16-06-2014, 04:04 PM
A SEMINAR EDDY CURRENT TESTING
[attachment=64942]
WORKING PRINCIPLE
In standard eddy current a probe coil carrying ac current is placed near the specimen which produces alternating magnetic flux around the coil.
The alternating magnetic flux links with the testing material induces eddy currents.
Then the variation of the magnitude and phase of these eddy current is monitored by using a search coil or by the same coil by measuring the change in current or impedance of primary search coil
THE PHYSICS BEHIND ECT
Farday’s laws of electromagnetic induction say that when ever there is a rate of change of flux linkage with a coil ,it produces an e.m.f and there by a current In case of a closed coil .The process is known as electro-magnetic induction.
PROPERTIES OF COILS
The two basic properties of coil which are of very special interest are-
1.self inductance
2.mutual inductance
Self inductance
In brief it is the property of a coil where a change in current causes a change in voltage in the same circuit
FORMATION OF EDDY CURRENT
In ECT the eddy currents are generated in the testing material due to the phenomenon of mutual inductance
The ac current in the probe coil produces alternating magnetic flux and these flux produce eddy current in the taste material
PROPRTIES OF EDDY CURRENTS
Eddy currents are closed loops of induced currents circulating in planes perpendicular to magnetic flux
They are confined to the area of inducing magnetic field
Eddy currents concentrate near the surface adjacent to an excitation coil and strength decreases with distance from the coil
Penetration depth
Due to skin effect the magnitude of eddy current decreases as depth of material increases
The depth of penetration is given by the formula written on the right side.
EFFECT OF EDDY CURRENT
The eddy current produce a magnetic flux which links with the magnetic flux of the exciting coil.
The amount of interaction between two fluxes determine the value of current or change in impedance of the primary coil and is a direct measurement of eddy current in the specimen.
MEASUREMENT OF EDDY CURRENT
The change in impedance of the probe can be easily detected by a high sensitive maxwell’s induction bridge.
It compares the inductance of the probe with some standard values and any change in this is reflected through a detecting coil or an oscilloscope
BASIC COMPONENTS OF ECT
PROBES
DETECTING INSTRUMENTS
DISPLAY UNITS
FREQUENCY GENERATOR
FILTERS
AUXILLIARIES
PROBES
DETECTING INSTRUMENTS
DISPLAY UNITS
FREQUENCY GENERATOR
FILTERS
AUXILLIARIES
APPLICATIONS OF ECT
Surface crack detection
Measurement of conductivity of materials
Thickness measurement of non conductive coatings
Tube inspection
Heat damage inspection
Material Identification
TUBE INSPECTION
In tube inspection the long tubes are inspeted through a long probe which is inserted into the tube and the condition of material inside the tube is determine from the impedance measurements
ADVACED TECHNOLOGY IN ECT
The advanced technology uses multiple frequency technique or swept frequency technique.
This technique simple involve collecting data at several frequencies and then comparing the data or mixing the data in some particular manner
LIMITATIONS OF ECT
Only conductive materials can be inspected
Surface must be accessible to the probe
Skill and training required is more extensive than other techniques
Reference standards needed for setup
Surface finish and roughness may interfere
Depth of penetration is limited
Flaws such as delimitations that lie parallel to the probe coil winding and probe scan direction are undetectable
SUMMARY
From the above discussion we can summaries that eddy current testing (ECT) is one of the most important testing technique which is now a days gaining popularity due to its low cost equipment, immediate results oriented testing ,accuracy of results and its wide range of application
[attachment=64942]
WORKING PRINCIPLE
In standard eddy current a probe coil carrying ac current is placed near the specimen which produces alternating magnetic flux around the coil.
The alternating magnetic flux links with the testing material induces eddy currents.
Then the variation of the magnitude and phase of these eddy current is monitored by using a search coil or by the same coil by measuring the change in current or impedance of primary search coil
THE PHYSICS BEHIND ECT
Farday’s laws of electromagnetic induction say that when ever there is a rate of change of flux linkage with a coil ,it produces an e.m.f and there by a current In case of a closed coil .The process is known as electro-magnetic induction.
PROPERTIES OF COILS
The two basic properties of coil which are of very special interest are-
1.self inductance
2.mutual inductance
Self inductance
In brief it is the property of a coil where a change in current causes a change in voltage in the same circuit
FORMATION OF EDDY CURRENT
In ECT the eddy currents are generated in the testing material due to the phenomenon of mutual inductance
The ac current in the probe coil produces alternating magnetic flux and these flux produce eddy current in the taste material
PROPRTIES OF EDDY CURRENTS
Eddy currents are closed loops of induced currents circulating in planes perpendicular to magnetic flux
They are confined to the area of inducing magnetic field
Eddy currents concentrate near the surface adjacent to an excitation coil and strength decreases with distance from the coil
Penetration depth
Due to skin effect the magnitude of eddy current decreases as depth of material increases
The depth of penetration is given by the formula written on the right side.
EFFECT OF EDDY CURRENT
The eddy current produce a magnetic flux which links with the magnetic flux of the exciting coil.
The amount of interaction between two fluxes determine the value of current or change in impedance of the primary coil and is a direct measurement of eddy current in the specimen.
MEASUREMENT OF EDDY CURRENT
The change in impedance of the probe can be easily detected by a high sensitive maxwell’s induction bridge.
It compares the inductance of the probe with some standard values and any change in this is reflected through a detecting coil or an oscilloscope
BASIC COMPONENTS OF ECT
PROBES
DETECTING INSTRUMENTS
DISPLAY UNITS
FREQUENCY GENERATOR
FILTERS
AUXILLIARIES
PROBES
DETECTING INSTRUMENTS
DISPLAY UNITS
FREQUENCY GENERATOR
FILTERS
AUXILLIARIES
APPLICATIONS OF ECT
Surface crack detection
Measurement of conductivity of materials
Thickness measurement of non conductive coatings
Tube inspection
Heat damage inspection
Material Identification
TUBE INSPECTION
In tube inspection the long tubes are inspeted through a long probe which is inserted into the tube and the condition of material inside the tube is determine from the impedance measurements
ADVACED TECHNOLOGY IN ECT
The advanced technology uses multiple frequency technique or swept frequency technique.
This technique simple involve collecting data at several frequencies and then comparing the data or mixing the data in some particular manner
LIMITATIONS OF ECT
Only conductive materials can be inspected
Surface must be accessible to the probe
Skill and training required is more extensive than other techniques
Reference standards needed for setup
Surface finish and roughness may interfere
Depth of penetration is limited
Flaws such as delimitations that lie parallel to the probe coil winding and probe scan direction are undetectable
SUMMARY
From the above discussion we can summaries that eddy current testing (ECT) is one of the most important testing technique which is now a days gaining popularity due to its low cost equipment, immediate results oriented testing ,accuracy of results and its wide range of application