Deep Penetrating Eddy Currents and Probes

Introduction

The eddy current technique is known as

an efficient surface and near surface in-

spection method avoiding any couplant

and able to penetrate thick non-conduct-

ing coatings. The probes mostly are easy

to handle and even mechanical probe

guiding and imaging techniques are

state of the art. These features bring up

new requirements. Why not use this

technique for subsurface inspection of

cracks, voids, corrosion or other mater-

ial anomalies? The weak point of eddy

currents is their limited penetration into

the conducting material. The magnetic

field of these currents is counteracting

the exciting magnetic field of the probe

thus lowering the eddy current density

with increasing depth. This behaviour

cannot be changed fundamentally but

the probes and the inspection parame-

ters can be optimized for maximum pen-

etration.

The following paragraphs analyse the

most significant influences on the pene-

tration behaviour of eddy currents and

present newly developed deep penetrat-

ing probes for application in aircraft

maintenance, nuclear and conventional

power plants and metal working indus-

try.

Standard and effectivepenetration depth

Figure 1 compares different definitions

of penetration depth. The standard pene-

tration depth δ defines the depth where

the eddy current density has decreased

down to 1/e of the surface density. At a

depth of 3δ the eddy current density de-

creases to about 5% of the surface den-

sity. The standard penetration depth

bases on the assumption of plane wave

behaviour of the penetrating magnetic

field. Actually, eddy current probes are

far from providing a plane magnetic field.

Following the model of Dodd and

Deeds [1] Mottl calculated the decrease

of eddy current density analytically [2].

For an air core probe he found that

the decrease of eddy current density

strongly depends on the probe diameter.

With small diameters (R/δ (δ ≈ 1) the

density decreases according to the

dashed line in Figure 1a and provides

significantly smaller values for the pen-

etration depth (δt) compared with the

plane wave. Only with rising diameter

up to R/δ > 10 the δt-values become sim-

ilar to δ.

Both values are theoretical values and

do not characterize the achievable in-

MP

2

EDDY CURRENT TESTING

Deep Penetrating Eddy Currentsand Probes

© Carl Hanser Verlag, München MP Materials Testing 49 (2007) 5

Gerhard Mook, Magdeburg,

Olaf Hesse, Nordhausen,

Germany, Valentin Uchanin,

Lviv, Ukraine

The eddy current skin-effect limits the detection of subsurface de-

fects and the range of thickness measurement. Traditional concepts

to estimate the penetration depth basing on plane wave propagation

into a conducting halfspace cannot describe the real depth of in-

spection achievable by state-of-the-art sensors and instruments.

The paper presents a more fruitful concept for estimating the noise

limited inspection depth. Here, the traditional parameters like

frequency, probe dimensions, conductivity and permeability are

analysed in combination with all sources of noise and disturbances

in eddy current technique. New low frequency eddy current probes

of inductive and magneto-resistive type are presented and charac-

terised. These probes combine deep penetration with comparatively

small size and good spatial resolution.

MP 100810 – Text geliefert

Figure 1. Definition of standard and effective penetration depth

spection depth. To fill this gap an effec-

tive penetration depth was defined. This

is the depth from where eddy current

signals can be received with a sufficient

signal to noise ratio. Obviously, this

depth cannot be calculated in general

but depends on the material and the de-

fect to be detected, the instrument and

probe parameters and the disturbing in-

fluences from the environment.

Figure 1b illustrates the effective pen-

etration depth for a signal to noise ratio

of 6 dB. Mostly the effective depth is

much greater than the calculated stan-

dard penetration depth.

Methods to increaseeffective penetration depth

Decrease of exciting frequency

Even from the equation for the standard

penetration depth and from the derived

diagram in Figure 2b can be seen that

the penetration increases with lowering

frequency. Figure 2a depicts the de-

crease of eddy current density for differ-

ent frequencies. Low frequencies seem

to be best suited for hidden defect detec-

tion.

This kind of analysis suffers from the

representation of eddy current density

as the ratio of the absolute density in a

defined depth to the surface current den-

sity. For the detection of hidden defects

the absolute current density is much

more important than the relative den-

sity. The absolute eddy current density

is a function of the field strength and the

frequency. So we have to consider, that

with lowering frequency the absolute

eddy current density lowers, too, due to

the lower rate of magnetic flux alter-

ation.

