays and gamma rays, can expose film.
Film is loaded in a light proof cassette in a
darkroom.
This cassette is then placed on the specimen
opposite the source of radiation. Film is often
placed between screens to intensify radiation.
 In order for the image to be viewed, the film must
be developed in a darkroom. The process is very
similar to photographic film development.
Film processing can either be performed
manually in open tanks or in an automatic
processor.
Once developed, the film is typically
referred to as a radiograph.
 One of the newest forms of radiographic
imaging is Digital Radiography.
Requiring no film, digital radiographic
images are captured using either special
phosphor screens or flat panels containing
micro-electronic sensors.
No darkrooms are needed to process film,
and captured images can be digitally
enhanced for increased detail.
Images are also easily archived (stored)
when in digital form.
There are a number of forms of digital
radiographic imaging including:
Computed Radiography (CR)
Real-time Radiography (RTR)
Direct Radiographic Imaging (DR)
Computed Tomography
Computed Radiography (CR) is a digital imaging
process that uses a special imaging plate which
employs storage phosphors.
Digital images are typically sent to a
computer workstation where specialized
software allows manipulation and
enhancement.
Examples of computed radiographs:
 Real-Time Radiography (RTR) is a term used to
describe a form of radiography that allows
electronic images to be captured and viewed in
real time.

Manipulating the part can be advantageous for

several reasons:
It may be possible to image the entire component with
one exposure.
Viewing the internal structure of the part from different
angular prospective can provide additional data for
analysis.
Time of inspection can often be reduced.
The equipment needed for an
RTR includes:
X-ray tube Computer with frame
Image intensifier or grabber board and
other real-time detector software
Camera Monitor
Sample positioning
system (optional)
 The image intensifier is a
device that converts the
radiation that passes through
the specimen into light.

The more radiation that

reaches the input screen, the
more light that is given off.
The image is very faint on the
input screen so it is
intensified onto a small
screen inside the intensifier
where the image is viewed
with a camera.
 A special camera A monitor is then connected
which captures the to the camera to provide a
light output of the viewable image.
screen is located near If a sample positioning
the image system is employed, the part
intensifying screen. can be moved around and
The camera is very rotated to image different
sensitive to a variety internal features of the part.
of different light
intensities.
 Direct radiography (DR) is a
form of real-time radiography
that uses a special flat panel
detector.
The panel works by converting
penetrating radiation passing
through the test specimen into
minute electrical charges.
The panel contains many
micro-electronic capacitors.
The capacitors form an
electrical charge pattern
image of the specimen.
Each capacitors charge is
converted into a pixel which
forms the digital image.
Computed Tomography (CT) uses a real-time
inspection system employing a sample
positioning system and special software.
 Many separate images are saved (grabbed) and
complied into 2-dimensional sections as the
sample is rotated.
2-D images are them combined into 3-
dimensional images.

Real-Time Compiled 2-D Compiled 3-D

Captures Images Structure
Can you determine what object was
radiographed in this and the next three slides?
 Technique is not limited by material type or
density.
Can inspect assembled components.
Minimum surface preparation required.
Detects both surface and subsurface
defects.
Provides a permanent record of the
inspection.
 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.
Cracking can be detected in a radiograph only the crack is
propagating in a direction that produced a change in thickness that
is parallel to the x-ray beam. Cracks will appear as jagged and
often very faint irregular lines. Cracks can sometimes appearing as
"tails" on inclusions or porosity.
Burn through (icicles) results when too much heat causes
excessive weld metal to penetrate the weld zone. Lumps of
metal sag through the weld creating a thick globular condition
on the back of the weld. On a radiograph, burn through
appears as dark spots surrounded by light globular areas.
 Radio Isotope (Gamma) Sources
It is the most energetic form of electromagnetic radiation, with a
very short wavelength of less than one-tenth of a nano-meter. Gamma rays
are essentially very energetic x-rays emitted by excited nuclei. Two of the
more common industrial Gamma-ray sources are Iridium-192 and Colbalt-
60.

These isotopes emit radiation in two or three discreet wavelengths. Cobalt

60 will emit a 1.33 and a 1.17 MeV gamma ray, and iridium-192 will emit
0.31, 0.47, and 0.60 MeV gamma rays.
Advantages of gamma ray sources include portability and the ability to
penetrate thick materials in a relativity short time.
Disadvantages include shielding requirements and safety considerations.
X-ray production

X-rays are produced when high energetic

electrons interact with matter.

The kinetic energy of the electrons is

converted into electromagnetic energy by atomic
interactions .
The classical X-ray tube requires:

electron source
electron acceleration potential
target for X-ray production
The major components of the modern X-ray tube are:

cathode (electron source)

anode (acceleration potential)

rotor/stator (target device)

glass/metal envelope (vacuum tube)

 X-ray have a wave length of 0.01nm
to10nm.
X-ray have a frequency b/w (3x1016 to
3x1019 Hz).
X-ray have a energy range b/w (100eV
100 KeV).
X-ray with photon energies above
5-10KeV are called hard x-ray. Below
This energy limit are called soft x-ray.

