Chapter 6 - Special Applications

8/12/2019 Chapter 6 - Special Applications

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CHAPTER - 6

SPECIAL APPLICATIONS

GENERAL

Due to rapid technological advances in the space industry, there is an

increasing need for the development of methods where by specific types of

information can be monitored and obtained. Eddy currents have become one of

the major tools for obtaining data on an operating vehicles. A variety of

techniques using the favourable characteristics of eddy currents have been

developed. Since, eddy currents testing does not require physical contact with

the article measurements can be made in hostile environments such asextremely high temperatures, cryogenic temperatures, high pressure, or in an

electrically conductive media. uture applications of eddy current may actually

ta!e place in space or under simulated space conditions, i.e., vaccum, high and

low temperatures, etc.

"his chapters is intended to simulate interest in eddy current applications

so that similar creativity may be applied to the solution of li!e problems.

DIMENSION MEASUREMENTS

1) NON-CONTACTING

#ne of the main advantage of eddy current testing is that an air gap can

be used as the couplant between the probe and the article. "his advantage

permits the development of a wide range of tests which were not previously

possible.

$here the environment is hostile, such as radioactive, high temperature,

high pressure, vacuum or ultra low temperature, the test coils can be constructed

from special materials and operated remotely.

igure %&' illustrates the use of eddy currents to monitor movements in the

order of (x ')&%inches.

igure %&* illustrates the use of eddy currents in a radioactive, high

temperature environment.

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Figure 6-1: Measureme! "# Sma$$ M"%eme!

Figure 6-&: E''( Curre! Measureme! i Ra'i"a!i%e E%ir"me!

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&) NONCONDUCTI*E THIC+NESS MEASUREMENT

Space and aerospace applications have required the use of numerous

nonconductive coatings. "hese coatings may range from micro films to macro

deposits. "he use of eddy current testing is being universally used as anondestructive means for controlling this thic!ness. Such coatings may be

produced by vacuum or electroplating, spraying, dipping, cladding, etc.

a. Mir" "a!ig Figure%&+ illustrates a very thin coating of a nonconductive

material bac!ed by a conductive or magnetic material. Since eddy current

measurement is primarily lift off, a high degree of accuracy is possible.

b. Mar" "a!igs igure %& illustrates the measurements of a thic!

nonconductive material by bac!ing with a conductive or magnetic material.

"hic!ness up to + inches can be successfully measured- thic!ness greater

than this are unreliable due to fall&off the magnetic field.

Figure 6-,: Measurig TiN""'u!i%e C"a!igs

Figure 6-.: Measureme! "# Ti/N""'u!i%e C"a!igs

CONDUCTI*IT0 MEASUREMENTS

1) THIN MATERIALS

As discussed in chapter , one of the more common uses of eddy current

testing is in the measurement of conductivity. owever, conductivity in thin

gauges /.)') inches and less0 is exceedingly difficult to measure because the

measurement becomes sensitive to dimension change when the depth of field

penetration exceeds the thic!ness of the material.

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igure %&(, vies A, illustrates a commonly used method for measuring the

conductivity of materials. 1n this case, however thhe depth of penetration

probably exceeds the material thic!ness thus giving inaccurate conductivity

measurements.

2iew 3 illustrates a two method for measuring the conductivity of

extremely thin gauges of material. "he two coils can be balanced out against a

standard, similar to the differentially coil technique. #nce this is accomplished

the accurate measurement of conductivity in other gauges of material is possible.

"he only disadvantage to this method is the need for access to both sides of the

material.

Figure 6-: Measurig Ti/ess "# Ti Ma!eria$s

&) 2ELDING 3UALIT0 CONTROL

or years other method of non&destructive testing has been used

successfully to determine weld quality. owever, in each situation the weld

article must be cooled to ambient temperature before testing.

1n eddy current testing the coil does not have to be in contract with the

article. "his enables the design of a system in which the weld quality can be

monitored while cooling. "he only limitation is the thic!ness of the weld.

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igure %&% illustrates a typical eddy current coil designed for evaluating

elements.

Figure 6-6: E''( Curre! Tes!ig "# 2e$'s

EDGE DISCONTINUIT0 DETECTION

1) EDGE LOCATIONS

"he detection of discontinuities or measurement of properties along the

edge of an article has always presented difficulty in eddy current testing due to

the edge effects.

owever, a shaped probe coil which rides on the lip or edge of the article

/figure %&40 in a fixed relation to the edge, can detect discontinuities in the article

since the edge effect would be contant.

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Figure 6-4: Tes!ig #"r Dis"!iui!ies a! E'ge "# Ar!i$e

&) INSIDE HOLE LOCATION

Discontinuity detection within holes can become very difficult especially

when the location of the discontinuity must be determined.

igure %&5 illustrates an eddy current probe designed to swept circularly

within the hole. "his type of unit detects and locates the discontinuity within the

opening.

