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Acoustic emission signal characterisation of failure mechanisms in CFRP composites using dual-sensor approach and spectral clustering technique

Šofer, Michal · 2023

Abstract

The characterisation of failure mechanisms in carbon fibre-reinforced polymer (CFRP) materials using the acoustic emission (AE) technique has been the topic of a number of publications. However, it is often challenging to obtain comprehensive and reliable information about individual failure mechanisms. This situation was the impetus for elaborating a comprehensive overview that covers all failure mechanisms within the framework of CFRP materials. Thus, we performed tensile and compact tension tests on specimens with various stacking sequences to induce specific failure modes and mechanisms. The AE activity was monitored using two different wideband AE sensors and further analysed using a hybrid AE hit detection process. The datasets received from both sensors were separately subjected to clustering analysis using the spectral clustering technique, which incorporated an unsupervised k-means clustering algorithm. The failure mechanism analysis also included a proposed filtering process based on the power distribution across the considered frequency range, with which it was possible to distinguish between the fibre pull-out and fibre breakage mechanisms. This functionality was particularly useful in cases where it was evident that the above-mentioned damage mechanisms exhibited very similar parametric characteristics. The results of the clustering analysis were compared to those of the scanning electron microscopy analysis, which confirmed the conclusions of the AE data analysis.

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Acoustic emission signal characterisation of failure mechanisms in CFRP composites using dual-sensor approach and spectral clustering technique

Author: Šofer, Michal
Publisher: MDPI
Year: 2023
DOI: 10.3390/polym15010047
Source: https://dspace.vsb.cz/bitstreams/7f33b5a7-6bb2-4cc5-ba8a-2e962b5c5e56/download
Ci a ion: Šo e , M.; Šo e , P.; Pagáˇc,
M.; Voloda skaja, A.; Babiuch, M.;
G uˇn, F. Acous ic Emission Signal
Cha ac e isa ion o Failu e
Mechanisms in CFRP Composi es
Using Dual-Senso App oach and
Spec al Clus e ing Technique.
Polyme s 2023,15, 47. h ps://
doi.o g/10.3390/polym15010047
Academic Edi o : Mau o Za elli
Recei ed: 16 No embe 2022
Re ised: 16 No embe 2022
Accep ed: 15 Decembe 2022
Published: 22 Decembe 2022
Copy igh : © 2022 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/).
polyme s
A icle
Acous ic Emission Signal Cha ac e isa ion o Failu e
Mechanisms in CFRP Composi es Using Dual-Senso App oach
and Spec al Clus e ing Technique
Michal Šo e 1,*, Pa el Šo e 2, Ma ek Pagáˇc 3, Anas asia Voloda skaja 4, Ma ek Babiuch 2and Filip G uˇn 5
1
Depa men o Applied Mechanics, Facul y o Mechanical Enginee ing, VSB—Technical Uni e si y o Os a a,
17. lis opadu 2172/15, 708 00 Os a a, Czech Republic
2Depa men o Con ol Sys ems and Ins umen a ion, Facul y o Mechanical Enginee ing, VSB—Technical
Uni e si y o Os a a, 17. lis opadu 2172/15, 708 00 Os a a, Czech Republic
3Depa men o Machining, Assembly and Enginee ing Technology, Facul y o Mechanical Enginee ing,
VSB—Technical Uni e si y o Os a a, 17. lis opadu 2172/15, 708 00 Os a a, Czech Republic
4Depa men o RMSTC, Facul y o Ma e ials Science and Technology, VSB—Technical Uni e si y o Os a a,
17. lis opadu 2172/15, 708 00 Os a a, Czech Republic
5BREBECK Composi e s. .o., Volenská1718, 739 34 Šeno u Os a y, Czech Republic
*Co espondence: [email p o ec ed]; Tel.: +420-731-664-248
Abs ac :
The cha ac e isa ion o ailu e mechanisms in ca bon ib e- ein o ced polyme (CFRP)
ma e ials using he acous ic emission (AE) echnique has been he opic o a numbe o publica ions.