24 Nov 1998
62
Filmis a media that makes a
permanent record of the
image.
Image recorded on film is
caused by exposure to
photons
 An interaction of a light wave with an
object that causes the light to be
redirected.
Scattering is a general physical process
where some forms of radiation, such as
light, sound, or moving particles, are
forced to deviate from a straight
trajectory by one or more localized non-
uniformities in the medium through
which they pass.
64
 Scattering can be broadly defined as the
redirection of radiation out of the original
direction of propagation, usually due to
interactions with molecules and
particles.
Reflection, refraction, diffraction etc. are
actually all just forms of scattering

65
1 - Thomson scattering
2 - Compton scattering x-ray scattering
3 - Raman scattering
Compton scattering is observed in x-
rays passing through a solid or gas. The
essential interaction is between higher
photon energy and individual electrons.
whether or not that electron is bond to
atomic nucleus.
66
24 Nov 1998
67
Principles of Sound
A mechanical vibration
The vibrations create Pressure Waves
Sound travels faster in more elastic
materials
Number of pressure waves per second is
the Frequency
Speed of travel is the Sound velocity
 Wavelength :
The distance required to complete a
cycle
Measured in Meter or mm
Frequency
:
The number of cycles per unit time
Measured in Hertz (Hz) or Cycles per second (cps)
Velocity
:
How quick the sound travels
Distance per unit time
Measured in meter / second (m / sec)
Wavelength Velocity

V

f

Frequency
 Sound Waves
Sound waves are the vibration of particles in solids liquids or gases

Particles vibrate about a mean position

In order to vibrate they require mass and resistance to change

One cycle
 Sound cannot travel
in vacuum
Sound energy to be
transmitted /
transferred from one
particle to another

SOLID LIQUID GAS

 The velocity of sound in a particular material is CONSTANT
It is the product of DENSITY and ELASTICITY of the
material
It will NOT change if frequency changes
Only the wavelength changes
Examples:
V Compression in steel : 5960 m/s
V Compression in water : 1470 m/s
V Compression in air : 330 m/s

5 M Hz

STEEL WATER AIR

 Sound : mechanical vibration

What is Ultrasonic?
Very High Frequency sound above 20 KHz
20,000 cps
 Frequency : Number of cycles per
second

1 second 1 second 1 second

1 cycle per 1 3 cycle per 1 18 cycle per 1

second = 1 Hertz second = 3 Hertz second = 18
THE HIGHER THE FREQUENCY THE SMALLER THE
Hertz
WAVELENGTH
 Pg
21

1 Hz = 1 cycle per second

1 Kilohertz = 1 KHz = 1000Hz
1 Megahertz = 1 MHz = 1000 000Hz

20 KHz = 20 000 Hz
5 M Hz = 5 000 000 Hz
 ULTRASONIC TESTING
Very High Frequency
5 M Hz

Glass
High
Frequency
DRUM BEAT 5 K Hz
Low Frequency Sound
40 Hz
 The higher the frequency the smaller the
wavelength
The smaller the wavelength the higher
the sensitivity
Sensitivity : The smallest
detectable flaw by the
system or technique
In UT the smallest detectable flaw is
(half the wavelength)
Ultrasound is generated with a transducer.
A piezoelectric element
in the transducer
converts electrical
energy into mechanical
vibrations (sound), and
vice versa.

The transducer is
capable of both
transmitting and
receiving sound
energy.
 Ultrasonic
waves are introduced into a material
where they travel in a straight line and at a constant
speed until they encounter a surface.
Atsurface 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.
 Ultrasonic testing is a very versatile inspection method,
and inspections can be accomplished in a number of
different ways.
Ultrasonic inspection techniques are commonly divided
into three primary classifications.
Pulse-echo and Through Transmission
(Relates to whether reflected or transmitted energy is used)
Normal Beam and Angle Beam
(Relates to the angle that the sound energy enters the test
article)
Contact and Immersion
(Relates to the method of coupling the transducer to the test
article)
 Piezoelectric transducers are used for
converting electrical pulses to
mechanical vibrations and vice versa
Commonly used piezoelectric
materials are quartz, Li2SO4, and
polarized ceramics such as BaTiO3 and
PbZrO3.
Usually the transducers generate
ultrasonic waves with frequencies in
the range 2.25 to 5.0 MHz
 Wave Propagation Direction
Longitudinal or
compression
waves

Shear or transverse
waves

Surface
or
Rayleigh waves

Plate
or Lamb
waves
Symmetrical Asymmetrical
 Longitudinal waves
Similar to audible sound
waves
the only type of wave
which can travel through
liquid
Shear waves
generated by passing the
ultrasonic beam through
the material at an angle
Usually a plastic wedge is
used to couple the
transducer to the material
 Surface waves
travel with little attenuation in the direction of
propagation but weaken rapidly as the wave
penetrates below the material surface
particle displacement follows an elliptical orbit
Lamb waves
observed in relatively thin plates only
velocity depends on the thickness of the
material and frequency
Piezoelectric Transducers
The active element of most acoustic
transducers is piezoelectric ceramic.
This ceramic is the heart of the
transducer which converts electrical
to acoustic energy, and vice versa.
A thin wafer vibrates with a
wavelength that is twice its thickness,
therefore, piezoelectric crystals are
cut to a thickness that is 1/2 the
desired radiated wavelength. Optimal
impedance matching is achieved by a Direction of wave
matching layer with thickness 1/4 propagation
wavelength.
 Characteristics of Piezoelectric Transducers
Transducers are classified into groups according to the application.
Contact: are used for direct
contact inspections. Coupling
materials of water, grease, oils, or
commercial materials are used to
smooth rough surfaces and
prevent an air gap between the
transducer and the component Contact type
inspected.
Immersion: do not contact the
component. These transducers
are designed to operate in a
liquid environment and all
connections are watertight.
Wheel and squirter transducers
are examples of such immersion
applications.
immersion
 Dual Element: contain two independently
operating elements in a single housing.
One of the elements transmits and the
other receives. Dual element transducers
are very useful when making thickness
measurements of thin materials and when
inspecting for near surface defects.
Dual element
Angle Beam: and wedges are typically
used to introduce a refracted shear wave
into the test material. Transducers can be
purchased in a variety of fixed angles or in
adjustable versions where the user
determines the angles of incident and
refraction. They are used to generate
surface waves for use in detecting defects
on the surface of a component.
Angle beam
Test Techniques - Pulse-Echo
In pulse-echo testing, a transducer sends out a pulse of energy
and the same or a second transducer listens for reflected energy
(an echo).
Reflections occur due to the presence of discontinuities and the
surfaces of the test article.
f
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
crack
plate
0 2 4 6 8 10
UT Instrument Screen
 Digital display
showing signal
generated from
sound reflecting
off back surface.

