EDDY CURRENT TESTING
ASME BPVC Section V
Sub Sec A - Article 8
Eddy current testing works on the principal of “Electromagnetic
Induction”
When a high frequency alternating current is applied to the conductor,
a magnetic field develops in and around the conductor.
AC current
Conductor
Magnetic field
This magnetic field expands and collapses as the alternating current
rises to maximum and reduced to zero.
If another electrical conductor is brought into the proximity of this
changing magnetic field, the reverse effect will occur.
Magnetic field cutting through the second conductor will cause an
“induced” current to flow in this second conductor.
Primary Conductor
(Probe / Coil)
Primary Magnetic field
Secondary Conductor Secondary Magnetic field
(Part)
Eddy Current
The secondary magnetic field is just opposite to primary magnetic field
This interaction produces an back e.m.f in the primary coil.
It can be measured by measuring the impedance (Z) of the coil.
Impedance (Z) of a coil is the total opposition to current flow which is
made up of capacitance (C), resistance (R) and reactance (L)
Impedance change is a Vector value
1. Magnitude Z L R 2 2
L
2. Angle / Direction Tan
R
Z is predominantly decided by Presence of flaws affect the thickness,
conductivity and permeability of the part in
Coil / probe properties their vicinity
Frequency of alternating current
Lift – off distance
Hence, they affect the strength of eddy
Part properties – currents.
Conductivity
Permeability This in turn changes the impedance (Z) of
Thickness the probe/coil.
Presence of any flaws
The magnitude and direction changes of
Impedance (Z) is measured
Impedance change due to flaws From this the defect characteristics are
determined.
Part with out Flaw Part with Flaw
Probe Z1 Probe Z2
Flaw
Part Part
SKIN EFFECT
Eddy currents are strongest at the surface of the material and
decrease in strength below the surface
Probe
Eddy
Part
The depth at which the eddy currents are only 37% as strong as
they are on the surface is known as the standard depth of
penetration (d) or skin depth.
100% 100%
37%
37%
Standard depth (δ) is decided by:
i. Probe frequency
ii. Part conductivity
iii. Part permeability
1
d
fms
Where:
d = Standard Depth of Penetration (mm)
f = Test Frequency (Hz)
m = Magnetic Permeability (H/mm)
s = Electrical Conductivity (% IACS)
Standard Depth
of
Depth
Depth
Penetration
(Skin Depth)
1/e or 37 %
of surface density
High Frequency Low Frequency
High Conductivity Low Conductivity
High Permeability Low Permeability
Selection of Test Frequency
For flaw detection = 37%
For conductivity measurements = 100%
Lift –off Distance
Probe
Space between the probe and the part
Even small changes can hide important details Eddy
Automation or Plastic caps
Part
Inspection Frequency
20 Hz – 1 kHz = Ferro Magnetic Materials
1 kHz – 5 MHz = Non -Ferro Magnetic Materials
Equipment
Equipment for eddy current inspection is very diversified.
Proper equipment selection is important if accurate
inspection data is desired for a particular application.
As a minimum, at least three basic pieces of equipment are
needed for any eddy current examination:
Instrument
Probes
Reference Standards
Portable Eddy Scopes
Portable eddy scopes present the inspection data in the form of an
impedance plane diagram.
Vector Point display
Elliptical display
Eddy Current Probes
Probe Classification
– Surface probes
– Inside Diameter (I.D.) or Bobbin Probes
– Outside Diameter (O.D.) or Encircling Coils
Surface probes are coils that are typically mounted close to one end of a
plastic housing.
Inside Diameter (I.D.) probes, also known as bobbin probes, are coils that
are usually wound circumferentially around a plastic housing.
Outside Diameter (O.D.) probes are coils that are wound the
circumference of a hollow fixture.
Probe operation modes
– Absolute Mode
– Differential Mode
– Reflection Mode
– Hybrid Mode
Absolute Probes
They have a single test coil that is used to both generate and
sense changes in the eddy current field.
Can be used for flaw detection, material sorting and thickness
measurements.
They are widely used due to their versatility.