Increase of exciting field strength

Another way of increasing the penetra-

tion is to increase the field strength of

the exciter. This can be achieved, for in-

stance, by stronger current in the excit-

ing coil. The exciting current is only lim-

ited by the properties of the coils wind-

ings and the thermal and magnetic prop-

erties of the flux guides.

To increase the exciting field strength

and prevent coil heating the pulsed eddy

current technique is used [3, 4]. With

constant thermal load the energy of

many sine periods is concentrated in

one large pulse followed by a break.

Selecting deep penetrating field tra-

jectories

The use of non-axial send-receiver

probes offers the opportunity to opti-

mize the distance between the transmit-

ting and the receiving coil. These probes

sometimes are called half-transmission

or remote field eddy current probes. Fig-

ure 4 brings up the principle of those

probes. The magnetic field of the excit-

ing coil penetrates accordingly to the

well known rules of alternating field

spreading into the material. The receiv-

ing coil only picks up this part of the

flux, which has penetrated deeply into

the material. The larger the distance be-

tween the two coils the deeper the de-

tected flux lines have penetrated the ma-

terial but the lower becomes the mea-

surement signal. This system of two

non-axial coils may be considered as an

axial coil system with a diameter corre-

sponding to the coil distance of the non-

axial system. With increasing distance

(or diameter) of the coils the defect vol-

ume decreases relatively to the volume

of interaction lowering the signal ampli-

tude. One has to trade off between these

parameters.

Changing material’s properties

In some cases the change of the electro-

magnetic properties of the material un-

der inspection can increase the penetra-

tion. Very rarely it is possible to decrease

the conductivity and/or the magnetic

permeability by heating the material.

More often the permeability of ferro-

magnetic materials can be decreased by

a superimposed dc-field. Figure 5 illus-

trates this idea.

EDDY CURRENT TESTING MP

349 (2007) 5

Figure 2. Inspection frequency vs. penetration Figure 3. Field strength vs. penetration

Figure 4. Selection of deep penetrating

magnetic field trajectories (eddy currents not

shown)

Figure 5. Incremental

permeability vs. bias

DC field

The DC field may reduce the incre-

mental permeability down to μ0, i.e. the

relative permeability decreases to 1.

This way, the ferromagnetic material is

transformed into a non-magnetic mater-

ial from the eddy current point of view.

Increasing sensor sensitivity at lower

frequencies

Sources of noise at eddy current inspec-

tion are the exciting signal, the ambient

fields, the sensor, the electronic cir-

cuitry, the handling systems and in a

more common sense the material itself.

Along with choosing a less noisy eddy

current instrument (at low frequencies

high gain values become necessary) and

a sophisticated sensor handling (avoid-

ing vibration of the probe, small lift-off,

signal filtering) the sensor itself helps to

reduce noise signals.

At lower frequencies common induc-

tive sensors seem to be less advanta-

geous due to the decreasing measure-

ment voltage. The following paragraphs

describe the results of optimizing induc-

tive sensors and the search for alterna-

tive solutions by newly developed mag-

netic dc-field sensors.

Deep penetratingeddy current probes

Improved inductive pick-up coils

Although inductive pick-up coils show

decreasing sensitivity at lower frequen-

cies they can successfully be used for

sensitive low frequency eddy current

testing. There are several means for

increasing the sensitivity of inductive

pick-up coils. Larger coil diameter in-

creases the coupling flux with the mate-

rial and brings the field nearer to the

plane wave but lowers the lateral resolu-

tion. The increase of the number of turns

demands very thin enamelled copper

wire with down to 20 μm diameter. An

example of a pick-up coil with 1000

turns is shown in Figure 6. Additionally,

well compensated differential arrange-

ments of pick-up coils guarantee the op-

timal usage of the dynamic range of the

read out electronics. A special shielding

may lower the influence of external elec-

tromagnetic noise sources.

The usage of inductive pickup coils

has some clear advantages over the mag-

netic field sensors. They perform very

linear, they only have a very small hys-

teresis and they do not saturate even at

quite large excitation levels. That per-

mits high flexible sensor configurations.