Digital display
showing the presence
of a reflector midway
through material, with
lower amplitude back
surface reflector.
The pulse-echo technique allows testing when access to only one
side of the material is possible, and it allows the location of
reflectors to be precisely determined.
Test Techniques Through-Transmission
Two transducers located on 11
opposing sides of the test
specimen are used. One T R

transducer acts as a transmitter,

the other as a receiver.
Discontinuities in the sound path T R

will result in a partial or total loss 2

of sound being transmitted and
be indicated by a decrease in the
received signal amplitude.
Through transmission is useful in
11

detecting discontinuities that are

not good reflectors, and when 2
signal strength is weak. It does
not provide depth information.
0 2 4 6 8 10
Test Techniques Through-Transmission

Digital display
showing received
sound through
material
thickness.

Digital display
showing loss of
received signal
due to presence
of a discontinuity
in the sound field.
Pulse-echo ultrasonic measurements can
determine the location of a discontinuity in
a part or structure by accurately
measuring the time required for a short
ultrasonic pulse generated by a
transducer to travel through a thickness of
material, reflect from the back or the
surface of a discontinuity, and be returned
to the transducer. In most applications,
this time interval is a few microseconds or
less.
d = vt/2 or v = 2d/t
where d is the distance from the surface
to the discontinuity in the test piece, v is
the velocity of sound waves in the
material, and t is the measured round-trip
transit time.
Angle Beam Transducers and wedges are typically used to
introduce a refracted shear wave into the test material. An
angled sound path allows the sound beam to come in from
the side, thereby improving detectability of flaws in and
around welded areas.

Can be used for testing

flat sheet and plate or
pipe and tubing
Angle beam units are
designed to induce
vibrations in Lamb,
longitudinal, and shear
wave modes
 Data Presentation
Ultrasonic data can be collected and displayed
in a number of different formats. The three most
common formats are know in the NDT world as
A-scan, B-scan and C-scan presentations.
Each presentation mode provides a different
way of looking at and evaluating the region of
material being inspected. Modern computerized
ultrasonic scanning systems can display data in
all three presentation forms simultaneously
 A-Scan
The A-scan presentation displays the amount of received
ultrasonic energy as a function of time. The relative amount of
received energy is plotted along the vertical axis and elapsed
time (which may be related to the sound energy travel time
within the material) is display along the horizontal axis.

Relative discontinuity size

can be estimated by
comparing the signal
amplitude obtained from an
unknown reflector to that
from a known reflector.
Reflector depth can be
determined by the position
of the signal on the
horizontal sweep.
 B-Scan
The B-scan presentations is a profile (cross-sectional) view of the a
test specimen. In the B-scan, the time-of-flight (travel time) of the
sound energy is displayed along the vertical and the linear position
of the transducer is displayed along the horizontal axis. From the B-
scan, the depth of the reflector and its approximate linear
dimensions in the scan direction can be determined.

The B-scan is typically

produced by establishing a
trigger gate on the A-scan.
Whenever the signal intensity
is great enough to trigger the
gate, a point is produced on
the B-scan. The gate is
triggered by the sound
reflecting from the backwall
of the specimen and by
smaller reflectors within the
material.
 C-Scan:
The C-scan presentation provides a plan-type view of the location
and size of test specimen features. The plane of the image is parallel
to the scan pattern of the transducer.
C-scan presentations are produced with an
automated data acquisition system, such as a
computer controlled immersion scanning
system. Typically, a data collection gate is
established on the A-scan and the amplitude
or the time-of-flight of the signal is recorded
at regular intervals as the transducer is
scanned over the test piece. The relative
signal amplitude or the time-of-flight is
displayed as a shade of gray or a color for
each of the positions where data was
recorded. The C-scan presentation provides
an image of the features that reflect and
scatter the sound within and on the surfaces
of the test piece.
 Flaw detection (cracks, inclusions, porosity, etc.)
Erosion & corrosion thickness gauging
Assessment of bond integrity in adhesively
joined and brazed components.
Estimation of void content in composites and
plastics
Estimation of grain size in metals.
 Ultrasonic Inspection
Advantages Disadvantages
Rapid results Trained and skilled
Sub-surface detection operator required
Safe Requires high operator

Can detect planar defect skill

Good surface finish
Capable of measuring the

depth of defects required

Difficulty on detecting
May be battery powered
volumetric defect
Portable
1. Visual Inspection
Most basic and common
inspection method.

