LEARNING UNIT V

PROCESS INSTRUMENTATION III

Learning Outcome Analyse and describe a variety of measurement
techniques using Radio-Active Sources and a variety of
non-destructive testing techniques for the purposes of
process control applications.
Prerequisites Learning Unit IV
Duration Week
After completion of 1. Describe and sketch the different Radio-Active
this learning unit one Source measuring techniques used for process
control.
should be able to:
2. Describe and sketch a variety of Non-Destructive
Testing methods that are used for process control.
Assessment 1. Online Test
2. Semester Test (Combined online test)
3.

MEASUREMENT WITH RADIO-ACTIVE SOURCES

5.1 INTRODUCTION
All elements are built up from atoms and an atom can be further divided into:

1. Protons

2. Neutrons

3. Electrons

5.1.1. Protons
Protons form a part of the nucleus of an atom. They have a unit positive charge and a
unit mass. Every atom has protons in its nucleus. The number of protons in an atom
varies, however, from one element to another.

5.1.2. Neutrons
Neutrons are normally also found in the nucleus of an atom. A neutron has no electric
charge but its mass is the same as that of a proton. The hydrogen atom, which is the
least complex atom, is the only one that does not have neutrons in its nucleus. Its
nucleus consists of a single proton, only.

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5.1.3. Electrons
Electrons rotate around the nucleus in different orbits or shells. The electron has a
negative charge of 1 unit and a mass of 1/1840 units.

The orbit or shell which is nearest the nucleus is the K-shell and has only two
electrons. The next one is the L-shell and has a maximum of 8 electrons. Then come
the M-shell with a maximum of 18 and the N-shell with a maximum of 32 electrons.

5.2 ISOTOPES
The chemical characteristics of an element are determined by the number of electrons
rotating around the nucleus of the atom. All the atoms of a specific element have the
same number of electrons. In an electrically neutral atom, the number of electrons is
equal to the number of protons and neutrons in the nucleus and its symbol is A. The
number of neutrons in the nucleus is indicated by the symbol N. Therefore one can
write:

A= N×Z (5.1)

A - Is the mass number

Z - The atomic number

If X represents the chemical symbol for an element, any element may be represented
as:

any element = ZAX (5. 2)

For example, consider the element hydrogen, which is represented by the chemical
symbol:

hydrogent element = 11H (5. 3)

It has only one proton and no neutrons in each nucleus of each atom, and hence it has
an atomic number of 1 and an atomic mass of 1 too.

There are however other hydrogen atoms e.g.

hydrogent deuterium element = 12H (5.4)

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hydrogent tritium element = 13H (5.5)

Those atoms of hydrogen are called isotopes of hydrogen. These are atoms of
hydrogen which have acquired neutrons in their nuclei. These atoms have the same
atomic number, i.e. 1 for 1 proton in each nucleus, but the mass number may be 1, 2
or 3.

In general then, we may say that nuclides of elements having the same atomic number
but different atomic masses are isotopes of the element.

5.3 RADIO ISOTOPES

Up to now we have seen that the nuclei of all the atoms, except hydrogen, consist of
protons and neutrons. Stable nuclei of atoms with atomic masses up to 40 have
roughly the same number of protons and neutrons. Atoms with atomic masses of
greater than 40 tend to have more neutrons than protons in their nuclei, within limits.
Atoms which do not fall within these limits are radio-active. Radio isotopes may
radiate one or more of the following types of particles:

1. Alpha Particles

2. Beta Particles

3. Gamma Rays

5.3.1. Alpha Particles

Alpha Particles consist of two protons and two neutrons. An alpha particle has an
atomic mass of 4 and an atomic number of 12 – i.e. exactly the same as the nucleus of
a helium atom. An alpha particle has a double positive charge (2 protons), and a mass
of approximately 7000 times that of an electron. The penetrating properties of alpha
particles are very limited, because an alpha particle is physically large in comparison
with an electron.

Alpha particles can easily ionise other atoms when they collide with them, because
they travel at high speed and are relatively massive and therefore they are radiated at
relative high kinetic energy levels.

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Alpha particle radiation can be screened off quite easily, even by means of a thin
piece of paper because the particles are physically large.

