Eddy current testing uses electromagnetic induction to detect flaws and changes in conductivity or permeability in conductive materials. When an alternating current is passed through a coil near a conductive material, it induces eddy currents in the material. Disruptions to eddy current flow caused by defects or material changes are then detected by a second coil. Eddy current testing is used for non-destructive testing of conductive materials to find cracks, thickness variations, and differences in material properties. It is commonly used in industries like steel production to inspect pipes, bars, and tubes for flaws.

1. 1
Mookambigai College of Engineering
Modern NDT Techniques
IV-UNIT
Arunprakash S
Assistant Professor
School of Mechanical Engineering

2. Eddy Current Testing
Introduction
Eddy current inspection is based on the principle of
electromagnetic induction and is used to identify
physical, structural and metallurgical condition of
materials
The method does not require direct electrical contact
with the part being inspected

3. Principle of testing
When an alternating current is made to flow in a coil, it
produces an alternating magnetic field around the coil
This coil, when brought close to the electrically conducting
surface of a metallic material to be inspected, induces an eddy
current flow in the material due to electromagnetic induction
These eddy current are generated parallel to the coil winding
The presence of any defect or discontinuity in the material
disturbs the eddy current flow
These eddy current, in turn, generate an alternating magnetic
field in the opposite direction which can be detected by
measuring voltage across a second coil

4. Factors affecting ECT
•The impedance change is affected by
•Electrical conductivity
•Magnetic permeability
•Geometry of the material
•Test frequency
•Spacing between the coil and the material

5. Advantages
Low cost methods
High speed
Large scale inspection
Automation and mass inspection is possible

6. Application
•To identify conditions and properties such as
electrical conductivity, magnetic permeability, grain
size, hardness and physical dimensions
•To detect seams, laps, cracks, voids and inclusions
•To measure the thickness of nonconductive coating
on a conductive metal
•To detect differences in the composition,
microstructure and other properties

7. Methods of detection of Eddy current intensity and flow pattern
The eddy current induced in a material generate their own
magnetic field
The magnitude, time lag, phase angle and flow patterns of the
eddy current within the test material are detected by another
set of sensing coils
In some cases, the magnetizing coils serves as signal pickup
devices or detectors of eddy current
Usually, the magnetizing coil system and the detectors or pick
up devices are combined into a single probe system
The output signals from eddy current test coils and probes are
analyzed by circuits and instrumentation

8. Control of EC penetration depths in test materials
Eddy current has the tendency to be concentrated
near material surfaces
Low excitation frequencies are used to penetrate
deeper within a conductive test material
High excitation frequencies are used for selective
examination of near surface regions of thin material
of low electric conductivity

9. Limitations of ECT
•ECT are applicable only to test material with
electrical conductivity
•ECT are very sensitive for surface and near surface
layers of test material
•It is difficult or impossible to penetrate to the center
of thick specimens because of attenuation of the
electromagnetic field at certain depths below the
surface

10. Industrial application of ECT
It has the ability to measure the thickness of metallic foils,
sheets, plates tube walls by non contacting means
To measure the thickness of coating over the base
material where these have different electrical or magnetic
properties
To identify materials by composition or structure
For detecting material discontinuities such as cracks,
seams, laps etc
For locating hidden metallic objects such as buried
bombs, metallic objects accidentally packed in foodstuffs

11. Eddy current transducers
I Classification on the basis of mode of operation
1. Absolute Eddy current transducers
It consists of single coil and the impedance in the coil
is measured directly
They are simple to construct and used in material
sorting and detection of long seams in steel rods

12. 2. Differential Eddy current transducers
It consists of a pair of coils connected in opposition so
that the net measured impedance is cancelled out when
both coils experience same conditions
Their sensitivity to discontinuities in materials is higher
than absolute transducers
3. Absolute and Differential Eddy current array transducers
In absolute mode, a number of small coils on the outer
surface of a transducer could be connected in series
In differential mode, a pair of coils are connected in
opposition to each other

