Ndt03 - Eddy Current Inspection

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EDDY CURRENT

INSPECTION

Department of Metallurgy

University of Indonesia

NDT Laboratory


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NDT laboratory

Introduction

Also known as Foucault currents or inducedcurrents

Use a varying magnetic field produced by a test

coil to induce small, circulating currents called

eddy currentsinto electrically conductive materials

Any change in the eddy currents is reflected by a

change in the test coil impedance

The effect is analogous to a transformer, withspecimen acting as transformer core

Most widely applied to non-magnetic materials,

because the relative permeability is unity


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History

H.C. Oersted 1819 Change of electric current affected a magnet

W. Sturgeon 1823 Copper wire around a horseshoe produced an electromagnet

Gamby 1824 Oscillations of suspended bar magnet damped by presence of metal

Plate

J. B. Foucault 1830 Demonstrated existence of eddy currents

M. Faraday 1832 Law of electromagnetic induction

D.E. Hughes 1879 Electric pulses from a microphone coil to induce eddy currents in

metals for NDT

F. Krantz 1920 Wall thickness measurements

C. Farrow 1925 Eddy current inspection testing of steel tubes on an industrial scale

Reutlingen

Institute, Germany

1948 Development of eddy current instrumentation

H.G. Doll 1949 Eddy current in geology

F. Forster 1954 Impedance plane diagram. Used model of mercury conductor with

plastic trips as discontinuities


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Basic Principles

Faraday's law states that, whenever a magnetic fieldcuts a conductor, an electrical current will flow in the

conductor, if a closed path is provided over which current

can circulate

The alternating current flowing through the test coilproduces an alternating magnetic field around the coil

Material

Eddy

Current

path

Coils

Magnetic field

Test coilAs coils magnetic field alternates

eddy currents flow in one direction

and then the other


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Basic Principles

The flow of eddy currents in the material causesa fluctuating magnetic field of its own

This magnetic field is always in opposition to the

coil's magnetic field

Test materials

Eddy currents

Direction of coils field

Direction of eddy currents field

Indicating

instrument

Test coils


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Basic Principles

V= i R

(for dc) (Ohms law) V = i Z(for ac)

I=I0sin t

=2

V L di/dt = i R

VL= -L di/dt = - L i0cos t = - XLi0cos t

V - XLi0cos t = i0R sin t

V = i0( R sin t + XLcos t )

Z = R sin t + XLcos t

R = resistanceL = inductance

Z = impedance

= angular frequency


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Basic Principles

When the test coil is placed on conductive material,the strength of the coil's magnetic field is reduced

This change in the magnetic field causes a change

inthe impedance of the coilwhich, in turn, causes

a change in the current flowing through the coil.

This change is detected by a meter placed in the

test circuit

anything that affects the eddy currents will affect theimpedance of the coil and, thus, be detectable by

the meter.


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Objectives/Applications

Surface and sub-surface discontinuities inmetallic surfaces, cracks, pits, scratches,

Intergranullar corrosion in tubes and pipes

depending on metals involvesHeat treatment crack in non-ferrous

surface

Conductivity measurement for determiningfire damaged area

Coating and metal sheet thicknesses


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Advantages

Instantaneous result Sensitive to a range of physical properties

Contact between inspection coil and

specimen not required Equipment small and self contained

Can be miniaturized to observe

discontinuities as small as 1 mm3


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Limitations

Specimen must conductor materials

Access to materials surface required

Special probe required for each applications

Dept of penetration restricted Trained and experienced operator required

Sensitive to combinations and variations in

materials No permanent records

Reference standard required


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NDT laboratory

Inspection System

1. Source of varying magnetic field, for example,a coil carrying an alternating current of

frequencies ranging from well below 1 kHz to

above 10 MHz (a pulsed source may also be

used)

2. Sensor to detect minute changes in the

magnetic field (~ 0.01%), for example,

inspection coil or Hall gaussmeter3. Electronic circuitry to aid the interpretation of

the magnetic field change


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Inspection System


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Inspection Coils Types

Flat pancake coils

Inside/bobbin coils

Encircling coils


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Inspection Coils Types

Flat surface coils

Encircling coils

Bobbin coils


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Inspection Coils System

Single Coil as a Combined Induction-Receiver. Thechange of impedance of the coil (or coils) is determined

in the same coil (or coils) used to generate the magnetic

field (fig a.)