5.3.2. Beta Particles

Beta particles consist of electrons and can have either a nett positive or nett negative
charge. The nucleus of an atom has no free electrons however, so the question is:
Where do the electrons, which are radiated from the nucleus of a radio isotope, come
from?

The answer is that a neutron can split into an electron and a proton. The proton then
remains in the nucleus while the electron is radiated. This increases the atomic
number of the isotopes by one unit. The electron is radiated as either a positive or a
negative beta particle.

Beta particles are very small like electrons and have either unit positive charge or unit
negative charge. The mass of a beta particle is approximately 7000 times smaller than
an alpha particle and therefore the penetrating properties of the beta particle are much
more effective than is the case with alpha particles, but even so they can only
penetrate a relatively thin layer of skin, and because of the small mass the kinetic
energy of a moving beta particle is not all that big. Beta particles cannot therefore
easily cause isolation.

Beta particles can be screened easily with about a 1 cm thickness of Perspex.

5.3.3. Gamma Radiation

Gamma radiation is electromagnetic radiation, which falls within the wavelength
range 3 × 10 −13 to 3 × 10 −14 of the electromagnetic spectrum.

Since gamma rays are electromagnetic waves, they have no mass and also no electric
charge. When alpha and beta decay occurs, large amounts of energy are liberated.
Some of this energy is left behind in the atom and this is then radiated in the form of
gamma rays from the unstable atom.

Since gamma radiation is electromagnetic radiation with no mass, it is able to

penetrate deeply. Gamma radiation is less likely to cause ionisation than is the case
with alpha or beta particles.

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Gamma radiation can be screened by lead, concrete or water.

5.4 RADIATION UNITS

The units which use to be used were the curie, the roentgen, the rad and rem, but they
have now been replaced by SI-Radiation Units. These SI-Radiation Units are listed
below as:

• Unit of Becquerel

• Unit of Exposure

• Unit of Absorbed Dose

• Unit of Equivalent Dose

5.4.1. Becquerel (Bq)

The SI-Unit of activity (radio-active disintegration rate) is the Becquerel, and is
defined as the activity of a quantity of radioactive material in which one nucleus
decays per second. Equation (5.6) represents the Becquerel definition.

1 Bq = 1 s −1 (5.6)

5.4.2. Unit of Exposure

The SI unit of exposure to ionizing radiation is 1 coulomb per kilogram (1 C/kg), and
is the amount of electromagnetic radiation (of x-rays or gamma rays), which produces
in 1 kg pure dry air ion-pairs, with a charge of 1 coulomb having positive and
negative charges.

5.4.3. Unit of Absorbed Dose

The unit of absorbed dose is called the gray (Gy). Absorbed dose is 1 Gy when the
energy per unit mass, given to material by ionisation, is 1 joule per kilogram (1 J/kg).

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5.4.4. Unit of Equivalent Dose
The different types of radiation cause different effects in human tissue. Thus a
weighted unit of absorbed dose is used and these are known as units of equivalent
dose. The dose equivalents are used to compare the effects of different types of
radiation of human tissue. The dose equivalent is the product of the absorbed dose
and a weighted factor (wR), which is determined by the type of radiation involved.

Table 5.1 shows the weighted factors of different types of radiation.

Table 5.1
Weighted Factor
Type of Radiation
(wR)
X-Rays 1
Gamma Rays 1
Beta Particles 1
Thermal Neutrons 3
Alpha Particles 10
Fast Neutrons 10

The Sievert (symbol: Sv) is the SI derived unit of dose equivalent.

The Sievert is the dose equivalent when the absorbed dose of ionisation radiation,
multiplied by the appropriate factor amount to 1 joule per kilogram (1 J/kg). This unit
used to be called the rem.

5.5 RADIO-ACTIVE DECAY

A radio-active isotope decays very slowly over a period of time and release energy in
this way. It is impossible to precisely predict when an unstable nucleus will decay,
and when there are a large number of unstable nuclides of the same type, it is possible
to measure the average atoms present at that instant. It is also possible to measure the
decay-factor of the applicable radio-active atoms. Activity can be expressed as shown
in Eq. (5.7).

dN
A= = −λN (5.7)
dt

dN
- Rate of Decay
dt

λ - Decay Factor

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N – Number of unstable nuclides

Where the rate of decay is a measure of the number of disintegrations per second, and
is known as the activity of the isotope. The numerical sign indicates that N gets
smaller as time passes.

