Eddy current inspection uses electromagnetic induction to detect flaws in conductive materials. Alternating current in a probe coil generates a magnetic field that induces eddy currents in the material. Changes in conductivity due to flaws disturb the eddy currents, and are detected by measuring impedance changes in the coil. Eddy current probes come in different configurations and operate in absolute, differential, reflection, or hybrid modes to optimize detection of different flaw types or minimize interference from other factors.

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Module -4
Eddy current inspection
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 proximity of this changing magnetic field, the
reverse effect will occur. Magnetic field cutting through the second conductor will cause
an “induced” current to flow in this second conductor, called eddy current. Eddy currents
induce electrical currents that flow in a circular path.
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 in the coil at a frequency chosen by the technician for the type of test
involved. A dynamic expanding and collapsing magnetic field forms in and around the
coil as the alternating current flows through the coil. When an electrically conductive
material is placed in the coil’s dynamic magnetic field electromagnetic, induction will
occur and eddy currents will be induced in the material.
Eddy current probes are classified by the configuration and mode of operation of the
test coils. The configuration of the probe refers to the way the coils are packaged to
best "couple" to the test area of interest. The mode of operation refers to the way the

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coils are wired and interface with the test equipment. The mode of operation of a probe
generally falls into one of four categories:
1. Absolute
2. Differential
3. Reflection and
4. Hybrid
Absolute Probes
Absolute probes generally have a single test coil that is used to generate the eddy
currents and sense changes in the eddy current field.AC is passed through the coil and
this sets up an expanding and collapsing magnetic field in and around the coil. When
the probe is positioned next to a conductive material, the changing magnetic field
generates eddy currents within the material. The generations of the eddy currents take
energy from the coil and this appears as an increase in the electrical resistance of the
coil. The eddy currents generate their own magnetic field that opposes the magnetic
field of the coil and this changes the inductive reactance of the coil. By measuring the
absolute change in impedance of the test coil, much information can be gained about
the test material
Since absolute probes are sensitive to things such as conductivity, permeability, liftoff
and temperature, steps must be taken to minimize these variables when they are not
important to the inspection being performed. Absolute coils can be used for flaw

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detection, conductivity measurements and thickness measurements. They are widely
used due to their versatility.
Differential probes
Differential probes have two active coils usually wound in opposition. When the two
coils are over a flaw-free area of test sample, there is no differential signal developed
between the coils since they are both inspecting identical material. However, when one
coil is over a defect and the other is over good material, a differential signal is produced.
They have the advantage of being very sensitive to defects yet relatively insensitive to
slowly varying properties such as gradual dimensional or temperature variations. Probe
wobble signals are also reduced with this probe type.
The disadvantage to using differential probes is that the signals may be difficult to
interpret. For example, if a flaw is longer than the spacing between the two coils, only
the leading and trailing edges will be detected due to signal cancellation when both coils
sense the flaw equally.
Reflection Probes
Reflection probes have two coils similar to a differential probe, but one coil is used to
excite the eddy currents and the other is used to sense changes in the test material.
Probes of this arrangement are often referred to as driver/pickup probes. The
advantage of reflection probes is that the driver and pickup coils can be separately
optimized for their intended purpose. The driver coil can be made so as to produce a
strong and uniform flux field in the vicinity of the pickup coil, while the pickup coil can be
made very small so that it will be sensitive to very small defects

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Hybrid Probes
This probe has a driver coil that surrounds two D shaped sensing coils. It operates in
the reflection mode but additionally, its sensing coils operate in the differential mode.
This type of probe is very sensitive to surface cracks. Another type of a hybrid probe is
one that uses a conventional coil to generate eddy currents in the material but then
uses a different type of sensor to detect changes on the surface and within the test
material. Another type of a hybrid probe is one that uses a Hall effect sensor to detect
changes in the magnetic flux leaking from the test surface. Hybrid probes are usually
specially designed for a specific inspection application.
Principle of ECT
In ECT, an AC current (1kHz-2MHz) is made to flow in a coil(probe),which produces an
alternating magnetic field around it. The coil when brought to an electrically conducting
surface (to be inspected), induces an eddy current flow in the surface due to
electromagnetic induction. These eddy currents are parallel to coil winding. The
presence of defects disturbs the eddy current flow. Then the eddy currents generate a
magnetic field, which is detected either as voltage across second coil or as a deviation
in impedance in original coil. Sine wave oscillator generates sinusoidal signals that pass
through test coils. Balancing is done to reduce voltage difference between two test coils.

