ma e ials
A icle
Damage Analysis o Composi e CFRP Tubes Using Acous ic
Emission Moni o ing and Pa e n Recogni ion App oach
Michal Šo e 1,*, Jakub Cienciala 1, Ma in Fusek 1, Pa el Pa líˇcek 1and Richa d Mo a ec 2


Ci a ion: Šo e , M.; Cienciala, J.;
Fusek, M.; Pa líˇcek, P.; Mo a ec, R.
Damage Analysis o Composi e CFRP
Tubes Using Acous ic Emission
Moni o ing and Pa e n Recogni ion
App oach. Ma e ials 2021,14, 786.
h ps://doi.o g/10.3390/ma14040786
Academic Edi o : Michele Bacciocchi
Recei ed: 12 No embe 2020
Accep ed: 28 Janua y 2021
Published: 7 Feb ua y 2021
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2021 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
1
Depa men o Applied Mechanics, Facul y o Mechanical Enginee ing, VŠB—Technical Uni e si y o Os a a,
17. lis opadu 2172/15, 708 00 Os a a, Czech Republic; [email p o ec ed] (J.C.);
[email p o ec ed] (M.F.); [email p o ec ed] (P.P.)
2
Ha el Composi es CZ s. .o., S ésedlice 67, 783 54 Pˇ ásla ice, Czech Republic; mo a ec @ha el-composi es.com
*Co espondence: [email p o ec ed]; Tel.: +420-731-664-248
Abs ac :
The acous ic emission me hod has been adop ed o de ec ion o damage mechanisms in
ca bon- ibe - ein o ced polyme composi e ubes du ing he h ee-poin bending es . The damage
e olu ion p ocess o he indi idual samples has been moni o ed using he acous ic emission me hod,
which is one o he non-des uc i e me hods. The ob ained da a we e hen subjec ed o a wo-s ep
echnique, which combines he unsupe ised pa e n ecogni ion app oach u ilizing he sho - ime
equency spec a wi h he bounda y cu e enabling he al eady clus e ed da a o be addi ionally
il e ed. The bounda y cu e iden i ica ion has been ca ied ou on he basis o p elimina y ensile
es s o he ca bon ibe shea s, whe e, by o e lapping he o ce e sus ime dependency by he
acous ic emission ac i i y e sus ime dependency, i was possible o iden i y he bounda y which
will sepa a e he signals o igina ing om he ibe b eak om unwan ed seconda y sou ces. The
applica ion o he p esen ed wo-s ep me hod esul ed in he iden i ica ion o he ailu e mechanisms
such as ma ix c acking, ibe b eak, decohesion, and debonding. Besides he compa ison o he
esul s wi h al eady published esea ch pape s, he s udy p esen s he comp ehensi e pa ame ic
acous ic emission signal analysis o he indi idual clus e s.
Keywo ds:
acous ic emission; CFRP composi e ube; unsupe ised lea ning app oach; ailu e mechanism
1. In oduc ion
O e he pas decades, ca bon- ibe - ein o ced polyme (CFRP) composi es ha e
shown a cons an inc ease in a a ie y o applica ions such as ca o ai c a componen s,
spo s and medical equipmen [
1
], and ecen ly also addi i e manu ac u ing [
2
–
4
]. Thei
main bene i lies p ima ily in he ela i ely high s eng h/weigh a io o he abili y o
cus omize he ma e ial p ope ies o dedica ed pu poses by changing he s acking se-
quence and ela ed ibe o ien a ion. A ela i e d awback o CRFP composi es is he lack
o duc ile-like beha io and he co esponding absence o p e-wa ning phase be o e he
s uc u al collapse [
5
,
6
] leading o he b i le ailu e. CRFP composi es a e also cha ac-
e ized by he accumula ion o damage inside he s uc u e wi hou any e idence on he
s uc u e su ace [
6
] hus leading o a ela i ely challenging damage assessmen . The e
a e many non-des uc i e es ing app oaches, which can be applied on composi e s uc-
u es, namely in a ed omog aphy [
7
], eddy cu en es ing [
8
], ul asonic es ing [
9
], and
X- ay omog aphy [10].
One o he mos p omising app oaches, especially coupled wi h o he me hods [
11
]
such as Scanning Elec on Mic oscopy (SEM), is he acous ic emission (AE) me hod, which
is also used in a ious applica ions as a eal ime moni o ing ool [
6
]. The AE me hod
exhibi s g ea sensi i i y including conside able eliabili y o ac i e c acks de ec ion [
12
],
e en in he case o ini ia ion phase [
13
]. The AE echnique is e en capable o de ec ing
he onse o plas ic de o ma ion [
14
], which has he cha ac e o whi e noise wi h low
ene gy [
15
]. Fo gaining a mo e de ailed insigh in o he damage moni o ing p ocess wi hin
Ma e ials 2021,14, 786. h ps://doi.o g/10.3390/ma14040786 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2021,14, 786 2 o 16
he meaning o AE sou ce cha ac e iza ion, i is a o able o inco po a e an adequa e signal
analysis ool. The supe ised/unsupe ised pa e n ecogni ion (UPR) app oach [
16
] has
become a e y sui able and p omising app oach o ackle a wide a ie y o p oblems
such as a igue es s [
17
], s uc u e heal h moni o ing [
18
], and condi ion assessmen o
p essu e essels [
19
] and p essu e componen s in ope a ion [
20
]. Nume ous s udies [
21
–
26
]
ha e been conduc ed in o de o assess cha ac e is ic ea u es o he AE ansien s o igi-
na ing om a ious ailu e mechanism in he CFRP composi es such as ma ix c acking,
delamina ion, ibe b eak, and debonding (see Table 1 o u he explana ion).
Table 1.
Basic cha ac e iza ion o damage mechanisms occu ing in ca bon- ibe - ein o ced polyme
(CFRP) composi es.