5.6 HALF-LIFE TIME OF A RADIO ISOTOPE

The half-life of a radio isotope is the time it takes for the activity of the particular
isotope to decrease to 50% of its original value. Half-life can be expressed as shown
in Eq. (5.7).

0.693
t= (5.8)
λ
t - Half-life time of the isotope in question
The half-life of an isotope can vary from a fraction of a second to hundreds of years,
dependant on the type of isotope involved.

5.7 SAFETY WITH A RADIO-ACTIVE ISOTOPE

Radio-active sources are very usefully employed in medical technology and also in
industry. Radio-activity can however be severely detrimental to health. The human
body can absorb radio-activity through contaminated food, by inhalation or by direct
ionisation from the source. Radiation can be controlled and limited by limiting the
amount of time spent near the source, by controlling the distance between the source
and the person and by proper screening.

The time spent near the source must be as short as possible. The distance between the
person and the source should be as large as possible. The inverse-square rule is
applicable here.

By using the correct screening material, a person can be protected against radiation.

5.8 RADIO-ACTIVE DETECTORS

The following section will explain the working principles of detecting radio-activity
by means of different types of radio-active detectors. They are listed below as:

• Ionisation Chambers

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• Proportional Counters

• Geiger-Müller Counters

• Simple Counting Statistics

5.8.1. Ionisation Chambers

Nuclear-radiation particles have sufficient energy to ionise gases. An electron and an
atom, which has lost an electron, becomes what are known as an ion-pair (i.e. a
positive ion and a negative electron).

Normally, positive and negative charges tend to recombine, but this can be prevented
by using a positive electrode to attract the negative electrons and a negative electrode
to attract the positive ions.

ALPHA
PARTICLES
++
++
+
+
ANODE + +

+ - +
-
-
+ - +
-
- + -
+
CATHODE -

GALVANOMETER

Figure 5.1: Ionisation Chamber

A galvanometer connected to the electrodes will then show a current which will be
proportional to the intensity of the radiation, the type of gas which fills the chamber
and the voltage applied to the electrodes. The voltage applied to the electrodes must
be high enough to attract the ions to the electrodes. Ionisation chambers can be used
to detect alpha particles, beta particles, gamma radiation and neutrons. Figure 5.3
shows the relationship between the current and voltage when ionisation occurs when
using an Ionisation Chamber.

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The number of pulses registered on the galvanometer is an indication of the intensity
of the radiation. A rate-counter can also be used as an indicating instrument as shown
in Fig. 5.2.

Each pulse received, charges C and then discharges through R. The magnitude of the
current then represents the intensity of the radiation. By changing the time constant R
and C, the reading can be made to fluctuate less between pulses, but the sensitivity
decreases if there is a sudden change in the pulse rate.

PULSES

RESERVOIR C
CAPACITOR

GALVANOMETER

Figure 5.2: Rate Counter

5.8.2. Proportional Counters

When the voltage, applied to the electrodes of the ionisation chamber, is increased
substantially, secondary ionisation occurs as shown in Fig. 5.3. The light, fast-
moving electrons are then attracted to the positive electrode even more quickly and
cause further ionisation on the way.

A single ionisation particle can then cause a large pulse. The magnitude of the pulse
is then proportional to the energy of the original particle that caused the ionisation.
The positive ions, on the other hand, moves too slowly to cause secondary ionisation.

5.8.3. Geiger-Müller Counters

The construction of the proportional counter and the Geiger-Müller counter are
similar in construction, but the gas in the proportional counter is approximately at
atmospheric pressure while the gas in the Geiger-Müller counter also contains a
damping gas together with the ionisation gas.

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By increasing the voltage between the cathode and the anode of the proportional
counter still further, as indicated in Fig. 5.3, each primary electron produces a large
number of secondary electrons. This type of ionisation is known as avalanche
ionisation. Pulses of constant amplitude are produced. When the slowly moving
positive ions reach the cathode, they may dislodge electrons from the cathode and
make the ionisation process begin all over again. The damping gas has the function of
neutralising the positive ions, thus preventing further ionisation from taking place.

