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CHAPTER 5 - EDDY CURRENT TESTING - Coggle Diagram
CHAPTER 5 - EDDY CURRENT TESTING
Introduction
Eddy current inspection is a method that use the principal of "electromagnetism" as the basis for conducting examinations
Eddy current NDT is a technique that can test metals for flaws either during the manufacturing process or as a consequence of age or environment
It is a highly sensitive technique & completely nondestructive
It is quick, safe & cost-effective to use
Factor Affecting Eddy
Current Response
Electrical conductivity of the test material
Magnetic permeability of the test material
Frequency
Geometry
Proximity
Depth of penetration
Generation of
Eddy Currents
Eddy current are induced electrical current that flow in a circular path
Eddies: it formed when a liquid/gas flow in a circular path around the obstacles when conditions are met
AC is allow to flow in the coil at a frequency chosen by the officer/technician for the type of the test involved
Dynamic expanding & collapsing magnetic field form in & around coil as the AC flow through the coil
When AC run through the coil & the conductor material is placed, induction will occur & eddy current will be induced to & in the material
Eddy current flow in material will generate their own secondary magnetic field which will oppose the coil primary magnetic field
Electromagnetic
Induction
Eddy current are created through a process called electromagnetic induction
Theory-Electromagnetic induction
Current passing through a coil creates a magnetic field
A moving magnetic field would induce a voltage in an electrical conductor
When alternating current (AC) is applied to the conductor, such as copper wire, a magnetic field develops in & around the conductor
The magnetic field expands as the AC rises to maximum & collapses as the current is reduced to zero
Eddy Current
Theory: Testing
When AC flow in a coil at close proximity to a conducting surface, magnetic field of the coil will induce circulating eddy current in the conductor surface
The magnitude & phase of the eddy current will affect the loading on the coil & thus their impedance
Deep crack in the surface will interrupt & reduce the eddy current flow where it will decrease the loading of coil & increase the effective impedance
Monitor the voltage across the coil, so that we can detect changes in the test (conductor) material
Cracks must interrupt the surface eddy current flow to be detected
Material Thickness
Measurement
Thickness measurement are possible with eddy current inspection within certain limitations
Certain amount of eddy current can form a given volume of material
The thicker material will support more eddy current than the thinner material
The strength of eddy current can be measured & relate it to the material thickness
Thickness measurement using eddy current inspection is used in aviation industries to inspect the material loss due to corrosion & erosion
The basic steps in performing an
inspection with a surface probe
Select & setup the instrument & probe
Select frequency
adjust the instrument to obtain recognizable defect response using calibration standard / setup specimen
Place the probe (coil) on the component surface
Scan the probe over the surface in a pattern which provide a complete coverage of the area that are inspected
Monitor the signal for local changes
Advantages of Eddy Current Inspection
Sensitive to small cracks & other defects
Surface & near surface defect can be detected
The inspection give immediate results
The equipment is very portable
Method can be used for more than flaw detection
Minimum parts preparation is required
The test probe does not need to contact the parts
Can inspect any complex shapes
Limitations of Eddy Current Inspection
Only conductive material can be inspected
Surface must be accessible to the probe
Skills & training is required
Surface finish & roughness may interfere
Reference standard really needed
Depth of penetration is limited
Parallel defect with the probe coil is undetectable