Intro to Eddy Current (1) by Raj Deep Gupta

8/4/2019 Intro to Eddy Current (1) by Raj Deep Gupta

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National Institute Of Technology,warangal

Presented By:Raj Deep Gupta

103405Materials Technology

M.Tech (1st)


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Methods of NDT

Visual


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Introduction

Eddy current inspection is one of severalmethods that use the principal ofelectromagnetism as the basis forconducting examinations. Several other

methods such as Remote Field Testing(RFT), Flux Leakage and Barkhausen Noisealso use this principle.


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Outline

Electromagnetic induction

Generation of eddy currents

Inspection applications

Equipment utilized in eddy current inspection Probes/Coils

Instrumentation

Reference standard

Advantages and Limitations

Glossary of Terms


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Electromagnetic Induction

Eddy currents are created through a processcalled electromagnetic induction.

Faradays law When alternating current is appliedto the conductor, such as copper wire, a

magnetic field develops in and around theconductor.

This magnetic field expands as the alternatingcurrent rises to maximum and collapses as thecurrent is reduced to zero.


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Electromagnetic Induction (cont.)

If another electrical conductor is brought into theproximity of this changing magnetic field, thereverse effect will occur. Magnetic field cuttingthrough the second conductor will cause an

induced current to flow in this second conductor.Eddy currents are a form of induced currents!

Current Flow


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Generation of Eddy Currents

Eddy currents are induced electrical currents thatflow in a circular path. They get their name fromeddies that are formed when a liquid or gas flowsin a circular path around obstacles when conditions

are right.

Test Probe

Eddy Currents


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Generation of Eddy Currents (cont.)

In order to generate eddy currents for an inspectiona probe is used. Inside the probe is a length ofelectrical conductor which is formed into a coil.


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Generation of Eddy Currents(cont.)

Alternating current is allowed to flow in the coil at afrequency chosen by the technician for the type oftest involved.


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A dynamic expanding and collapsing magnetic fieldforms in and around the coil as the alternatingcurrent flows through the coil.


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When an electrically conductive material is placed inthe coils dynamic magnetic field electromagnetic,induction will occur and eddy currents will beinduced in the material.


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Eddy currents flowing in the material will generatetheir own secondary magnetic field which willoppose the coils primary magnetic field.


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This entire electromagnetic induction process toproduce eddy currents may occur from severalhundred to several million times each seconddepending upon inspection frequency.


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Alternate current induces a primary field in a coil

The primary field induces Eddy Currents in the tube wall

The Eddy Currents induce a secondary field, which has an affect to the

coil impedance Defects in the tube wall are changing the secondary field and

consequently the impedance of the coil

The change of the coil impedance can be measured in input/outputphase and amplitude


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D

epth

Eddy Current Density

Depth

Eddy Current Density

Low FrequencyLow ConductivityLow Permeability

High FrequencyHigh ConductivityHigh Permeability

Standard Depthof

Penetration(Skin Depth)

1/e or 37 %of surface density


Eddy currents are strongest at the surface of the materialand decrease in strength below the surface. The depththat the eddy currents are only 37% as strong as they areon the surface is known as the standard depth ofpenetration or skin depth. This depth changes with

probe frequency, material conductivity and permeability.


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Eddy Current Density &

Depth of Penetration


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Effect of liftoff or fill factor

Effect of conductivity

Effect of magnetic permeability Effect of geometry


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Inspection Data (cont.)

R~

XL

Test

Coil

Impedance (Z) in an eddy current coil isthe total opposition to current flow. In acoil, Z is made up of resistance (R) andinductive reactance (XL).

Definitions: Resistance - The opposition of current

flow, resulting in a change of electricalenergy into heat or another form ofenergy.

Inductive Reactance (XL) - Resistance toAC current flow resulting fromelectromagnetic induction in the coil.

Impedance (Z) - The combinedopposition to current flow resulting from

inductive reactance and resistance.

In an AC coil, induction fromthe magnetic field of one loopof the coil causes a secondarycurrent in all other loops. Thesecondary current opposes the

primary current.


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Inspection Applications

One of the major advantages of eddy current as anNDT tool is the variety of inspections that can beperformed. The following slides depict some of thethese capabilities.


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Material Thickness Measurement

Thickness measurements are possible with eddycurrent inspection within certain limitations.

Only a certain amount of eddy currents can formin a given volume of material.

Therefore, thicker materials will support moreeddy currents than thinner materials.

The strength (amount) of eddy currents can bemeasured and related to the material thickness.

Eddy Currents

Magnetic Field

From Probe

Test

Material


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Material Thickness Measurement(cont.)

Eddy current inspection is often used in the aviationindustries to detect material loss due to corrosionand erosion.


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Material Thickness Measurement(cont.)

Eddy current inspection is used extensively toinspect tubing at power generation andpetrochemical facilities for corrosion and erosion.


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Crack Detection

Crack detection is one of the primary uses of eddycurrent inspection. Cracks cause a disruption in thecircular flow patterns of the eddy currents andweaken their strength. This change in strength atthe crack location can be detected.

Magnetic FieldFrom Test Coil

Magnetic FieldFrom

Eddy Currents

Eddy Currents

Crack


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Crack Detection (cont.)

Eddy current inspection is exceptionally well suitedfor the detection of cracks, with an especially highsensitivity to detection of surface breaking cracks.


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Nonconductive CoatingMeasurement (cont.)

The photo to the left shows an aircraft panel paintthickness inspection. On the right, the display of adigital eddy current inspection instrument shows thedifferent signals obtained by measuring eightdifferent thicknesses of paint on aluminum.