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When the coils are brought near the defects, a small unbalanced signal is produced
which will be amplified ,filtered and displayed on an XY monitor, after converting to DC
Factors affecting eddy current testing
Test parameters
1. Frequency
2. Type and geometry of test coil
Test object properties
1. Electrical Conductivity
2. Magnetic permeability
3. Dimensions
4. Temperature

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Limitations
• Only electrically conductive materials are inspected
• E C T is not effective when more than one variables is present
• Only metallic components up to thickness 6 mm can be inspected
• It is expensive
Acoustic Emission Testing
Acoustic Emission may be defined as a transient elastic wave generated by the rapid
release of energy within 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 rock bursts to the initiation and growth of cracks, slip and dislocation
movements, melting, twinning, and phase transformations in metals.
Detection and analysis of AE signals can supply valuable information regarding the
origin and importance of a discontinuity in a material. Instead of supplying energy to the
object under examination, AET simply listens for the energy released by the object. AET
deals with dynamic processes, or changes, in a material. This is particularly meaningful
because only active features (e.g. crack growth) are highlighted.
Acoustic waves

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Acoustic waves are a type of longitudinal waves that propagate by means of adiabatic
compression and decompression. Longitudinal waves are waves that have the same
direction of vibration as their direction of travel. Acoustic waves travel with the speed of
sound which depends on the medium they're passing through
TESTING
Materials "talk" when they are in trouble: with Acoustic Emission equipment you can
"listen" to the sounds of cracks growing, fibers breaking and many other modes of
active damage in the stressed material.
Transducer for AET
AET usually employees a Piezo electric transducer with frequency of the order 30KHZ-
2MHZ. Resonant types are used with narrow band instrumentation and non – resonant
types are used with wide band instrumentation

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Pre amplification and filtering
Pre amplifier follows the transducer. It should have low noise ,high power gain and input
impedance matching the transducer .Filters are designed for different band widths and
can b e plugged into meet specific requirements
Post amplification and threshold
Pre amplification output is of low level and hence to be amplified further before handling
with the processing unit. Further amplification with selectable gain is incorporated in this
unit. In order to eliminate the background noise from analysis, only signal exceeding
certain threshold voltage are detected and analyzed
Data acquisition
Two types of systems are in common use for acquisition of AE data for off line analysis,
namely video recorders and analog magnetic recorders. When real-time decision
making is important with capability for on-line processing and analysis, transient
recorders with computer interface are used
Processing and analysis
The processing instrumentation required for AET depends on the form and quantity of
data. Its function is to convert analog data into digital form
Display devices
A variety of display devices are used for displaying and recording analyzed data. The
simplest and most commonly used recording device is the X-Y recorder
Leak detection
Leakage refers to the fluid that has flowed through a leak. Leak rate refers to the rate of
fluid flow per unit of time under a given set of conditions, and is properly expressed in
units of mass per unit of time. The minimum detectable leak refers to the smallest hole
or discrete passage that can be detected and minimum detectable leak rate refers to the
smallest detectable fluid flow rate.

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Leaks are of two types
1) Real leaks: a localized leak ,ie a discrete passage through which fluid
flow( eg : hole ,crack)
2) Virtual leaks: leaks involve gradual desorption of gases from surfaces or
components within vacuum systems.
LEAK DETECTION
Leak detection may be carried out by visual inspection using soap bubbles , mass
spectrometers etc. Among the various method bubble testing is most widely used
Bubble testing
Immersion of pressurized components in water is used to check the leak tightness .In
this method Sensitivity can be made high if high pressures are used. The main reason
for insensitivity of testing in water is that comparatively large bubbles are formed which
will take long time to appear that they can easily be missed. Low surface tension liquids
are most preferred as they will readily form small diameter bubbles. Best combinations
of pressurizing medium are hydrogen and ether. Methanol and helium is used in
detection of smaller leaks. Magnification glass, and good lighting, dark background aids
in case of very small leaks. Ideal liquid for bubble testing should have low surface
tension and low viscosity. Bubble size depends on the viscosity of liquid, pressure and
diameter of leak
Advantages
• Inexpensive
• Can be carried out by relatively less experienced person
• Is rapid
• Gives accurate location of leak
• The whole specimen is inspected simultaneously

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Limitation
• Cannot locate very small leakage
Application
For checking integrity of pressure vessels, valve instruments ,piping circuits,
containments, condenser, heat exchanges , pumps, cylinders etc
Helium Leak Detector
For very sensitivity leak test ,mass spectrometer based helium leak detector is used. It
can detect the presence of less than one part of helium in 10 million parts of air. A mass
spectrometer is an instrument for separating or sorting atoms of different mass. Gas
molecules entering the mass spectrometer are bombarded by electrons emitted from a
heated filament. The ion beam produced by the electron bombardment is accelerated
into the form of a narrow beam by means of an electric field.

