7/25/2019 UT NDT

1/20

NDT.net- June 1999, Vol. 4No. 6

Ultrasonic Inspection 2

Training For Nondestructive Testing -

Variables Affecting Test Results

Ed GinzelEmail:eginzel@mri.on.ca

Homepage: http://www.mri.on.caHome study correspondence courses (UT, ET, !"# and $"#% including

&'T "rolem )ol*er )o+tware (sol*ing standard euations in UT, -T andET%

Introduction

TABLE OF CONTENTS

#ntroduction

#nstrument "er+ormance

Transducer "er+ormance

$aterial ariations

)ur+ace -oughness

)ur+ace oatings

ouplant ondition

"art )ize and Geometry

#nternal )tructure

'e+ect ariation

'e+ect )ize and Geometry

!ocation with -espect to d0acent )ur+aces

1rientation o+ $a0or 2is

Type o+ 'iscontinuity and onditions o+ -e+lection

http://www.ndt.net/index.htmlhttp://www.ndt.net/index.htmlhttp://www.ndt.net/v04n06.htmmailto:eginzel@mri.on.camailto:eginzel@mri.on.cahttp://www.mri.on.ca/http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#1http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#2http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#3http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#4http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#41http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#42http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#43http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#44http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#45http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#5http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#51http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#52http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#53http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#54http://www.ndt.net/v04n06.htmmailto:eginzel@mri.on.cahttp://www.mri.on.ca/http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#1http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#2http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#3http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#4http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#41http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#42http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#43http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#44http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#45http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#5http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#51http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#52http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#53http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm#54http://www.ndt.net/index.html

7/25/2019 UT NDT

2/20

Ultrasonic tests can provide information about several aspects of a material

suc as! tic"ness, attenuation, sape, presence of defects, si#e and teir

orientation. Tese rel$ on t%o main measurements! amplitude of si&nal and

time of te si&nal arrival. To a lesser e'tent te fre(uenc$ content of te si&nal

can also provide useful information but its application is not so common.

)e ma"e certain assumptions about te test conditions and presume tat

can&es in time or amplitude are caused b$ variation in te parameter of

interest. Te assumptions made are based on all parameters bein& constant

e'cept te one %e are interested in measurin& can&es in. *or e'ample, %en

performin& a tic"ness measurement %e assume te acoustic velocit$ of te

test piece %e are measurin& is te same as te acoustic velocit$ in te

calibration piece. )e furter assume tat te temperature at %ic tests and

calibrations are made are not important. +et eiter or bot of te parameters

assumed fi'ed materials velocit$ and temperature can affect our test results.

Variables affectin& te test results %ill be divided into 4 &roups!

1. instrument performance

. transducer performance

/. material variations

4. defect variations

0noter factor relatin& to te results of an inspection is te uman *actor, tisis a %idel$ debated sub2ect. Te sub2ect is not discussed in tis capter nor is

te related sub2ect of 3robabilit$ of Detection. *or more information on 3D

please refer to earlier publications on NDTnet

Instrument Performance

5n apter 7, performance verification of instrumentation %as discussed. 8cope

displa$ and pulserreceiver variations can affect time, amplitude and

fre(uenc$ content of ultrasonic si&nals.

Scope -Te primar$ variable in te scope is te linearit$ of te time base.

Verification metods %ill usuall$ re(uire a tolerance in accurac$ to a

percenta&e of te total screen ran&e t$picall$ :- ;. Tis ensures no distance

measured %ill be in error b$ more tan ;, e.&. for a

7/25/2019 UT NDT

3/20

Pulser-Receiver - 0mplitude uncertainties %ill result from variations in te

linearit$ of te vertical deflection of te scope or due to inaccuracies in te

amplitude control. 8cope vertical linearit$ ensures tat te relationsip bet%een

t%o si&nals of different amplitudes is maintained over te entire ran&e of te

screen ei&t. Tis is done b$ comparin& te relative ei&t of t%o ecoes at

different screen ei&ts. e.&. settin& t%o ecoes 6 d> apart startin& %it one at?=; *8, te oter at 4=; *8 ad2ustments are made to first increase te ?=;

*8 si&nal to 9=; and 1==;. Te lo%er si&nal sould be 4 B = lo& 001 can&in& te

d> &ain b$ a fi'ed amount sould can&e te ratio of te si&nals. Tis allo%s

us to e'pect a si&nal at is added

to te receiver &ain. 08C code re(uires scannin& of a %eld to be done usin&

14d> over reference. Tis means a si&nal tat %as =; of te reference

amplitude at reference &ain %ould ten come up to te reference level denoted

b$ te D0. 5f te receiver &ain is not linear te smallest recordable indication

ma$ be &reater or less tan te intended level. Tis %ill be anoter source of

incorrectl$ si#in& a defect %it respect to a reference.

Te effect of bandpass filters on displa$ed si&nal amplitude as been discussed

in apter 6. Te effect on amplitude is to reduce si&nal ei&t if te centre

fre(uenc$ of te received si&nal is be$ond &reater or less tan te bandpass

re&ion of ma'imum response. Tis can be a factor %en it is "no%n tat te

reflected or re-transmitted si&nal off a defect as a fre(uenc$ spectrum

determined b$ te caracteristics of te defect tis is te basis of acoustic

spectrum anal$sis or fre(uenc$ anal$sis.

