Theory and Application of Precious Ultrasonic Thickness Gaging

8/9/2019 Theory and Application of Precious Ultrasonic Thickness Gaging

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Theory and Application of PreciousUltrasonic Thickness Gaging

by Kenneth A. Fowler, Gerry M. lfbau!, and Tho!as ".#elligan

Ultrasonic nondestructi$e testing %#&T' ( a !ethod of characteri)ing !aterial

thickness, integrity, or other physical properties by !eans of high fre*uency sound

wa$es ++ is a widely used techni*ue for product testing and *uality control. n thickness

gaging applications, ultrasonic techni*ues per!it *uick and reliable !easure!ent of

thickness without re*uiring access to both sides of a part. -alibrated accuracies as

high as / !icro!eters or 0.0001 inch are achie$able in so!e applications. Most

engineering !aterials can be !easured ultrasonically, including !etals, plastic,

cera!ics, co!posites, epo2ies, and glass, as well as li*uid le$els and the thickness of

certain biological speci!ens. 3nline or in+process !easure!ent of e2truded plastics or

rolled !etal is often possible, as is !easure!ent of indi$idual layers or coatings o$er

substrates in !ultilayer !aterials. Modern hand+held digital gages are si!ple to use

and highly reliable.

-o!!ercial ultrasonic thickness gages are generally di$ided into two types4 corrosion

gages and precision gages. -orrosion gages are specifically designed for !easuring

the re!aining wall thickness of !etal pipes, tanks, structural parts, and pressure

$essels that are sub5ect to internal corrosion that cannot be seen fro! the outside.

They e!ploy signal processing techni*ues that are opti!i)ed for detecting the

!ini!u! re!aining thickness in a rough, corroded test piece, and they use speciali)ed

dual ele!ent transducers for this purpose. -orrosion gages are outside the scope of

this paper. The precision gages discussed here use single ele!ent transducers and

are reco!!ended for all other applications %including s!ooth !etal'. 6ith !any

different types of transducers a$ailable, precision gages are e2tre!ely $ersatile andcan !easure to higher accuracy than corrosion gages.

1. Measure!ent Principles

Precision ultrasonic thickness gages usually operate at fre*uencies between 700 K8)

and /0 M8), using broadband, well da!ped pie)oelectric transducers that when

e2cited by electrical pulses generate bursts of sound wa$es, and in recei$ing !ode

con$ert sound wa$es back into electrical pulses. A wide $ariety of transducers with

$arious acoustic characteristics ha$e been de$eloped to !eet the needs of di$erseindustrial applications. Typically, lower fre*uencies will be used to opti!i)e penetration


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when !easuring thick, highly attenuating, or highly scattering !aterials, while higher

fre*uencies will be reco!!ended to opti!i)e resolution in thinner, non+attenuating,

non+scattering !aterials. A broadband design is typically used for precision thickness

gaging applications to that !a2i!i)e near surface resolution.

A pulse+echo ultrasonic thickness gage deter!ines the thickness of a part or structure

by accurately !easuring the ti!e re*uired for the short ultrasonic pulse generated by a

transducer to tra$el through the thickness of the !aterial, reflect fro! the back or inside

surface, and return to the transducer. n !ost applications this ti!e inter$al is only a

few !icroseconds or less. The !easured round trip transit ti!e is di$ided by two to

account for the down+and+back tra$el path, and then !ultiplied by the $elocity of sound

in the test !aterial. The result is e2pressed in the well+known relationship4

d = Vt/2

where d 9 the thickness of the test piece

: 9 the $elocity of sound wa$es in the !aterial

t 9 the !easured round+trip transit ti!e

Additionally, in actual practice, a )ero offset is usually subtracted fro! the !easured

ti!e inter$al to account for certain fi2ed electronic and !echanical delays. n the

co!!on case of !easure!ents in$ol$ing direct contact transducers, the )ero offset

co!pensates for the transit ti!e of the sound pulse through the transducer;s wearplate

and the couplant layer, as well as any electronic switching ti!e or cable delays. This)ero offset is set as part of instru!ent calibration procedures and is necessary for

highest accuracy and linearity.