Howe e , i is o en challenging o ob ain comp ehensi e and eliable in o ma ion abou indi idual
ailu e mechanisms. This si ua ion was he impe us o elabo a ing a comp ehensi e o e iew
ha co e s all ailu e mechanisms wi hin he amewo k o CFRP ma e ials. Thus, we pe o med
ensile and compac ension es s on specimens wi h a ious s acking sequences o induce speci ic
ailu e modes and mechanisms. The AE ac i i y was moni o ed using wo di e en wideband AE
senso s and u he analysed using a hyb id AE hi de ec ion p ocess. The da ase s ecei ed om
bo h senso s we e sepa a ely subjec ed o clus e ing analysis using he spec al clus e ing echnique,
which inco po a ed an unsupe ised k-means clus e ing algo i hm. The ailu e mechanism analysis
also included a p oposed il e ing p ocess based on he powe dis ibu ion ac oss he conside ed
equency ange, wi h which i was possible o dis inguish be ween he ib e pull-ou and ib e
b eakage mechanisms. This unc ionali y was pa icula ly use ul in cases whe e i was e iden ha
he abo e-men ioned damage mechanisms exhibi ed e y simila pa ame ic cha ac e is ics. The
esul s o he clus e ing analysis we e compa ed o hose o he scanning elec on mic oscopy analysis,
which con i med he conclusions o he AE da a analysis.
Keywo ds: acous ic emission; CFRP; ailu e mechanism; spec al clus e ing
1. In oduc ion
Composi e ma e ials in many o ms ha e become inc easingly popula in ecen
decades, especially in he medical, au omo i e, and ae ospace indus ies. In pa icula ,
ca bon- ib e- ein o ced polyme composi es ha e been used in a wide ange o spo ing
ac i i ies such as cycling (bicycle ames and accesso ies), yach ing (hull, mas , and o he
cons uc ion elemen s), o mo o spo s (bonne , bike ame, and wheel ims). The main
ad an ages o CFRP composi es a e ha hey ha e a e y high s eng h/weigh a io and
ha hei p ope ies can be cus omised o a speci ic pu pose by changing he s acking
sequence o he indi idual plies [
1
]. Howe e , hei main d awback is hei inabili y o
ail in a duc ile manne [
2
]. Mo eo e , he inal ac u e occu s as a esul o g adual
accumula ing damage, which is ba ely isible on he ou side su ace [
3
]. This is one o
he main easons o adop ing a sui able me hod o he comp ehensi e analysis and
cha ac e isa ion o ailu e mechanisms in CFRP composi es.
Polyme s 2023,15, 47. h ps://doi.o g/10.3390/polym15010047 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2023,15, 47 2 o 25
Cu en ly, nume ous non-des uc i e es ing (NDT) me hods, such as ul asonic es -
ing [
4
], eddy cu en es ing [
5
], and X- ay omog aphy [
6
], ha e been applied o de ec
and/o localise de ec s in composi es. Howe e , hese me hods ha e se e al limi a ions
ega ding eal- ime e alua ion, di icul easibili y o he oluminal po ion o he ma e ial,
which can be es ed in eal ime. Hence, he acous ic emission me hod [
7
] has been used in a
la ge numbe o s udies [
8
–
11
] because i o e s a p ac ical and e ec i e solu ion compa ed
o he me hods lis ed abo e. The AE me hod belongs o a g oup o NDT me hods ha
a e ully suppo ed by in e na ional s anda ds o applica ions in echnical p ac ice. The
me hod is highly e ec i e o es ing me allic p essu e equipmen [
12
], such as pipelines,
boile s, seamless s eel essels, au ocla es, and composi e essels, whe e he modal AE
echnique is o en used [
13
]. The AE me hod is based on he de ec ion o acous ic s ess
wa es o igina ing om a ma e ial as a esul o sudden s ess edis ibu ion owing o he
p esen mic os uc u al damage, which is igge ed by he applied load [
12
]. The cap u ed
AE signals a e hen analysed, and hey may exhibi di e en deg ees o complexi y. These
di e en deg ees o complexi y a e pa icula ly impo an in he case o composi e ma e ials,
which ha e se e al ypes o ailu e mechanisms [
14
], such as ib e b eakage, ib e/ma ix
debonding, delamina ion, o ma ix c acking.