Tools include
fiberscopes,
borescopes, magnifying
glasses and mirrors.

Portable video inspection

unit with zoom allows
inspection of large tanks
and vessels, railroad tank
cars, sewer lines.
Robotic crawlers permit
observation in hazardous or
tight areas, such as air
ducts, reactors, pipelines.
 2.1 Introduction
A nondestructive testing method used for defect detection. Fast and
relatively easy to apply and part surface preparation is not as critical as
for some other NDT methods. MPI one of the most widely utilized
nondestructive testing methods.
MPI uses magnetic fields and small magnetic particles, such as iron
filings to detect flaws in components. The only requirement from an
inspectability standpoint is that the component being inspected must be
made of a ferromagnetic material such as iron, nickel, cobalt, or some of
their alloys. Ferromagnetic materials are materials that can be
magnetized to a level that will allow the inspection to be affective.
The method is used to inspect a variety of product forms such as castings,
forgings, and weldments. Many different industries use magnetic particle
inspection for determining a component's fitness-for-use. Some examples
of industries that use magnetic particle inspection are the structural steel,
automotive, petrochemical, power generation, and aerospace industries.
Underwater inspection is another area where magnetic particle
inspection may be used to test such things as offshore structures and
underwater pipelines.
 2.2 Basic Principles
In theory, magnetic particle inspection (MPI) is a relatively
simple concept. It can be considered as a combination of
two nondestructive testing methods: magnetic flux leakage
testing and visual testing.
Consider a bar magnet. It has a magnetic field in and
around the magnet. Any place that a magnetic line of force
exits or enters the magnet is called a pole. A pole where a
magnetic line of force exits the magnet is called a north pole
and a pole where a line of force enters the magnet is called
a south pole.
Interaction of materials with an external
magnetic field
When a material is placed within a magnetic field, the
magnetic forces of the material's electrons will be affected.
This effect is known as Faraday's Law of Magnetic
Induction.
However, materials can react quite differently to the
presence of an external magnetic field. This reaction is
dependent on a number of factors such as the atomic and
molecular structure of the material, and the net magnetic
field associated with the atoms. The magnetic moments
associated with atoms have three origins. These are the
electron orbital motion, the change in orbital motion
caused by an external magnetic field, and the spin of the
electrons.
 Diamagnetic, Paramagnetic, and
Ferromagnetic Materials
Diamagnetic metals: very weak and negative susceptibility
to magnetic fields. Diamagnetic materials are slightly
repelled by a magnetic field and the material does not retain
the magnetic properties when the external field is removed.
Paramagnetic metals: small and positive susceptibility to
magnetic fields. These materials are slightly attracted by a
magnetic field and the material does not retain the magnetic
properties when the external field is removed.
Ferromagnetic materials: large and positive susceptibility
to an external magnetic field. They exhibit a strong
attraction to magnetic fields and are able to retain their
magnetic properties after the external field has been
removed.
Ferromagnetic materials become magnetized when the
magnetic domains within the material are aligned. This can be
done by placing the material in a strong external magnetic field or
by passes electrical current through the material. Some or all of
the domains can become aligned. The more domains that are
aligned, the stronger the magnetic field in the material. When all
of the domains are aligned, the material is said to be magnetically
saturated. When a material is magnetically saturated, no
additional amount of external magnetization force will cause an
increase in its internal level of magnetization.

Unmagnetized material Magnetized material

General Properties of Magnetic Lines of Force
Follow the path of least resistance
between opposite magnetic poles.
Never cross one another.
All have the same strength.
Their density decreases (they
spread out) when they move from an
area of higher permeability to an
area of lower permeability.
Their density decreases with
increasing distance from the poles.
flow from the south pole to the
north pole within the material and
north pole to south pole in air.
When a bar magnet is broken in the center of its length, two
complete bar magnets with magnetic poles on each end of
each piece will result. If the magnet is just cracked but not
broken completely in two, a north and south pole will form at
each edge of the crack.
The magnetic field exits the north
pole and reenters the at the south
pole. The magnetic field spreads out
when it encounter the small air gap
created by the crack because the air
can not support as much magnetic
field per unit volume as the magnet
can. When the field spreads out, it
appears to leak out of the material
and, thus, it is called a flux leakage
field.
If iron particles are sprinkled on a cracked magnet, the particles will
be attracted to and cluster not only at the poles at the ends of the
magnet but also at the poles at the edges of the crack. This cluster
of particles is much easier to see than the actual crack and this is
the basis for magnetic particle inspection.
 Magnetic Particle Inspection
The magnetic flux line close to the surface of a
ferromagnetic material tends to follow the surface
profile of the material
Discontinuities (cracks or voids) of the material
perpendicular to the flux lines cause fringing of
the magnetic flux lines, i.e. flux leakage
The leakage field can attract other ferromagnetic
particles
The magnetic particles form a
ridge many times wider than
the crack itself, thus making
the otherwise invisible crack
visible

Cracks just below the

surface can also be
revealed
MPI is not sensitive to shallow
and smooth surface defects

The effectiveness of MPI

depends strongly on the
orientation of the crack
related to the flux lines
 Cleaning
Demagnetization
Contrast dyes (e.g. white paint for dark
particles)
Magnetizing the object
Addition of magnetic particles
Illumination during inspection (e.g. UV lamp)
Interpretation
Demagnetization - prevent accumulation of iron
particles or influence to sensitive instruments
 Indirect magnetization: using a strong external magnetic
field to establish a magnetic field within the component

