Inspecting with Eddy Currents

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Inspecting with Eddy Currents

• Theory
• Practical Testing
• Aerospace Applications
• Industrial Applications
• WeldScan
• Review of Current Equipment
• Probe Range
• Introducing Locator 2

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H NDT Ltd., St. Albans, UK

• Manufacturers of Non-Destructive Testing (NDT)
  Equipment
• Leaders in the Field of Eddy Current Technology

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Eddy Current Products

• Portable instruments
• Analogue Meter displays
• Analogue and Digital screen display
• Digital Conductivity meter
• Dynamic rotating inspection
• Systems
• Automated in-line and off-line inspection
• Aircraft wheel inspection
• Condenser and heat exchanger tubing
• Probes
• Wide range of standard and special probes

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Product History

• 1968 - Amlec for Royal Navy
• 1970 - Halec
• 1971 to 1980 - Phasec D4 and D5
• 1983 - Locator UH for RAF
• 1984 - Locator UH-B for USAF
• 1986 - AV10b/AV100
• 1988 - AutoSigma 2000
• 1990 - Phasec 1.1/WheelScan
• 1991 - Phasec 3.4/2.21
• 1993 - MiniPhasec
• 1995 - Phasec 2200
• 1998 - Phasec D62
• 2000 - Locator 2 for RAF

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Part 1Theory
HOCKING eddy current training programme
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Introduction - Historical Context

• 1879 - Hughes sorted metals of different
  permeability and conductivity
• 1930s used for metal sorting.
• 1940s crack detection applications developed.
• 1950s 60s techniques developed in Aviation
  and nuclear industries.

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Basic Eddy Current Theory Simple Coil above a
metal surface

• AC Field induces circulating eddy currents
• Eddy currents load coil
• Loading affects coil impedance

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Basic Eddy Current Theory Simple Coil above a
metal surface

• Crack in surface reduces eddy current flow
• Loading on coil changes
• Coil impedance changes

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Principle Of Eddy Current Inspection

• An AC magnetic field induces circulating eddy
  currents in a conductive material
• Changes in the properties of the material change
  the sensor impedance

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Basic Eddy Current Theory Simple Coil above a
metal surface

• Monitor voltage across coil
• Coil impedance changes
• Voltage across coil changes
• Detect changes in eddy current flow

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Basic Eddy Current Theory Simple Coil above a
metal surface
Crack parallel to eddy
currents - not detected
Crack interrupts eddy
currents - detected
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Basic Eddy Current Theory Depth of Penetration

• Eddy current density is greatest at surface
• Reduces exponentially with depth
• At standard D of P 1/e (37) of surface value

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Basic Eddy Current Theory Depth of Penetration
Depth (mm)
Depth (in)

• Decreases with an increase in frequency
• Decreases with an increase in conductivity
• Decreases with an increase in permeability

100
4
Titanium
10
0.4
Aluminium
Copper
1
0.04
Steel
0.1
0.004
0.01
0.0004
100Hz
10MHz
1MHz
100kHz
10kHz
1kHz
Frequency
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Basic Eddy Current Theory The impedance plane

• Resistance (X) vs. Reactance (Y)
• Values unique to probe and frequency, but general
  form is the same.

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Basic Eddy Current Theory The impedance plane
Titanium
Crack in Aluminium
Lift-Off
Aluminium
Increasing conductivity of Test
Sample
Copper
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Basic Eddy Current Theory The impedance plane

• Typical instrument display is a Window on
  impedance plane
• Rotate and Zoom to suit application

cracks
Lift-Off

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Basic Eddy Current Theory Factors affecting
eddy current response

• Conductivity
• Measured in IACS or MSm-1
• Greater Conductivity -gt Greater current flow on
  the surface - Less penetration
• Conductivity is often measured using eddy
  currents.
• Permeability (relative)
• one for Nonferrous, up to hundreds for Ferrous.
• Higher permeability reduces penetration into
  metal and gives much larger EC response.
• Permeability variations may mask defects

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Basic Eddy Current Theory Factors affecting
eddy current response

• Frequency
• Very significant effect on response
• The one thing that we can totally control!
• Geometry
• CRACKS!!!!
• Curvature, edges, grooves etc. all affect
  response
• Generally try and scan along line of constant
  geometry
• Thickness relevant if less than depth of
  penetration.

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Basic Eddy Current Theory Factors affecting
eddy current response

• Lift-off
• Closer probe to surface -gt greater effect
• Lift-off signal as spacing varies
• reduction in sensitivity as spacing increases.
• All of these factors will affect the response
  accurate assessment of one requires that the
  others be held constant or their influence
  minimised

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Basic Eddy Current Theory Coil Configurations

• Three main groups
• Surface probes - used mostly with the probe axis
  normal to the surface, includes pencil probes and
  fastener hole probes
• Encircling coils - e.g. in-line inspection of
  round products
• ID probes - e.g. in-service inspection of heat
  exchangers.

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Basic Eddy Current Theory Coil Configurations

• Absolute probe
• Single coil (mostly)
• Metal sorting and crack detection
• Sensitive also to material variations,
  temperature changes etc.

