inspection is commonly called a "headshot.

"

Central Conductor:

When testing hollow parts such as pipes, tubes and fittings, a conductive circular bar can be placed between
the heads with the part suspended on the bar (the "central conductor") as shown in Figure 6. The part is then wetted
down with the bath solution and the current is applied, travelling through the central conductor rather than through
the part. The ID and OD of the part can then be inspected. As with a head shot, the magnetic field is perpendicular
to the current flow, wrapping around the test piece, so indications running axially down the length of the part can be
found using this technique.

Ultrasonic Testing (UT):

Ultrasonic testing uses the same principle as is used in naval SONAR and fish finders. Ultra-high frequency
sound is introduced into the part being inspected and if the sound hits a material with a different acoustic impedance
(density and acoustic velocity), some of the sound will reflect back to the sending unit and can be presented on a

8
visual display. By knowing the speed of the sound through the part (the acoustic velocity) and the time required for
the sound to return to the sending unit, the distance to the reflector (the indication with the different acoustic
impedance) can be determined. The most common sound frequencies used in UT are between 1.0 and 10.0 MHz,
which are too high to be heard and do not travel through air. The lower frequencies have greater penetrating power
but less sensitivity (the ability to "see" small indications), while the higher frequencies don't penetrate as deeply but
can detect smaller indications.

The two most commonly used types of sound waves used in industrial inspections are the compression (longitudinal)
wave and the shear (transverse) wave, as shown in above figure . Compression waves cause the atoms in a part to
vibrate back and forth parallel to the sound direction and shear waves cause the atoms to vibrate perpendicularly
(from side to side) to the direction of the sound. Shear waves travel at approximately half the speed of longitudinal
waves.

Sound is introduced into the part using an ultrasonic transducer ("probe") that converts electrical impulses
from the UT machine into sound waves, then converts returning sound back into electric impulses that can be
displayed as a visual representation on a digital or LCD screen (on older machines, a CRT screen). If the machine is
properly calibrated, the operator can determine the distance from the transducer to the reflector, and in many cases,
an experienced operator can determine the type of discontinuity (like slag, porosity or cracks in a weld) that caused
the reflector. Because ultrasound will not travel through air (the atoms in air molecules are too far apart to transmit
ultrasound), a liquid or gel called "couplant" is used between the face of the transducer and the surface of the part to
allow the sound to be transmitted into the part.

UT Techniques:
Straight Beam:-

Fig: Straight Beam

Straight beam inspection uses longitudinal waves to interrogate the test piece as shown at the right. If the
sound hits an internal reflector, the sound from that reflector will reflect to the transducer faster than the sound
coming back from the back-wall of the part due to the shorter distance from the transducer. This results in a screen
9
display like that shown at the right in Figure 11. Digital thickness testers use the same process, but the output is
shown as a digital numeric readout rather than a screen presentation.

Angle Beam:

Angle beam inspection uses the same type of transducer but it is mounted on an angled wedge (also called a
"probe") that is designed to transmit the sound beam into the part at a known angle. The most commonly used
inspection angles are 45 o, 60o and 70o, with the angle being calculated up from a line drawn through the thickness of
the part (not the part surface). A 60o probe is shown in above Figure. If the frequency and wedge angle is not
specified by the governing code or specification, it is up to the operator to select a combination that will adequately
inspect the part being tested.

In angle beam inspections, the transducer and wedge combination (also referred to as a "probe") is moved
back and forth towards the weld so that the sound beam passes through the full volume of the weld. As with straight
beam inspections, reflectors aligned more or less perpendicular to the sound beam will send sound back to the
transducer and are displayed on the screen.

Immersion Testing

Immersion Testing is a technique where the part is immersed in a tank of water with the water being used as the
coupling medium to allow the sound beam to travel between the transducer and the part. The UT machine is
mounted on a movable platform (a "bridge") on the side of the tank so it can travel down the length of the tank. The
transducer is swivel-mounted on at the bottom of a waterproof tube that can be raised, lowered and moved across the
tank. The bridge and tube movement permits the transducer to be moved on the X-, Y- and Z-axes. All directions of
travel are gear driven so the transducer can be moved in accurate increments in all directions, and the swivel allows
the transducer to be oriented so the sound beam enters the part at the required angle. Round test parts are often
mounted on powered rollers so that the part can be rotated as the transducer travels down its length, allowing the full
circumference to be tested. Multiple transducers can be used at the same time so that multiple scans can be
performed.