Damage Mechanism Cha ac e iza ion
Fibe b eak Disin eg a ion o single and/o mul iple ca bon ibe s
Delamina ion Sepa a ion o wo adjacen plies (In e ace ailu e)
Debonding In eg i y ailu e be ween ibe and ma ix (In e ace ailu e)
Ma ix c acking
Nuclea ion and u he p opaga ing o (mic o)c acks in he ma ix
Al hough Chou [
21
] poin s o a disc epancy conce ning, in pa icula , he signal
ampli ude, du a ion as well as equency spec a o he indi idual damage mechanisms, i
was possible o compile a gene al o e iew, which is gi en in he ollowing able (Table 2).
Table 2.
Summa y o he acous ic emission (AE) signal cha ac e is ics o gi en damage mechanisms
in CFRP composi es
Damage Mechanism AE Signal Cha ac e is ics *
Fibe b eak A: 50–100 dBAE, D: 100–10,000 µs, = 300–700 kHz
Ma ix mic o c acks A: 30–40 dBAE, D: <1000 µs, = 100–250 kHz
Ma ix mic o c acks (p opaga ion) A: 40–80 dBAE, D: 1000–10,000 µs, = 100–250 kHz
Delamina ion A: >70 dBAE, D: 1000–10,000 µs, = 250–300 kHz
Debonding A: <60 dBAE, ∼
=300 kHz
* A—ampli ude, D—du a ion.
In he las decade, se e al/nume ous s udies u ilizing ad anced echniques o classi-
ica ion o ailu e modes, such as he use o s a is ical analysis o wa ele coe icien s [
27
] o
in a ed he mog aphy (IT) [
28
], ha e been conduc ed. Ano he in e es ing app oach can
be ound in he wo k published by Munoz e al. [
29
], who iden i ied and u he cha ac e -
ized he damage mechanisms in he unidi ec ional CFRP composi es subjec ed o axis and
o -axis s a ic ensile es s using he acous ic emission me hod and in a ed he mog aphy.
Fu he u iliza ion o unsupe ised pa e n ecogni ion echnique oge he wi h he IT
me hod esul ed in he iden i ica ion o he ailu e mechanisms such as ma ix c acking,
ibe b eakage, and in e ace ailu e, o which he cha ac e iza ion in e ms o he signal
ampli ude o ene gy has been pe o med. In 2011, Gu kin e al. published an ex ensi e
esea ch [
30
], in which he AE signal da a om a ious es con igu a ions we e analyzed
by h ee di e en pa e n ecogni ion app oaches. The analysis esul ed in cha ac e is ic
equency spec a o ma ix c acking, delamina ion, debonding, ibe pull-ou , and ibe
ailu e. I has o be no ed ha he gi en indings in e ms o he equency spec a a e o
some ex en simila o he esul s summa ized in Table 2and he e o e con i ms he ac ual
accu acy o he s udy [30].
The main objec i e o his s udy is o in es iga e and comp ehensi ely desc ibe he AE
signal cha ac e is ics o he damage mechanisms in h ee di e en ypes o CFRP composi e
ubes using a wo-s ep me hod combining he unsupe ised pa e n ecogni ion app oach
wi h he u iliza ion o he bounda y cu e. The cons uc ion o he bounda y cu e has
been conduc ed on he da a om he p elimina y ca bon ibe shea ensile es s. The
al eady iden i ied bounda y cu e has hen been used o u he e inemen o he da a
ac oss indi idual clus e s. Using he p esen ed app oach, i was possible o iden i y a
Ma e ials 2021,14, 786 3 o 16
o al o ou damage mechanisms p esen ed in Table 1wi h subsequen compa ison o he
ob ained esul s wi h he al eady published esea ch pape s. The pa o he s udy is also
he comp ehensi e AE wa e o m analysis o he ep esen a i e signals belonging o he
indi idual clus e s.
2. Expe imen al P ocedu e
2.1. Tes Sample Cha ac e iza ion
The expe imen s we e ca ied ou on h ee ypes o CFRP ubes wi h a di e en
numbe o laye s, hei o ien a ion, and wo en ibe densi y o he used ma e ial (see
Figu e 1), whe e each ype o CFRP ube has been ep esen ed by h ee es samples. The
samples labeled “A” we e manu ac u ed using ou laye s o unidi ec ional ca bon wo en
ab ic wi h densi y o 200 g/m
2
and one laye o a amid/ca bon wo en ab ic (0–90
◦
) wi h
densi y o 175 g/m
2
and a e age wall hickness o 1.45 mm. The p oduc ion o samples
labeled “B” included he use o wo laye s o unidi ec ional ca bon wo en ab ic wi h
densi y 300 g/m
2
and one laye o ca bon wo en ab ic (0–90
◦
) wi h densi y 280 g/m
2
wi h a e age wall hickness o 0.9 mm, while samples labeled “C” we e manu ac u ed
using solely ou laye s o unidi ec ional ca bon wo en ab ic wi h densi y o 300 g/m
2
wi h a e age wall hickness o 1.42 mm. Table 3summa izes he speci ica ion o he es ed
CFRP ubes.
Ma e ials 2021, 14, x FOR PEER REVIEW 4 o 16
Figu e 1. CFRP composi e ubes unde es ; A—A se ies sample, B—B se ies sample, C—
C se ies sample.
2.2. Th ee-Poin Bending Tes
The h ee conside ed ypes o CFRP composi e ubes unde es a e being used o
paddle p oduc ion; he e o e, he h ee-poin bending es has been selec ed in o de o
simula e as much as possible he eal na u e o he loading p ocess du ing he use o he
gi en spo s equipmen . The expe imen s we e ca ied ou on he uni e sal Tes ome ic
M500-50CT es ing machine (The Tes ome ic Company L d., Rochdale, UK) wi h
dedica ed weldmen , which enables i s geome y o be modi ied wi h i s mo ing pa s o
a wide a ie y o such expe imen s. The dis ance be ween he suppo s was equal o 1040
mm wi h he o ce ac ing poin in a dis ance o 440 mm om AE senso #1 (see Figu e 2).
