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material, result in increased energy absorption and ap- ture maps over extended surfaces. The sensitivity ofpear as regions of higher temperature. Temperature this technique is shown in Fig. 7. Thermal scanningsignals, corresponding to the defects or flaws, are de- has been applied to the determination of insulationtected by a radiometer and indicated on a recorder defects by heating one side of an insulation panel and(see Fig. 6). scanning the other side. Regions of poor insulation

The use of scanning techniques with the basic ra- (higher thermal conductivity) are detected as areas ofdiometer has provided a means of obtaining tempera- higher temperature.

Nondestructive Eddy Current TestingGLENN 0. McCLURGt

One of the outstanding new nondestructive quality- variation of alloy content will affect both the perme-sensing methods introduced to industry recently goes ability and conductivity; and variation of dimensionsby the name "Eddy Current Testing." Introduced in a will affect the degree of coupling.primitive way during World War II, this method has de-veloped rapidly and shows great promise for the fu- COIL TYPESture. Some automatic equipment is already in use and

Two general types of coils are in common use. Onemore will be in use soon.Since the method is new, this paper will give a is a circumferential coil through which the test part

brief explanation of its basic principles, will discuss passes; this type is called the feed-through coil. The

some equipment currently available, and will indicate second type of coil is a small coil which can be

its present and future usefulness in metal cutting prob- placed on the surface of the part to be tested such

lems. that the axis of the coil is perpendicular to the sur-

face; this second type is called the probe coil. The

BASIC PRINCIPLES theory of the feed-through coil method has been workedout in detail and verified by experience. The problem

In eddy current testing, information concerning the of the probe coil has a partial theoretical solution and

test object is obtained from the change in the imped- much is known about it from empirical data. In each

ance of a test coil supplied with an alterating cur- case, the properties of the test object are inferred

rent of suitable frequency. The coil induces in a part from the impedance of the exciting coil or from theunder test a varying field which, in turn, generates impedance of one or more secondary coils as a pickupeddy currents in the part. The eddy currents react on device.the exciting coil (or upon a secondary coil used as asensing device) and affect its impedance. Variationsin the eddy currents may be caused by: Feed-through coils are particularly suited to the

1) the electrical conductivity; testing of cylindrical objects such as barstock and2) the magnetic permeability; tubing. For a nonmagnetic rod which completely fills3) the dielectric susceptibility; the test coil, theory and experience show that the im-4) the size and shape of the test object; pedance of the coil varies as shown in Fig. 1. On5) the frequency of the varying field; and this curve, the abscissa values are given in terms of6) the degree of coupling between the device excit- the normalized resistance R/IJLo and the ordinate

ing the field, the test object, and the pick-up de- values in terms of the normalized reactance L/coLo,vice. where R and aXL are resistive and reactive compo-

Thus, any property of the test object which can be nents of the test coil impedance with the sample in-related to these variables may be detected and meas- serted and coL0 is the reactance of the empty coil.ured, at least in theory. For example, internal stresses The test object is characterized by a number havingin magnetic material affect the permeability; cracks the dimensions of frequency and is given byand seams or voids will affect the local conductivity; 56

tMagnaflux corp., Chicago, Ill. gyd (1)

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NONDESTRUCTIVE EDDY CURRENT TESTING

The dashed curves in Fig. 2 join the points represent-XLL6 I | Iing the impedance of the coil when the only change in

the coil and test object system is a change in thediameter of the test object. The direction of increasingdiameter is indicated in the figure.

0,8 4 Fig. 2 shows that the change in voltage across thefg- 5060 test coil produced by a diameter change of the test

_ d_k object differs in phase from the change in voltagecaused by a change in conductivity. Thus it is possi-

0,6L i6,25 ble to distinguish between changes in dimensions andchanges in conductivity of the test object by means of

____ |___ ____ N! phase-sensitive circuits.The preceeding discussion has concerned nonmag-

0,4 l A / f netic material. In Fig. 3, the normalized impedance04r fg for the test coil is given for various values of perme-

ability, conductivity, and diameter for a fill factor 7772 of one half. Along the curve corresponding to a perme-

ability of 100 gauss per oersted, values of f/fg are0.2 49 marked. The dashed curves in this figure join the

100 points representing the impedance of a coil when theonly change in the coil and test object system is a

/I400 t change in permeability of the test object. For mag-

0 a _ onetic materials, a change in diameter causes an im-

0 0,2 0,4A Lo pedance change in the same direction as that causedFig. 1-Normalized impedance curve of the test coil as a by a change in permeability. For low values of f/fg

function of f/f . the change in impedance due to a conductivity changeis approximately at right angles to the change due to

where a is the conductivity in m/ohm mm2 and d isthe diameter of the rod in cm. Fig. 1 assumes that the aLresistance of the empty coil is negligible. Points wLo_along the curve are marked at various values of theratio f/fg of the exciting frequency to the character- 2 ____listic frequency of the test object. \ \ \