Figure 6-5: Tes!ig #"r Dis"!iui!ies isi'e H"$e

,) MAGNETIC AND NONMAGNETIC ARTICLES

1t should be reali6ed that eddy current testing of conductive, nonmagnetic

articles for discontinuities is fairly straight forward. owever, detection of

discontinuities in magnetic articles can be very difficult as the permeability will

mas! measurements. the permeability effect can be reduced by a steady state

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magnetic field. "his magnetic field can be provided by a wire wound coil

energi6ed by direct current, or it may be sufficient to use a permanent magnet

shaped to cover the small area of the article under test. /igure %&70.

Figure 6-: Use "# Mage!i Fie$' !" O%er"me E##e!s "# Permea7i$i!(

.) END-EFFECT

End effects are so pronounced that they can often be used to detect

movements- ma!e measurements, count articles, etc. you will not in igure %&')

how the spo!e brea!s the field as the wheel rotates. "his produces the descried

end effect. 1f such wheels are mounted within the flow of a liquid, the wheel

would rotate in direct proportion the liquid flow.

"he reaction of the spo!es on the probe outside the container

continuously indicates the number of rotations of the wheel. "he speed of

rotation measures the flow. Electronic integrating counters can measure the

liquid passing the measuring point.

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Figure 6-18: N"!e' 2ee$ C"u!er !" Re"r' Li9ui' F$"

) LO2 CONDUCTION MATERIAL

8raphite and certain other semiconductor materials present a problem in

measurement of receptivity and purity- however, by proper coil design and

experimentation it is possible to assign values to these materials. 9easurements

by eddy current techniques removes the necessity for contact with the material

and eliminates self testing. 3y proper design of the test coil, extremely small a

real be evaluated- however, the frequency should be high so that magnetic field

does not completely penetrate the article.

6) CONDUCTI*E LI3UIDS

"he problem of measurements of liquids may be one of nondestructive

testing, and will be briefly discussed. "hose liquids which conduct electrons

can be measured by eddy current.

1) CONCENTRATION OF LI3UID

"he ability of a liquid to conduct electrons is a functions of its conductivity

and concentration. 1n a given test area we can measure this conductivity and

use this information as an indication of concentration igure %&'' illustrates such

a test.

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Figure 6-11: N""'u!i%e Pi;e Se!i" i Li9ui' F$" S!ream

&) FLUID LE*EL

Eddy current can be used to penetrate a container and observe the level

of conductive fluid. "his measurements can be made even under conditions of

high temperature or high pressure in the liquid environment. igure %&'*

illustrates the measurements of fluid level.

Figure 6-1&: De!ermia!i" "# F$ui' Le%e$

CONDUCTI*E GAS

1t is !nown that gases can be conductive under certain conditions of

pressure, temperature, and ion concentration. Since eddy current s can be

induced under these conditions, some form of measurements can be made.

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1) CONCENTRATION

9easuring the ability of a gas to carry electrons can be used determine

pressure, temperature, or concentration of the gas. "his would serve as a

means to control or monitor an ioni6ed gas stream. /igure %&'+.0

Figure 6-1,: C"'u!i%i!( Measureme! "# i"i


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&) =OUNDAR0 LOCATION

Since even a very wea! conductibility of a gas to electrons can be

detected by eddy current means, it is possible to detect lift&off changes. "his lift&

off measurements can define the boundary of such a conductive gas, e.g.,envelope control of plasma /igure %&'0.

Figure 6-1.: ="u'ar( De!ermia!i" i I"i


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CHAPTER > 6

LE*EL II - 3UESTIONNAIRE

1? C"i$ s;aig " 'i##ere!ia$ ;r"7es #"r geera$ is;e!i" ;ur;"ses "#!u7ig is usua$$(a0 defect depthb0 wall thic!nessc0 both a and bd0 not important

&? L"g gra'ua$ 'e#e!s a 7e misse' 7( usig @@@@@@@@@ ;r"7es?a0 encirclingb0 differentialc0 bobbin

d0 absolute

,? 2i "# !e #"$$"ig is a a'%a!age "# !e 'i##ere!ia$ ;r"7e"m;are' !" !e a7s"$u!ea0 sensitive to gradual dimensional changesb0 low sensitivity to probe wobblec0 easily interpreted signalsd0 all of the above

.? E##e!s "# !em;era!ure 'ri#! are re'ue' 7( usiga0 differential probes

b0 probe pre&heatc0 liquid nitrogen bathsd0 gap probes

? Te mai reas" a e''( urre! "i$ a 'e!e! su;;"r! ;$a!es i ea!eBagers e !es!ig !u7es #r"m !e isi'e 'iame!er isa0 support plates are always ferro&magneticb0 support plates are always the same material as the tubec0 magnetic flux is not restricted by the tube walld0 support plates act as resonance amplifiers in the circuit

6? A ;r"7e "se ";era!ig im;e'ae is "! 7e!ee &8 a' &88 "msi$$ m"s! $i/e$( resu$! ia0 decreased signal to noise ratiob0 decreased signal amplitudec0 both a and bd0 none of the above, probe impedance matching to instrument impedance isnot important

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CHAPTER > 6

LE*EL II > ANS2ER

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Chapter 6 - Special Applications

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