Figure 1 – Typical Gage Block Diagram


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Figure 1 represents a block diagra! of a typical ultrasonic thickness gage. The pulser,

under the control of the !icroprocessor, pro$ides a broadband spike or tuned s*uare

wa$e $oltage i!pulse to the transducer, generating the outgoing ultrasonic wa$e.

choes returned fro! the test piece are recei$ed by the transducer and con$erted back

into electrical signals, which in turn are fed into the recei$er a!plifier and then digiti)ed.

The !icroprocessor+based control and ti!ing logic both synchroni)es the pulser and

selects the appropriate echoes that will be used for the ti!e inter$al !easure!ent.

Auto!atic gain control is co!!only utili)ed to nor!ali)e echo a!plitude.

f echoes are detected, the ti!ing circuit will precisely !easure a ti!e inter$al in one of

the !odes discussed in the ne2t section of this paper, and then typically repeat this

process se$eral ti!es to obtain an a$eraged reading. The !icroprocessor then uses

this ti!e inter$al !easure!ent along with progra!!ed sound $elocity and )ero offset

$alues to calculate thickness. Finally, the thickness is displayed and updated at a

selected rate.

Many gages incorporate an internal datalogger and are capable of storing se$eral

thousand thickness !easure!ents along with identification codes and setup

infor!ation in !e!ory. These stored readings !ay be recalled to the gage;s display or

uploaded to a printer or co!puter for further analysis or archi$ing.

/. Measure!ent Modes and Transducer


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ele!ent fro! direct contact with hot test pieces, and delay lines can be shaped or

contoured to i!pro$e sound coupling into sharply cur$ed or confined spaces.

3. Immersion transducers: !!ersion transducers use a colu!n or bath of water to

couple sound energy into the test piece. They can be used for on+line or in+process!easure!ent of !o$ing product, for scanned !easure!ents, or for opti!i)ing sound

coupling into sharp radiuses, groo$es, or channels.

f we classify the !easure!ent techni*ues by the choice of echoes used in !aking the

transit ti!e !easure!ent, we find that there are again three basic classifications or

!odes4

ode 1 + n Mode 1, !easure!ent is !ade between an e2citation pulse and the first

backwall echo fro! the test piece, using contact+type transducers. t is a general

purpose test !ode and is nor!ally reco!!ended for use unless one of the conditions

described under Modes / or = is present.

ode 2 + n Mode /, !easure!ent is !ade between an interface echo representing

the near surface of the test piece and the first backwall echo, using delay line or

i!!ersion transducers. n plastics, Mode / can i!pro$e !ini!u! thickness resolution

o$er Mode 1. t is also used for !easure!ents on sharp conca$e or con$e2 radiuses or

in confined spaces with delay line or i!!ersion transducers, for on+line !easure!ent

of !o$ing !aterial with i!!ersion transducers, and for high+te!perature

!easure!ents.

ode 3 + n Mode =, !easure!ent is !ade between two successi$e backwall echoes,

using delay line or i!!ersion transducers. t !ay be e!ployed only when clean

!ultiple backwall echoes appear, which typically li!its its use to !aterials of relati$ely

low attenuation and high acoustic i!pedance such as fine+grained !etals, glass, and

cera!ics. Mode = typically offers the highest !easure!ent accuracy and the best

!ini!u! thickness resolution in a gi$en application, at the e2pense of penetration, and

it is co!!only used when accuracy and>or resolution re*uire!ents cannot be !et in

Mode 1 or /.

These classifications are su!!ari)ed in Figure 2 , which gi$es a sche!atic

representation of the three !odes of ti!ing and the types of transducers that can be

e!ployed for each.


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? Thickness ranges assu!e a sound $elocity of appro2i!ately 7.@ !!> < or ./= in> < and

further assu!e that !a2i!u! range is not li!ited by scattering or sound attenuation in the

!aterial.

Figure 2 – !recision ultrasonic gaging tec"ni#ues classi$ied %y t"e ec"oes used

to make t"e measurements.