Last not least inductive coils may be eas-

ily adapted to commercial eddy current

instruments.

Most significant disadvantages of in-

ductive pickup coils are the limited re-

producibility and the very time consum-

ing technology of their production re-

sulting in a quite high price.

Such highly sensitive coils can not be

used in absolute arrangement because

they are very sensitive to environmental

electromagnetic noise. This can be over-

come by well compensated differential

arrangements. Noise resulting from dis-

tant environmental sources will be can-

celled while material inhomogeneities

will produce field gradients detectable

by the gradiometric sensing element.

Further it is necessary to avoid direct

coupling of the excitation field into the

sensing element. This will result in a

very small output signal in the case of

homogeneous material maintaining

high sensitivity to disturbances in the

material under test. Figure 7 displays

some sensor configurations for low fre-

quency eddy current testing.

Magnetic field sensors in eddy cur-

rent probes

As mentioned above the sensitivity of

pick-up coils decreases significantly

with lower testing frequencies. In this

case dc-field sensors should be consid-

ered, e.g. magneto-resistors, Hall ele-

ments, flux gates or even SQUIDs (su-

perconducting quantum interference de-

vices). Very good results on deep

penetration eddy current testing were re-

ported using flux gates and SQUIDs [5-

7]. But testing systems described there

can hardly be used in real industrial ap-

plications because of the complexity and

costs of such systems, their insufficient

robustness and poor lateral resolution.

In our study we successfully used

commercially available AMR and GMR

sensors in eddy current probes for low

frequency eddy current testing. The ad-

ditional read out electronics for these

magnetoresistive type sensing elements

are quite simple and can easily be placed

into the sensor housing together with

the power supply necessary for sensor

excitation and read out electronics.

Anisotropic magneto-resistive sensors

For AMR (anisotropic magneto-resis-

tive) sensors we have to keep in mind

their limited dynamic range, the influ-

ence of magnetic field changes in the

sensitive direction of the sensor element

on the demodulated signal and their sen-

sitivity to heterogeneity of permanent

magnetic fields with direction perpen-

dicular to the sensitive direction and in

plane with the permalloy sensor stripes,

which can lead to strong disturbances of

sensor characteristics.

This situation can be overcome by fol-

lowing means. First, use these sensors

with zero detector read out electronics

(negative magnetic field feedback); sec-

ond, substitute absolute arrangements

by gradiometric of at least two sensing

elements; third, apply a stabilising mag-

netic field with direction perpendicular

to the sensitive direction and in plane

with the permalloy sensor stripes;

fourth, avoid direct coupling of the ex-

citation field to the sensing element.

An integrated sensor module nor-

mally used for non-contact current mea-

surement is available. It includes gradio-

metric layout of the AMR sensing ele-

ment, zero detection readout electronics

with on-chip field feedback inductors

and a sensor stripe pre-magnetisation

by calibrated permanent magnets pre-

cisely placed onto the sensor module.

The module has an acceptable size for

MP

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EDDY CURRENT TESTING

49 (2007) 5

Figure 6. Highly sensitive pick up coil, 0.5 mm

diameter, 3 mm length, number of turns

about 1000

Figure 7. Differential eddy current probes.

Sensing for both probes: Two differentially

connected pick up coils with about 3000

turns on ferrite rods 5 × 1.2 mm and μr ≈ 600.

Excitation: a) U-shaped ferrite yoke with ex-

citation coil, b) Two coils with axis parallel to

the test surface and differential orientation

a) b)

integration into eddy current sensors.

The functional scheme of the module is

shown in Figure 8a.

The base length of the gradiometer in

this module is 3 mm. To detect deep

buried defects we have to work on very

low excitation frequencies. This will

lead to a more and more blurred field

disturbance because the defect produces

weaker field gradients. Modelling of this

situation is required to get a clearer un-

derstanding how to optimise the gra-

diometer layout. It seems to be obvious

to increase the base length when defects

with greater underlying have to be de-

tected. This problem could be solved ex-

perimentally, but it is quite difficult to

produce a gradiometer module on dis-

crete elements as well compensated and

balanced as on the CMS2000 module.

Figure 8b and 8c describe some sensor

configurations for low frequency eddy

current testing using the described

AMR sensor module.