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Through Transmission:

Through transmission inspections are performed using two transducers, one on each side of the part as shown in
Figure 13. The transmitting transducer sends sound through the part and the receiving transducer receives the sound.
Reflectors in the part will cause a reduction in the amount of sound reaching the receiver so that the screen
presentation will show a signal with a lower amplitude (screen height).

Phased Array:

Phased array inspections are done using a probe with multiple elements that can be individually activated. By
varying the time when each element is activated, the resulting sound beam can be "steered", and the resulting data
can be combined to form a visual image representing a slice through the part being inspected.

Time of Flight Diffraction:

Time of Flight Diffraction (TOFD) uses two transducers located on opposite sides of a weld with the transducers set
at a specified distance from each other. One transducer transmits sound waves and the other transducer acting as a
receiver. Unlike other angle beam inspections, the transducers are not manipulated back and forth towards the weld,
but travel along the length of the weld with the transducers remaining at the same distance from the weld. Two
sound waves are generated, one travelling along the part surface between the transducers, and the other travelling
down through the weld at an angle then back up to the receiver. When a crack is encountered, some of the sound is
diffracted from the tips of the crack, generating a low strength sound wave that can be picked up by the receiving
unit. By amplifying and running these signals through a computer, defect size and location can be determined with
much greater accuracy than by conventional UT methods.

11
Radiographic Testing (RT):

Industrial radiography involves exposing a test object to penetrating radiation so that the radiation passes through the
object being inspected and a recording medium placed against the opposite side of that object. For thinner or less
dense materials such as aluminum, electrically generated x-radiation (X-rays) are commonly used, and for thicker or
denser materials, gamma radiation is generally used.
Gamma radiation is given off by decaying radioactive materials, with the two most commonly used sources
of gamma radiation being Iridium-192 (Ir-192) and Cobalt-60 (Co-60). IR-192 is generally used for steel up to2-1/2
- 3 inches, depending on the Curie strength of the source, and Co-60 is usually used for thicker materials due to its
greater penetrating ability.
The recording media can be industrial x-ray film or one of several types of digital radiation detectors. With
both, the radiation passing through the test object exposes the media, causing an end effect of having darker areas
where more radiation has passed through the part and lighter areas where less radiation has penetrated. If there is a
void or defect in the part, more radiation passes through, causing a darker image on the film or detector, as shown in
above figure.

RT Techniques:
Film Radiography
Film radiography uses a film made up of a thin transparent plastic coated with a fine layer of silver bromide on one
or both sides of the plastic. When exposed to radiation these crystals undergo a reaction that allows them, when
developed, to convert to black metallic silver. That silver is then "fixed" to the plastic during the developing process,
and when dried, becomes a finished radiographic film.

To be a usable film, the area of interest (weld area, etc.) on the film must be within a certain density (darkness) range
and must show enough contrast and sensitivity so that discontinuities of interest can be seen. These items are a
function of the strength of the radiation, the distance of the source from the film and the thickness of the part being
12
inspected. If any of these parameters are not met, another exposure ("shot") must be made for that area of the part.

Computed Radiography:

Computed radiography (CR) is a transitional technology between film and direct digital radiography. This technique
uses a reusable, flexible, photo-stimulated phosphor (PSP) plate which is loaded into a cassette and is exposed in a
manner similar to traditional film radiography. The cassette is then placed in a laser reader where it is scanned and
translated into a digital image, which take from one to five minutes. The image can then be uploaded to a computer
or other electronic media for interpretation and storage.