The transducers used in precision thickness gaging are nor!ally broadband single

ele!ent designs. An additional co!!on type of transducer is the dual ele!ent, or

BdualB, which is nor!ally used for corrosion sur$ey applications rather than the

precision gaging work that is the sub5ect of this paper. As their na!e i!plies, dual

ele!ent transducers use a pair of separate pie)oelectric ele!ents, one for trans!itting

and one for recei$ing, bonded to separate delay lines. Thickness !easure!ent is

!ade in a !odified Mode 1 !ethod, reading to the first backwall echo and subtracting

a )ero offset e*ual to the transit ti!e through the delays. &ual ele!ent transducers are

typically rugged and able to withstand e2posure to high te!peratures, and are highly

sensiti$e to detection of pitting or other locali)ed thinning conditions. 8owe$er they are

generally not reco!!ended for precision thickness gaging applications because of thepossibility of )ero drift and ti!ing errors related to :+path correction. For further

infor!ation on the use of dual ele!ent transducers, contact 3ly!pus


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general, the highest fre*uency and s!allest dia!eter transducers that will gi$e

acceptable results o$er the re*uired range would be reco!!ended.

transducers are !ore easily coupled to the test !aterial and per!it the thinnest

couplant layer at a gi$en contact pressure. Further!ore, higher fre*uency transducers

produce echoes of faster rise ti!e and thereby enhance the precision of thickness

!easure!ents. 3n the other hand, the acoustic properties or surface condition of the

test !aterial !ay re*uire large, low fre*uency transducers to o$erco!e poor coupling

or signal losses due to scattering or attenuation.


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transducers re*uire less coupling force to s*uee)e out the e2cess couplant than larger

dia!eter transducers. n all !odes, tilting the transducer will distort echoes and cause

inaccurate readings, as noted below.

d' -ur$ature of the test piece4 A related issue in$ol$es the align!ent of the transducerwith respect to the test piece. 6hen !easuring on cur$ed surfaces, it is i!portant that

the transducer be placed appro2i!ately on the centerline of the part and held as nearly

nor!al to the surface as possible. n so!e cases a spring+loaded :+block holder !ay

be helpful for !aintaining this align!ent. n general, as the radius of cur$ature

decreases, the si)e of the transducer should be reduced, and the !ore critical

transducer align!ent will beco!e. For $ery s!all radiuses, an i!!ersion approach will

be necessary. n so!e cases it will be useful to obser$e a wa$efor! display as an aid

in !aintaining opti!u! align!ent. 3ften practice with the aid of a wa$efor! display will

gi$e the operator a proper BfeelB for the best way to hold the transducer.

3n cur$ed surfaces it is i!portant to use only enough couplant to obtain a reading.

2cess couplant will for! a fillet between the edges of the transducer and the radiused

test surface where sound will re$erberate and possibly create spurious signals that

!ay trigger false readings.

%e' Taper or eccentricity4 f the contact surface and back surface of the test piece are

tapered or eccentric with respect to each other, the return echo will be distorted due to

the $ariation in sound path across the width of the bea!. Accuracy of !easure!ent willbe reduced. f the !isalign!ent between outer and inner surfaces is significant, no

!easure!ent will be possible.

%f' Acoustic properties of the test !aterial4 There are se$eral conditions found in certain

engineering !aterials that can potentially li!it the accuracy and range of ultrasonic

thickness !easure!ents4

•


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increases with te!perature. The !a2i!u! thickness that can be !easured in

these !aterials will often be li!ited by attenuation.

• :elocity :ariations ++ An ultrasonic thickness !easure!ent will be accurate only

to the degree that !aterial sound $elocity is consistent with gage calibration.fiber ratio. Many plastics and rubbers show a rapid change

in sound $elocity with te!perature, re*uiring that $elocity calibration be

perfor!ed at the te!perature where !easure!ents are to be !ade.

%g' Phase De$ersal or Phase &istortion ++ The phase or polarity of a returning echo is

deter!ined by the relati$e acoustic i!pedances %density 2 $elocity' of the boundary

!aterials. -o!!ercial gages typically assu!e the custo!ary situation where the test

piece is backed by air or a li*uid, both of which ha$e lower acoustic i!pedances than

!etals, cera!ics, or plastics. 8owe$er, in so!e speciali)ed cases %such as

!easure!ent of glass or plastic liners o$er !etal, or copper cladding o$er steel' this

i!pedance relationship is re$ersed, and the echo appears phase re$ersed. To !aintain

accuracy in these cases it is necessary to change the appropriate echo detection

polarity.