(a) permanent magnets

(b) Electromagnets

(c) coil shot

 British Standards
BS M.35: Aerospace Series: Magnetic Particle Flaw
Detection of Materials and Components
BS 4397: Methods for magnetic particle testing of welds
ASTM Standards
ASTM E 709-80: Standard Practice for Magnetic Particle
Examination
ASTM E 125-63: Standard reference photographs for
magnetic particle indications on ferrous castings etc.
 One of the most dependable and sensitive methods
for surface defects
fast, simple and inexpensive
direct, visible indication on surface
unaffected by possible deposits, e.g. oil, grease or
other metals chips, in the cracks
can be used on painted objects
surface preparation not required
results readily documented with photo or tape
impression
 Only good for ferromagnetic materials
sub-surface defects will not always be indicated
relative direction between the magnetic field and
the defect line is important
objects must be demagnetized before and after the
examination
the current magnetization may cause burn scars on
the item examined
Examples of visible dry magnetic particle indications

Indication of a crack in a saw blade Indication of cracks in a weldment

Indication of cracks running between

Before and after inspection pictures of attachment holes in a hinge
Examples of Fluorescent Wet Magnetic
Particle Indications
Magnetic particle wet fluorescent
indication of a cracks in a drive shaft

Magnetic particle wet

fluorescent
indication of a crack
in a bearing

Magnetic particle wet fluorescent indication

of a cracks at a fastener hole
Liquid penetrant inspection (LPI) is one of the
most widely used nondestructive evaluation
(NDE) methods. Its popularity can be attributed
to two main factors, which are its relative ease
of use and its flexibility. LPI can be used to
inspect almost any material provided that its
surface is not extremely rough or porous.
Materials that are commonly inspected using
LPI include metals (aluminum, copper, steel,
titanium, etc.), glass, many ceramic materials,
rubber, and plastics.
 3.1 Introduction
Liquid penetration inspection is a method that is used to reveal
surface breaking flaws by bleedout of a colored or fluorescent
dye from the flaw.
The technique is based on the ability of a liquid to be drawn
into a "clean" surface breaking flaw by capillary action.
After a period of time called the "dwell," excess surface
penetrant is removed and a developer applied. This acts as a
"blotter." It draws the penetrant from the flaw to reveal its
presence.
Colored (contrast) penetrants require good white light while
fluorescent penetrants need to be used in darkened conditions
with an ultraviolet "black light". Unlike MPI, this method can be
used in non-ferromagnetic materials and even non-metals
Modern methods can reveal cracks 2m wide
Standard: ASTM E165-80 Liquid Penetrant Inspection Method
 Why Liquid Penetrant Inspection?
To improves the detectability of flaws
There are basically two ways that a
penetrant inspection process
makes flaws more easily seen.
(1) LPI produces a flaw indication
that is much larger and easier for
the eye to detect than the flaw
itself.
(2) LPI produces a flaw indication
The advantage that a liquid with a high level of contrast
penetrant inspection (LPI) offers between the indication and the
over an unaided visual inspection is background.
that it makes defects easier to see
for the inspector.
 3.2 Basic processing steps of LPI
1. Surface Preparation: One of the most critical steps of a liquid
penetrant inspection is the surface preparation. The surface must
be free of oil, grease, water, or other contaminants that may prevent
penetrant from entering flaws. The sample may also require
etching if mechanical operations such as machining, sanding, or
grit blasting have been performed. These and other mechanical
operations can smear the surface of the sample, thus closing the
defects.

2. Penetrant Application: Once the surface has been thoroughly

cleaned and dried, the penetrant material is applied by spraying,
brushing, or immersing the parts in a penetrant bath.

3. Penetrant Dwell: The penetrant is left on the surface for a

sufficient time to allow as much penetrant as possible to be drawn
from or to seep into a defect. The times vary depending on the
application, penetrant materials used, the material, the form of the
material being inspected, and the type of defect being inspected.
Generally, there is no harm in using a longer penetrant dwell time
as long as the penetrant is not allowed to dry.
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. Depending on the penetrant
system used, this step may involve cleaning with a solvent, direct
rinsing with water, or first treated with an emulsifier and then
rinsing with water.

5. Developer Application: A thin layer of developer is then applied

to the sample to draw penetrant trapped in flaws back to the
surface where it will be visible. Developers come in a variety of
forms that may be applied by dusting (dry powdered), dipping, or
spraying (wet developers).

6. Indication Development: The developer is allowed to stand on

the part surface for a period of time sufficient to permit the
extraction of the trapped penetrant out of any surface flaws. This
development time is usually a minimum of 10 minutes and
significantly longer times may be necessary for tight cracks.
7. Inspection: Inspection is then performed under appropriate
lighting to detect indications from any flaws which may be present.