Vector Point display
Differential Probes
They have two active coils usually wound in opposition or in addition.
When the two coils are over a flaw-free area of test sample, there is no
differential signal developed between the coils since they are both
inspecting identical material.
However, when one coil is over a defect and the other is over good
material, a differential signal is produced.
Elliptical display
Reflection Probes
Reflection probes have two coils similar to a differential probe, but one
coil is used to excite the eddy currents and the other is used to sense
changes in the test material.
Probes of this arrangement are often referred to as driver/pickup probes.
Vector Point display
Hybrid probes
Split D, differential probes.
This probe has a driver coil that surrounds two D shaped sensing coils.
Elliptical display
driver coil sensing coils
Reference Standards
• In order to give the inspector useful data while conducting an
inspection, signals generated from the test specimen must be
compared with known values.
• Reference standards are typically manufactured from the same or very
similar material as the test specimen.
• Many different types of standards exist for due to the variety of eddy
current inspections performed.
Material thickness Standards
Used to help determine such things as material thinning
caused by corrosion or erosion.
Crack Standards
ASME Tubing Pit Standard
Nonconductive coating (paint) standard
Inspection Applications
1. Crack Detection
Cracks cause a disruption in the circular flow patterns of the
eddy currents and weaken their strength.
This change in strength at the crack location can be detected.
Magnetic Field
From Test Coil
Magnetic Field
From
Eddy Currents
Crack
Eddy Currents
1. Keep the probe in the air and find out the Zair point
in the display
Zair
Probe
XL
R
2. Keep the probe just over the surface of a calibration block
(or) over the flaw free region of the part by maintaining a
gap between probe and part – Lift off distance
3. Now find out the Zcal point in the display
Probe
XL
Lift off distance Zcal
Calibration
Block
R
4. Bring the vector point at left side middle of the display by
small adjustments - Nullification
Probe
Zcal XL
Calibration
Block
R
5. Now calibrate the probe and instrument for flaws of known
sizes that present in the calibration block
2
Probe
1
XL
0
0 1 2
3
R
6. Then keep the probe just over the surface of the part and
scan the entire surface – Scanning
7. Now find out the new Z point in the display and measure
the change in its magnitude and direction
2
Probe
1
XL
0
Part
R
2. Material Thickness Measurement
Thickness measurements are possible with eddy current
inspection within certain limitations.
Only a certain amount of eddy currents can form in a given
volume of material.
Therefore, thicker materials will support more eddy
currents than thinner materials.
The strength (amount) of eddy currents can be measured
and related to the material thickness.
Magnetic Field
From Probe
Test
Material
Eddy Currents
3. Corrosion & Erosion Damage
Eddy current inspection is often used in the aviation
industries to detect material loss due to corrosion and
erosion.
Eddy current inspection is used extensively to inspect tubing
at power generation and petrochemical facilities for corrosion
and erosion.
4. Nonconductive Coating Thickness Measurement
Nonconductive coating thickness on electrically conductive
substrates can be measured very accurately with ET
The coating displaces the eddy current probe from the
conductive base material and this weaken the strength of the
eddy currents.
This reduction in strength can be measured and related to
coating thickness.
Nonconductive
Coating
Conductive
Base Metal
Eddy Currents
5. Monitoring Conductivity and Permeability
Variations
Eddy current inspection is sensitive to changes in a
material’s electrical conductivity and magnetic permeability.
This “sensitivity” allows the inspection method to be used for
• Material Identification & Sorting
• Determination of heat damage
• Cladding and plating thickness measurement
• Heat treatment monitoring
Advantages
• Sensitive to small cracks and other defects
• Detects surface and near surface defects
• Inspection gives immediate results
• Equipment is very portable
• Method can be used for much more than flaw detection
• Minimum part preparation is required
• Test probe does not need to contact the part
• Inspects complex shapes and sizes of conductive materials
Limitations
• Only conductive materials can be inspected
• Surface must be accessible to the probe
• Skill and training required is more extensive than other techniques
• Surface finish and roughness may interfere
• Reference standards needed for setup
• Depth of penetration is limited