13. II Classification on the basis of method of sensing
1. Impedance method
Here, the driving coil is monitored and the change in
coil current or voltage is due to the impedance
changes in coil
It is possible to use this method for sensing any
material parameters that results in impedance
changes

14. 2. Transmit-Receive method
Here, there are separate driving coil and pickup coil
and the induced voltage across the pickup coil is
measured
Both the methods may be used with absolute or
differential eddy current transducers

15. III Classification on the basis of usage
1. Encircling transducers
It is used to test outer surface of products such as
tubes,, bars, wires and bearing components
It is also called as annular, circumferential or feed
through coils

16. 2. Surface transducers
It consists of flat coils and are used to test flat
surfaces or surfaces with relatively large curvature
3. Forked transducers
It is used to test flat, thin sheets of metals. The test
material is passed between the coils
They are used to test thin sheets as they have
excellent liftoff characteristics and very sensitive to
material thickness

17. Factors affecting Eddy current transducers
1. Electrical conductivity
All materials have a resistance to the flow of
electricity. Those with highest resistivity are
insulators.
Those having intermediate resistivity are
semiconductors and those having a very low
resistivity are conductors
The test object for ECT should be a good conductor of
electricity

18. 2. Magnetic permeability
Magnetic permeability is not a constant for a given
material but depend on the strength of the magnetic
field acting upon it
Magnetic permeability will remain constant when a
ferromagnetic material is saturated

19. 3. Coil impedance
In using direct current, the magnetic reaches a
constant level and the electrical resistance of wire is
the only limitation to the flow of current
In using alternating current, two limitations are
electrical resistance of the wire and inductive
reactance
The alternating current resistance of an empty coil
operating at low frequency or having small wire
diameter is equal to direct current resistance of the
wire of the coil

20. 4. Liftoff curve
When a inspection coil is energized in air, it give some
indication even if there is no conductive material in
the vicinity of the coil
This initial indication will begin to change as the coil is
moved closer to a conductor
These changes in indication with changes in spacing
between the coil and the conductor is called as “lift
off”

21. 5. Fill factor
In an encircling coil, a condition comparable to lift off
is known as “ fill factor”
It is a measure of how well the part being inspected
fills the coil.
The variation in outside diameter must be controlled
as small changes can give large indications
The fill factor curve is same as the lift off curve

22. 6. Skin effect
Eddy current induced on the test object are not
uniformly distributed throughout the material
The Eddy current is dense at the surface and its
density reduces with depth in the material
The distance that Eddy current penetrates the
material is called as the depth of penetration

23. 7. Edge effect
When an inspection coil approaches the end or edge
of a part being inspected, the eddy current are
distorted as they are unable to flow beyond the edge
of the part
This distortion of eddy current is shown as an
indication known as edge effect

24. Applications of Eddy Current Testing
1. Eddy current application in Pipe wall
Figure 3: A number of pipes are tested to determine the capability of Eddy
current technique
Pipe samples of length 1.8 m and 76 mm diameter were used in the first six
tests
24 scans were used to produce the inner wall electromagnetic field map and at
15 degree radial spacing, the scans were kept at about 10 mm apart
The detector was a small coil that produce localized indications of field
strength
The effects of simulated pits and cracks on the exterior of the pipe are shown
in figure
Discontinuity depths ranges from 10 to 67 % of the 76 mm wall thickness

25. Figure 4: Wall thinning over a uniform machined
section of pipe is shown in the figure
The pipe has it outside diameter reduced at about
0,.4 mm. Since the reduction is uniformly distributed
around the circumference, the response of both
exciter and detector is seen
Half of the deflection occurs when the exciter passes
over the shoulder portion of the machined section.

26. Figure 6: The figure shows a hard spot in a pipe
sample. This was created by heating a small area to
red heat ad quenching it with water.
No dimensional variations were created. The exciter
response give a more pronounced indications that the
detector, but the detector gives more detail on small
incremental areas with in the hard spot.
Variations in magnetic permeability are the cause of
the indications

27. 2. Eddy current application in Steel industry
Round bar inspection system
Steel industry uses a portable commercial EC instrument with a
rotary straightener to inspect high alloy round bar 13 to 76 mm
in diameter
The inline rotary straightener provides rotation and a follower is
necessary to position a probe transducer at a constant distance
from the bar surface and perpendicular to the bar
The figure shows the machined notches 0.38 to 0.76 mm deep
and a natural seam 1.3 to 2 mm deep
For the sensitivity shown, the deep natural seam saturates the
system but 0.38 mm deep notch gives a response 4 times the
noise level.