8. Clean Surface: The final step in the process is to thoroughly clean

the part surface to remove the developer from the parts that were
found to be acceptable.
A penetrant must possess a number of important characteristics. A
penetrant must
spread easily over the surface of the material being inspected to
provide complete and even coverage.
be drawn into surface breaking defects by capillary action.
remain in the defect but remove easily from the surface of the
part.
remain fluid so it can be drawn back to the surface of the part
through the drying and developing steps.
be highly visible or fluoresce brightly to produce easy to see
indications.
must not be harmful to the material being tested or the inspector.
Dye penetrants Fluorescent penetrants
The liquids are coloured so that Liquid contain additives to
they provide good contrast give fluorescence under UV
against the developer Object should be shielded
Usually red liquid against white from visible light during
developer inspection
Observation performed in Fluorescent indications are
ordinary daylight or good easy to see in the dark
indoor illumination

Standard: Aerospace Material

Specification (AMS) 2644.
According to the method Based on the strength or
used to remove the excess detectability of the indication
penetrant from the part, the that is produced for a
penetrants can be classified number of very small and
into: tight fatigue cracks,
Method A - Water Washable penetrants can be classified
Method B - Post into five sensitivity levels are
Emulsifiable, Lipophilic shown below:
Method C - Solvent Level - Ultra Low
Removable Sensitivity
Method D - Post Level 1 - Low Sensitivity
Emulsifiable, Hydrophilic Level 2 - Medium Sensitivity
Level 3 - High Sensitivity
Level 4 - Ultra-High
Sensitivity
 Emulsifiers
When removal of the penetrant from the defect due to over-
washing of the part is a concern, a post emulsifiable
penetrant system can be used. Post emulsifiable penetrants
require a separate emulsifier to break the penetrant down
and make it water washable.
Method B - Lipophilic Emulsifier,
Method D - Hydrophilic Emulsifier
Lipophilic emulsification systems are
oil-based materials that are supplied
in ready-to-use form. Hydrophilic
systems are water-based and supplied
as a concentrate that must be diluted
with water prior to use .
 Developer
The role of the developer is to pull the trapped penetrant material
out of defects and to spread the developer out on the surface of
the part so it can be seen by an inspector. The fine developer
particles both reflect and refract the incident ultraviolet light,
allowing more of it to interact with the penetrant, causing more
efficient fluorescence. The developer also allows more light to be
emitted through the same mechanism. This is why indications are
brighter than the penetrant itself under UV light. Another function
that some developers performs is to create a white background
so there is a greater degree of contrast between the indication
and the surrounding background.
 Developer Types
Dry powder developer the least sensitive but
inexpensive
Water soluble consist of a group of chemicals
that are dissolved in water and form a developer
layer when the water is evaporated away.
Water suspendible consist of insoluble
developer particles suspended in water.
Nonaqueous suspend the developer in a
volatile solvent and are typically applied with a
spray gun.
Using dye and developer from different
manufacturers should be avoided.
 3.4 Primary Advantages
The method has high sensitive to small surface discontinuities.
The method has few material limitations, i.e. metallic and
nonmetallic, magnetic and nonmagnetic, and conductive and
nonconductive materials may be inspected.
Large areas and large volumes of parts/materials can be inspected
rapidly and at low cost.
Parts with complex geometric shapes are routinely inspected.
Indications are produced directly on the surface of the part and
constitute a visual representation of the flaw.
Aerosol spray cans make penetrant materials very portable.
Penetrant materials and associated equipment are relatively
inexpensive.
 3.5 Primary Disadvantages
Only surface breaking defects can be detected.
Only materials with a relative nonporous surface can be inspected.
Precleaning is critical as contaminants can mask defects.
Metal smearing from machining, grinding, and grit or vapor
blasting must be removed prior to LPI.
The inspector must have direct access to the surface being
inspected.
Surface finish and roughness can affect inspection sensitivity.
Multiple process operations must be performed and controlled.
Post cleaning of acceptable parts or materials is required.
Chemical handling and proper disposal is required.
Radiography involves the use of penetrating
gamma- or X-radiation to examine material's
and product's defects and internal features. High Electrical Potential
An X-ray machine or radioactive isotope is
used as a source of radiation. Radiation is Electrons
directed through a part and onto film or other
+
media. The resulting shadowgraph shows the -
internal features and soundness of the part.
Material thickness and density changes are
X-ray Generator or
indicated as lighter or darker areas on the Radioactive Source
film. The darker areas in the radiograph Creates Radiation
below represent internal voids in the
component.

Radiation
Penetrate
the Sample

Exposure Recording Device

 X-rays or gamma radiation is used
4.1.1 x-ray source
Properties and Generation of X-ray
X-rays are electromagnetic
radiation with very short
wavelength ( 10-8 -10-12 m)
The energy of the x-ray can
be calculated with the
equation
E = h = hc/
e.g. the x-ray photon with
wavelength 1 has energy
12.5 keV
 X-rays are produced
whenever high-speed
electrons
W collide with a metal
target.
A source of electrons hot
W filament, a high
accelerating voltage
target X-rays Vacuum (30-50kV) between the
cathode (W) and the anode
and a metal target.
The anode is a water-cooled
block of Cu containing
desired target metal.
 4.1.2 Radio Isotope (Gamma) Sources
Emitted gamma radiation is one of the three types of natural radioactivity. It
is the most energetic form of electromagnetic radiation, with a very short
wavelength of less than one-tenth of a nano-meter. Gamma rays are
essentially very energetic x-rays emitted by excited nuclei. They often
accompany alpha or beta particles, because a nucleus emitting those
particles may be left in an excited (higher-energy) state.
Man made sources are produced by introducing an extra neutron to atoms
of the source material. As the material rids itself of the neutron, energy is
released in the form of gamma rays. Two of the more common industrial
Gamma-ray sources are Iridium-192 and Colbalt-60. These isotopes emit
radiation in two or three discreet wavelengths. Cobalt 60 will emit a 1.33
and a 1.17 MeV gamma ray, and iridium-192 will emit 0.31, 0.47, and 0.60
MeV gamma rays.
Advantages of gamma ray sources include portability and the ability to
penetrate thick materials in a relativity short time.
Disadvantages include shielding requirements and safety considerations.
 The part is placed between the
radiation source and a piece of 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
X-ray film test object.
Defects, such as voids, cracks,
inclusions, etc., can be detected.
= less exposure
= more exposure
Top view of developed film
 Contrast and Definition
Contrast
The first subjective criteria for determining radiographic quality is
radiographic contrast. Essentially, radiographic contrast is the
degree of density difference between adjacent areas on a
radiograph.

