Eddy Current

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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NDT Ltd.,NDT Ltd., St. Albans, UKSt. 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

Part 1:

Theory

HOCKINGeddy currenttraining programme

Introduction - Historical Context

• 1879 - Hughes sorted metals of different permeability and conductivity

• 1930’s used for metal sorting.

• 1940’s crack detection applications developed.

• 1950’s & 60’s techniques developed in Aviation and nuclear industries.

• AC Field induces circulating eddy currents

• Eddy currents load coil

• Loading affects coil impedance

Basic Eddy Current Theory: Simple Coil above a metal surface

• Crack in surface reduces eddy current flow

• Loading on coil changes

• Coil impedance changes


• An AC magnetic field induces circulating eddy currents in a conductive material

• Changes in the properties of the material change the sensor impedance

Principle Of Eddy Current Inspection

• Monitor voltage across coil

• Coil impedance changes

• Voltage across coil changes

• Detect changes in eddy current flow



Crack parallel to eddy

currents - not detected

Crack interrupts eddy

currents - detected

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

Basic Eddy Current Theory: Depth of Penetration

• Decreases with an increase in frequency

• Decreases with an increase in conductivity

• Decreases with an increase in permeability

50 ( . )f r

Copper

Steel

Aluminium

Titanium

Frequency

0.01

0.1

1

10

100

Depth (mm)

100Hz 10MHz1MHz100kHz10kHz1kHz

Depth (in)

0.0004

0.004

0.04

0.4

4

Basic Eddy Current Theory : The impedance plane

• Resistance (X) vs. Reactance (Y)

• Values unique to probe and frequency, but general form is the same.

Steel

XL

R

Probe inAir

PROBE COIL IMPEDANCE - Z

NON-MAGNETIC

MAGNETIC

Lift-Off

Crack in Steel

Ferrite

Titanium

Crack in Aluminium

Aluminium

Increasing conductivity of TestSample

Copper


Lift-Off

Titanium

Crack in Aluminium

Aluminium

Increasing conductivity of Test

Sample

Copper


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

Lift-Off

“cracks”

• Conductivity– Measured in %IACS or MSm-1

– Greater Conductivity -> 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

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.


response


response•Lift-off

– Closer probe to surface -> 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

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.


• Absolute probe– Single coil (mostly)– Metal sorting and crack detection – Sensitive also to material variations,

temperature changes etc.


• 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“wobble”

“cracks”


• 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

Basic Eddy Current Theory: Coil Connections

• Bridge Probes– When the bridge is balanced the measured voltage

will be zeroDifferential Amplifier

Phase Sensitive Detector &balance circuitry

Main Amplifier

Basic Eddy Current Theory: Coil Connections

• Reflection (Driver/Pickup) ProbesDifferential Amplifier

Phase Sensitive Detector &balance circuitry

Main Amplifier

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.

Part 2:

Practical Testing

HOCKINGeddy currenttraining programme

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.

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

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

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 > 100 kHz) tests:– Little penetration, – Find small flaws, More signals from scratches etc.

• Low Frequency (typically <10kHz) Test– Deep Penetration: Find Thickness variations etc.– Insensitive to signals from small flaws and scratches

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


• 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


• 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


• 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


• 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.


• 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.

Eddy Current

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