Eddy Current Array Tutorial

8/16/2019 Eddy Current Array Tutorial

1/13


2/13

3.' 8ivet nspection

3.+ Corrosion 1etection

*eneral ntroduction to Eddy Current (EC)

Testing

Eddy current (EC) testing is a no contact method for the inspection of metallic parts. Eddy currents are fields

of alternating magnetic current that are created when an alternating electric current is passed through one or

more coils in a probe assembly. hen the probe is lin#ed with the part under inspection, the alternating

magnetic field induces eddy currents in the test part. 1iscontinuities or property variations in the test part

change the flow of the eddy current and are detected by the probe in order to ma#e material thic#ness

measurements or to detect defects such as crac#s and corrosion.

9ver the years, probe technology and data processing have advanced to the point where eddy current

testing is recogni&ed as being fast, simple, and accurate. The technology is now widely used in the

aerospace, automotive, petrochemical, and power generation industries for the detection of surface or near-

surface defects in materials such as aluminum, stainless steel, copper, titanium, brass, nconel:, and even

carbon steel (surface defects only).

istory of Eddy Current Testing

http://www.olympus-ims.com/ndt-tutorials/eca-tutorial/applications/rivethttp://www.olympus-ims.com/ndt-tutorials/eca-tutorial/applications/corrosionhttp://www.olympus-ims.com/data/Image/eca-tutorial/OmniScan_Aero_08.jpghttp://www.olympus-ims.com/ndt-tutorials/eca-tutorial/applications/rivethttp://www.olympus-ims.com/ndt-tutorials/eca-tutorial/applications/corrosion


3/13

The phenomenon of eddy currents was discovered by ;rench physicist $eon ;oucault in '


4/13

9mni7can ECA

0asic Concepts

Eddy current array and conventional eddy current technology share the same basic principle. Alternating

current inected into a coil creates a magnetic field (in blue). hen the coil is placed over a conductive part,

opposed alternating currents (eddy currents, in red) are generated. 1efects in the part disturb the path of the

eddy currents (in yellow). This disturbance can be measured by the coil.

Eddy current array (ECA) technology provides the ability to electronically drive multiple eddy current coils

placed side by side in the same probe assembly. 1ata ac!uisition is performed by multiple%ing the eddy

current coils in a special pattern to avoid mutual inductance between the individual coils.

ost conventional eddy current flaw detection techni!ues can be reproduced with an ECA inspection. ith

the benefits of single-pass coverage, and enhanced imaging capabilities, ECA technology provides a

remar#ably powerful tool and significant time savings during inspections.

Major advantages of ECA testing are the followingF

$arger area can be scanned in a single-probe pass, while maintaining a high resolution

8educed need for comple% robotics to move the probe" a simple manual scan is often enough

mproved flaw detection and si&ing with C-scan imaging

http://www.olympus-ims.com/omniscan-eca/http://www.olympus-ims.com/data/Image/eca-tutorial/Principe_CF_04.gifhttp://www.olympus-ims.com/data/Image/eca-tutorial/OmniScan_ECA_2003_07_2.jpghttp://www.olympus-ims.com/data/Image/eca-tutorial/OmniScan_ECA_2003_07.jpghttp://www.olympus-ims.com/omniscan-eca/


5/13

nspection of comple% shapes using probes customi&ed to the profile of the part being inspected

The 9mni7canD ECA test configuration supports + sensor coils (up to 63 with an e%ternal multiple%er)

wor#ing in bridge or transmit-receive mode. The operating fre!uency ranges from + & to 6 & with the

option of using multiple fre!uencies in the same ac!uisition.

1epth of 2enetration

Eddy current density does not remain constant across the depth of a material. The density is greatest at the

surface and decreases e%ponentially with depth (the Gs#in effectG). The standard depth of penetration

e!uation (shown to the right) is used to e%plain the penetration capability of eddy current testing, whichdecreases with increasing fre!uency, conductivity, or permeability. ;or a material that is both thic# and

uniform, the standard depth of penetration is the depth at which the eddy current density is >H of the

material surface value. To detect very shallow defects in a material, and also to measure the thic#ness of

thin sheets, very high fre!uencies are used. 7imilarly, in order to detect subsurface defects, and to test

highly conductive, magnetic, or thic# materials, lower fre!uencies must be used.

http://www.olympus-ims.com/data/Image/eca-tutorial/Principe_ECA_Raster_02C.gif


6/13


7/13


8/13

Benefits of Eddy Current Array esting

Compared to single-channel eddy current technology, eddy current array technology provides the following

benefitsF

1rastically reduces inspection time

Covers a large area in one single pass8educes the comple%ity of mechanical and robotic scanning systems