The es ed CFRP ube wi h a ached AE senso s was addi ionally placed in a plas ic pipe
o p e en damage o he AE senso s and o he equipmen due o sudden s uc u al
in eg i y iola ion. The suppo s we e co e ed by hin el o allow ee mo emen o he
ube du ing i s bending. The es has been de o ma ion-con olled wi h he uppe an il
speed equal o 10 mm/min.
(a)
(b)
Figu e 2. (a) In-Si u pho og aph o he es ig including he specimen equipped wi h AE senso s; (b) Schema ic
ep esen a ion o he h ee-poin bending es se up.
2.3. Acous ic Emission Moni o ing
The acous ic emission ac i i y has been moni o ed using he Vallen AMSY-6 AE
sys em (Vallen Sys eme GmbH, Icking, Ge many) wi h wo u ilized measu ing channels
(ASIP-2A dual channel signal p ocesso ca d), equipped wi h he AEP5H 34 dB
p eampli ie s and he b oad-band Vallen VS-900 AE senso s. The senso s we e a ached
on o he ube wi h he use o oil-based plas icine. The sampling equency o he AE da a
Figu e 1.
CFRP composi e ubes unde es ; A—A se ies sample, B—B se ies sample, C—C
se ies sample.
Table 3. Speci ica ion o he es ed CFRP ubes
P ope y A Se ies B Se ies C Se ies
Wall h. (mm)/Diame e (mm)
1.45/32 0.9/32 1.42/32
Fab ica ion
4 laye s o 200 g/m2unidi .
ca bon ab ic1 laye o 175
g/m2a amid/ca bon ab ic
(0◦–90◦)
2 laye s o 300 g/m2unidi .
ca bon ab ic1 laye o 280
g/m2ca bon ab ic (0◦–90◦)
4 laye s o 300 g/m2unidi .
ca bon ab ic
Ma e ials 2021,14, 786 4 o 16
2.2. Th ee-Poin Bending Tes
The h ee conside ed ypes o CFRP composi e ubes unde es a e being used o
paddle p oduc ion; he e o e, he h ee-poin bending es has been selec ed in o de o
simula e as much as possible he eal na u e o he loading p ocess du ing he use o he
gi en spo s equipmen . The expe imen s we e ca ied ou on he uni e sal Tes ome ic
M500-50CT es ing machine (The Tes ome ic Company L d., Rochdale, UK) wi h dedica ed
weldmen , which enables i s geome y o be modi ied wi h i s mo ing pa s o a wide
a ie y o such expe imen s. The dis ance be ween he suppo s was equal o 1040 mm
wi h he o ce ac ing poin in a dis ance o 440 mm om AE senso #1 (see Figu e 2). The
es ed CFRP ube wi h a ached AE senso s was addi ionally placed in a plas ic pipe o
p e en damage o he AE senso s and o he equipmen due o sudden s uc u al in eg i y
iola ion. The suppo s we e co e ed by hin el o allow ee mo emen o he ube du ing
i s bending. The es has been de o ma ion-con olled wi h he uppe an il speed equal o
10 mm/min.
Ma e ials 2021, 14, x FOR PEER REVIEW 4 o 16
Figu e 1. CFRP composi e ubes unde es ; A—A se ies sample, B—B se ies sample, C—
C se ies sample.
2.2. Th ee-Poin Bending Tes
The h ee conside ed ypes o CFRP composi e ubes unde es a e being used o
paddle p oduc ion; he e o e, he h ee-poin bending es has been selec ed in o de o
simula e as much as possible he eal na u e o he loading p ocess du ing he use o he
gi en spo s equipmen . The expe imen s we e ca ied ou on he uni e sal Tes ome ic
M500-50CT es ing machine (The Tes ome ic Company L d., Rochdale, UK) wi h
dedica ed weldmen , which enables i s geome y o be modi ied wi h i s mo ing pa s o
a wide a ie y o such expe imen s. The dis ance be ween he suppo s was equal o 1040
mm wi h he o ce ac ing poin in a dis ance o 440 mm om AE senso #1 (see Figu e 2).
The es ed CFRP ube wi h a ached AE senso s was addi ionally placed in a plas ic pipe
o p e en damage o he AE senso s and o he equipmen due o sudden s uc u al
in eg i y iola ion. The suppo s we e co e ed by hin el o allow ee mo emen o he
ube du ing i s bending. The es has been de o ma ion-con olled wi h he uppe an il
speed equal o 10 mm/min.
(a)
(b)
Figu e 2. (a) In-Si u pho og aph o he es ig including he specimen equipped wi h AE senso s; (b) Schema ic
ep esen a ion o he h ee-poin bending es se up.
2.3. Acous ic Emission Moni o ing
The acous ic emission ac i i y has been moni o ed using he Vallen AMSY-6 AE
sys em (Vallen Sys eme GmbH, Icking, Ge many) wi h wo u ilized measu ing channels
(ASIP-2A dual channel signal p ocesso ca d), equipped wi h he AEP5H 34 dB
p eampli ie s and he b oad-band Vallen VS-900 AE senso s. The senso s we e a ached
on o he ube wi h he use o oil-based plas icine. The sampling equency o he AE da a
Figu e 2.
(
a
) In-Si u pho og aph o he es ig including he specimen equipped wi h AE senso s; (
b
) Schema ic ep esen a-
ion o he h ee-poin bending es se up.
2.3. Acous ic Emission Moni o ing
The acous ic emission ac i i y has been moni o ed using he Vallen AMSY-6 AE sys em
(Vallen Sys eme GmbH, Icking, Ge many) wi h wo u ilized measu ing channels (ASIP-2A
dual channel signal p ocesso ca d), equipped wi h he AEP5H 34 dB p eampli ie s and
he b oad-band Vallen VS-900 AE senso s. The senso s we e a ached on o he ube wi h
he use o oil-based plas icine. The sampling equency o he AE da a was se o 10 MHz
while he ansien da a (wa e ansien s) we e sampled wi h 20 MHz in he equency
ange be ween 50 and 1100 kHz. The de ec ion h eshold has been se o 32 dB owing o
a ela i ely g ea e dis ance o bo h senso s om he a ea in which he b each will mos
likely occu and ela ed highe a enua ion o he AE signal in composi es. Only localized
AE e en s, which all wi hin he
h
250,650
i
(mm) (see Figu e 2b) in e al o he x coo dina e
will be included o u he da a p ocessing.