A !ore co!ple2 situation can occur in anisotropic or inho!ogeneous !aterials such as

coarse+grain !etal castings or certain co!posites, where !aterial conditions result in

the e2istence of !ultiple sound paths within the bea! area. n these cases phase

distortion can create an echo that is neither cleanly positi$e nor negati$e. -areful

e2peri!entation with reference standards is necessary in these cases to deter!ine

effects on !easure!ent accuracy. f the effect is consistent it will usually be possible to

co!pensate by !eans of a )ero offset ad5ust!ent, but if echo shape is $ariable, highly

accurate thickness !easure!ents will not be possible.

E. -ouplants

A wide $ariety of couplant !aterials !ay be used in ultrasonic gaging. Propylene glycol

and glycerin are co!!only used and are suitable for !ost applications. n applications

where !a2i!u! transfer of sound energy is re*uired, as with $ery thick or attenuating

!aterials, glycerin is reco!!ended. 8owe$er, on so!e !etals glycerin can pro!ote

corrosion by !eans of water absorption and thus !ay be undesirable. 3ther suitable

couplants for !easure!ents at nor!al te!peratures !ay include water, $arious oilsand greases, gels, and silicone fluids.


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n so!e applications in$ol$ing s!ooth surfaces, it is possible to substitute in place of

li*uid couplant a thin co!pliant !e!brane %such as a thin piece of polyurethane'

between the face of the transducer or delay line and the test piece. This approach will

often re*uire changes to gage setup para!eters and re*uires that the transducer be

pressed fir!ly to the surface of the test piece.

As noted below, !easure!ents at ele$ated te!peratures will re*uire specially

for!ulated high te!perature couplants.

7. 8igh Te!perature Measure!ents

Measure!ents at ele$ated te!peratures %higher than appro2i!ately 70 -elsius or

1/7 Fahrenheit' represent a special category. First, it is i!portant to note that

standard direct contact transducers will be da!aged or destroyed by e2posure tote!peratures higher than this li!it. This is due to the $arying ther!al e2pansion

coefficients of the !aterials used to construct the!, which will cause disbonding at

ele$ated te!peratures. &irect contact transducers should ne$er be used on a surface

that is too hot to co!fortably touch with bare fingers. Thus, high te!perature

!easure!ents should always be done in Mode / or Mode = with either a delay line

transducer %with an appropriate high te!perature delay line' or an i!!ersion

transducer.


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in a water bath. Measure!ent is nor!ally perfor!ed in Mode / or =, although in a few

special cases sliding direct contact transducers working in Mode 1 ha$e been

e!ployed. For accurate on+line ultrasonic !easure!ent, !aterial te!perature !ust be

stable to a$oid errors due to $elocity $ariations.


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Ultrasonic thickness !easure!ents utili)ing direct contact transducers in Mode 1 areoften the si!plest to i!ple!ent and can be used in the !a5ority of co!!on

applications. For !ost !aterials the contact !ethod of !easure!ent pro$ides the

highest coupling efficiency of ultrasound fro! the transducer into the test piece. Mode

1 contact !easure!ents are co!!only reco!!ended when !ini!u! !aterial

thickness does not fall below appro2i!ately 0./7 !! %0.010 inches' of plastic or 0.7

!! %0.0/0 inches' of !etal, precision re*uired is not better than >+/7 !icro!eters

%0.001 inch', test !aterial is at or close to roo! te!perature, and geo!etry per!its

contact coupling.