2rovides real-time cartography of the inspected region, facilitating data interpretation

s well suited for comple% part geometries

mproves reliability and probability of detection (291)


9/13

Eddy Current Testing

By om !elligan and Cynthia Calderwood

agnetism, the underlying principle behind electric motors and generators, relays and stereo spea#ers, is

also the force that enables an important category of 41T tools called eddy current instruments. Eddy current

testing is widely used in the aerospace industry and in other manufacturing and service environments that

re!uire inspection of thin metal for potential safety-related or !uality-related problems. n addition to crac#

detection in metal sheets and tubing, eddy current can be used for certain metal thic#ness measurements

such as identifying corrosion under aircraft s#in, to measure conductivity and monitor the effects of heat

treatment, and to determine the thic#ness of nonconductive coatings over conductive substrates. 0oth field

portable and fi%ed system instruments are available to meet a wide variety of test needs.

Eddy current 41T can e%amine large areas very !uic#ly, and it does not re!uire use of coupling li!uids. n

addition to finding crac#s, eddy current can also be used to chec# metal hardness and conductivity in

applications where those properties are of interest, and to measure thin layers of nonconductive coatingsli#e paint on metal parts. At the same time, eddy current testing is limited to materials that conduct electricity

and thus cannot be used on plastics. n some cases, eddy current and ultrasonic testing are used together

as complementary techni!ues, with eddy current having an advantage for !uic# surface testing and

ultrasonics having better depth penetration.

"ow it wor#s

Eddy current testing is based on the physics phenomenon of electromagnetic induction. n an eddy current

probe, an alternating current flows through a wire coil and generates an oscillating magnetic field. f the

probe and its magnetic field are brought close to a conductive material li#e a metal test piece, a circular flow

http://www.olympus-ims.com/data/Image/EddyCurrent_works.jpg


10/13

of electrons #nown as an eddy current will begin to move through the metal li#e swirling water in a stream.

That eddy current flowing through the metal will in turn generate its own magnetic field, which will interact

with the coil and its field through mutual inductance. Changes in metal thic#ness or defects li#e near-surface

crac#ing will interrupt or alter the amplitude and pattern of the eddy current and the resulting magnetic field.

This in turn affects the movement of electrons in the coil by varying the electrical impedance of the coil. The

eddy current instrument plots changes in the impedance amplitude and phase angle, which can be used bya trained operator to identify changes in the test piece.

Eddy current density is highest near the surface of the part, so that is the region of highest test resolution.

The standard depth of penetration is defined as the depth at which the eddy current density is >H of its

surface value, which in turn can be calculated from the test fre!uency and the magnetic permeability and

conductivity of the test material. Thus, variations in the conductivity of the test material, its magnetic

permeability, the fre!uency of the AC pulses driving the coil, and coil geometry will all have an effect on test

sensitivity, resolution, and penetration.

There are many factors that will affect the capabilities of an eddy current inspection. Eddy currents traveling

in materials with higher conductivity values will be more sensitive to surface defects but will have less

penetration into the material, with penetration also being dependent on test fre!uency. igher test

fre!uencies increase near surface resolution but limit the depth of penetration, while lower test fre!uencies

increase penetration. $arger coils inspect a greater volume of material from any given position, since the

magnetic field flows deeper into the test piece, while smaller coils are more sensitive to small defects.

Lariations in permeability of a material generate noise that can limit f law resolution because of greater

bac#ground variations.

hile conductivity and permeability are properties of the test material that are outside of the operator=s

control, the test fre!uency, coil type, and coil si&e can be chosen based on test re!uirements. n a given test,

resolution will be determined by the probe type while detection capability will be controlled by material and

e!uipment characteristics. 7ome inspections involve sweeping through multiple fre!uencies to optimi&e

results, or inspection with multiple probes to obtain the best resolution and penetration re!uired to detect allpossible flaws. t is always important to select the right probe for each application in order to optimi&e test

performance.

Impedance plane displays

hile some older eddy current instruments used simple analog meter displays, the standard format now is

an impedance plane plot that graphs coil resistance on the %-a%is versus inductive reactance on the y-a%is.

Lariations in the plot correspond to variations in the test piece. ;or e%ample, the display below shows a

setup for inspection for surface crac#s in aluminum. The top curve represents a .3G deep surface crac#,

the middle curve is a .+G deep crac#, and the smallest curve is a .