The il e ed da a was ollowingly analyzed wi h he Vallen VisualClass so wa e
package (Vallen Sys eme GmbH, Icking, Ge many), which uses he pa e n ecogni ion
me hod [
15
] o associa e simila wa e o m ypes in o sepa a e g oups. Due o he na u e
o he ask, an unsupe ised lea ning app oach was chosen. The p ocedu e s a s wi h
loading he selec ed da abase o AE ansien s in o Vallen VisualClass so wa e, whe e he
numbe o ime windows including hei span and he s a ing poin o he segmen a ion
analysis in he ime domain mus be speci ied (see Figu e 3).
Ma e ials 2021,14, 786 5 o 16
Ma e ials 2021, 14, x FOR PEER REVIEW 5 o 16
was se o 10 MHz while he ansien da a (wa e ansien s) we e sampled wi h 20 MHz
in he equency ange be ween 50 and 1100 kHz. The de ec ion h eshold has been se o
32 dB owing o a ela i ely g ea e dis ance o bo h senso s om he a ea in which he
b each will mos likely occu and ela ed highe a enua ion o he AE signal in
composi es. Only localized AE e en s, which all wi hin he 〈250,650〉 (mm) (see Figu e
2b) in e al o he x coo dina e will be included o u he da a p ocessing.
The il e ed da a was ollowingly analyzed wi h he Vallen VisualClass so wa e
package (Vallen Sys eme GmbH, Icking, Ge many), which uses he pa e n ecogni ion
me hod [15] o associa e simila wa e o m ypes in o sepa a e g oups. Due o he na u e
o he ask, an unsupe ised lea ning app oach was chosen. The p ocedu e s a s wi h
loading he selec ed da abase o AE ansien s in o Vallen VisualClass so wa e, whe e he
numbe o ime windows including hei span and he s a ing poin o he segmen a ion
analysis in he ime domain mus be speci ied (see Figu e 3).
Figu e 3. Se ing up he Hamming windowed ime segmen s wi h co esponding esul s in he equency domain.
The cu en analysis uses he ollowing se ings o he AE ansien s wi h ela ion o
he VisualClass so wa e package: Numbe o ime segmen s: 5; Size o single ime
segmen in e ms o poin s: 4096; Rel. igge o se : −256 poin s; min/max equency limi :
50/800 kHz. The so wa e hen pe o ms he assembly o mul idimensional ea u e ec o ,
he size o which depends on he chosen numbe o ime segmen s including hei size.
The pa e n ecogni ion analysis will esul in he basic ea u e space iden i ica ion, which
is hen linea ly ans o med o maximizing in e -class dis ance and minimizing he in a-
class ex ension, a he same ime (see Figu e 4).
Figu e 4. T ans o med ea u es p ojec ion— esul s o ou selec ed clus e s.
Figu e 3. Se ing up he Hamming windowed ime segmen s wi h co esponding esul s in he equency domain.
The cu en analysis uses he ollowing se ings o he AE ansien s wi h ela ion
o he VisualClass so wa e package: Numbe o ime segmen s: 5; Size o single ime
segmen in e ms o poin s: 4096; Rel. igge o se :
−
256 poin s; min/max equency
limi : 50/800 kHz. The so wa e hen pe o ms he assembly o mul idimensional ea u e
ec o , he size o which depends on he chosen numbe o ime segmen s including hei
size. The pa e n ecogni ion analysis will esul in he basic ea u e space iden i ica ion,
which is hen linea ly ans o med o maximizing in e -class dis ance and minimizing he
in a-class ex ension, a he same ime (see Figu e 4).
Ma e ials 2021, 14, x FOR PEER REVIEW 5 o 16
was se o 10 MHz while he ansien da a (wa e ansien s) we e sampled wi h 20 MHz
in he equency ange be ween 50 and 1100 kHz. The de ec ion h eshold has been se o
32 dB owing o a ela i ely g ea e dis ance o bo h senso s om he a ea in which he
b each will mos likely occu and ela ed highe a enua ion o he AE signal in
composi es. Only localized AE e en s, which all wi hin he 〈250,650〉 (mm) (see Figu e
2b) in e al o he x coo dina e will be included o u he da a p ocessing.
The il e ed da a was ollowingly analyzed wi h he Vallen VisualClass so wa e
package (Vallen Sys eme GmbH, Icking, Ge many), which uses he pa e n ecogni ion
me hod [15] o associa e simila wa e o m ypes in o sepa a e g oups. Due o he na u e
o he ask, an unsupe ised lea ning app oach was chosen. The p ocedu e s a s wi h
loading he selec ed da abase o AE ansien s in o Vallen VisualClass so wa e, whe e he
numbe o ime windows including hei span and he s a ing poin o he segmen a ion
analysis in he ime domain mus be speci ied (see Figu e 3).
Figu e 3. Se ing up he Hamming windowed ime segmen s wi h co esponding esul s in he equency domain.
The cu en analysis uses he ollowing se ings o he AE ansien s wi h ela ion o
he VisualClass so wa e package: Numbe o ime segmen s: 5; Size o single ime
segmen in e ms o poin s: 4096; Rel. igge o se : −256 poin s; min/max equency limi :
50/800 kHz. The so wa e hen pe o ms he assembly o mul idimensional ea u e ec o ,
he size o which depends on he chosen numbe o ime segmen s including hei size.
The pa e n ecogni ion analysis will esul in he basic ea u e space iden i ica ion, which
is hen linea ly ans o med o maximizing in e -class dis ance and minimizing he in a-
class ex ension, a he same ime (see Figu e 4).
Figu e 4. T ans o med ea u es p ojec ion— esul s o ou selec ed clus e s.