Giant magneto-resistive sensors

Several restrictions have to be kept in

mind when using GMR (giant magneto-

resistive) sensors in eddy current

probes. First, the dynamic range is lim-

ited and only has a quite narrow linear

branch; second, the influence of magnetic

field variations on the demodulated sig-

nal in the sensitive direction of the sensor

element; third, the loss of information

about the field direction due to the V-

shaped sensor characteristics; fourth, the

hysteresis of sensor characteristics.

Despite the lower field limited resolu-

tion of GMRs compared to AMRs there

are promising advantages for eddy cur-

rent applications. They are insensitive

to magnetic fields perpendicular to

their sensitive direction and their char-

acteristics do not depend on strong

magnetic fields the sensor is exposed

to. More robust eddy current probes for

noisy industrial environment may be

expected.

For GMR sensors in eddy current

probes it is desirable to work in gradio-

metric arrangement and with zero detec-

tion read out electronics. Furthermore, a

biasing dc-field reduces non-linear dis-

tortions in the output signal and max-

imises the ac-field sensitivity.

The characteristics of commercially

available GMRs of one type differ signif-

icantly, so it is difficult to balance them

in gradiometric arrangement. Additional

problems result from the non-linearity of

their characteristics and the hysteresis.

Figure 9 depicts some sensor configu-

rations for low frequency eddy current

testing using GMR sensors.

Experimental Results

All eddy current probes were tested with

three reference pieces containing differ-

ent open and hidden defects. For imag-

ing a stepper driven 2D-scanner was

used.

Figure 10 compares the results of the

inductive and the GMR sensor at an alu-

minium sheet with open and hidden

slots. Best results could be obtained with

a differential inductive eddy current

probe using two well-compensated in-

ductive coils with number of turns of

8000 each. Working frequencies of down

to 350 Hz were used for deep penetra-

tion. All slots could be detected.

Good results could be obtained by

GMR sensors in absolute probe arrange-

ment. Gradiometer configurations are

under test now and probably may im-

prove these results. The attempt to use

the more sensitive GMR sensor type has

not been very successful yet, probably

because of their very non-linear behav-

iour.

Nevertheless, the signal-to-noise-ratio

of the inductive sensor is better than

that of the GMR sensor.

The second reference piece was a hid-

den gap between two 2 mm aluminium

sheets under 15 mm aluminium cover-

age. Figure 11 shows that both the in-

ductive and the GMR sensor are able to

visualize the gap.

EDDY CURRENT TESTING MP

549 (2007) 5

Figure 8. a) Functional

scheme of the AMR mod-

ule Sensitec CMS2000

and its application in

a b) normally and

c) tangentially excited

arrangement

b)a) c)

Figure 9. Eddy current probes using GMR

sensors (NVE AAH002-02). Excitation:

a) Inductive coil with perpendicular field ori-

entation (dashed line) around the GMR sen-

sor, b) Two differentially connected coils with

parallel orientation to the test surface

Figure 10. Comparison of the performance

of an inductive and a GMR sensor at open

and hidden slots

Figure 11. Comparison of the performance

of an inductive and a GMR sensor at a hid-

den gap

a) b)

The AMR modules were used in our

first attempt to integrate industrial

available magnetic field sensors into

eddy current probes. Due to some

specifics of these modules we obtained

rather good but not overwhelming re-

sults, which were much improved by

GMR and inductive sensors.

The third reference piece was an alu-

minium block with holes parallel to the

surface. The eddy current images are

shown in Figure 12. Again both the in-

ductive and the GMR sensor brought up

all defects.

Multi-differential eddycurrent probes with inductivepick-up coils

Features of the probes

In our previous papers the LEOTEST

family of low-frequency eddy current

probes with multi-differential secondary

coil joining, designed in Leotest-

Medium-Center (Lviv, Ukraine), were

presented [8-11]. The most attractive

features of this type of probes are its

high sensitivity to long cracks and local

flaws like pores and pitting, its high pen-

etration and detection performance for

hidden defects, its good lift-off compen-

sation and high spatial resolution and its

good sensitivity to flaws under very

thick dielectric protection coating.

The good penetration features were

obtained for probes with comparatively

small size. We define this feature as a

high ratio of penetration to probe size.