∞
CURRENT

GEIGER-MüLLER
COUNTER

PROPORTIONAL
IONISATION COUNTER
CHAMBER

0 ∞
VOLTAGE

Figure 5.3: O/P Current vs. Voltage for Radio-Active Detectors

5.8.4. Simple Counting Statistics

Radio-active decay is an arbitrary process. The time it takes for a specific atom to
decay cannot be predicted and is independent of the rate of decay of other atoms.
Using statistical methods however, the most probable count can be determined. The
actual count will normally be a little more than, or a little less than the average
calculated count. The count is then given as the average count plus or minus the
standard deviation. The standard deviation is defined as the square-root of the
average count. When standard deviations are used, 31.7% of the large number of
readings will have a deviation greater than 31.7%.

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Example 5.1
100 Counts were registered in 5 seconds in a certain experiment. Determine the count
rate and standard deviation.

n n
Count Rate = ± 2
t t
100 100
= ±
5 52
= 20 ± 2

Therefore, the Count rate is 20 counts per second with a standard deviation of ± 2
counts per second. The count therefore lies between 18 and 22.

Usually the count will be influenced by a background count which will always be
present, i.e. cosmic radiation etc.

In order to obtain a more accurate count, the background count must be measured
beforehand and this must then be subtracted from the count produced by the source
and background together.

Example 5.2
6720 Counts were registered with a Geiger-Müller counter in 4 minutes from a
source. The background radiation is measured for 10 minutes and the count was 480.
Determine the corrected count rate and the standard deviation.

n b n b
Count Rate = − ±  2 + 2 
t1 t 2  t1 t 2 
6720 480 6720 480
= − ± −
240 600 240 2 600 2
= 27.2 ± 0.34 per sec ond

5.9 INSTRUMENTATION WITH RADIO-ACTIVE SOURCES

The following different measuring methods using instrumentation which makes use of
radio-active sources will be explained in the next sections:

• Measurement of Level

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• Measurement of Density

• Measurement of Moisture in Solid Materials

• Measurement of Thickness

• Measurement of Coating Thickness

5.9.1. Measurement of Level

Basically, there are three methods used to measure level:

1. Change of distance between the source and the detector.

2. Absorption of energy by the medium being measured.

3. The “switch” principle, where a large amount of energy is absorbed by

the medium.

Radio-active methods are usually used to measure the level of dangerous liquids,
where other methods are unsuitable (e.g. molten glass, molten iron, etc.). Radio-
active methods also have the advantage that flanges do not have to be mounted on the
containers which hold the liquid. The liquid can not block the impulse lines. High
temperatures cannot influence the source or the detector, because they do not make
contact with the liquid.

Figure 5.4 represents a level measuring technique by means of absorption of energy

by the medium being measured.

DETECTOR AMPLIFIER

SOURCE

Figure 5.4: Absorption of Energy

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Figure 5.5 represents a level measuring technique by means of the change in distance
between the source and the detector.

DETECTOR AMPLIFIER

SOURCE

Figure 5.5: Change of Distance

Figure 5.6 represents a level measuring technique by means of the “Switch Principle”.

SOURCE
DETECTOR AMPLIFIER

Figure 5.6: The Switch Principle

The disadvantages of these methods are that a reading is influenced when the density
of the liquid changes. The change in the reading is also non-linear with respect to the
level. Radio-active sources involve a health risk, and a special license has to be
obtained by users of radio-active sources.

5.9.2. Measurement of Density

The principle used to measure density is the same as that used for level measurement.
The energy absorbed is proportional to the change in density of the medium being
measured.

Two installations methods are shown in Fig. 5.7 and 5.8 respectively for pipe
diameters smaller than 150 mm, and those greater than 150 mm.

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LARGE PIPE
> 150 MM

SOURCE

DETECTOR AMPLIFIER

FLOW

Figure 5.7: Pipe Diameter Greater than 150 mm

SOURCE

FLOW

SMALL PIPE
< 150 MM

DETECTOR AMPLIFIER

Figure 5.8: Pipe Diameter Smaller than 150 mm

5.9.3. Measurement of Mass

The actual mass of the material moving past a certain point is a function of the
amount of material on the belt and the belt speed. The tachometer produces a signal
which is proportional to the speed of the belt. The belt speed signal and the mass-
signal of the multiplier indicate the actual mass of the material. The total flow of
material can be shown, using an integrator. Any change of density of the material,
will however, influence the readings. The absorption method used to measure mass is
shown in Fig. 5.9.