n this !ode of !easure!ent, the ti!e inter$al between the e2citation pulse and the

first returned echo includes a s!all ti!e incre!ent representing pulse transit ti!e

through the transducer wearplate and the coupling fluid, as well as cable delay and any

offset due to rise ti!e or fre*uency content of the detected echo. n order to

co!pensate for these factors, gages are pro$ided with a )ero offset function, which

effecti$ely subtracts fro! the total !easured ti!e inter$al a period e*ui$alent to the

su! of these $arious fi2ed delays. Cero offset nor!ally !ust be ad5usted whene$er the

transducer type is changed. This !ay be done with the aid of a reference standard of

known thickness and sound $elocity, or, if $elocity is unknown, two standards of

different known thicknesses which can be used to establish both $elocity and )ero.

ode 2: Inter$ace &c"o to First Back(all &c"o


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Measure!ents between the first two echoes following the e2citation pulse arecategori)ed as Mode /. #or!ally this in$ol$es !easure!ent fro! an interface echo

%representing the boundary between a delay line or water path and the outside surface

of the test piece' and a backwall echo representing the inside surface.

There are se$eral conditions that !ust be considered in !aking Mode /

!easure!ents, based on the fact that they re*uire two $alid echoes, interface and

backwall. First, it is necessary to insure that an interface echo e2ists. There are certain

cases in$ol$ing i!!ersion !easure!ents of low i!pedance !aterials such as soft

plastics and silicones where the acoustic i!pedance of the test !aterial is $ery si!ilarto that of water. A si!ilar situation can occur when a delay line transducer is used on a

!aterial %typically a poly!er' whose i!pedance nearly !atches that of the delay line.

n such cases the i!pedance !atch between the water or delay line and the test

!aterial !ay reduce the interface echo to such low a!plitude that it cannot reliably be

detected. 6ith delay line transducers the difficulty can usually be re!edied by

switching to a different delay line !aterial, for e2a!ple fro! the co!!on polystyrene

delays to an epo2y or polyi!ide. 6hen the proble! occurs in i!!ersion

!easure!ents, there !ay be no easy solution, since it is rarely possible to use li*uids

other than water as effecti$e i!!ersion couplants.

The !a2i!u! thickness that can be !easured in a Mode / setup is deter!ined by the

length of the delay line or water path, since the backwall echo fro! the test piece !ust

arri$e before !ultiples of the interface echo. n so!e cases range can be e2tended by

lengthening the delay line or water path, howe$er Mode / is generally not well suited

for !easure!ent of thick !aterials.

6hen working in Mode / it is also necessary to !onitor the phase or polarity of both

interface and backwall echoes, and ad5ust instru!ent detection polarity and>or )erooffset to co!pensate as necessary for in$ersions. A plastic delay line coupled to a


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!etal test piece represents a low+to+high i!pedance boundary, seen as a positi$e

interface echo, while the sa!e delay line coupled to !any poly!er !aterials can

represent a high+to+low relationship of relati$e acoustic i!pedance representing a

negati$e interface echo. The interface echo polarity re$erses between these two

situations, and if the gage is not properly ad5usted a !easure!ent error will result. This

can happen when a gage with a delay line transducer is set up on !etal reference

blocks and then used to !easure plastics. Additionally, a rough !etal surface !ay

create a negati$e interface echo due to the couplant gap under the transducer, while

high i!pedance plastics can produce a positi$e interface. Ly obser$ing the echoes an

operator can select the correct detection polarity in a gi$en case. Figure = shows so!e

e2a!ples of co!!only encountered conditions.

Figure 3 – &c"o polarities in ode 2 measurements

nterface and backwall echo phase distortions can also occur in setups in$ol$ing

sharply radiused !aterial, where co!ple2 interactions between bea! shape and front

and back surface cur$ature can significantly affect echo shape. n such applications it

is essential to set up the instru!ent on reference standards representing the actual

!aterial shape to be !easured, so that the effects of any phase distortion can be

co!pensated with )ero offset.