We suppose that this parameter is very

important and determinant for some

practical applications, especially in air-

craft structures with small distance be-

tween fasteners. The LEOTEST probes

can be connected to commercial eddy

current instruments and have shown

their outstanding performance in many

applications like the detection of differ-

ent flaws in inner parts of multi-layer

aircraft structures, the detection of flaws

in ferromagnetic tubes and welds cov-

ered with thick protection layers, in alu-

minium aircraft wings from the inner

surface without sealing removal, the de-

tection of cracks in hidden layers in

multi-layer aircraft structures under dif-

ferent type of fastener heads, the detec-

tion of the deeply underlying pores in

copper canisters and the detection of

subsurface cracks in 15 mm thick stain-

less steel tubes [12-16]. The probe de-

sign and its Point Spread Function is

shown in Figure 13.

Figure 14 presents an eddy current

image obtained by 2D-scanning a spe-

cimen with hidden flat bottom holes of

4 mm diameter. The Point Spread Func-

tion of the probe produces a highly sig-

nificant pattern of up to 6 mm under-

lying.

Investigation into the ultimate depth

of inspection

Let us consider the limits of eddy cur-

rent probes to detect subsurface flaws

using the concept of quasi-infinite crack

[17]. Figure 15 presents a special speci-

men with artificial quasi-infinite crack.

The specimen consists of two parts: the

bottom aluminium alloy plate with flaw

installed on the base made from the

same material and upper sandwich type

part. Mechanical matching of two finely

milled and grinded D16T aluminium al-

loy plates created the artificial flaw. The

artificial crack width is negligible like in

real fatigue cracks. The plate thickness

and the corresponding crack depth were

25 mm, i.e. deep enough for further

crack depth growth. Bottom sides of the

plates do not influence the signal re-

sponse. The artificial crack was oriented

perpendicular to the tested sample sur-

face. The length of this crack is much

larger then the eddy current probe diam-

eter. That way, the influence of flaw size

(depth and length) on the signal re-

sponse was eliminated. These flaw fea-

tures allow calling it a quasi-infinite

crack. This quasi-infinite crack was cov-

ered by upper sandwich type stack con-

sisting of 0.9 mm thick D16T aluminium

alloy sheets. The quantity n of sheets on

top of the crack was changed from 0 to

32 to simulate different depth of flaw un-

derlying (Hr).

Two low frequency eddy current

probes developed in Leotest-Medium-

Center were investigated – Leotest MDF

1701 and Leotest MDF 3301. The Leotest

MDF 1701 and MDF 3301 probes have

a working surface diameter of 17 and

33 mm, respectively. To estimate the

limiting underlying depth of detectable

flaws the concept of noise limited pene-

tration depth was used. The number of

covering sheets was increased step by

step to study the signal behaviour of the

MP

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EDDY CURRENT TESTING

49 (2007) 5

Figure 12. Comparison of the performance

of an inductive and a GMR sensor at hidden

holes parallel to the surface

Figure 14. Aluminium block with flat bottom

holes to simulate hidden defects and its eddy

current image

Figure 15. Specimen with a quasi-infinite

crack

Figure 13.

Multi-differential eddy

current probe and its

Point Spread Function

quasi-infinite crack. The noise signal

also was investigated and compared

with the flaw signal by scanning the

sandwich specimen in the flaw-free re-

gion. With increasing flaw underlying

the complex plane was rotated to adjust

the flaw signal to be oriented in vertical

direction.

Figure 16 presents the signal re-

sponses in the complex plane obtained

with MDF 1701 probe at an inspection

frequency of 100 Hz.

In the upper part of Figure 16 the com-

plex plane signals are presented. The

bottom part of Figure 16 chart diagrams

of the signal’s Y-component is shown to

estimate the signal-to-noise-ratio.

According to the noise limited pene-

tration depth concept the presented re-

sults allow estimating the ultimate un-

derlying depth of detectable flaws. For

reliable crack detection a signal to noise

ratio of more than 6 dB was supposed.