It is essential that sufficient

contrast exist between the defect
of interest and the surrounding
area. There is no viewing
technique that can extract
information that does not
already exist in the original
radiograph

low kilovoltage high kilovoltage

 Definition
Radiographic definition is the abruptness of change in going from
one density to another.

good poor
High definition: the detail portrayed in the radiograph is equivalent to
physical change present in the part. Hence, the imaging system
produced a faithful visual reproduction.
 Can be used in any situation when one wishes to view
the interior of an object
To check for internal faults and construction defects,
e.g. faulty welding
To see through what is inside an object
To perform measurements of size, e.g. thickness
measurements of pipes

Standard:
ASTM
ASTM E94-84a Radiographic Testing
ASTM E1032-85 Radiographic Examination of Weldments
ASTM E1030-84 Radiographic Testing of Metallic Castings
 There is an upper limit of thickness through
which the radiation can penetrate, e.g. -ray
from Co-60 can penetrate up to 150mm of steel
The operator must have access to both sides of
an object
Highly skilled operator is required because of
the potential health hazard of the energetic
radiations
Relative expensive equipment
Cracking can be detected in a radiograph only the crack is
propagating in a direction that produced a change in thickness that
is parallel to the x-ray beam. Cracks will appear as jagged and
often very faint irregular lines. Cracks can sometimes appearing as
"tails" on inclusions or porosity.
Burn through (icicles) results when too much heat causes
excessive weld metal to penetrate the weld zone. Lumps of
metal sag through the weld creating a thick globular condition
on the back of the weld. On a radiograph, burn through
appears as dark spots surrounded by light globular areas.
Gas porosity or blow holes Sand inclusions and dross
are caused by accumulated are nonmetallic oxides,
gas or air which is trapped by appearing on the radiograph
the metal. These as irregular, dark blotches.
discontinuities are usually
smooth-walled rounded
cavities of a spherical,
elongated or flattened shape.
 5.1 Introduction
In ultrasonic testing, high-frequency sound
waves are transmitted into a material to
detect imperfections or to locate changes
in material properties.
The most commonly used
ultrasonic testing technique is
pulse echo, whereby sound is
introduced into a test object and
reflections (echoes) from internal
imperfections or the part's
geometrical surfaces are returned
to a receiver. The time interval
between the transmission and
reception of pulses give clues to
the internal structure of the
material.
High frequency sound waves are introduced into a
material and they are reflected back from surfaces or
flaws.
Reflected sound energy is displayed versus time, and
inspector can visualize a cross section of the specimen
f
showing the depth of features that reflect sound.

initial
pulse

back surface
echo
crack
echo

crack

0 2 4 6 8 10 plate

Oscilloscope, or flaw
detector screen
Electrical currents are generated in a conductive material by an
induced alternating magnetic field. The electrical currents are
called eddy currents because the flow in circles at and just
below the surface of the material. Interruptions in the flow of
eddy currents, caused by imperfections, dimensional changes,
or changes in the material's conductive and permeability
properties, can be detected with the proper equipment.
Eddy current testing can be used on all electrically conducting
materials with a reasonably smooth surface.
The test equipment consists of a generator (AC power supply), a
test coil and recording equipment, e.g. a galvanometer or an
oscilloscope
Used for crack detection, material thickness measurement
(corrosion detection), sorting materials, coating thickness
measurement, metal detection, etc.
 When a AC passes through a
test coil, a primary magnetic
field is set up around the coil
The AC primary field induces
eddy current in the test
object held below the test
coil
A secondary magnetic field
arises due to the eddy
current
 Mutual Inductance
(The Basis for Eddy Current Inspection)
The magnetic field produced by circuit 1
will intersect the wire in circuit 2 and
create current flow. The induced current
flow in circuit 2 will have its own
magnetic field which will interact with
the magnetic field of circuit 1. At some
point P on the magnetic field consists of
a part due to i1 and a part due to i2. These
fields are proportional to the currents
producing them.

The flux B through circuits as the sum of two parts.

B1 = L1i1 + i2M
B2 = L2i2 + i1M
L1 and L2 represent the self inductance of each of the coils. The constant
M, called the mutual inductance of the two circuits and it is dependent on
the geometrical arrangement of both circuits.
 The strength of the
secondary field depends on
electrical and magnetic
properties, structural
integrity, etc., of the test
object
If cracks or other
inhomogeneities are
present, the eddy current,
and hence the secondary
field is affected.
 The changes in the
secondary field will be a
feedback to the primary coil
and affect the primary
current.
The variations of the
primary current can be
easily detected by a simple
circuit which is zeroed
properly beforehand
 6.2 Eddy Current Instruments
Voltmeter
Coil's
Coil magnetic field

Eddy current's
magnetic field
Eddy
currents

Conductive
material
 Depth of Penetration
Eddy currents are closed loops of induced current circulating in planes
perpendicular to the magnetic flux. They normally travel parallel to the
coil's winding and flow is limited to the area of the inducing magnetic field.
Eddy currents concentrate near the surface adjacent to an excitation coil
and their strength decreases with distance from the coil as shown in the
image. Eddy current density decreases exponentially with depth. This
phenomenon is known as the skin effect.