5.9.4. Measurement of Moisture in Solid Materials

Neutrons have no charge and therefore they are deflected when materials are
bombarded with neutrons and they collide with the nucleus of an atom. The energy

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emitted by the collision is a function of the mass of the nucleus with which they
collide.

AMP MULTIPLIER INTEGRATOR

DETECTOR

IDLERS
OR
ROLLERS

SPEED
SIGNAL

CONVEYER
BELT
SOURCE

TACHOMETER

Figure 5.9: Absorption Method of Mass Measurement

When the mass of the nucleus is equal to the mass of the colliding neutrons, the
neutron is brought to a complete stand-still. Now the mass of the nucleus of a
hydrogen atom is equal to the mass of the neutron. Therefore, water is the most
efficient absorber of neutrons.

Fig. 5.10 shows how the moisture in solid materials is measured.

TO RECORDER
MOISTURE DENSITY

THERMAL FAST
NEUTRON NEUTRON GAMMA GAMMA
DETECTOR SOURCE DETECTOR SOURCE

SCRAPER

FLOW

Figure 5.10: Moisture Measurement in Solid Materials

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The density of the sample affects the number of neutrons that are deflected. The
density of the material is measured using a gamma source. The signals obtained from
the neutron detector and the gamma detector is scaled to indicate the amount of
moisture in the substance.

5.9.5. Measurement of Thickness

The mass-measurement method can also be used to measure the thickness. An
example of thickness measurement of a strip of metal is shown in Fig. 5.11.

AMP RECORDER

DETECTOR

MATERIAL
BEING
MEASURED

SOURCE

Figure 5.11: Thickness Measurement

5.9.6. Measurement of Coating Thickness

Sometimes it is required to measure the thickness of a coating on, say a metal strip.
Fig. 5.12 shows how the thickness of the coating may be measured using radio-active
methods up to an accuracy of 1% or more.

DETECTOR 1 DETECTOR 2
COATING

SOURCE 1 SOURCE 2

Figure 5.12: Thickness Measurement

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Source 1 and detector 1 are used to measure the thickness of the material, and source
2 and detector 2 are used to measure the thickness of the material plus the thickness of
the coating. Subtraction of the results gives the thickness of the coating.

5.10 ANALYTICAL METHODS

For this course only two analytical methods will be discussed and they are listed
below as:

• X-Ray Analyses

• Neutron-Activation Analyses

5.10.1. X-Ray Analyses

When the electron is removed from one of the inner shells of an atom, an electron
from an outer shell with higher energy will take the place of the one removed from the
inner shell with lower energy, in order to stabilise the atomic situation. The excess
energy is given off as an X-ray.

By bombarding the substance with X-rays, it is possible to cause the above-mentioned

electron displacement. The substance then gives off X-rays which are unique to the
elements in the substance. The spectrum of the X-rays is then analysed, and the type
of element can be identified.

5.10.2. Neutron-Activation Analyses

When a sample is bombarded with neutrons, the nuclei of the atoms of the element
take up the neutrons. Most elements become radio-active by virtue of the balance
between protons and neutrons. The atomic number of the element remains
unchanged, but the atomic mass changes. Immediately after the neutrons are
absorbed, energy in the form of a gamma ray is emitted. The radio-activity of the so-
called, radio-isotope begins to decay, and the radio-activity given off is unique to the
particular element. By measuring this decay the element can be identified.

Example 5.1
23
Na + n → 24
Na + 8

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Where n is the uptake of neutrons and 8 is the energy that is emitted over time as
shown in the Decay process below.

Decay:

0 1 beta particle → energy 1.389 MeV 0

↓ 1 gama ray → energy 1.369 MeV ↓
Time Time
↓ 1 gama ray → energy 2.754 MeV ↓
∞ ∞

Where eV is the unit for energy and 1 eV = 1.602 176 53(14)×10−19 J.

