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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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