Nume ous s udies ha e been de o ed o he cha ac e isa ion and me hodology o
ailu e mechanisms. The mos basic app oach is pa ame e -based analysis [
15
] o he
de ec ed AE signals using ea u es such as ampli ude, ise ime, ene gy, du a ion, and
equency pa ame e s in he o m o equency cen oids o peak equencies [
16
]. This
app oach is less e ec i e o composi e s uc u es, mos ly because o he o e all complexi y
o he ailu e mechanisms, which may occu simul aneously. As a esul , he de ec ed AE
signals may be associa ed wi h mo e han one ailu e mechanism [
2
,
17
]. One o he majo
d awbacks o pa ame e -based analysis is he conside able pa ame e sensi i i y o he
geome y, senso ype, o dis ance be ween he senso and he AE sou ce ( he in luence o
signal a enua ion) [2].
A mo e ecen app oach is signal-based analysis [
16
], whose de elopmen and subse-
quen applica ion ha e been highly condi ioned by AE ins umen a ion. In his app oach,
indi idually de ec ed AE wa e o ms a e eco ded and subjec ed o equency analysis,
mos ly in he o m o he Fou ie ans o m [3], wa ele ans o m [18,19], wa ele packe
ans o m [20], o Hilbe –Huang ans o m [21].
In he las wo decades, supe ised/unsupe ised clus e ing and pa e n ecogni ion
algo i hms [
22
] based on mul i-pa ame e analysis [
2
] ha e been adop ed in a la ge num-
be o s udies because o hei abili y o u ilise ea u es om bo h pa ame e -based and
signal-based analyses. In he supe ised app oach, he inpu signals a e associa ed wi h a
gi en class o clus e , which, o example, ep esen s a pa icula ailu e mechanism using
he aining da ase . In he unsupe ised app oach, signals wi h simila cha ac e is ics a e
g ouped wi hou using he aining da ase . The mos commonly used clus e ing me hod
among he unsupe ised app oaches is k-means [
23
–
25
], ollowed by i s a ian s such
as k-means++ clus e ing [
26
], uzzy c-means clus e ing [
27
,
28
], o gene ic k-means clus-
e ing [
29
], which may be combined wi h o he app oaches such as sel -o ganising maps
(SOMs) [30] o p incipal componen analysis (PCA) o imp o e hei e ec i eness [31].
Al hough he e is a ela i ely ex ensi e knowledge base o a ious me hodologies
o iden i ying ailu e mechanisms in CFRP composi es, ce ain issues s ill a ise ega ding
hei o e all cha ac e isa ion. Fo example, i he ailu e mechanisms a e classi ied based
on he peak equency, some inconsis en esul s a e ob ained, al hough he same ype o
AE senso (WD senso ; Physical Acous ics Co po a ion) is used [
32
] (see Table 1). This
can be a ibu ed o a numbe o ac o s, such as he es ed ma e ial, specimen geome y,
a ia ions in he senso esponse, o p ocessing me hodology. The g ea es a ia ions in
e ms o peak equency occu in he case o ib e b eakage o ib e pull-ou , ollowed by
delamina ion and ib e/ma ix debonding.
Polyme s 2023,15, 47 3 o 25
Table 1.
Peak equencies o AE signals (in kHz) o di e en ailu e mechanisms in CFRP composi es
( he esul s a e ob ained wi h a WD senso ).
Re e ence Ma ix C acking Delamina ion Fib e/Ma ix Debonding Fib e B eakage Fib e Pull-Ou
Peak F equency [kHz]
[3] 80–120 120–170 170–200 ~250 -
[33] 90–180 - 240–310 >300 180–240
[34] <50 50–150 200–300 400–500 500–600
[30] <150 150–300 - >400 -
[35] 60–120 120–210 - 200–350 -
The aim o his s udy is o in es iga e he cha ac e is ics o he ailu e mechanisms o
CFRP composi e ma e ials. Two es se ies a e conduc ed in his s udy: he i s es se ies
in ol es only one ailu e mechanism, which is ully cha ac e ised, and he subsequen es
se ies in ol es se e al ypes o ailu e mechanisms. Two wideband senso s [
22
] a e used o
moni o he AE ac i i y. Unlike mul i esonan / esonan senso s, wideband senso s p o ide
a mo e accu a e cha ac e isa ion o AE signals in e ms o hei pa ame e s. The collec ed
AE signals a e hen analysed using he spec al clus e ing echnique, which u ilises he
k-means unsupe ised pa e n ecogni ion app oach. Subsequen ly, he clus e ing esul s
a e associa ed wi h he indi idual ypes o es s pe o med, ollowed by a discussion ela ed
o he ailu e mechanism cha ac e isa ion, which is addi ionally suppo ed by scanning
elec on mic oscopy (SEM) esul s.