Computed Tomography:

Computed tomography (CT) uses a computer to reconstruct an image of a cross sectional plane of an object as
opposed to a conventional radiograph, as shown in Figure 9. The CT image is developed from multiple views taken
at different viewing angles that are reconstructed using a computer. With traditional radiography, the position of
internal discontinuities cannot be accurately determined without making exposures from several angles to locate the
item by triangulation. With computed tomography, the computer triangulates using every point in the plane as
viewed from many different directions.

Digital Radiography:

Digital radiography (DR) digitizes the radiation that passes through an object directly into an image that can be
displayed on a computer monitor. The three principle technologies used in direct digital imaging are amorphous

13
 silicon, charge coupled devices (CCDs), and complementary metal oxide semiconductors (CMOSs). These images
are available for viewing and analysis in seconds compared to the time needed to scan in computed radiography
images. The increased processing speed is a result of the unique construction of the pixels; an arrangement that also
allows a superior resolution than is found in computed radiography and most film applications.

INTRODUCTION TO ACOUSTIC EMISSION TESTING:

Acoustic Emission Testing (AE):

Acoustic Emission Testing is performed by applying a localized external force such as an abrupt mechanical load
or rapid temperature or pressure change to the part being tested. The resulting stress waves in turn generate short-
lived, high frequency elastic waves in the form of small material displacements, or plastic deformation, on the part
surface that are detected by sensors that have been attached to the part surface. When multiple sensors are used, the
resulting data can be evaluated to locate discontinuities in the part.

Acoustic Emission (AE) refers to the generation of transient elastic waves produced by a sudden redistribution of
stress in a material. When a structure is subjected to an external stimulus (change in pressure, load, or temperature),
localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are
recorded by sensors. With the right equipment and setup, motions on the order of picometers (10 -12 m) can be
identified. Sources of AE vary from natural events like earthquakes and rockbursts to the initiation and growth of
cracks, slip and dislocation movements, melting, twinning, and phase transformations in metals. In composites, matrix
cracking and fiber breakage and debonding contribute to acoustic emissions. AE‟s have also been measured and
recorded in polymers, wood, and concrete, among other materials.
Detection and analysis of AE signals can supply valuable information regarding the origin and importance of a
discontinuity in a material. Because of the versatility of Acoustic Emission Testing (AET), it has many industrial
14
applications (e.g. assessing structural integrity, detecting flaws, testing for leaks, or monitoring weld quality) and is
used extensively as a research tool.
Acoustic Emission is unlike most other non destructive testing (NDT) techniques in two regards. The first
difference pertains to the origin of the signal. Instead of supplying energy to the object under examination, AET simply
listens for the energy released by the object. AE tests are often performed on structures while in operation, as this
provides adequate loading for propagating defects and triggering acoustic emissions.
The second difference is that AET deals with dynamic processes, or changes, in a material. This is particularly
meaningful because only active features (e.g. crack growth) are highlighted. The ability to discern between developing
and stagnant defects is significant. However, it is possible for flaws to go undetected altogether if the loading is not
high enough to cause an acoustic event. Furthermore, AE testing usually provides an immediate indication relating to
the strength or risk of failure of a component. Other advantages of AET include fast and complete volumetric
inspection using multiple sensors, permanent sensor mounting for process control, and no need to disassemble and
clean a specimen.
Unfortunately, AE systems can only qualitatively gauge how much damage is contained in a structure. In order to
obtain quantitative results about size, depth, and overall acceptability of a part, other NDT methods (often ultrasonic
testing) are necessary. Another drawback of AE stems from loud service environments which contribute extraneous
noise to the signals. For successful applications, signal discrimination and noise reduction are crucial.

Guided Wave Testing (GW):

Guided wave testing on piping uses controlled excitation of one or more ultrasonic waveforms that travel along the
length of the pipe, reflecting from changes in the pipe stiffness or cross sectional area. A transducer ring or exciter
coil assembly is used to introduce the guided wave into the pipe and each transducer/exciter. The control and analysis
software can be installed on a laptop computer to drive the transducer ring/exciter and to analyze the results. The
transducer ring/exciter setup is designed specifically for the diameter of the pipe being tested, and the system has the
advantage of being able to inspect the pipe wall volume over long distances without having to remove coatings or
insulation. Guided wave testing can locate both ID and OD discontinuities but cannot differentiate between them.
Laser Testing Methods (LM);
Laser Testing includes three techniques, Holography, Shearography and Profilometry. As the method name implies,
all three techniques user lasers to perform the inspections.
LM Techniques:
Holographic Testing
Holographic Testing uses a laser to detect changes to the surface of a part as it deforms under induced stress which
can be applied as mechanical stress, heat, pressure, or vibrational energy. The laser beam scans across the surface of
the part and reflects back to sensors that record the differences in the surface created by that stress. The resulting
image will be a topographical map-like presentation that can reveal surface deformations in the order of 0.05 to 0.005