Increasing paintthickness


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Equipment

Equipment for eddy current inspection is verydiversified. Proper equipment selection is importantif accurate inspection data is desired for a particularapplication.

As a minimum, at least three basic pieces ofequipment are needed for any eddy currentexamination:

Instrumentation

Probes Reference Standards


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Instrumentation - Meters

Meters are typicallythe simplest form ofeddy currentinstrumentation.

The two generalcategories of metersare digital and analog.


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Digital Meters

Digital meters are typically designed to examine onespecific attribute of a test component such asconductivity or nonconductive coating thickness.These meters tend to have slightly higher accuracy

than analog devices.


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Analog Meters

Analog meters can beused for many differentinspection applicationssuch as crack detection,

material thicknessmeasurements,nonconductive coatingmeasurements orconductive coating

measurements.


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Portable Eddy Scopes


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Eddy Current Probes (cont.)

Probes selection is critical to acquiring adequateinspection data.

Several factors to consider include: Material penetration requirements (surface vs.

subsurface) Sensitivity requirements

Type of probe connections on eddy currentinstrument (many variations)

Probe and instrument impedance matching (willprobe work with instrument)

Probe size (smaller probes penetrate less)

Probe type (absolute, differential, reflection or

hybrid)


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Due the the large variety of probes in eddy currenttesting there are many different systems ofclassification.

Three of the most common classifications are: Surface probes Inside Diameter (I.D.) or Bobbin Probes

Outside Diameter (O.D.) or Encircling probes


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Some surface probes are specifically designed forcrack detection of fastener holes. These includesliding probes, ring probes and hole probes.


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Inside Diameter (I.D.) probes, also known asbobbinprobes, are coils that are usually woundcircumferentially around a plastic housing. Theseprobes are primarily designed for inspection inside oftubular materials.


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Outside Diameter (O.D.) probes are coils that arewound the circumference of a hollow fixture. Thecoil is designed such that the test part is ranthrough the middle of the coil. These probes can beused to inspect bars, rods as well as tubes.


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Magnetooptic eddy currentimaging


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Schematic setup


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Advantages of Eddy CurrentInspection

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 flawdetection

Minimum part preparation is required

Test probe does not need to contact the part

Inspects complex shapes and sizes of conductivematerials


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Limitations of Eddy CurrentInspection

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 and roughness may interfere

Reference standards needed for setup

Depth of penetration is limited

Flaws such as delaminations that lie parallel to theprobe coil winding and probe scan direction areundetectable

It can inspact with reasonable senstivity metallic


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Glossary of Terms

Alternating Current: electrical current that regularlyreverses direction.

Analog: being or relating to a mechanism in whichdata is represented by continuously variable physical

quantities such as a watch with hour and minutehands.

ASME: acronym for American Society of MechanicalEngineers. This society is highly involved inestablishing and maintaining industrial standards.


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Glossary of Terms

CRT: acronym for Cathode Ray Tube. Vacuum tubethat uses one or more electron guns for generatingan image.

Calibration: adjustment of a test systems responseusing known values so that unknown quantities maybe derived.

Conductor: material capable of allowing electricalcurrent to flow through it.

Discontinuity: an interruption in the physicalstructure of a part. Cracks are examples ofdiscontinuities.

EDM: acronym for Electrical Discharge Machine.


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Glossary of Terms

EDM: acronym for Electrical Discharge Machine.Machining technique which uses an electrode andelectrical current to remove metal. Sometimes usedto prepare calibration standards for eddy currenttesting.

Electromagnetic Induction: process which createselectrical current flow when a dynamic magnetic fieldis brought into close proximity with an electricalconductor.

Extrapolation: to project or predict unknown valuesfrom know quantities.


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Glossary of Terms

I.A.C.S.: acronym for International Annealed CopperStandard. Standard unit of measurement of electricalconductivity in eddy current testing with pureannealed copper as the standard, measuring 100%at 20 degrees Celsius.

Impedance Plane Diagram: A diagram that depictsthe changes in electrical impedance that occur in aneddy current coil as test variables change.

Multiplexing: use of a time sharing system in which

a coil is stimulated at several different frequenciesone after another for a certain amount of time.Results from each stimulation can then be processedand displayed.


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Glossary of Terms

Permeability: the ease with which a material can bemagnetized.

Probe: common term used in eddy current inspectionthat refers to the test coil.

RAM: acronym for Random Access Memory. Mostmodern eddy current instruments have some form ofmemory used as a data buffer to store information.


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Some questions

1)on which principle ECT works?

Electromagnetic induction

2)Which type of current we use in ECT?a) DC b) AC

3)What is the difference between lift-off

and fill factor?4)What is the standard depth of

penetraion?

63% of the intensity at the surface


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5) on which path eddy currents flow?

Circular

6)How we can find the the direction ofsecondary magnetic field?

right hand thumb rule

7)ECT basically is not useful for whichtype of material?

Ferromagnetic


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8)Which type of material suitable forECT?

a)Conductors b)semiconductorsc)insulators

9) Cracks which are parallel to eddycurrents cant be detected.

10)Intensity of eddy currents inversilyproportional to depth


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11)What is the relation between conductivity andresistivity?

Conductivity is inverse of resistivity

12)Conductivity depends on-1:heat treatment

2:chemical composition

3:temperature

13)How you can change the depth of penetration?By changing freequency,conductivity and magnetic

permiability of material.

Documents

Intro to Eddy Current (1) by Raj Deep Gupta