2. Expe imen s
The expe imen s in ol ed wo es s: ensile and compac ension (CT) es s. The
ensile es was conduc ed on ca bon ib e bundle (CFB) (Ha el Composi es CZ Company
L d., Czech Republic) and c oss-ply specimens, whe eas he CT es was conduc ed on bulk
esin (BRCT), unidi ec ional (UCT), and c oss-ply (CPCT) specimens, which a e shown in
Figu e 1. No e ha each sample ype was ep esen ed by ou pieces. The BRCT specimens
we e made o LH 385 epoxy esin wi h cu ing agen H512 (Ha el Composi es CZ Company
L d., Czech Republic). The UCT and CPCT specimens we e made o high-pe o mance
ca bon/epoxy p ep eg CM-P eg T-C-230/600 CP004 39 wi h a nominal ply hickness o
0.25 mm. The BRCT and UCT/CPCT pla es we e cu ed based on he schedule p o ided
by he manu ac u e . De ailed in o ma ion abou he indi idual specimens is p esen ed in
Tables 2and 3.
Polyme s 2022, 13, x FOR PEER REVIEW 3 o 25
e ms o peak equency occu in he case o ib e b eakage o ib e pull-ou , ollowed by
delamina ion and ib e/ma ix debonding.
Table 1. Peak equencies o AE signals (in kHz) o di e en ailu e mechanisms in CFRP compo-
si es ( he esul s a e ob ained wi h a WD senso ).
Re e ence Ma ix C acking Delamina ion Fib e
/
Ma ix Debonding Fib e B eakage Fib e Pull-Ou
Peak F equency [kHz]
[3] 80–120 120–170 170–200 ~250 -
[33] 90–180 - 240–310 >300 180–240
[34] <50 50–150 200–300 400–500 500–600
[30] <150 150–300 - >400 -
[35] 60–120 120–210 - 200–350 -
The aim o his s udy is o in es iga e he cha ac e is ics o he ailu e mechanisms o
CFRP composi e ma e ials. Two es se ies a e conduc ed in his s udy: he i s es se ies
in ol es only one ailu e mechanism, which is ully cha ac e ised, and he subsequen es
se ies in ol es se e al ypes o ailu e mechanisms. Two wideband senso s [22] a e used
o moni o he AE ac i i y. Unlike mul i esonan / esonan senso s, wideband senso s p o-
ide a mo e accu a e cha ac e isa ion o AE signals in e ms o hei pa ame e s. The col-
lec ed AE signals a e hen analysed using he spec al clus e ing echnique, which u ilises
he k-means unsupe ised pa e n ecogni ion app oach. Subsequen ly, he clus e ing e-
sul s a e associa ed wi h he indi idual ypes o es s pe o med, ollowed by a discussion
ela ed o he ailu e mechanism cha ac e isa ion, which is addi ionally suppo ed by
scanning elec on mic oscopy (SEM) esul s.
2. Expe imen s
The expe imen s in ol ed wo es s: ensile and compac ension (CT) es s. The en-
sile es was conduc ed on ca bon ib e bundle (CFB) (Ha el Composi es CZ Company
L d., Czech Republic) and c oss-ply specimens, whe eas he CT es was conduc ed on
bulk esin (BRCT), unidi ec ional (UCT), and c oss-ply (CPCT) specimens, which a e
shown in Figu e 1. No e ha each sample ype was ep esen ed by ou pieces. The BRCT
specimens we e made o LH 385 epoxy esin wi h cu ing agen H512 (Ha el Composi es
CZ Company L d., Czech Republic). The UCT and CPCT specimens we e made o high-
pe o mance ca bon/epoxy p ep eg CM-P eg T-C-230/600 CP004 39 wi h a nominal ply
hickness o 0.25 mm. The BRCT and UCT/CPCT pla es we e cu ed based on he schedule
p o ided by he manu ac u e . De ailed in o ma ion abou he indi idual specimens is
p esen ed in Tables 2 and 3.