Figu e 4. T ans o med ea u es p ojec ion— esul s o ou selec ed clus e s.

Ma e ials 2021,14, 786 6 o 16
The esul s a e hen ans e ed in o VisualAE so wa e o u he pos p ocessing.
Fou clus e s we e chosen o subsequen analysis, since he addi ional inc ease o he
numbe o clus e s did no lead o be e di e en ia ion o indi idual ansien s. One sample
om each se ies has been subjec ed o he a enua ion measu emen o he AE signal using
Hsu-Nielsen sou ce [
31
] (pencil lead diame e : 0.35 mm; ha dness: 2H) in o de o p ope ly
e alua e he eal AE signal ampli ude in subsequen da a analysis. The esul s o he
a enua ion measu emen s a e shown in Table 4. The p opaga ion eloci y has been
de e mined expe imen ally using Hsu-Nielsen sou ce wi h alue a ying be ween 3200
and 3300 m/s ac oss A/B/C se ies samples.
Table 4. A enua ion measu emen on A/B/C se ies o CFRP composi e ubes
Sample Se ies Nea Field A enua ion (dB/m) Fa ield A enua ion (dB/m)
A 90 33.3
B 66.6 33.2
C 222.2 36.2
The p elimina y measu emen s also included a se ies o six ensile es s o ca bon ibe
shea s (see Figu e 5) in o de o cons uc he abo e-men ioned bounda y cu e, which will
be u he used o he de ec ion o he ca bon ibe b eaks ac oss he iden i ied clus e s.
The de o ma ion-con olled es s (uppe an il speed equal o 0.5 mm/min) we e ca ied
ou on he uni e sal Tes ome ic M500-50CT es ing machine equipped wi h 100 N load
cell. The ela i ely small scale o he load cell enabled us o de ec in ime he e en s
co esponding o he ailu e o ce ain numbe o ibe s hanks o he egis e ed o ce d op.
The AE ac i i y has been moni o ed using he Vallen AMSY-6 AE sys em wi h h ee u ilized
measu ing channels (ASIP-2S dual channel signal p ocesso ca d), equipped wi h he
AEP5H 34 dB p eampli ie s and he b oad-band DAKEL MIDI AE senso s, whe e he op
and bo om senso ac ed as gua d elemen s, while he middle senso has been used o
he da a acquisi ion. The ibe shea s we e glued on hei ends hus p o iding a clamping
suppo o he a achmen in o he jaws.
Ma e ials 2021, 14, x FOR PEER REVIEW 6 o 16
The esul s a e hen ans e ed in o VisualAE so wa e o u he pos p ocessing.
Fou clus e s we e chosen o subsequen analysis, since he addi ional inc ease o he
numbe o clus e s did no lead o be e di e en ia ion o indi idual ansien s. One
sample om each se ies has been subjec ed o he a enua ion measu emen o he AE
signal using Hsu-Nielsen sou ce [31] (pencil lead diame e : 0.35 mm; ha dness: 2H) in
o de o p ope ly e alua e he eal AE signal ampli ude in subsequen da a analysis. The
esul s o he a enua ion measu emen s a e shown in Table 4. The p opaga ion eloci y
has been de e mined expe imen ally using Hsu-Nielsen sou ce wi h alue a ying
be ween 3200 and 3300 m/s ac oss A/B/C se ies samples.
Table 4. A enua ion measu emen on A/B/C se ies o CFRP composi e ubes
Sample Se ies
Nea Field A enua ion (dB/m)
Fa ield A enua ion (dB/m)
A
90
33.3
B
66.6
33.2
C
222.2
36.2
The p elimina y measu emen s also included a se ies o six ensile es s o ca bon
ibe shea s (see Figu e 5) in o de o cons uc he abo e-men ioned bounda y cu e,
which will be u he used o he de ec ion o he ca bon ibe b eaks ac oss he iden i ied
clus e s. The de o ma ion-con olled es s (uppe an il speed equal o 0.5 mm/min) we e
ca ied ou on he uni e sal Tes ome ic M500-50CT es ing machine equipped wi h 100
N load cell. The ela i ely small scale o he load cell enabled us o de ec in ime he e en s
co esponding o he ailu e o ce ain numbe o ibe s hanks o he egis e ed o ce d op.
The AE ac i i y has been moni o ed using he Vallen AMSY-6 AE sys em wi h h ee
u ilized measu ing channels (ASIP-2S dual channel signal p ocesso ca d), equipped wi h
he AEP5H 34 dB p eampli ie s and he b oad-band DAKEL MIDI AE senso s, whe e he
op and bo om senso ac ed as gua d elemen s, while he middle senso has been used
o he da a acquisi ion. The ibe shea s we e glued on hei ends hus p o iding a
clamping suppo o he a achmen in o he jaws.
Figu e 5. Expe imen al se up o ensile es o he ca bon ibe s.
The basic equency analysis o he AE signal is, besides he exploi ed pa e n
ecogni ion app oach, ela i ely e icien and powe ul ool o il e ing he AE signal,
which can be hen a ilia ed o di e en ailu e mechanisms [32]. Chou in his wo k [21]
s a es ha ibe b eakages in he case o ca bon ibe /glass ibe composi e sys ems
p oduce ex ensional wa e signals wi h equencies be ween 350–700 kHz, while ma ix
c acks gene a e lexu al wa e modes wi h equencies up o 350 kHz. I has o be no ed
ha he gi en inding has been e i ied on he p elimina y ensile es s o he ca bon
ibe s, whe e he AE signals o igina ing om ibe ailu e exhibi a highe powe ac ion
in he 300(350)–600 kHz equency in e al. Based on his conside a ion, he e will be
Figu e 5. Expe imen al se up o ensile es o he ca bon ibe s.