28. Rotating pipe inspection system
Another steel industry uses a rotary straightener to create
a multi channel EC device
The system is a five channel for inspecting seamless tube
with 100 to 355 outside diameter at a normal speed. The
inspection frequency is 50 KHz
RPIS result were compared with magnetic particle testing
for a group of 101 seamless pipe ranging in size from 140
to 245 mm outside diameter
Only significant discontinuities are reported by RPIS
The figure 22 shows that the discontinuities shown by
RPIS is 5 times greater than magnetic particle testing

29. 3. Eddy current application in Automobile industry
Master brake cylinder inspection
Master brake cylinder is inspected two at a time by a
semiautomatic test system for detecting porosity,
cracks and 6 holes
This is done using a rotary probe excited at 100 KHz. A
computer is programmed to verify the presence of
the holes by means of amplitude of signals and to
verify the absence of cracks and porosity by the
absence of signals between the holes

30. Porosity is indicated by the signal in addition to the 6
hole signals
The time required for the test is 8 seconds where
loading and unloading is not counted
The system is calibrated using a master part that has 6
required holes and 0.25 mm deep porosity
The sensitivity level is sufficient to detect detrimental
cracks as well as porosity

31. 4. Eddy current application in Bolt hole inspection
The diameter and the depth of hole to be inspected is
determined from engineering drawing
Proper size probe and reference standard are selected
The instrument is calibrated with respect to reference
standard
Corrosion is checked visually in the holes and it is
removed by reaming or drilling

32. Then the discontinuities are scanned by rotating the
probe at constant depth at intervals not exceeding 1.5
mm in depth.
Scanning is conducted at one revolutions intervals
If suspect areas are noted near the end of scan, the
area is reset so that the suspect area are located in
the middle portion of the second scan
Crack depth are estimated from the amplitude of
eddy current reading

33. 2. Acoustic Emission Testing
Principle of AE technique
Acoustic emission is defined as the high frequency
stress waves generated by the rapid release of strain
energy that occurs within a material during crack
growth, plastic deformation or phase transformation
This energy may originate from stored elastic energy
as in crack propagation or from stored chemical free
energy as in phase transformation
Acoustic emission is also called as stress wave
emission and microseism

34. Sources of acoustic emission for generating stress waves
in material are the local dynamic movements of cracks,
twinning, slip, sudden reorientation of grain boundaries
The stresses in a metallic system will be well below the
elastic limit but the region near the flaw or crack may
undergo plastic deformation and result in premature
failure
AE inspection detects and analyze minute acoustic
emission signal generated by discontinuities in materials
under applied stress
Proper analysis of these signals can provide information
regarding the location and structural significance of the
detected discontinuities

35. Kaiser effect
An important feature of AE testing is its irreversibility
If a material is loaded to a given stress level and then
unloaded, no emission will be observed upon
immediate reloading unless the reloading does not
exceed the previous load
This is known as Kaiser effect and it is due to the fact
that AE is closely related to plastic deformation ad
fracture

36. This property is very much useful in detecting sub
critical growth of flaws such as fatigue crack growth,
stress corrosion cracking etc
The AE technique is capable of detecting growing
flaws and is capable of locating one or more
discontinuities while they are growing

37. Types of Acoustic Emissions
Continuous acoustic emission: The waveform is of small
amplitude and the amplitude varies with the acoustic
emission activity
In metals and alloys, this form of emission is due to the
dislocation movements in the grains
Burst acoustic emission: It is of short duration pulse and
discrete release of strain energy
This type is generated due to twinning, micro cracks and
have a greater amplitude than the continuous type