ode 3: &c"o to &c"o Follo(ing Inter$ace


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The Mode = !easure!ent techni*ue as defined here in$ol$es the !easure!ent of ati!e inter$al between successi$e back echoes following an interface echo. This !ode

is nor!ally reser$ed for situations where the test !aterial is relati$ely thin, and where

the highest le$el of accuracy is re*uired. Mode = !easure!ent is best applied to low

attenuation engineering !aterials ha$ing an acoustic i!pedance greater than 1 2 10

g!>c!/+sec %which includes !ost !etals, cera!ics, and glass'. n !aterials of this

type, successi$e re$erberations are all of the sa!e polarity, and the relati$e a!plitude

of successi$e echoes is deter!ined by the trans!ission coefficient of the sound energy

out of the !aterial into either polystyrene or water. 0.00 B up to 1/.7!!>0.7B, depending on

fre*uency and delay line length. As with direct contact transducer !easure!ents, the

dia!eter or acti$e ele!ent si)e of the delay line should be reduced as the radius of

cur$ature is reduced. For radiuses s!aller than appro2i!ately =!!>0.1/7B, i!!ersion

transducers will pro$ide better coupling and are preferred.

The !a2i!u! thickness that can be !easured in a Mode = setup is deter!ined by the

length of the delay line or water path, since two backwall echoes fro! the test piece

!ust arri$e before !ultiples of the interface echo. n so!e cases range can be

e2tended by lengthening the delay line or water path, howe$er Mode = is not well

suited for !easure!ent of thick !aterials.

f accurate thickness !easure!ents are re*uired on !achined surfaces ha$ing a

surface finish of appro2i!ately = !icrons DM


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transducer. This is due to the fact that successi$e echo re$erberations tend to subtract

out the $ariable thickness of the couplant layer that adds to the ti!e inter$al !easured

using a direct contact transducer. The sa!e general principle applies to painted

surfaces, where !ultiple echoes will represent re$erberations in the !etal or other

high+i!pedance !aterial, not the paint. 8owe$er, there are li!itations on what sort of

surfaces will per!it Mode = !easure!ent, and in the case of se$ere roughness or

corrosion this techni*ue will not be applicable. At least two clean backwall echoes are

re*uired for a Mode = !easure!ent, and as conditions get worse the signal losses due

to roughness will e$entually obliterate the second echo.

6hen using focused i!!ersion transducers for Mode = !easure!ents, and>or when

!easuring certain radiuses, it is always necessary to obser$e the wa$efor! during

initial setup. The ad$antage of focused as opposed to unfocused i!!ersion

transducers of the sa!e fre*uency and si)e is that they often tolerate !ore bea!

angulation or !isalign!ent, as well as i!pro$e coupling into radiused test pieces.


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Ea. Proper !easure!ent of clean echoes

Eb. rror + !easure!ent of successi$e lobes of single echo

Ec. rror + !easure!ent of !ode con$erted shear wa$e echo

Figure ) – &'amples o$ ode 3 *a+e$orms

,ppendi' 1 su!!ari)es so!e typical applications where these !easure!ent !odes

are applied. Please note that the thickness range, accuracy, and transducer

reco!!endations are intended only as a general guide. Lecause of possible $ariations

in !aterial acoustic properties and the effects of geo!etry, the e2act range and

accuracy in a gi$en application should always be $erified with the aid of reference

standards of the !aterial in *uestion. n so!e cases !easure!ent will be possible


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o$er greater ranges than indicated in the table, and in other cases less. And although

transducer reco!!endations are shown, it will often be possible to use two or !ore

different transducers with essentially e*ui$alent results o$er a specified range.

-onclusionThis paper has su!!ari)ed so!e of the !a5or aspects of precision ultrasonic

thickness gaging. For further infor!ation on any of the points discussed, or

reco!!endations for specific e*uip!ent, contact 3ly!pus


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inch0.5 – 20 mm

inch0.3 – 6 mm

10 MHz, 0.25" delay line 3 0.012 - 0.500inch0.3 - 10 mm

!

5 MHz, 0.5" delay line 2 0.050 -1.00 inch1.25 – 25 mm

0.040 - 0.500inch1- 10 mm

5 MHz, 0.5" delay line 3 0.050-0.500 inch1.25 – 10 mm

!

2.25 MHz, 0.5" delay line 2 0.080 -1.00 inch2 – 25 mm

0.050 -0.500 inch1.25 – 10 mm

2.25 MHz, 0.5" delay line 3 0.060 -0.500inch

2.5 – 10 mm

!

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Theory and Application of Precious Ultrasonic Thickness Gaging