Neutron-Activation Analysis has the following advantages:

1. The method is very sensitive.

2. It is a very selective method.

3. The influence of other elements during analysis is minimal.

NON-DESTRUCTIVE TESTING

5.11 INTRODUCTION TO NON-DESTRUCTIVE TESTING

Three of the most common causes of failures in engineering materials/components
are: inadequate design, incorrect use or presence of defects (flaws) in the materials.
Non-destructive testing methods are used to detect structural deficiencies in materials.

5.12 VISUAL TECHNIQUES

Defects are sometimes plainly fusible. For recording purposes, photographic methods
can be used. Dye penetrants may be used to make defects more visible. A liquid
containing the colouring agent (dye) is applied to the surface, and penetrates into the
material wherever cracks are present. After a few minutes, the surface is wiped clean
and coated with a powder that has the ability to absorb the dye, in the same way as
blotting paper absorbs ink. The powder thus becomes stained around the defects.

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5.13 MAGNETIC FLUX METHOD

If a ferromagnetic surface is magnetised, the magnetic flux will tend to confine itself
within the material. If a defect is present on the surface, the flux is deflected as shown
in Fig. 5.13.

FLUX LEAKAGE

Figure 5.13: Magnetic Flux Method

Flux leakages can only be detected if a high level of magnetisation can be obtained.
For a round bar of steel with a diameter of 25 mm, an electric current of more than
100 A may be required.

Magnetic flux leakage can also be used for Non-Destructive Testing (NDT) by
applying finely divided ferric-oxide particles to the surface. The particles tend to
accumulate around the point of flux leakage, just as iron filings accumulate at the tips
(poles) of a horse-shoe magnet. It is not easy to obtain a permanent record of the
flaws using this method.

5.14 POTENTIAL DROP TECHNIQUES

A discontinuity in the surface of a conducting material can be detected by measuring
the resistance of the material. A four-point probe can be used as shown in Fig. 5.14.

V1 V2

V2 > V1

Figure 5.14: Potential Drop Method

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The potential difference across the crack is measured by the two inner probes, and the
magnitude of the potential difference is proportional to the depth of the crack.

5.15 EDDY CURRENT TECHNIQUES (NDT Resource Center, 2001)

The eddy current technique is a very effective and convenient method of detecting
flaws, as well as determining the properties of materials.

Eddy current inspection is one of several NDT methods that use the principal of
“electromagnetism” as the basis for conducting examinations. Several other methods
such as Remote Field Testing (RFT), Flux Leakage and Barkhausen Noise also use
this principle.

Eddy currents are created through a process called electromagnetic induction. When
alternating current is applied to the conductor, such as copper wire, a magnetic field
develops in and around the conductor. This magnetic field expands as the alternating
current rises to maximum and collapses as the current is reduced to zero. If another
electrical conductor is brought into the close proximity to this changing magnetic
field, current will be induced in this second conductor. Eddy currents are induced
electrical currents that flow in a circular path. They get their name from “eddies” that
are formed when a liquid or gas flows in a circular path around obstacles when
conditions are right.

In order to generate eddy currents for an inspection, a probe is used. Inside the probe
is a length of electrical conductor which is formed into a coil. Alternating current is
allowed to flow through the coil at a frequency determined by the type of test
involved. An expanding and collapsing magnetic field (dynamic) forms in the coil as
the alternating current flows through the coil. When an electrically conductive
material is placed in the coils dynamic magnetic field, electromagnetic induction will
occur and eddy currents will be induced in the material. Eddy Currents flowing in the
material will generate their own secondary magnetic field which will oppose the coils
primary magnetic field as shown in Fig. 5.15 (a). This entire electromagnetic
induction process to produce Eddy Currents may occur from several hundred to
several million times each second depending on the frequency.

When there is a flaw within the material the Eddy Currents are disrupted as shown in
Fig. 5.15 (b).