Transducer Performance

0s %it te pulserreceiver, transducer performance is cec"ed and monitored

for can&e. odes and standards suc as tose discussed in apter 7 cover te

details for verif$in& instrument, transducer and s$stem performance. >ut in

addition to ensurin& tese aspects are %itin tolerances allo%ed initiall$, te$

must all be monitored on a re&ular basis to ensure no si&nificant can&es occur.

7/25/2019 UT NDT

4/20

>8 4//1 3art /E, recommends te follo%in& probes$stem performance cec"sF

IT! !"NIT"RIN# FR$UN%&

probe inde' dail$ on rou& surfaces, suc as castin&s, t%ice dail$

beam an&le

beam s"e%s(uint

beam profilemontl$ and %en lar&e can&es in probe an&le or inde' are

observed

dominant

fre(uenc$montl$ and %enever repairs ave been made to eiter probe orinstrument and if one instrument is replaced %it anoter

pulse len&t

dead #one

near field

si&nal-to-

noise ratio overall

s$stem &aindail$ and after repairs or replacement as above

resolvin&

po%ermontl$ and after repairs or replacement as above

E >8 4//1 3art / %ill be replaced b$ N 166?-, Non-destructive testin& - aracteri#ation and

verification of ultrasonic e'amination e(uipment- 3art ! 3robes.

Te above monitorin& items appl$ to contact testin& probes. Te %ear

e'perienced b$ movement on metal surfaces tends to accelerate can&es in

performance. 8ome of te can&es introduced b$ %ear can alter test resultssi&nificantl$. 0s an e'ample, consider beam an&le can&e. 5f at te start of te

da$ a nominal 6=G probe %as found to ave an actual an&le of 6G, an

indication is found %it a soundpat of 1

7/25/2019 UT NDT

5/20

3igure 456

8imilar errors in lateral positional plottin& can result from s"e%in& of te beam. )en

plottin& an indication %it an an&le beam %e assume te beam e'tends directl$ aead

in-line %it te probe ousin& but %ear on one side or te

oter of te %ed&e %ill steer te beam a%a$ from te

centre-line. 5f %e use te previous e'ample %it te 6G

probe findin& an indication at 1

7/25/2019 UT NDT

6/20

peratin& probes in %arm %ater I

7/25/2019 UT NDT

7/20

plate scale, dirt sand and rou& cast surfaces from sand castin&. Tese

irre&ularities %ill cause some points of contact to pus a%a$ te couplant and

force it into te lo%er areas around te probe. 5f te couplant is not sufficientl$

viscous it %ill drain a%a$ (uic"l$ and fail to couple te probe to te test piece.

8ee *i&ure ?-/.

3igure 458

5n addition to reduced couplin&, %ic %ill reduce si&nal amplitudes, te rou&

surface increases te rate of %ear on te probe. n an oter%ise smoot surface

isolated protrusions suc as %eld spatter can inder or stop probe motion or in

te case of mecani#ed s$stems tere ma$ be sufficient force to move te probe

past te obstruction but tis could result in dama&in& te probe b$ eiter

tearin& it from its mountin& or severel$ scorin& te plastic %ed&e. )en te

dirt on te test piece is ver$ fine similar to a flour te'ture couplin& can beprevented due to surface tension preventin& te li(uid couplant penetratin& to

te metal. Unless a transfer value as been establised bet%een test piece and

calibration piece, tis could &o undetected.

5n addition to affectin& couplin&, surface rou&ness tends to reduce si&nal

amplitude b$ scatterin& and focusin& te beam. Tis applies to bot contact and

immersion testin&.

)eter uniform or irre&ular, a rou& surface as te potential to present a

scatterin& effect at an interface %ere a beam impin&es. Te de&ree ofscatterin& is based on te ratio of rou&ness to %avelen&t. )en rou&ness is

less tan about 11= a %avelen&t, scatter %ill be ne&li&ible. To reduce si&nal

losses due to scatterin& an operator can select a lo%er fre(uenc$ probe. )it a

%avelen&t of =./7mm in %ater for a 4C# probe, si&nal loss due to scatter

can occur for irre&ularities as small as about =.=4mm. 5n addition to si&nal

reduction anoter effect of surface irre&ularities is to redirect and mode convert

some ener&$ %ic %en returned to te probe can be te source of spurious

si&nals. 5n contact testin& false indications from standin& %aves resultin& from

scatter on rou& surfaces %ill normall$ ave sort soundpats. Te$ can be

eliminated as true fla%s b$ failin& to locate an$ trace of indication from te fulls"ip or from te opposite side.

Unless done properl$, removal of surface rou&ness b$ mecanical means can

result in furter scatterin& problems. 8mall curved &ou&es left b$ a &rindin&

%eel used to remove spatter or macinin& &rooves can form small lenses. Te

affect of &rindin& can be unpredictable. 8ome of te lensin& ma$ concentrate

7/25/2019 UT NDT

8/20

te beam tereb$ increasin& si&nal amplitude, or, te lens effect ma$ be a de-

focusin& of te beam, a&ain resultin& in lo%er tan e'pected si&nal amplitudes.