We can see that the amplitude of signal

response for MDF 1701 probe from quasi-

infinite cracks under the 25 sheets

(Hr = 22.5 mm) is approximately 6 dB

larger than noise. That way, the ultimate

underlying depth of cracks for MDF 1701

probe can be estimated as 22.5 mm. For

MDF 3301 probe at a frequency of 50 Hz

the ultimate underlying depth of detect-

ed crack was determined as 28.8 mm.

Conclusions

The effective depth of penetration is lim-

ited by the noise signal and depends on

the instrument, the probe, the environ-

ment and on the flaw to be detected. It

cannot be described by the standard

depth of penetration basing on the plane

wave theory but has to be evaluated ex-

perimentally using test specimens.

Opportunities to increase the effective

depth of penetration are offered by in-

creasing the eddy current density and

the lowering density decrease with in-

creasing depth. On the receiver side new

perfectly balanced inductive sensors

have handling and performance advan-

tages over AMR and GMR sensors. The

advantage of GMR sensors is the price

and the readiness for sensor arrays.

References

1 Z. Mottl: The quantitative relations be-

tween true and standard depth of penetra-

tion for air-cored probe coils in eddy cur-

rent testing. NDT International, 23 (1990)

No. 1, pp. 11-18

2 C. V. Dodd, W.E. Deeds: Analytical solu-

tions to eddy-current probe-coil problems.

Journal of Applied Physics, 39 (1968) No. 6,

pp. 2829-2838

3 G. Wittig; G. Grigulevitsch: Ein Beitrag

zu den theoretischen und experimentellen

Grundlagen des Impuls-Wirbelstromver-

fahrens. Materialprüfung 20 (1978) Nr. 12,

pp. 449-454

4 R. A. Smith, D. Edgar, J. Skramastad, J.

Buckley: Enhanced transient eddy current

detection of deep corrosion, Insight – Non-

Destructive Testing and Condition Monitor-

ing 46 (2004) No. 2, pp. 88-91

5 R. Hohmann: SQUID-System mit Joule-

Thompson-Kühlung zur Wirbelstromprü-

fung von Flugzeugfelgen, Dissertations-

schrift, Justus-Liebig-Universität Gießen,

1999

6 Gabor Vertesy, Antal Gasparics: Fluxset

Sensor Analysis. Journal of Electrical

Engineering, 53 (2002) pp. 53-55

7 M. v. Kreutzbruck, K. Allweins, C. Heiden:

Wirbelstromprüfsystem mit integriertem

Fluxgate-Magnetometer. DACH-Tagung

DGZfP, ÖGfZP, SGZP, Innsbruck,

29.–31.5.2000, BB 73.2, pp. 871-881

8 V. Uchanin, G. Mook, T. Stepinski: The

investigation of deep penetrating high

resolution EC probes for subsurface flaw

detection and sizing. Proceedings of 8th

ECNDT, Barcelona. 2002, CDrom, paper 27

9 V. Uchanin: New type multi-differencial

eddy current probes for surface and sub-

surface flaw detection. Zeszyty proble-

mowe Badania nieniszcz_ce. Warszawa,

(2001) 6, pp. 201-204

10 G. Mook, H. Bauke, V. Uchanin: Wirbel-

stromprüfung mit hohen Eindringtiefen –

Theorie und Praxis. DACH-Tagung der

DGZfP, ÖGfZP und SGZP 2000, Innsbruck,

BB 73 Band 1, pp. 145-154

11 S. Cherepov, D. Gilga, O. Hesse, G. Mook,

V. Uchanin, S. Pankratyev: Niederfrequente

Wirbelstromprüfung mit hoher Ein-

dringfähigkeit und verbesserter Ortsauf-

lösung – vergleichende Untersuchungen

verschiedener Sensorkonfigurationen.