Ma e ials 2021,14, 786 7 o 16
The basic equency analysis o he AE signal is, besides he exploi ed pa e n ecogni-
ion app oach, ela i ely e icien and powe ul ool o il e ing he AE signal, which can
be hen a ilia ed o di e en ailu e mechanisms [
32
]. Chou in his wo k [
21
] s a es ha ibe
b eakages in he case o ca bon ibe /glass ibe composi e sys ems p oduce ex ensional
wa e signals wi h equencies be ween 350–700 kHz, while ma ix c acks gene a e lexu al
wa e modes wi h equencies up o 350 kHz. I has o be no ed ha he gi en inding has
been e i ied on he p elimina y ensile es s o he ca bon ibe s, whe e he AE signals
o igina ing om ibe ailu e exhibi a highe powe ac ion in he 300(350)–600 kHz
equency in e al. Based on his conside a ion, he e will be de ined a a iable deno ed as
p
ac o , which will ela e he powe ac ion o he AE signal in a ce ain equency band
o he AE signal powe in he en i e conside ed equency ange:
P =P(350–800)kHz
P(50–800)kHz 100(%)(1)
whe e P
(350–800)kHz
ep esen s he powe ac ion o he AE signal in he 350–800 kHz
equency ange and P
(50–800)kHz
is he powe o he AE signal in he en i e conside ed
equency ange, i.e., 50–800 kHz. No e ha he powe o he AE signal in he gi en
equency in e al has been calcula ed using Pa se al’s heo em. The bounda y cu e hen
s a es he p
ac o and he AE signal ampli ude in ela ion wi h subsequen in en ion o
il e he AE signal o igina ing om he ibe ailu e om he o he ailu e mechanisms
o he seconda y AE signals, which igu e as noise (in e ac ion o he indi idual ibe s
be ween each o he ( ubbing) and/o AE ac i i y a ising om he sample a achmen
poin s). The iden i ica ion p ocedu e o he bounda y cu e is based on he o e lapping he
o ce e sus ime dependency by he acous ic emission ac i i y e sus ime dependency,
whe e he de ec ed o ce d ops caused by he ailu e o he indi idual/mul iple ibe s can
be di ec ly ma ched o he eme ged AE signals.
The ollowing igu es display he dependency be ween he o ce and ampli ude o
indi idual AE hi s on ime (Figu e 6) and he ela ion be ween he p
ac o and he AE
signal ampli ude o he en i e se ies (Figu e 7). No e ha he AE signal ampli ude is being
e e ed o dB
AE
uni , hus exp essing he ol age ampli ude o non-ampli ied signal as a
gain ela ed o 1
µ
V. The blue line in Figu e 7 ep esen s he bounda y line sepa a ing AE
signals belonging o he ibe b eak om signals o igina ing om he in e ac ion be ween
he indi idual ibe s and he o he in e e ing AE sou ces. A his poin , i has o be
no ed ha he high- equency ange appea ing in nume a o in Equa ion (1) has been o
u he applica ion o he CFRP ubes ex ended o 300–800 kHz ange due o a equency
a enua ion a ising om he geome ic dimensions o he CFRP ubes and he mu ual
dis ance be ween he AE sou ce and he AE senso s.
Ma e ials 2021, 14, x FOR PEER REVIEW 7 o 16
de ined a a iable deno ed as p ac o , which will ela e he powe ac ion o he AE
signal in a ce ain equency band o he AE signal powe in he en i e conside ed
equency ange:
p =P(350–800)kHz
P(50–800)kHz
100 (%),
(1)
whe e P(350–800)kHz ep esen s he powe ac ion o he AE signal in he 350–800 kHz
equency ange and P(50–800)kHz is he powe o he AE signal in he en i e conside ed
equency ange, i.e., 50–800 kHz. No e ha he powe o he AE signal in he gi en
equency in e al has been calcula ed using Pa se al’s heo em. The bounda y cu e
hen s a es he p ac o and he AE signal ampli ude in ela ion wi h subsequen in en ion
o il e he AE signal o igina ing om he ibe ailu e om he o he ailu e mechanisms
o he seconda y AE signals, which igu e as noise (in e ac ion o he indi idual ibe s
be ween each o he ( ubbing) and/o AE ac i i y a ising om he sample a achmen
poin s). The iden i ica ion p ocedu e o he bounda y cu e is based on he o e lapping
he o ce e sus ime dependency by he acous ic emission ac i i y e sus ime
dependency, whe e he de ec ed o ce d ops caused by he ailu e o he
indi idual/mul iple ibe s can be di ec ly ma ched o he eme ged AE signals.
The ollowing igu es display he dependency be ween he o ce and ampli ude o
indi idual AE hi s on ime (Figu e 6) and he ela ion be ween he p ac o and he AE
signal ampli ude o he en i e se ies (Figu e 7). No e ha he AE signal ampli ude is being
e e ed o dBAE uni , hus exp essing he ol age ampli ude o non-ampli ied signal as a
gain ela ed o 1 µV. The blue line in Figu e 7 ep esen s he bounda y line sepa a ing AE
signals belonging o he ibe b eak om signals o igina ing om he in e ac ion be ween
he indi idual ibe s and he o he in e e ing AE sou ces. A his poin , i has o be no ed
ha he high- equency ange appea ing in nume a o in Equa ion (1) has been o u he
applica ion o he CFRP ubes ex ended o 300–800 kHz ange due o a equency
a enua ion a ising om he geome ic dimensions o he CFRP ubes and he mu ual
dis ance be ween he AE sou ce and he AE senso s.
Figu e 6. Fo ce and AE ac i i y as he unc ion o ime o he selec ed sample.
Figu e 6. Fo ce and AE ac i i y as he unc ion o ime o he selec ed sample.
Ma e ials 2021,14, 786 8 o 16
Ma e ials 2021, 14, x FOR PEER REVIEW 8 o 16
Figu e 7. Dependency be ween p ac o and AE signal ampli ude o he en i e se ies.