15
microns without damage to the part. By comparing the test results with an undamaged reference sample, holographic
testing can be used to locate and evaluate cracks, delaminations, disbonds, voids and residual stresses.

Laser Profilometry:

Laser Profilometry uses a high-speed rotating laser light source, miniature optics and a computer with high-speed
digital signal processing software. The ID surface of a tube is scanned in two dimensions and the reflected light is
passed through a lens that focuses that light onto a photo-detector, generating a signal that is proportional to the
spot's position in its image plane. As the distance from the laser to the ID surface changes, the position of the focal
spot on the photo-detector changes due to parallax, generating a high resolution three-dimensional image of the part
surface that represents the surface topography of the part. This technique can be used to detect corrosion, pitting,
erosion and cracks in pipes and tubes.
Laser Shearography:
Laser Shearography applies laser light to the surface of the part being tested with the part at rest (non-stressed) and
the resulting image is picked up by a charge-coupled device (CCD) and stored on a computer. The surface is then
stressed and a new image is generated, recorded and stored. The computer then superimposes the two patterns and if
defects such as voids or disbonds are present, the defect can be revealed by the patterns developed. Discontinuities as
small as a few micrometers in size can be detected in this manner.
Leak Testing (LT):
Leak Testing, as the name implies, is used to detect through leaks using one of the four major LT techniques: Bubble,
Pressure Change, Halogen Diode and Mass Spectrometer Testing. These techniques are described below.
LT Techniques Bubble Leak Testing

16
Bubble Leak Testing, as the name implies, relies on the visual detection of a gas (usually air) leaking from a
pressurized system. Small parts can be pressurized and immersed in a tank of liquid and larger vessels can be
pressurized and inspected by spraying a soap solution that creates fine bubbles to the area being tested. For flat
surfaces, the soap solution can be applied to the surface and a vacuum box can be used to create a negative pressure
from the inspection side. If there are through leaks, bubbles will form, showing the location of the leak.

Pressure Change Testing:

Pressure Change Testing can be performed on closed systems only. Detection of a leak is done by either pressurizing
the system or pulling a vacuum then monitoring the pressure. Loss of pressure or vacuum over a set period of time
indicates that there is a leak in the system. Changes in temperature within the system can cause changes in pressure,
so readings may have to be adjusted accordingly.

Halogen Diode Testing:

Halogen Diode Testing is done by pressurizing a system with a mixture of air and a halogen-based tracer gas. After a
set period of time, a halogen diode detection unit, or "sniffer", is used to locate leaks.

Mass Spectrometer Testing:

Mass Spectrometer Testing can be done by pressurizing the test part with helium or a helium/air mixture within a test
chamber then surveying the surfaces using a sniffer, which sends an air sample back to the spectrometer. Another
technique creates a vacuum within the test chamber so that the gas within the pressurized system is drawn into the
chamber through any leaks. The mass spectrometer is then used to sample the vacuum chamber and any helium
present will be ionized, making very small amounts of helium readily detectable.

Magnetic Flux Leakage (MFL):

17
 Magnetic Flux Leakage detects anomalies in normal flux patterns created by discontinuities in ferrous material
saturated by a magnetic field. This technique can be used for piping and tubing inspection, tank floor inspection and
other applications. In tubular applications, the inspection head contain is made up of drive and sensor coils and a
position transducer that is connected by cable back to the power source and signal processing computer. This head

is placed around the pipe or tube to be inspected and the drive coil is energized, creating a magnetic field in the part.
As the head travels along the length of the part, variations in the wall thickness due to corrosion, erosion, pitting etc.,
will cause a change in the magnetic flux density can be picked up by the sensor and sent back to the computer. The
location of this signal is sent by the position transducer so that the area detected can be marked for further evaluation.
This technique can be done without removing the insulation, resulting in a fast, economic way to inspect long runs of
pipe or tubing.