Separate Induction-Receiver Coils. The inducedmagnetic field is measured by a separate coil.

Decreasing the size of the inspection coils is an

advantage, and also the coil can be enclosed in a

magnetic shield using mu-metal, when the coil is

considered focused. (fig b. and c.)

Hall effect device is used to sense the eddy current

magnetic field


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Inspection Coils System


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Detector System


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Effect of Discontinuity

The presence of a non-conducting discontinuity such asa crack or non-metallic inclusion, is to impede andeffectively reduce the eddy currents.

This will result in an increase in impedance which will bedetectable by the instrument

The instrument is then telling us is that there issomething present in the surface which has caused aneffective decrease in conductivity - no matter what themanufacturer of the instrument may call it, it is notessentially a "crack detector" but rather a "change inelectrical/magnetic properties detector".

The decision as to whether a crack is present is made bythe inspector and not the instrument!


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Oscilloscope Displays


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Oscilloscope Displays


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Impedance Test

Measures the magnitude of the impedance with no information

about the phase change

The balanced bridge method is set up so that there is no signal

through the meter when the inspection coil is against the surface of

a specimen of good condition

When the inspection coil is in the presence of a discontinuity, thebridge is now unbalanced resulting in a potential difference across

the meter


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Frequencies

At the lower frequency, depth of penetration is relatively high, but

sensitivity is relatively low


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Frequencies

Selection factors (depends on specimen) :

Electrical conductivity

Magnetic permeability

Dimension

Cylindrical specimen ; characteristic frequency (vc )

Thin-walled tubes

0

22

1

r

c

rv

0

2

1

r

v


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NDT laboratory

Specimens

Fundamental properties of materials that affectthe eddy currents : The electrical conductivity of the material

The dimensions of the material

The magnetic permeability of the material Metal condition (alloy, hardness, homogeneity, grain

size)

Discontinuities in specimen

Testing conditions : Distance between coil and specimen lift-off

Alternating current frequency, coil size, number ofturn


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Conductivity

The ability of the materialto carry electrical current

the IACS system the

conductivity of unalloyed

(pure) annealed copper is

arbitrarily selected as the

standard

Each type of material has

an inherent conductivity

that is different from that ofother types of material

The higher the conductivity,

the more sensitive the test

Metal or Alloy Conductivity (%IACS)

Silver 105

Copper, annealed 100

Gold 70

Aluminium 61

Aluminium alloys:6061-T6 42

7075-T6 32

2024-T4 40

Magnesium 37

70-30 Brass 28Phosphor Bronze 11

Monel 3.6

Titanium 3.1

Ti-6Al-4V Alloy 1.0

304 Stainless Steel 2.5

NDT l b t


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Conductivity

There are some internal factors that affecting theconductivity of material : Alloying :Each metal or chemical element has an

individual effect on the conductivity of the base metal.The conductivity of the base metal is changed to a

value related to the composition of the alloy Heat-treatment or Hardness :The change in

hardness is brought about by an internal change inthe material

Temperature and Residual Stress :An increase intemperature normally results in a decrease inconductivity

Conductive Coatings

NDT l b t


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Dimensional Factors

Material Thickness Eddy currents do not penetrate throughout a thick material but

tend to be concentrated near the surface

Thus, there is a finite, or limited depth of penetration

NDT l b t


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Dimensional Factors

Lift-off Factor any space that occurs between the test coil and the specimen under test

This effect is greatest when the coil is close to the surface, when very

small changes in lift-off can result in relatively large instrument

responses, which can swamp other test indications

NDT laboratory


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Dimensional Factors

Edge Effect eddy currents are distorted when the end, or an edge, of a part is

approached with the test coil since the eddy currents can only flow in

the test article

A similar effect is apparent at the junctions between sections

NDT laboratory


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Depth of penetration of eddy currents below the surface islimited

intensity decreases exponentially with depth

The "standard depth of penetration" is defined as that depth atwhich their intensity has fallen to 1/e (where e is the naturallogarithm) of their value at the surface

S = the standard depth of penetration

= the conductivity of the material

= the frequency

r= the relative magnetic permeability

K = a constant depending on the units used

Standard for Depth of Penetration

r

KS

Ndt03 - Eddy Current Inspection

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