3. Resul s and Discussion
3.1. Mechanical P ope ies and Basic AE Signal Analysis
Figu e 8 shows ela ion be ween o ce and displacemen o he an il o indi idual
A/B/C se ies p oduc ion samples. As expec ed, he highes s i ness is being eached by
he C se ies samples, ollowed by A and B se ies samples. All h ee manu ac u ing
modi ica ions show wi hin hei g oup e y simila end in e ms o he o ce-
displacemen cou se excep he A se ies samples, namely A2 sample, which exhibi s
ma ginally lowe s i ness, mos likely due o he ab ica ion p ocess, which is no in he
o m o he au oma ed p oduc ion. The abo e-men ioned consis ency in e ms o he
sample s i ness is, howe e , no alid o he maximum o ce ac oss indi idual se ies,
whe e di e ences om 10 o 28 pe cen ela ed o he maximum achie ed o ce in each
p oduc ion se ies can be obse ed. Again, he eason o such esul s a ia ion can be
ound in he p oduc ion o m i sel . A somewha simila end can be egis e ed in he
case o he numbe o loca ed AE e en s ac oss he 0–Fmax ange o he indi idual samples
(see Figu e 9), whe e a ela i ely la ge a ia ion has been egis e ed. Howe e , e en
despi e his ac , A/B se ies epo conside ably highe le el o he loca ed AE e en s,
which is mos likely caused due o he p esence o (0°–90°) ab ic igu ing as a op laye .
No e ha he gi en s a emen is cu en ly a hypo hesis, which needs o be e i ied in he
u u e.
Figu e 8. Fo ce as he unc ion o displacemen o indi idual A/B/C se ies p oduc ion samples
Figu e 7. Dependency be ween p ac o and AE signal ampli ude o he en i e se ies.
3. Resul s and Discussion
3.1. Mechanical P ope ies and Basic AE Signal Analysis
Figu e 8shows ela ion be ween o ce and displacemen o he an il o indi idual
A/B/C se ies p oduc ion samples. As expec ed, he highes s i ness is being eached by
he C se ies samples, ollowed by A and B se ies samples. All h ee manu ac u ing mod-
i ica ions show wi hin hei g oup e y simila end in e ms o he o ce-displacemen
cou se excep he A se ies samples, namely A2 sample, which exhibi s ma ginally lowe
s i ness, mos likely due o he ab ica ion p ocess, which is no in he o m o he au o-
ma ed p oduc ion. The abo e-men ioned consis ency in e ms o he sample s i ness is,
howe e , no alid o he maximum o ce ac oss indi idual se ies, whe e di e ences om
10 o 28 pe cen ela ed o he maximum achie ed o ce in each p oduc ion se ies can be
obse ed. Again, he eason o such esul s a ia ion can be ound in he p oduc ion o m
i sel . A somewha simila end can be egis e ed in he case o he numbe o loca ed AE
e en s ac oss he 0–F
max
ange o he indi idual samples (see Figu e 9), whe e a ela i ely
la ge a ia ion has been egis e ed. Howe e , e en despi e his ac , A/B se ies epo
conside ably highe le el o he loca ed AE e en s, which is mos likely caused due o he
p esence o (0
◦
–90
◦
) ab ic igu ing as a op laye . No e ha he gi en s a emen is cu en ly
a hypo hesis, which needs o be e i ied in he u u e.
Ma e ials 2021, 14, x FOR PEER REVIEW 8 o 16
Figu e 7. Dependency be ween p ac o and AE signal ampli ude o he en i e se ies.
3. Resul s and Discussion
3.1. Mechanical P ope ies and Basic AE Signal Analysis
Figu e 8 shows ela ion be ween o ce and displacemen o he an il o indi idual
A/B/C se ies p oduc ion samples. As expec ed, he highes s i ness is being eached by
he C se ies samples, ollowed by A and B se ies samples. All h ee manu ac u ing
modi ica ions show wi hin hei g oup e y simila end in e ms o he o ce-
displacemen cou se excep he A se ies samples, namely A2 sample, which exhibi s
ma ginally lowe s i ness, mos likely due o he ab ica ion p ocess, which is no in he
o m o he au oma ed p oduc ion. The abo e-men ioned consis ency in e ms o he
sample s i ness is, howe e , no alid o he maximum o ce ac oss indi idual se ies,
whe e di e ences om 10 o 28 pe cen ela ed o he maximum achie ed o ce in each
p oduc ion se ies can be obse ed. Again, he eason o such esul s a ia ion can be
ound in he p oduc ion o m i sel . A somewha simila end can be egis e ed in he
case o he numbe o loca ed AE e en s ac oss he 0–Fmax ange o he indi idual samples
(see Figu e 9), whe e a ela i ely la ge a ia ion has been egis e ed. Howe e , e en
despi e his ac , A/B se ies epo conside ably highe le el o he loca ed AE e en s,
which is mos likely caused due o he p esence o (0°–90°) ab ic igu ing as a op laye .
No e ha he gi en s a emen is cu en ly a hypo hesis, which needs o be e i ied in he
u u e.
Figu e 8. Fo ce as he unc ion o displacemen o indi idual A/B/C se ies p oduc ion samples
Figu e 8. Fo ce as he unc ion o displacemen o indi idual A/B/C se ies p oduc ion samples
Ma e ials 2021,14, 786 9 o 16
Ma e ials 2021, 14, x FOR PEER REVIEW 9 o 16
Figu e 9. Cumula i e numbe o loca ed e en s as he unc ion o o ce o indi idual A/B/C se ies samples
The ene gy o accumula ed AE e en s e sus o ce (Figu e 10) is ano he impo an
dependency, which can b ing us close o he o e all s uc u e beha io . The maximum
alue o he eleased AE ene gy is o all samples be ween 5 × 108 and 109 aJ. The
di e ence, howe e , lies in he cha ac e how he ene gy is being eleased du ing he
loading p ocess. The A and B se ies specimen exhibi almos g adual AE ene gy elease,
wi h he di e ence in he inal loading s age. While he A se ies specimen end o
g adually con inue wi h he cumula ion o he AE e en s and g adual elease o he AE
ene gy, he B se ies samples end o suddenly lose in eg i y wi hou any signi ican
wa ning phase. A comple ely di e en beha io can be ound in he case o he C se ies
samples, which ha e conside ably la ge ene gy pe e en a io wi h a lack o any wa ning
phase be o e he in eg i y lose.