(b)
(a) (c)
Figu e 1. Geome y o he es ed specimens [mm]: (a) BRCT, CPCT, and UCT samples; (b) CPT
sample; (c) CPTS sample.
Figu e 1.
Geome y o he es ed specimens [mm]: (
a
) BRCT, CPCT, and UCT samples; (
b
) CPT
sample; (c) CPTS sample.
Polyme s 2023,15, 47 4 o 25
Table 2. Ma e ial p ope ies o he es specimens.
Sample Iden i ica ion Ma e ial Pa ame e s
Bulk esin (BRCT) LH 385 epoxy esin wi h cu ing agen H512
E = 3.1 GPa (ex), Rm= 58 MPa (ex)
Unidi ec ional (UCT)
C oss-ply (CPT, CPCT, CPTS)
ca bon/epoxy p ep eg CM-P eg T-C-230/600 CP004 39
nominal ply hickness = 0.25 mm, esin con en = 39%
E(0◦) = 135 GPa (mmd), Rm(0◦) = 1900 MPa (mmd)
Ca bon ib e bundle (CFB) E = 240 GPa (mmd), Rm= 4 GPa (mmd)
No es: E—Young’s modulus, Rm—Ul ima e ensile s eng h, ex—expe imen ally ob ained, mmd—manu ac u e ’s
ma e ial da a.
Table 3. Cha ac e isa ion o he es specimens.
Type o Tes Name/Numbe o Specimens S acking Sequence Thickness [mm]
Tensile es Ca bon ib e bundle (CFB)/4 - -
Tensile es C oss-ply (CPT)/4 (90◦, 0◦)4S 2
Tensile es C oss-ply (CPTS)/4 (45◦,−45◦)4S 2
Compac ension Unidi ec ional (UCT)/4 (90◦)41
Compac ension C oss-ply (CPCT)/4 (90◦, 0◦)4S 2
Compac ension Bulk esin (BRCT)/4 - 5
Bo h es s we e conduc ed on he Tes ome ic M500-50CT uni e sal es ing machine in
de o ma ion-con olled mode wi h an uppe g ip speed o 1 mm/min o he ensile es o
he CFB specimens and 0.5 mm/min o he o he es s. Addi ionally, a load cell o 100 N
was u ilised o he ensile es o he CFB specimens and he CT es o he UCT specimens,
whe eas a 50 kN load cell was u ilised o he CT es o he BRCT and CPCT specimens, as
well as he ensile es o he CPT and CPTS specimens.
2.1. Acous ic Emission Moni o ing
The AE ac i i y was eco ded using wo wideband AE senso s, DWC 454 and Fujice a
1045S, which we e di ec ly connec ed o a Vallen AEP5H 40 dB p eampli ie . The al eady
ampli ied signal is hen ans e ed o he AE uni , whe e basic signal p ocessing akes place,
ollowed by a ans e o he AE da a o he PC o he en i e da a p ocessing p ocedu e
(see Figu e 2). I mus be emphasised ha he maximum dis ance be ween he axis o he
senso and he a ea in which he AE ac i i y occu ed was no g ea e han 25 mm. Thanks
o his, i was possible o la gely neglec he dependence o he equency spec um on he
dis ance om he AE sou ce. Bo h u ilised AE senso s a e designed as wideband senso s
wi h a usable ange om 50 o 1200 kHz, while in he case o Fujice a, i is possible o coun
on an e en wide band due o inc eased sensi i i y a equencies abo e 1 MHz.
Polyme s 2022, 13, x FOR PEER REVIEW 4 o 25
Table 2. Ma e ial p ope ies o he es specimens.
