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Eddy Current Inspection
It is based on the principles of ElectromagneticInduction.
It can be used to monitor following in electricallyconductive parts
Physical Condition Structural Condition Metallurgical Condition
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Applications
1.To Measure Electrical conductivity, Magnetic permeability,
Grain size, Heat treatment condition, Physical dimensions.
2. To Detect Following Flaws
Seams, Laps, Cracks, Voids, Inclusions.
3. To Sort Metals and Differentiate Them According to
Composition, Microstructure and Other properties.
4 .To Measure Thickness of Coatings on Metals.
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PRINCIPLE OF METHOD
An electric coil through which
alternating current is passed is
brought near the object being
inspected or the part is placed insidethe coil through which current flow
as shown in figure. Exciting current
flowing through the coil causes eddy
current to flow in the part being
inspected due to electromagnetic
induction. The eddy current flow inloops and its strength is dependent on
Strength of the primary field
Electrical properties of the part
Electromagnetic fieldestablished due to current
flowing through the part being
inspected.
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ADVANTAGES
Does not require any contact with partbeing inspected
Can be used for high speed inspection.
Entire production output can be
inspected
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LIMITATIONS
1.Method is based on indirectmeasurement.
2.Correlation between the instrument
reading and characteristics andserviceability of actual part is to beestablished
3.Some inherent properties of the
material may lead to irrelevantindications
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EDDY CURRENT SYSTEMEssential components of the Eddy Current system
are1. Engineering of the inspection coil.
2. Modulation of the inspection coil output signal
by the part being inspected.
3. Processing of the signal.4. Amplification of the above signal.
5. Detection of the signal and analysis.
6. Display of signal.
7. Movement of part through the coil& assembly
required for the same.
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TYPES OF COILS USED FOR EDDY CURRENT INSPECTION(a)Probe type coil : Inspection of cracks in a flat plate.(b) U-shape Coil : For detection of laminar flaw in a flat surface
(c) Encircling coil: For detecting defects parallel tocoil axis in a bar or tube.(d) Bobbin coil : For internal flawsin a tube, which are parallel to axis of coil.
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(e) Multiple coil: Used as absolute coil arrangement and
differential coil arrangements variation in crosssections
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TYPES OF EDDY CURRENT INSTRUMENTS
A. A simple arrangement in which voltage across the coil is
measured.B. Typical impedance bridge- here inspection coil is in one leg
and balancing impedance in other leg.C. Impedance bridge with a dual coil other coil is identical
inspection coil.D.Impedance with dual coil and a reference sample in the second
coil.
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E. Induction bridge probe- It consists of transmitter coil and twodetector coils wound in opposite direction are near the objectact as inspection coil and top one as reference coil. Thiseliminates the effect of transmitter coil and allows the detectorcoil to pick up the signal from the part being inspected.
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INSPECTION OF TUBES MORE THAN 3 INCH DIAMETERMultiple probe is used mounted on a wear resistant shoe which is pressed against thesurface of tube . Each probe is having independent electrical circuit and are mechanicallyseparated. These probes are 2 long and square. These probes cover the whole
circumference and over lapping is provided. Signal from these probes is fed to separatedisplay device. It gives higher sensitivity and helps in localizing the discontinuity positionwithin a sector.
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Setup and coil arrangement for eddy-current inspection of longitudinalweld in ferromagnetic welded tubing
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TRANSMISSION METHOD:
c. Exciting coil inside and receiving coil outside. Sensitive to both inside &outside surface defects and more sensitive to outside defect. Used for
inspection six finned tubes.d. Exciting coil outside & receiving coil inside More sensitive to inner surfacedefects. Used for inspection of six finned tubes
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