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PRIMARY DYNAMIC MAGNETIC FIELD

EDDY CURRENTS
SECONDARY DYNAMIC MAGNETIC FIELD

ALTERNATING ALTERNATING
CURRENT CURRENT

(a) (b)

Figure 5.15: Eddy Current Testing

One of the major advantages of eddy current as an NDT tool is the variety of
inspections and measurements that can be performed. In the proper circumstances,
eddy currents can be used for:

• Crack detection

• Material thickness measurements

• Coating thickness measurements

• Conductivity measurements for:

o Material identification

o Heat damage detection

o Case depth determination

o Heat treatment monitoring

Some of the advantages of eddy current inspection include:

• Sensitive to small cracks and other defects

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• 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

Some of the limitations of eddy current inspection include:

• 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

5.16 ULTRASONIC METHODS

Ultrasonic waves are generated by a transducer mounted on a probe. The transducer
expands and contracts when an ac voltage is applied to it and vice versa, due to the
piezoelectric effect. A coupling material is placed between the probe and the surface
to be tested, in order to transmit the ultrasonic energy from one to the other.

Testing is carried out in one of three ways:

• Transmission

• Pulse-echo

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• Resonance

In the transmission method a beam of ultrasonic energy is passed through the sample.
An ultrasonic transmitter is placed on one face and an ultrasonic receiver is placed on
the other face. Flaws cause reduction of the amplitude of the received signal. No
indication of the depth of the crack can be obtained in this way.

If the pulse-echo method is used, the transmitter and receiver can be mounted on the
same probe (not necessarily, but for convenience). Access to one surface only is then
necessary. Waves are reflected back to the probe from within the sample, and the
time it takes for a pulse to travel from the transmitter to a flaw where it is reflected
back towards the probe, and the time taken to travel back to the probe is accurately
measured.

The resonance method differs from the pulse-echo method, in that there is a certain
frequency which is characteristic of a given material, for which the reflected pulse is
amplified by the material. The material is made to vibrate at its characteristic natural
frequency, and at this point resonance is said to exist. Resonance can be obtained if
the path length of the pulse is equal to ½ the wavelength of the pulse. The frequency
of the ultrasonic energy is thus adjusted to obtain maximum sensitivity, (at resonance)
and thickness can thus be measured from one side of the material under test, at
various points.

5.17 ULTRASONIC SPECTROSCOPY TECHNIQUES

Ultrasonic spectroscopy is a technique which can be used to analyse the frequency
spectrum of ultrasonic signals. Short pulses of ultrasonic energy which contain a
whole spectrum of different frequencies are used. The effect of the sample on the
signal can provide information about geometry, thickness, changes in
structure/microstructure of the material, etc. The interpretation of the spectra is rather
complicated. Two of the most important factors which affect the results are:

1. The frequency response of the transducer used.

2. The output spectrum of the generator that excites the transducer.

The frequency response of individual transducers can differ considerably. In order to

obtain reliable results, both the response characteristics and the beam profile should

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be controlled. The first back wall echo from a sample of which the ultrasonic
attenuation has negligible dependence on frequency and which is relatively thin, is
analysed. The results can be used to compare the frequency response of different
transducers, by using the same sample in each case.

5.18 RADIOGRAPHIC METHODS

Radiography can be defined as: the technique of producing a photographic image of
an opaque specimen by means of the penetration of radiation such as gamma rays,
neutrons or charged particles.

The radiation is absorbed by the specimen, and the absorption depends on the
thickness, density and composition of the specimen. An image of the absorbed
radiation is registered by a photographic film adjacent to the specimen. The image is
also called a shadow graph or radiograph. An inverse in density at a certain position
will appear as a lighter point on the shadow graph at that specific point, etc. The
shape of the spot on the image will be the same as the shape of the spot itself, in the
same way as X-ray photographs of organs of human beings are obtained.

If neutron beams are used, the presence of elements with low atomic numbers such as
liquids, adhesives, rubber, plastic, etc. can be detected in metal samples.

A method which is very much the same as the radiographic method is called
fluoroscopic analysis. The apparatus used is the same as for radiography. Instead of
film, a specially constructed screen is used which fluoresces when X-rays fall on it.
The image is positive instead of negative as in the case of X-rays. This method
provides easier and faster interpretation of the results.

Assignment 5.1
Recommended Submission Date:
Problem to of
1. reference:
APPENDIX I. __________________
APPENDIX I

Recommended Submission Date:

Problem to of
2. reference:
APPENDIX I. __________________
APPENDIX I

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Recommended Submission Date:

Problem to of
3. reference:
APPENDIX I. __________________
APPENDIX I

Recommended Submission Date:

Online Assessment
3. reference: &
Test 5. __________________
APPENDIX I

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