Figu e 10. Ene gy as he unc ion o o ce o indi idual A/B/C se ies samples
3.2. AE Signal Analysis Using Pa e n Recogni ion App oach
The u ilized unsupe ised pa e n ecogni ion analysis esul ed in iden i ica ion o
ou clus e s o AE signals wi h he ollowing ea u es. The signals a ilia ed o he i s
clus e a e cha ac e ized by high ampli ude, in mos cases exceeding 90 dBAE wi h ene gy
alue usually abo e 1 × 106 aJ and equencies in he span om 50 kHz o 150 kHz (see
Figu e 11a), whe eas his clus e also pa ially con ains signals wi h equency con en
abo e 300 kHz. The second clus e is cha ac e ized by he ampli udes mos ly below 65
dBAE wi h AE ene gy in he o de o hund eds o he ens o housands o aJ and he
equency in he 50–450 kHz ange (see Figu e 11b). The hi d clus e is ep esen ed by
Figu e 9. Cumula i e numbe o loca ed e en s as he unc ion o o ce o indi idual A/B/C se ies samples
The ene gy o accumula ed AE e en s e sus o ce (Figu e 10) is ano he impo an
dependency, which can b ing us close o he o e all s uc u e beha io . The maximum
alue o he eleased AE ene gy is o all samples be ween 5
×
10
8
and 10
9
aJ. The di e ence,
howe e , lies in he cha ac e how he ene gy is being eleased du ing he loading p ocess.
The A and B se ies specimen exhibi almos g adual AE ene gy elease, wi h he di e ence
in he inal loading s age. While he A se ies specimen end o g adually con inue wi h he
cumula ion o he AE e en s and g adual elease o he AE ene gy, he B se ies samples end
o suddenly lose in eg i y wi hou any signi ican wa ning phase. A comple ely di e en
beha io can be ound in he case o he C se ies samples, which ha e conside ably la ge
ene gy pe e en a io wi h a lack o any wa ning phase be o e he in eg i y lose.
Ma e ials 2021, 14, x FOR PEER REVIEW 9 o 16
Figu e 9. Cumula i e numbe o loca ed e en s as he unc ion o o ce o indi idual A/B/C se ies samples
The ene gy o accumula ed AE e en s e sus o ce (Figu e 10) is ano he impo an
dependency, which can b ing us close o he o e all s uc u e beha io . The maximum
alue o he eleased AE ene gy is o all samples be ween 5 × 108 and 109 aJ. The
di e ence, howe e , lies in he cha ac e how he ene gy is being eleased du ing he
loading p ocess. The A and B se ies specimen exhibi almos g adual AE ene gy elease,
wi h he di e ence in he inal loading s age. While he A se ies specimen end o
g adually con inue wi h he cumula ion o he AE e en s and g adual elease o he AE
ene gy, he B se ies samples end o suddenly lose in eg i y wi hou any signi ican
wa ning phase. A comple ely di e en beha io can be ound in he case o he C se ies
samples, which ha e conside ably la ge ene gy pe e en a io wi h a lack o any wa ning
phase be o e he in eg i y lose.
Figu e 10. Ene gy as he unc ion o o ce o indi idual A/B/C se ies samples
3.2. AE Signal Analysis Using Pa e n Recogni ion App oach
The u ilized unsupe ised pa e n ecogni ion analysis esul ed in iden i ica ion o
ou clus e s o AE signals wi h he ollowing ea u es. The signals a ilia ed o he i s
clus e a e cha ac e ized by high ampli ude, in mos cases exceeding 90 dBAE wi h ene gy
alue usually abo e 1 × 106 aJ and equencies in he span om 50 kHz o 150 kHz (see
Figu e 11a), whe eas his clus e also pa ially con ains signals wi h equency con en
abo e 300 kHz. The second clus e is cha ac e ized by he ampli udes mos ly below 65
dBAE wi h AE ene gy in he o de o hund eds o he ens o housands o aJ and he
equency in he 50–450 kHz ange (see Figu e 11b). The hi d clus e is ep esen ed by
Figu e 10. Ene gy as he unc ion o o ce o indi idual A/B/C se ies samples
3.2. AE Signal Analysis Using Pa e n Recogni ion App oach
The u ilized unsupe ised pa e n ecogni ion analysis esul ed in iden i ica ion o
ou clus e s o AE signals wi h he ollowing ea u es. The signals a ilia ed o he i s
clus e a e cha ac e ized by high ampli ude, in mos cases exceeding 90 dB
AE
wi h ene gy
alue usually abo e 1
×
10
6
aJ and equencies in he span om 50 kHz o 150 kHz (see
Figu e 11a), whe eas his clus e also pa ially con ains signals wi h equency con en
abo e 300 kHz. The second clus e is cha ac e ized by he ampli udes mos ly below
Ma e ials 2021,14, 786 16 o 16
21.
Chou, H.-Y. Damage Analysis o Composi e P essu e Vessels Using Acous ic Emission Moni o ing. Ph.D. Thesis, School o
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Aus alia, 2011.
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Ono, K.; Kawamo o, K. Digi al signal analysis o acous ic emission om ca bon ibe /epoxy composi es. J. Acous . Emiss.
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Komai, K.; Minoshima, K.; Shibu ani, T. In es iga ions o he ac u e mechanism o ca bon/epoxy composi es by AE signal
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Bacca , D.; Sö ke , D. Iden i ica ion and classi ica ion o ailu e modes in lamina ed composi es by using a mul i a iable s a is ical
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P akash, R.V.; Maha ana, M. Damage de ec ion using in a ed he mog aphz in a ca bon- lax ibe hyb id composi e. P ocedia
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Munoz, V.; Valés, B.; Pe in, M.; Pas o , M.L.; Welemane, H.; Can a el, A. Damage de ec ion in CFRP by coupling acous ic
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