Uniform surface preparation b$ sand or sot blastin& usuall$ provides a &ood

surface for ultrasonic testin&. @emoval of e'cess metal b$ a and eld &rindin&

%eel is commonl$ used on %eld caps and roots. )en a pipe %eld as ad its

root &round flus and inspection can onl$ be performed from te outsidediameter, (ualit$ of &rindin& can result in unnecessar$ repair calls if &rindin&

as been alon& te %eld a'is. Te small &rooves made b$ te &rindin& %eel

run parallel to te root ed&e and are easil$ confused %it lac" of fusion, missed

ed&e or undercut defects.

Surface %oatings

8urface coatin&s are added to protect a surface from corrosion or to enance its

appearance. Tin films, suc as o'ide la$ers, anodi#in& la$ers or electroplated

finises, and te sli&tl$ tic"er coatin&s of paint or lac(uer are usuall$ %ellbonded to te surface. Kualit$ of bond ma$ be compared to te uncoated

reference bloc" b$ a simple transfer value. ven a sli&t loss due to te coatin&

ma$ be preferable to removin& te coatin& and tr$in& to inspect on te rou&

surface it ides.

)en tic"ness testin& is done on a painted surface te paint tic"ness can add

error to te readin&. *or e'ample!

0 nominal

7/25/2019 UT NDT

9/20

%ouplant %ondition

>ot contact and immersion metods utili#e intervenin& media to transfer

sound from te probe into te test piece and bac" to te receiver. )it

immersion metods it is accomplised b$ a sin&le fluid medium. 5n contact

testin& tere are nearl$ al%a$s at least t%o intervenin& mediaF te dela$line orprotective face and te tin film of couplin& fluid or &rease. 0ttenuation and

acoustic velocit$ are te t%o main properties tat dictate te performance of a

couplant. 0ttenuation affects amplitude of te si&nal and velocit$ %ill

determine bot transit time and refracted an&les.

>ut attenuation and velocit$ of couplants are not independent properties. ac

is a function of oter parameters. Unless tese parameters are controlled or in

some %a$ compensated for, &ross variations from te reference value or

calibration conditions can result.

0ttenuation of couplants varies %it material composition as %ould be

e'pected. 3ublised attenuation values are available for man$ materials as

indicated in te table belo%. 0ttenuation coefficients are often (uoted in nepers

%ic allo% for fre(uenc$ dependence. 1 Np B ?.6?6 ' fB d>cm. Table ?-1

indicates attenuation of some common li(uids.

Table ?-1

!aterial Attenuation (Np ) *+-*,

%ater mm attenuation is considered ne&li&ible. >ut for te eavier oils

attenuations == to

7/25/2019 UT NDT

10/20

fre(uenc$ content of transmitted %aveforms. 8ince te operator rarel$ "no%s

%at %ed&e material a manufacturer as used, little can be done to correct for

potential variations in periodic inspections %ere results of tests ta"en %it one

or more $ears separation are compared.

Table ?-

!aterialAttenuation d./cm

0 , !1

Acoustic

Velocit3

3le'i&las

acr$lic6.4 to 1.4 .7< to .61

le'an pol$

carbonate/. ./=

pol$st$rene 1.? to /.6 ./ to .4?

n$lon .? to 16 .6 to .77

0ttenuation is not a material constant. Under can&es in conditions it can

can&e. *or e'ample attenuation in %ater is inversel$ proportional to bot

temperature and pressure.

0t standard pressure and temperature 1 atmospere and =G attenuation in

%ater is

7/25/2019 UT NDT

11/20

Just as plastic compositions can&e in velocit$ so too does %ater. 3ure %ater at

=G and 1 atmospere pressure as a velocit$ of 14?=ms. >ut %ater is not

normall$ pure. 0s salinit$ increases as in sea %ater, acoustic velocit$ increases.

0t =G in sea %ater %it a /; salinit$ te acoustic velocit$ increases to about

1

7/25/2019 UT NDT

12/20

metanol 11=/ -/.

%ater pure 149? :.4

%ater sea 1$ comparison most metals ave a temperature dependence of bet%een -=.< to

-

7/25/2019 UT NDT

13/20

*i&ure ?-< illustrates te effect of temperature on refracted an&les in steel for tree

common fi'ed %ed&e an&les.

3igure 45

5ncident an&les indicated in te le&end in *i&ure ?-< are tose to produce

nominal 4

7/25/2019 UT NDT

14/20

5n te second fi&ure te probe ma"es contact on a flat surface but te beam

reflects off a conve' curve tereb$ redirectin& portions of te beam a%a$ and

reducin& te ma'imum possible reflected ener&$. Te &rap used does not

consider probe diameter, instead, ratio of surface curvature to metal pat

tic"ness is used. movin& verticall$ up from te ratio a'is ori#ontal a'is at

te appropriate ratio for $our %or" piece, te point on te vertical a'is %erete curve is intersected &ives te necessar$ correction factor.

3igure 45; 3igure 45