Sample Iden i ica ion Ma e ial Pa ame e s
Bulk esin (BRCT) LH 385 epoxy esin wi h cu ing agen H512
E = 3.1 GPa (ex), Rm = 58 MPa (ex)
Unidi ec ional (UCT)
C oss-ply (CPT, CPCT, CPTS)
ca bon/epoxy p ep eg CM-P eg T-C-230/600 CP004 39
nominal ply hickness = 0.25 mm, esin con en = 39%
E(0°) = 135 GPa (mmd), Rm(0°) = 1900 MPa (mmd)
Ca bon ib e bundle (CFB) E = 240 GPa (mmd), Rm = 4 GPa (mmd)
No es: E—Young’s modulus, Rm—Ul ima e ensile s eng h, ex—expe imen ally ob ained, mmd—
manu ac u e ’s ma e ial da a.
Table 3. Cha ac e isa ion o he es specimens.
Type o Tes Name/Numbe o Specimens S acking Sequence Thickness [mm]
Tensile es Ca bon ib e bundle (CFB)/4 - -
Tensile es
Tensile es
C oss-ply (CPT)/4
C oss-ply (CPTS)/4
(90o, 0o)4S
(45o, −45o)4S
2
2
Compac ension Unidi ec ional (UCT)/4 (90o)4 1
Compac ension C oss-ply (CPCT)/4 (90o, 0o)4S 2
Compac ension Bulk esin (BRCT)/4 - 5
Bo h es s we e conduc ed on he Tes ome ic M500-50CT uni e sal es ing machine
in de o ma ion-con olled mode wi h an uppe g ip speed o 1 mm/min o he ensile es
o he CFB specimens and 0.5 mm/min o he o he es s. Addi ionally, a load cell o 100
N was u ilised o he ensile es o he CFB specimens and he CT es o he UCT speci-
mens, whe eas a 50 kN load cell was u ilised o he CT es o he BRCT and CPCT spec-
imens, as well as he ensile es o he CPT and CPTS specimens.
2.1. Acous ic Emission Moni o ing
The AE ac i i y was eco ded using wo wideband AE senso s, DWC 454 and Fu-
jice a 1045S, which we e di ec ly connec ed o a Vallen AEP5H 40 dB p eampli ie . The
al eady ampli ied signal is hen ans e ed o he AE uni , whe e basic signal p ocessing
akes place, ollowed by a ans e o he AE da a o he PC o he en i e da a p ocessing
p ocedu e (see Figu e 2). I mus be emphasised ha he maximum dis ance be ween he
axis o he senso and he a ea in which he AE ac i i y occu ed was no g ea e han 25
mm. Thanks o his, i was possible o la gely neglec he dependence o he equency
spec um on he dis ance om he AE sou ce. Bo h u ilised AE senso s a e designed as
wideband senso s wi h a usable ange om 50 o 1200 kHz, while in he case o Fujice a,
i is possible o coun on an e en wide band due o inc eased sensi i i y a equencies
abo e 1 MHz.
Figu e 2. Schema ic diag am o he expe imen al se up.
Figu e 2. Schema ic diag am o he expe imen al se up.
Polyme s 2023,15, 47 5 o 25
Figu e 3displays he esul s o ace- o- ace ela i e calib a ion o bo h senso s using
he Vallen Senso Tes e
®
u ili y (Manu ac u e : Vallen Sys eme GmbH, Ge many). The
cap u ed signals we e hen p ocessed by a Vallen AMSY-6 AE sys em uni comp ising an
ASIP-2A signal p ocesso ca d wi h sampling equency and equency il e se o 10 MHz
and 50–1100 kHz, espec i ely.
Polyme s 2022, 13, x FOR PEER REVIEW 5 o 25
Figu e 3. Face− o− ace calib a ion esul s o DWC B454 and Fujice a 1045S AE senso s using he
Vallen Senso Tes e ® u ili y.
Figu e 3 displays he esul s o ace- o- ace ela i e calib a ion o bo h senso s using
he Vallen Senso Tes e ® u ili y (Manu ac u e : Vallen Sys eme GmbH, Ge many). The
cap u ed signals we e hen p ocessed by a Vallen AMSY-6 AE sys em uni comp ising an
ASIP-2A signal p ocesso ca d wi h sampling equency and equency il e se o 10
MHz and 50–1100 kHz, espec i ely.