The depth at which eddy current density has decreased to 1/e, or about 37%
of the surface density, is called the standard depth of penetration ().
 Thetest coils are
commonly used in
three configurations
Surface probe
Internal bobbin
probe
Encircling probe
 6.3 Result presentation

The impedance plane

diagram is a very useful
way of displaying eddy
current data. The strength
of the eddy currents and
the magnetic permeability
of the test material cause
the eddy current signal on
the impedance plane to
react in a variety of
different ways.
 6.4 Applications
Crack Detection
Material Thickness
Measurements
Coating Thickness
Measurements
Conductivity Measurements For:
Material Identification
Heat Damage Detection
Case Depth Determination
Heat Treatment Monitoring
 Surface Breaking Cracks
Eddy current inspection is an excellent
method for detecting surface and near
surface defects when the probable defect
location and orientation is well known.

Successful detection requires:

1. A knowledge of probable defect type, position, and
orientation.
2. Selection of the proper probe. The probe should fit the
geometry of the part and the coil must produce eddy
currents that will be disrupted by the flaw.
3. Selection of a reasonable probe drive frequency. For
surface flaws, the frequency should be as high as
possible for maximum resolution and high sensitivity. In the lower image, there is a
For subsurface flaws, lower frequencies are necessary flaw under the right side of
to get the required depth of penetration.
the coil and it can be see that
the eddy currents are weaker
in this area.
 Mainly for automatic
production control
Round bars, pipes, wires and
similar items are generally
inspected with encircling
probes
Discontinuities and
dimensional changes can be
revealed
In-situ monitoring of wires
used on cranes, elevators,
towing cables is also an
useful application
 Primarily for
examination of tubes
in heat exchangers
and oil pipes
Become increasingly
popular due to the
wide acceptance of
the philosophy of
preventive
maintenance
 6.5 Advantages of ET
Sensitive to small cracks and other defects
Detects surface and near surface defects
Inspection gives immediate results
Equipment is very portable
Method can be used for much more than flaw detection
Minimum part preparation is required
Test probe does not need to contact the part
Inspects complex shapes and sizes of conductive
materials
 Limitations of ET
Only conductive materials can be inspected
Surface must be accessible to the probe
Skill and training required is more extensive than other
techniques
Surface finish and and roughness may interfere
Reference standards needed for setup
Depth of penetration is limited
Flaws such as delaminations that lie parallel to the
probe coil winding and probe scan direction are
undetectable
Inspection of Raw Products
Inspection Following
Secondary Processing
In-Services Damage
Inspection
Forgings,
Castings,
Extrusions,
etc.
 Machining
Welding
Grinding
Heat treating
Plating
etc.
Cracking
Corrosion
Erosion/Wear
Heat Damage
etc.
Periodically, power plants are
shutdown for inspection.
Inspectors feed eddy current
probes into heat exchanger
tubes to check for corrosion
damage.

Pipe with damage Probe

Signals produced
by various
amounts of
corrosion
thinning.
Electromagnetic devices
and visual inspections are
used to find broken wires
and other damage to the
wire rope that is used in
chairlifts, cranes and other
lifting devices.
Robotic crawlers
use ultrasound to
inspect the walls of
large above ground
tanks for signs of
thinning due to
corrosion.

Cameras on
long
articulating
arms are used
to inspect
underground
storage tanks
for damage.
 Nondestructive testing is used
extensively during the
manufacturing of aircraft.
NDT is also used to find cracks
and corrosion damage during
operation of the aircraft.
A fatigue crack that started at
the site of a lightning strike is
shown below.
 Aircraft engines are overhauled
after being in service for a period
of time.
They are completely disassembled,
cleaned, inspected and then
reassembled.
Fluorescent penetrant inspection
is used to check many of the parts
for cracking.
 Sioux City, Iowa, July 19, 1989
A defect that went
undetected in an
engine disk was
responsible for
the crash of
United Flight 232.
The failure of a pressure vessel
can result in the rapid release of
a large amount of energy. To
protect against this dangerous
event, the tanks are inspected
using radiography and
ultrasonic testing.
Special cars are used to
inspect thousands of miles
of rail to find cracks that
could lead to a derailment.
 The US has 578,000
highway bridges.
Corrosion, cracking and
other damage can all
affect a bridges
performance.
The collapse of the Silver
Bridge in 1967 resulted in
loss of 47 lives.
Bridges get a visual
inspection about every 2
years.
Some bridges are fitted
with acoustic emission
sensors that listen for
sounds of cracks growing.
NDT is used to inspect pipelines
to prevent leaks that could
damage the environment. Visual
inspection, radiography and
electromagnetic testing are some
of the NDT methods used.

Remote visual inspection using

a robotic crawler.

Magnetic flux leakage inspection.

This device, known as a pig, is
placed in the pipeline and collects
data on the condition of the pipe as it
is pushed along by whatever is being
transported.
Radiography of weld joints.
Boeing employees in Philadelphia were given the
privilege of evaluating the Liberty Bell for damage using
NDT techniques. Eddy current methods were used to
measure the electrical conductivity of the Bell's bronze
casing at a various points to evaluate its uniformity.