Berichtsband des 12. Sommerkurses Werk-

stofftechnik, Universität Magdeburg,

3.-4.9.2004, pp. 225-246

12 V. Uchanin: Eddy current methods of defect

detection close to rivets in multi-layer air-

craft structures. In: Technical Diagnostics

and Non-Destructive Testing (2006) 2,

pp. 3-12 (in Russian)

13 V. Uchanin: Eddy current method of crack

detection from the inner side of wing pres-

sure tanks without seal removal. Proceed-

ings of 5th National Conference for Non-

Destructive Testing and Technical Diagnos-

tics, Kiev, 2006, pp. 184-187 (in Russian)

14 T. Stepinski: Deep Penetrating Eddy Cur-

rent for Detection Voids in Copper. Pro-

ceedings of 8th ECNDT, Barcelona, 2002,

CDrom, paper 74

15 V. Uchanin, G. Lutcenko, A. Nikonenko:

Automated Eddy Current System for Flaw

Detection and Sizing during In-service Stain-

EDDY CURRENT TESTING MP

749 (2007) 5

Figure 16. Signal responses of MDF 1701

probe at 100 Hz with flaw underlying of 18

mm and 22.5 mm compared to noise signal

Abstract

Der wirbelstromtechnische Nachweis verdeckter Fehler wird

durch den Skineffekt begrenzt. Die Standardeindringtiefe als Maß

zur Quantifizierung des Skineffektes lässt sich über die Ausbreitung

ebener Wellen im halbunendlichen leitfähigen Halbraum theoretisch

ermitteln. Sie kann jedoch die erreichbare Prüftiefe moderner Sen-

soren nicht hinreichend beschreiben. Der Beitrag stellt ein Konzept

zur Ermittlung einer rauschbegrenzten Eindringtiefe vor, die die

bekannten Einflussgrößen wie Prüffrequenz, Sensordimensionen,

elektrische Leitfähigkeit und magnetische Permeabilität des Werk-

stoffs in Bezug zu Rausch- und anderen Störquellen betrachtet.

Neue induktive und magnetoresistive Niederfrequenzsensoren wer-

den vorgestellt und ihr Leistungsvermögen ermittelt. Diese Sen-

soren kombinieren hohe Prüftiefen mit geringen Baugrößen und

guter lateraler Auflösung.

less Steel Tube Inspection. Proceedings of

9th ECNDT, Berlin, 2006, CDrom, poster 33

16 V. Uchanin: The investigation of low

frequency eddy current probes with super

high penetration (THP04). Abstracts of 16th

WCNDT, Montreal, August 30-September 3,

2004, p. 145

17 V. Uchanin: Development of eddy current

inspection methods: challenges, solutions

and outlook. Proceedings of 5th National

Conference for Non-Destructive Testing

and Technical Diagnostics, Kiev, 2006,

pp. 46-54 (in Russian)

The authors

Prof. Dr. Gerhard Mook (*1956) graduated in

Automatics and Telematics from the Odessa

Polytechnic Institute in 1980. After a postgrad-

uate study in Non-destructive Testing he ob-

tained his doctor degree from the Institute of

Materials Engineering and Technology of the

Otto-von-Guericke-Universität Magdeburg and

habilitated in 1990. Since 1992 he has been

responsible for research and education in NDT

at this institute. Prof. Mook is engaged in

national and international projects focused on

electromagnetic methods and the emerging

field of Structural Health Monitoring.

Dipl.-Ing. Olaf Hesse was born in 1964 in

Nordhausen/Germany. He studied physical

metallurgy at the Moscow Institute of Steel

and Alloys (Technical University) from 1983 to

1989. From 1985 to 2006 he has been working

in the material testing department of IMG

gGmbH Nordhausen. He was responsible for

several projects on application of highly sensi-

tive magnetic field sensors (AMR, GMR,

SQUID) in eddy current testing. Since 2006 he

is working in the material testing department

of TÜV Thüringen e.V.

Dr. Valentin Uchanin graduated in Electro-

physics from Lviv Politechnical Institute,

Ukraine in 1971. Since 1971 he has worked in

Physico-Mechanical Institute of Ukrainian

Academy of Sciences on projects including

eddy current and electrical material and prod-

uct testing. After postgraduate study in All-

Union (now All-Russian) Institute of Aviation

Materials (Moscow) he received his doctor

degree from Institute of Machinery Technology

(Moscow). He is the member of the board of

Ukrainian NDT Society as the Chairman of

Western branch, the member of editorial

board of Journal “Nondestructive Testing and

Technical Diagnostic” (Kiev) and organizer of

the annual NDT conferences LEOTEST.

MP

8

EDDY CURRENT TESTING

49 (2007) 5

You will find the article and additional material by entering the document number MP 100810

on our website at www.materialstesting.de