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Basic Eddy Current Theory Coil Configurations

• Differential probe
• Sensitive to small defects
• Insensitive to lift-off, temperature, geometry
  changes common to both coils
• Characteristic figure 8 response
• Probe / flaw orientation critical

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Basic Eddy Current Theory Coil Configurations

• Reflection (Driver/Pickup) Probes
• Primary winding driven from the oscillator
• Sensor winding(s) connected to the measurement
  circuit
• May give response equivalent to either an
  absolute (top) or differential probe(lower).
• Each winding can be optimised for its function
• Wider frequency range
• Better penetration
• Better sensitivity at large lift-off

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Basic Eddy Current Theory Coil Connections

• Bridge Probes
• When the bridge is balanced the measured voltage
  will be zero

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Basic Eddy Current Theory Coil Connections

• Reflection (Driver/Pickup) Probes

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Practical Testing Requirements

• Any practical Eddy current test will require the
  following
• An instrument with the necessary capabilities.
• A suitable probe
• A good idea of size, location and type of the
  flaws it is desired to find
• A suitable test standard to set up the equipment
  and verify correct operation
• A procedure or accept/reject criteria based on
  the above.
• The necessary operator expertise to understand
  and interpret the results.

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Part 2Practical Testing
HOCKING eddy current training programme
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Practical Testing Requirements

• Any practical Eddy current test will require the
  following
• An instrument with the necessary capabilities.
• A suitable probe
• A good idea of size, location and type of the
  flaws it is desired to find
• A suitable test standard to set up the equipment
  and verify correct operation
• A procedure or accept/reject criteria based on
  the above.
• The necessary operator expertise to understand
  and interpret the results.

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Practical Testing Typical Instrumentation

• Special Purpose(AutoSigma 3000 shown)
• Conductivity, Coating thickness etc.
• Simple digital readout
• Minimal operator training
• Crack Detectors (Locator UH shown)
• Meter or Bar-graph readout
• High frequency - Surface cracks and sorting
• Often absolute probe only

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Practical Testing Typical Instrumentation

• Portable impedance plane Eddy Current Flaw
  detectors(Phasec 2200 shown)
• Impedance plane display
• Wide frequency ranges
• extensive alarm facilities,
• rate filtering
• may have multifrequency operation,

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Advantages of Eddy Current Inspection

• High sensitivity to microscopic flaws at or near
  the metal surface
• High inspection speeds
• No surface preparation required
• Can detect flaws through paint layers
• Good discrimination between flaw types
• No couplant, no consumables, no radiation hazards
• No effluent treatment needed
• Ability to access the small and complex
  geometries
• Skills are easy to acquire
• Complementary to Ultrasonic technology

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Practical Testing Operating frequency

• Primary operator controlled variable.
• Determine Relative response from different flaws
  or Artefacts
• Mostly Determined by
• Probe,
• Material Type,
• Material thickness/Geometry
• High frequency ( typically gt 100 kHz) tests
• Little penetration,
• Find small flaws, More signals from scratches
  etc.
• Low Frequency (typically lt10kHz) Test
• Deep Penetration Find Thickness variations etc.
• Insensitive to signals from small flaws and
  scratches

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Practical Testing Applications

• Surface Crack Detection
• Pencil or Pancake probes
• High Frequencies
• Find cracks down to 0.1mm or so deep
• Normally Absolute probes, sometimes differential,
  but crack direction/probe orientation is critical

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Practical Testing Applications

• Metal Sorting
• Conductivity / Permeability Testing
• For NFe Conductivity meter may be a better
  choice
• Frequencies from few Hz to MHz depending on
  parameters / geometry
• N.B Same reading does not mean same metal
• Many factors can vary together,
• Check for correct Heat treatment or composition,
  Not both at once

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Practical Testing Applications

• Sub-Surface Crack/Corrosion Detection.
• Primarily Used in Airframe Inspection.
• Low Frequency,
• Usually Reflection Probes
• Penetrate Aluminium Structures (10mm)
• Detect Second and Third Layer Cracking or
  Corrosion

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Practical Testing Applications

• Heat exchanger tube testing
• Petrochemical or power generation Heat exchangers
  may have thousands of tubes, up to 20m long.
• Use a differential ID bobbin probe
• Test at high speed (up to 1 m/s or so with
  computerized data analysis.)
• Identifies cracks, inside or outside corrosion
• Pitting can be assessed to an accuracy of about
  5 of wall thickness.
• The operating frequency is determined by the tube
  material and wall thickness,
• Dual or multiple frequency inspections commonly
  used

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Practical Testing Applications

• In-Line inspection of Steel tubing
• Inspect using encircling coils .
• Magnetic material - two main problems
• High permeability - little or no penetration.
• Variations in permeability cause eddy current
  responses greater than those from defects.
• Overcome by magnetically saturating the tube
  using a strong DC field.
• Tubes up to around 170mm diameter
• Welded tubes tested using sector coils which only
  test the weld zone.

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Practical Testing Applications

• Ferrous weld inspection
• geometry and material variations prevent
  inspection with a conventional eddy current
  probe,
• Special purpose WeldScan probe has been
  developed
• Allows inspection of welded steel structures for
  fatigue-induced cracking,
• May be used in adverse conditions, or even
  underwater,
• Will operate through paint and other
  corrosion-prevention coatings.
• Cracks around 1mm deep and 6mm long can be found
  in typical welds.