From (1), it can be seen that fg decreases as a in- 0.8 4creases so that the ratio f/fg must increase with in- 4creasing a, and the curve in Fig. 1 is marked to indi-cate this fact. If the ratio f/fg is unity for a particularrod in the coil, then the coil impedance will be repre- .Th0,35sented by the point marked 1.00 on the curve. If the 0,6rod is now replaced by one which is identical to it ex- \cept that the conductivity of the new rod is four times 9larger, then the impedance of the coil will be repre- d\sented by the point marked 4.00. The change in coil 0.4impedance, AZ, will cause a change in the magnitude T1=0.63 I 16of the voltage across the coil and in its phase. Either t -or both of these changes can be detected by suitable dl 3 _means. 0.2

If the rod does not completely fill the test coil, ioothen the coil impedance will vary as shown in Fig. 2.The ratio of the cross-sectional area of the test object/to the cross-sectional area of the coil is called the _ 10 / 1fill-factor, v1. Fig. 2 gives the test coil impedance rl 0 0,2 0,4 ctOLo

curvs fo vaiousvales o i~.Thecurv corespnd- Fig. 2-Normalized impedance curves of the test coil foring to y -1 is the same curve as is shown in Fig. 1. various fill factors.

21

IRE TRANSACTIONS ON NUCLEAR SCIENCE

50 + 1 a permeability variation. In this range, it is possibleto distinguish between permeability and conductivity

l£L 41 effects.wLo 1X The impedance of the test coil when the test object

is tubing rather than rod is shown in Fig. 4 for an40< 79 0°> < f/fg ratio of 15. The curve on the extreme right shows

the variation in the impedance of the test coil as a

funiction of wall thickness. The values of wall thick-ness in percent of the outside diameter, are marked on

30 4:\,4 \\ fg_ this curve. Thus the test coil impedance for solidrod is represented by point A on Fig. 4. If a tubehaving the same outside diameter and a wall thicknessof ten percent of that diameter is now inserted in a

20- 1 I \ IJrV Icoil, the impedance of the coil will be represented bypoint B.

,0@fM q 5 The curve on the extreme left illustrates variation-t/> - of coil impedance for tubes having maximum eccen-

tricity. This curve is marked in terms of the ratio of0o- - the inside diameter to the outside diameter expressed

_ 36 as a percentage.1 - The dashed curves in Fig. 4 show the variation of

10 / 20 R test coil impedance for eccentric tubing. Thus the0 R_ impedance of tubing whose wall thickness is ten per

coLo cent of its outside diameter will vary along the dashedFig. 3-Normalized impedance curves of the test coil as a line beginning at point B, as the eccentricity varies

function of permeability (for a fill factor of one half). from zero to its maximum.

ig g

22,

0.7~~ ~ ~ ~ ~ ~ ~ 07-

_ _ _ _ _ _ _ _ ~~~~~ ~ ~ ~~ ~~~~~~0.6$- _

0.6- ~~~~~~~~~~~~~~~~~z

w 1

40~~~~~~~~

03- 0. 0. 0. 04

Fi. 4-Imeac aiain0fte etci hn h et Fg.5Vrain ofts olipdnc0asdb ue

NONDESTRUCTIVE EDDY CURRENT TESTING

4,1 _ "depth of crack in per cent." Thus, a longitudinal\ U f*15 surface crack having a depth equal to 20 per cent of

0.16 r - the rod diameter will cause the impedance of the coil1(1>\\ l|mto move from the point A to the point D. The curve

0.14 - - -.K\ marked "distance of crack from surface in per cent of6 diameter" shows how the impedance changes for sub-

0.12 3D) surface cracks. Thus, a thin crack located 3.3 per¢ffit 5 | \ \? 1 O*/// | |\-' cent below the surface and having a depth of 30 per

0.1 --%\4 0 811\ ( > 2 | cent of the diameter would cause the impedance to\ V /., , ,change to the point E. The remaining curve in Fig. 6

4 1AI illustrates the variation of the coil impedance for V-0.08 1 0 shaped defects whose ratios of width to depth are

marked on the curve.0.06 AL - The large circle whose center is at point A of Fig.

\1\ /j 5(T>n6 gives the necessary per cent change in each of the0.04 - 2\ \ VS \ _Z zL\ Jthree variables (diameter, conductivity, and defects)

to cause the same change in the magnitude of imped-ance. For example, a 1.0 per cent change in diameter

0.02 produces the same change in the magnitude of imped-ance as a 7.0 per cent change in conductivity or a 5.0

° t t \oSW Jotl 1 per cent depth surface crack. It is therefore obviousthat phase-sensitive circuits are required to distin-

/|___'-' _____ guish between these variables.0 0.02 0.04 0.06 Fig. 7 gives the over-all picture of changes in im-

pedance caused by these variables for various values

Fig. 6-Impedance variations caused by surface and sub- gsurface cracks for f/f = 15.g

Fig. 5 shows the variation of test coil impedancefor nonmagnetic tubes for various values of f/fgB. 0.