38. Features of Acoustic Emission inspection
In most conventional NDT, discontinuity is detected by
directing some form of energy such as ultrasonic
beam, radiographic wave, x-rays, gamma rays etc
But in AE technique, discontinuity is detected by the
released strain energy initially stored in the test
object
If a discontinuity is unstable and affected by loading,
the discontinuity will emit acoustical energy which
reveals its presence

39. If a discontinuity is stable and not affected by loading, the
discontinuity will not emit acoustical emission which
means that it does not affect the structural integrity of
the material being tested
Acoustic emission examination is non-directional. The
emitted energy is in the form of spherical wave front and
a sensor located anywhere in the vicinity of the emission
source can detect the resulting acoustic emission
This is in contrast with other methods of NDT which
depend on prior knowledge of the location and
orientation of a discontinuity in order to direct a beam of
energy through the area of interest

40. Advantages
It is a dynamic inspection method which provide
response to discontinuities under significant imposed
structural stress
It is possible to detect and analyze the defect during a
single test
Since only small area is required, the defect that
cannot be found by other NDT methods are found out
by acoustic emission technique

41. Applications
 Mechanical properties testing
 Pre-service proof testing
 Re-qualification testing
 Online monitoring
 In process weld monitoring
 Leak detection and location

42. Acoustic Emission Testing Equipment
The various components of the system includes
sensors, pre-amplifiers, filters and amplifiers to make
the signal measurable
The figure 1 shows the block diagram of basic four
channel acoustic emission system

43. Acoustic emission sensors
When an acoustic emission wave front impinges on the
surface of the test object, very minute displacements of
the surface molecules occur
A sensor’s function is to detect this mechanical
displacement and convert it into a usable electric signal.
Generally a piezoelectric transducer are used as
electromechanical conversion device
The considerations in sensor selection are operating
frequency, sensitivity, environmental and physical
characteristics

44. Pre-amplifiers and Frequency selection
The pre-amplifier should be close to the sensor and
usually it will be built in with the sensor housing
The preamplifier provides the required filtering, gain
and drive capability
Frequency is wide band width and ranges from
audible range to 50 MHz
The most common frequency is from 100 to 200 KHz

45. Microcomputers
Each acoustic signal is measured by hardware circuits
and the measured parameters are passed through the
central microcomputer to a disk file
After data storage, the system extracts data for
graphic displays and hard copy reports

46. System mainframe
The mainframe consists of main amplifier and
thresholds which are adjusted to determine the test
sensitivity
The sensitivity decreases with increasing distances
between the acoustic emission and the sensors

47. Applications of Acoustic Emission Testing
1. Resistance spot welding
The acoustic emission system consists of a high frequency
acoustic emission sensor attached to one of the spot
welding electrodes
Matching amplifiers and filters with a center frequency of
0.5 MHz may be used to counterbalance the effects of
mechanically induced low frequency signals
The initiation of each weld impulse is sensed and a
window is generated synchronous with the effective
length of the impulse

48. The window gates the acoustic emission counts into
the counter, display and digital comparator
For multiple impulse welds, the energy counts are
cumulative
The comparator output can activate a control circuit
to terminate the weld cycle or an alarm circuit can be
activated
The energy count is displayed on light emitting diode
readout

49. 2. Ceramic capacitor crack detector
Since the soldering process is continuous, a window or
gate period could not be used existing crack detector
could not be used effectively for the undersea soldering
operation
The required system have to monitor cracking during the
entire processing cycle
The crack detection system is in conjunction with the
rotational soldering operation that joins the metal caps to
the ends of the ceramic capacitor
The incoming acoustic emission signal from the
preamplifier is filtered and further amplified

50. The analog signal is then passed to both a threshold
detector and an envelope detector
The output of the threshold detector is a wave train of
pulses corresponding to each threshold crossing of the
filtered acoustic emission signal
The acoustic emission envelope is then passed to both a
threshold detection circuit and an envelope strength
circuit
The envelope strength circuit converts the acoustic
emission envelope into pulses whose number is
proportional to the amplitude and length (area) of the
envelope curve