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The ions then pass between the pole piece of a permanent magnet . The magnetic field
deflects the ions in circular path . Helium leak detector is adjusted so that only helium
ions are collected. Flow of helium ions to the collector induces a minute electrical
current. The current is detected, amplified and used to activate electrical meter to
control the pitch of audio signal generator
Different techniques used for leak detection are
• Probe technique
• Envelope vacuum technique
• Sniffer technique
• Envelope pressure technique
• Pressurization technique

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Probe technique
In this a fine jet of helium obtained from a hypodermic needle , is passed over the
exterior surface of the specimen. Helium gas will be drawn into any opening through the
walls of the specimen and register on the leak detector as a visible or audible indication
Envelope vacuum technique
This technique is used when it is necessary to find out the total magnitude of the leak.
The specimen containing helium air mixture is put into an evacuated chamber. The
chamber is evacuated using an auxiliary pump and the pump outlet is connected to the
leak detector for measuring the leak
Sniffer technique

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In this the specimen to be tested is filled with helium or a mixture of helium and air to
pressure greater than atmosphere. The surface of the test object is then scanned with a
sniffer connected to the leak detector. Helium flowing through any opening will be
sucked into a leak detector system by the sniffer and the leak rate is indicated
Envelope pressure technique
In this the test system is surrounded by a hood containing helium. The test system in
then evacuated. Helium will flow through any leaks into evacuated test system and then
to the leak detector. This technique gives the overall leak of the component
Pressurization technique
In this the component is first placed in a helium pressurization vessel and exposed to a
helium atmosphere. The component is removed from the pressurization vessel and
transferred to a second chamber which is connected to a vacuum pump and helium leak
detector
Thermal Imaging / Thermography
Thermal measurement technology measures temperatures of key equipment parts or
areas being monitored.
There are two types of equipment used in this technology.
Contact methods of temperature measurement, using thermometers and
thermocouples, are still commonly used for many applications. non-contact
measurement using infrared sensors has become an increasingly desirable alternative
to conventional methods.
Infrared thermography (IRT)
Infrared thermography is based on measuring the distribution of radiant thermal energy
(heat) emitted from a target surface and converting this to a surface temperature map or
thermogram. Infrared thermography is the technique of producing an image of invisible
infrared light emitted by objects due to their thermal condition. Thermal, or infrared

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energy, is light that is not visible because its wavelength is too long to be detected by
the human eye, it's the part of the electromagnetic spectrum. Unlike visible light, in the
infrared world, everything with a temperature above absolute zero emits heat. The
higher the object's temperature, the greater the IR radiation emitted.Infrared allows us to
see what our eyes cannot.
Thermal Imaging / Thermography
Thermal Imaging / Thermography is the conversion of Infrared Radiation into Thermal
images. A thermal image is an analogue pictorial representation or visualisation of
temperature differences. All objects above absolute zero (-273 degrees) emit radiation,
some of which is infra-red. Depending on temperature and emissivity, most objects in
the world can be thermally imaged. A special lens focuses the infrared light emitted by
all of the objects in view. The focused light is scanned by a phased array of infrared-
detector elements.
The detector elements create a very detailed temperature pattern called a
thermogram.This information is obtained from thousands of points in the field of view of
the detector array. The thermogram created by the detector elements is translated into
electric impulses.The impulses are sent to a signal-processing unit, a circuit board with
a dedicated chip that translates the information from the elements into data for the
display. The signal-processing unit sends the information to the display, where it

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appears as various colours depending on the intensity of the infrared emission. The
combination of all the impulses from all of the elements creates the image.
Potential Drop method
The potential drop technique relies upon the passage of a constant current through a
specimen and the subsequent measurement of the voltage generated across an area
(usually the crack site) on the specimen.
Two forms of the technique exist
AC potential drop (ACPD) in which small alternating currents are passed through
the specimen
DC potential drop (DCPD) in which large direct currents are used.
The techniques essentially measure resistance (DCPD) or impedance (ACPD). The
change in these quantities generated by a propagating defect usually results in an
increase in the potential drop being measured.

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Measurements are commonly done with a four-point probe which uses one pair of
electrodes to inject current on the structure to be tested and a second pair to measure
the resulting voltage difference between two points on the surface.By comparing the
signal with a reference (‘baseline’) obtained on the same specimen in a defect-free area
,the shape and size of the defect can be obtained. Disturbances like changes in
temperature, lack of stability of the input currents or other undesirable changes in
instrumentation can be eliminated to improve measurement accuracy
Direct current Potential Drop method
In this technique, electrical DC currents are injected into a conducting specimen through
one pair of electrodes, while a second pair is positioned near the crack (or a small
monitoring area where crack initiation is expected). The injecting electrodes should be
positioned at a sufficient distance to ensure field uniformity in the inspection area. As
the length or depth of the crack increases (or a new crack is initiated), the cross-
sectional area of the specimen is reduced, this causes an increase in resistance and
ultimately in the potential difference measured between the electrodes positioned in the
crack.

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The amplitude of the measured voltage depends not only on the properties of the
inspected specimen, such as conductivity and geometry, but also on several other
factors including the distance between the measuring electrodes. By comparing the
signal with a reference (‘baseline’) obtained on the same specimen in a defect-free area
,the shape and size of the defect can be obtained
Alternating current Potential Drop method
ACPDM Measures a difference of potential between two reference electrodes to
enables the detection of surface and subsurface flaws. Two contact electrodes placed
on the surface of the component injects an alternating current into the material. This has
the effect of creating a potential difference between the contacts. Spatial variation of
electrical conductivity caused by the presence of a flaw influence the potential
difference which leads to a change in the reference voltage. The voltage difference is
measured and the detected defect can be sized. The depth of crack is a function of
potential measured and the distance between the potential contacts