The data for impedance changes caused by cracks,both surface and subsurface, were obtained empirically.The changes which occur vary widely depending on 0.8 30% DEPTH

OF CRACKthe particular value of the f/fg ratio for the system.Fig. 6 shows the changes which might be expected if 0.7 Hthe operating point corresponds to an 1/Ig equal to 15.The point marked A in this figure represents the im- 0.6pedance of the test coil when the coil contains a goodtest object and corresponds to a value of f/fg equal /to 15 in Fig. 1. If the diameter of the sample is de- 0.5 41creased by 1.0 per cent, the normalized impedance /changes to the point marked B in Fig. 6. Larger 0.4 |changes in diameter cause the impedance to change 4:1in the same direction, and a line indicating the direc- 0.3 -___ X _/tion of these changes is marked in per cent. 4:j i

If, on the other hand, the conductivity should in- 1 Icrease by 5.0 per cent, the impedance will be repre- 0.2 - tosented by the point marked C. In general, the direc- Ition and magnitude of the normalized impedance o s-r 5changes caused by given per cent changes in con-/I I I Iductivity are indicated by points along the line desig- 0' -nated by ha. 0 0.1 0.2 0.3 0.4

The impedance changes due to longitudinal surface Fig. 7 Impedance variations caused by surface and sub-cracks are shown in this figure by the curve marked surface cracks as a function of f/fg.

23

IRE TRANSACTIONS ON NUCLEAR SCIENCE

It can be seen from a study of Figs. 2-7 that the x RTwlg

frequency required for a particular nondestructive testis governed by the type of defect, its location in thetest object, the conductivity of material, and its di-ameter. For general crack detection, a value between5 and 15 for the ratio of f/fg is best.

FEED-THROUGH COIL ARRANGEMENT

Since the information concerning the test objectappears as a change in the impedance of a test coilrather than in the actual impedance of the coil, it isusual to use two or more coils connected electricallyso that the impedance changes are detected. Two coilarrangements are in common use. In both arrange-ments, two coils are connected in series oppositionsuch that the combined voltage output is zero whenboth coils are empty. One arrangement is called the"absolute" coil method. In the absolute method, a

test object known to be good is placed in one coil and athe object to be tested is placed in the other. If theobject to be tested is also good, then the voltage out- Fig. 9-Photograph of instrument of Fig. 8.put of the combined coils will be zero. If the objectto be tested differs in some respect from the standardobject, then the impedance of the two coils will be is being made, then the absolute coil arrangementdifferent and this difference will be measured by the must be used.output voltage. The other coil arrangement is calledthe differential method. In the differential coil ar-rangement, the two coils are lined up coaxially so that The basic theory for probe coils has also beenthe object to be tested goes through both coils. Thus, worked out. The results differ in some details fromone section of the object is compared with an adjacent that of the circumferential coils but the differences dosection of the same object. not warrant our attention here.

Since diameter changes and alloy changes (conduc-tivity, permeability) usually take place slowly in EXAMPLES OF CURRENT INSTRUMENTSwires and bars, the differential coil arrangement haslow sensitivity to these changes but retains normal Fig. 8 is the block diagram of a probe-type instru-sensitivity to any change which effects only one coil, ment whose picture is shown in Fig. 9. In this case,such as a short crack or the beginning of a long seam. the sensing coil (labeled "Probe") is placed on theThis discrimination against gradual changes allows work, and the balancing coil is a fixed coil within thethe use of great amplification and consequently great instrument housing. The unit is designed particularlysensitivity to short cracks. Of course when diameter for the absolute measurement of conductivity. It isand alloy changes are the qualities for which the test also used for sorting mixed-up stock of sheet, bar, and

tube; for measuring hardness (where hardness has aone-to-one relation to conductivity); and for measuringthe thickness of conducting and nonconducting coatings.