11/13

This display would be considered the calibration of the instrument. 9nce the parameters are set, they should

not be changed during the inspection. The inspection measurements are dependent entirely on the

comparison of the signal against the reference calibration.

Another common test involves measurement of nonconductive coatings li#e paint over metals. The screen

display below shows a nonmetallic coating over aluminum. ;or this application, the probe is GnulledG

(balanced) in air and then placed on the sample. The top line shows the signal on aluminum without any

coating. The second line down is a .3G coating, then a .


12/13

Eddy current instruments can perform a wide variety of tests depending on the type of probe being used,

and careful probe selection will help optimi&e performance. 7ome common probe types are listed below.

Surface probes - Jsed for identifying flaws on and below metal surfaces, usually large diameter to

accommodate lower fre!uencies for deeper penetration, or for scanning larger areas.

Pencil probes - 7maller diameter probes housing coils built for high fre!uencies for high resolution of near

surface flaws.

Bolt hole probes - 1esigned to inspect the inside of a bolt hole. These probes can be rotated by hand or

automatically using a rotary scanner.

Donut probes - 1esigned to inspect aircraft fastener holes with fasteners in place.

Sliding probes - Also used in testing aircraft fastener holes, offering higher scan rates than donut probes.

ID probes - Jsed for inspection of heat e%changers and similar metal tubing from the inside, available in avariety of si&es.

OD probes - Jsed for inspection of metal tubing and bars from the outside, with the test piece passing

through the coil

$eference standards

An eddy current system consisting of an instrument and a probe must always be calibrated with appropriate

reference standards at the start of a test. This process involves identifying the baseline display from a given

test piece and observing how it changes under the conditions that the test is intended to identify. n flawdetection applications, this calibration process typically involves the use of reference standards of the same

material, shape, and si&e as the test piece, containing artificial defects such as saw cuts, drilled holes, or

milled walls to simulate flaws. n thic#ness measurement applications the reference standards would consist

of various samples of #now thic#ness. The operator observes the response from the reference standards

and then compares the indications from test pieces to these reference patterns to categori&e parts. 2roper

calibration with appropriate reference standards is an essential part of any eddy current test procedure.

http://www.olympus-ims.com/data/Image/EddyCurrent_ConductivityScreen.jpg


13/13

Common applications

Eddy current instruments can be used in a wide variety of tests. 7ome of the most common are listed below.

Weld Inspection - any weld inspections employ ultrasonic 41T for subsurface testing and a complimentaryeddy current method to scan the surface for open surface crac#s on weld caps and in heat affected &ones.

Conductivity Testing - Eddy current testing=s ability to measure conductivity can be used to identify and sort

ferrous and nonferrous alloys, and to verify heat treatment.

Surface Inspection - 7urface crac#s in machined parts and metal stoc# can be readily identified with eddy

current. This includes inspection of the area around fasteners in aircraft and other critical applications.

Corrosion Detection - Eddy current instruments can be used to detect and !uantify corrosion on the inside of

thin metal such as aluminum aircraft s#in. $ow fre!uency probes can be used to locate corrosion on second

and third layers of metal that cannot be inspected ultrasonically.

Bolt Hole Inspection - Crac#ing inside bolt holes can be detected using bolt hole probes, often with

automated rotary scanners.

Tubing inspection - 0oth in-line inspection of tubing at the manufacturing stage and field inspection of tubing

li#e heat e%changers are common eddy current applications. 0oth crac#ing and thic#ness variations can be

detected.

Eddy current arrays

Eddy Current Array testing, or ECA, is a technology that provides the ability to simultaneously use multiple

eddy current coils that are placed side by side in the same probe assembly. Each individual coil produces a

signal relative to the phase and amplitude of the structure below it. This data is referenced to an encoded

position and time and represented graphically as a C-scan image showing structures in a planar view. n

addition to providing visuali&ation through C-scan imaging, ECA allows coverage of larger areas in a single

pass while maintaining high resolution. ECA can permit use of simpler fi%turing, and can also simplify

inspection of comple% shapes through custom probes built to fit the profile of the test piece.

For further information

*eneral eddy current tutorial from owa 7tate JniversityF

httpFBBwww.olympus-ims.comBenBndt-tutorialsBeca-tutorialB

Tutorial on Eddy Current Array testing from 9lympus 41TF

httpFBBwww.olympus-ims.comBenBndt-tutorialsBeca-tutorialB

American 7ociety of 4ondestructive Testing, 4ondestructive Testing andboo#, Lolume 5, Electromagnetic

Testing (available through www.asnt.org)

Documents

Eddy Current Array Tutorial