Tank floor inspection applies the same principle, but uses a series of magnetic field generators ("bridges")
and sensors (as shown in Figure 16) located side by side across the front of a vacuum sweeper-like machine. The
bridges generate a magnetic field that saturates the tank floor, and any reduction in thickness or loss of material due
to pitting or corrosion will cause the field to "leak" upwards out of the floor material where it can be picked up by the
sensors. On very basic machines, each sensor will be connected to an audio and/or visual display that lets the
operator know there is an indication; more advanced machines can have both visual displays and recording capability
so that the results can be stored, analyzed and compared to earlier results to monitor discontinuity growth.

Neutron Radiographic Testing (NR):

Neutron radiography uses an intense beam of low energy neutrons as a penetrating medium rather than the gamma-
or x-radiation used in conventional radiography. Generated by linear accelerators, betatrons and other sources,
neutrons penetrate most metallic materials, rendering them transparent, but are attenuated by most organic materials
(including water, due to its high hydrogen content) which allows those materials to be seen within the component
being inspected. When used with conventional radiography, both the structural and internal components of a test
piece can be viewed.

Thermal/Infrared Testing (IR):

Thermal/Infrared Testing, or infrared thermography, is used to measure or map surface temperatures based on the
infrared radiation given off by an object as heat flows through, to or from that object. The majority of infrared
radiation is longer in wavelength than visible light but can be detected using thermal imaging devices, commonly
called "infrared cameras." For accurate IR testing, the part(s) being investigated should be in direct line of sight with
the camera, i.e., should not be done with panel covers closed as the covers will diffuse the heat and can result in false
readings. Used properly, thermal imaging can be used to detect corrosion damage, delaminations, disbonds, voids,
inclusions as well as many other detrimental conditions.
18
Vibration Analysis (VA):
Vibration analysis refers to the process of monitoring the vibration signatures specific to a piece of
rotating machinery and analyzing that information to determine the condition of that equipment. Three types of
sensors are commonly used: displacement sensors, velocity sensors and accelerometers.
Displacement sensors uses eddy current to detect vertical and/or horizontal motion (depending on
whether one or two sensors are used) and are well suited to detect shaft motion and changes in clearance
tolerances.
Basic velocity sensors use a spring-mounted magnet that moves through a coil of wire, with the outer case
of the sensor attached to the part being inspected. The coil of wire moves through the magnetic field, generating
an electrical signal that is sent back to a receiver and recorded for analysis. Newer model vibration sensors use
time-of- flight technology and improved analysis software. Velocity sensors are commonly used in handheld
sensors.
Basic accelerometers use a piezoelectric crystal (that converts sound waves to electrical impulses and
back) attached to a mass that vibrates due to the motion of the part to which the sensor casing is attached. As the
mass and crystal vibrate, a low voltage current is generated which is passed through a pre-amplifier and sent to the
recording device. Accelerometers are very effective for detecting the high frequencies created by high speed
turbine blades, gears and ball and roller bearings that travel at much greater speeds than the shafts to which they
are attached.

Guided Wave Testing (GW):

Guided wave testing on piping uses controlled excitation of one or more ultrasonic waveforms that travel
along the length of the pipe, reflecting from changes in the pipe stiffness or cross sectional area. A transducer ring
or exciter coil assembly is used to introduce the guided wave into the pipe and each transducer/exciter. The control
and analysis software can be installed on a laptop computer to drive the transducer ring/exciter and to analyze the
results. The transducer ring/exciter setup is designed specifically for the diameter of the pipe being tested, and the
system has the advantage of being able to inspect the pipe wall volume over long distances without having to
remove coatings or insulation. Guided wave testing can locate both ID and OD discontinuities but cannot
differentiate between them.

19