Li-SI SENS. Two versions of a unit designed particularly for

R METER magnetic materials are shown in Figs. 10 and 11. One1/ _{|=nJis "portable," and the other is a more ruggedly de-

signed instrument for production line use. This unit

PROBE iQQQJv uses a CRT display and presents the information as a

distorted sine wave. Two of the uses for this equip-WORK ment are shown in Figs. 12 and 13 (alloy and hardness

Fig. 8-Block diagram of a probe-type instrument. sorting). A modified version of this unit has been

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NONDESTRUCTIVE EDDY CURRENT TESTING_ | l , 1 w~~~~~~~~~~~~~~~~~5140 11Fig. 12.Alloy sorting using the instrument in Fig. 10.

temperature of the oven is controlled by the level ofFig. 10_A portable instrument for magnetic material, the trace on the CRT.

used for automatic control of the heat treatment of USEFULNESS IN METAL CUTTING PROBLEMSspring ribbon. In this application, the continuousribbon of spring stock passes through the measurement At present, eddy current techniques are useful as acoil of the unit after leaving the heat-treat oven. The means of assuring that the work presented to a metal

cutting operation has the characteristics required bythe specifications and for which the cutting operationwas designed. This is especially important for auto-matic machining operations and will be more importantin the future as machining operations controlled bypunched-tape, etc., become more common. For suchoperations, it is necessary that the hardness of thework be held within tolerances. A single piece whichis too hard may damage the tool and result in manydefective parts before the damage is discovered andthe tool replaced. Eddy current methods can sort thebarstock by alloy (and often even by heat) so thatinitial machining operations can be uniform. Inter-mediate heat treating operations can be adjusted ac-

Fig. 11_Ruggedized production line version of the instru- Fig. 13-Tensile strength sorting using the instrument inment in Fig. 10. Fig. 10.

25

IRE TRANSACTIONS ON NUCLEAR SCIENCE

cording to alloy so as to produce uniform hardness, the components do exist and there is no theoreticalthus avoiding difficulty with subsequent machining. If reason why the ideal instrument cannot be realized.necessary, an eddy current hardness test can be carriedout before machining to insure that proper heat treat- ACKNOWLEDGMENTment has been carried out.

Future instruments could be designed as an integral The material presented above is largely taken frompart of the machine tool. Ideally, it would: published and unpublished papers by Dr. Friedrich

1) measure the hardness; check the result against Foerster,' Institute Dr. Foerster, Reutlingen, Germany.the specification and reject the part if out oftolerance; 'Some of Dr. Foerster's most basic papers are:

2) monitor the part during machining for cracks, F. Foerster, "Theoretical and experimental basis for thethe operation and reject the part if cracked, nondestructive material testing with eddy current meth-

stop the operatlon ane relect tne part lr cracaea, ods. IV. Practical eddy current instrument with feed-and reset the machine for the next part; through coils for the quantitative nondestructive material

3) measure the depth of the cut by means of a coil testing," Z. Metallkunde, vol. 45, no. 4, pp 180-187;preceding the cutting tool and another coil fol- 1954.

F. Foerster and H. Breitfeld, "Theoretical and experi-lowing the tool, and check the result against the mental basis for the nondestructive material testing withspecification and adjust the tool if necessary; eddy current methods. V. Quantitative crack testing of

4) compare the dimensions of the part at the end of metallic materials with feed-through coil," Z Metall-kunde, vol. 45, no. 4, pp 188-193; 1954.

the operation against a standard and pass or re- F. Foerster and K. Stambke, "Theoretical and experi-ject the part. mental basis for the nondestructive material testing with

In conclusion, let it be said that the ideal instru- eddy current methods. III. Feed-through coil method forIn concusin,lt l be a1dthattheldea lntruquantitative nondestructive material testing," Z. Metall-ment described above does not exist today. However, kunde, vol. 45, no. 4, pp 166-179; 1954.

Electronic PhotographyM. L. SUGARMAN, JR.,t M. B. LEVINE, t AND

N. P. STEINERt

INTRODUCTION Electrographic Printing, or "Electrography"

The term "electronic photography" was first used In these systems, an electrostatic charge pattern,publicly by General David Sarnoff, of the Radio Corpo- comprising a facsimile of the material to be displayed,ration of America, in announcing RCA's development is laid down directly on a nonphotosensitive insu-of its system of magnetic recording of video images. lating surface. This charge image is made visible, orThis form of electronic photography, while it has been "developed," by means of a powder, particulate spray,the quickest to gain widespread public recognition, is or liquid suspension. To achieve development, theonly one method of using electronics to store or pro- particulate developing medium is attracted and held to

duce visible images temporarily or permanently. This the charge pattern by virtue of its being previouslypaper will outline the development and, briefly, the charged to an opposite polarity.current status of the art with regard to several addi-tional systems of utilizing electrons to produce photo- E lectrophotographygraphic images. As will be seen, the techniques de-scribed offer great potential in the future development In electrophotographic systems, the electronof photography, the graphic arts, computer read-out, (charge) image is produced by light incident directlyfacsimile devices, and information handling in general. on a photosensitive surface, and is subsequently de-

The systems to be discussed fall into two general veloped to a visible form by the same techniques usedcategories: electrography and electrophotography. in electrographic printing.

________________ ~~~~~~Fig. 1 illustrates the distinction between thesetAmerican Photocopy Equipment Co., Evanston, Ill. two systems. The two may overlap, as in the case of

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