Mos comme cially a ailable AE de ices use he ixed- h eshold p inciple o hi de-
ec ion. The main disad an age o his p inciple is he endency o a i icially combine hi s
ha eme ge in apid succession. The eason o his beha iou is ha he signal unde he
gi en ci cums ances does no all below he h eshold; he e o e, he hi is no in e up ed.
As a esul , such hi s con ain a numbe o o he wise sepa a ed hi s, leading o signi ican
inaccu acies in he cha ac e isa ion o AE signals in some cases ( ha is, specimen iola ion
du ing he ensile es ). Al hough his beha iou can be educed o some ex en by using
sui able pa ame e s o he hi sepa a ion p ocess o by inc easing he sensing h eshold,
hese s eps do no lead o he comple e elimina ion o his phenomenon.
In 2017, Sha e i e al. p oposed a oo mean squa e (RMS) AE hi de ec ion algo i hm
[15] ha can o e come he p oblems associa ed wi h he h eshold-based echnique by
using he in o ma ion s o ed in he RMS en elope o he AE signal. Subsequen ly, he
join - alley peak- inding algo i hm is hen applied o de ec sepa a e e en s. The unc-
ionali y o he en i e algo i hm is no desc ibed in his pape , as i has been explained in
de ail by Sha e i e al. [15]. Howe e , we modi ied his algo i hm o u he imp o e i s
pe o mance.
The RMS-based AE hi de ec ion algo i hm uses h ee unc ionali ies o ensu e he
de ec ion o possible AE hi s, namely wo h esholds: one o he RMS and he o he o
he o iginal AE signal, and he a io be ween he p esen peak and alley wi hin he RMS
en elope, which is hen used o alida e whe he he de ec ed AE hi is alid. Sha e i e
al. [15] de i ed an exp ession o calcula ing a gi en peak/ alley a io depending on he
RMS en elope and he o iginal ime signal h esholds. Howe e , we ound ha his a io
depends on a numbe o ac o s, such as he size o he ime window o calcula ing he
RMS alue, he ype o senso , na u e o he AE signal, and he geome y being es ed.
Addi ionally, o a mo e e icien sepa a ion o he AE hi s, we implemen ed a wo-
le el h eshold ( ixed and loa ing) o he RMS signal wa e o m. In p ac ice, his h esh-
old segmen s po en ial hi s in he i s phase using a p ede e mined ixed RMS h eshold,
and hen, depending on he RMS alue o he backg ound noise be o e he s a o he hi ,
he maximum ampli ude o he RMS wa e o m is se as a new RMS h eshold, allowing
mo e e icien hi segmen a ion.
The p ocedu e abo e can be applied o wa e ansien s, which a e ob ained by he
con inuous eco ding o wa e o ms o a gi en measu ing channel o by using a ixed
ampli ude h eshold by he p ocedu e below:
Figu e 3.
Face
−
o
−
ace calib a ion esul s o DWC B454 and Fujice a 1045S AE senso s using he
Vallen Senso Tes e ®u ili y.
Mos comme cially a ailable AE de ices use he ixed- h eshold p inciple o hi
de ec ion. The main disad an age o his p inciple is he endency o a i icially combine
hi s ha eme ge in apid succession. The eason o his beha iou is ha he signal
unde he gi en ci cums ances does no all below he h eshold; he e o e, he hi is no
in e up ed. As a esul , such hi s con ain a numbe o o he wise sepa a ed hi s, leading o
signi ican inaccu acies in he cha ac e isa ion o AE signals in some cases ( ha is, specimen
iola ion du ing he ensile es ). Al hough his beha iou can be educed o some ex en
by using sui able pa ame e s o he hi sepa a ion p ocess o by inc easing he sensing
h eshold, hese s eps do no lead o he comple e elimina ion o his phenomenon.
In 2017, Sha e i e al. p oposed a oo mean squa e (RMS) AE hi de ec ion algo-
i hm [15] ha can o e come he p oblems associa ed wi h he h eshold-based echnique
by using he in o ma ion s o ed in he RMS en elope o he AE signal. Subsequen ly, he
join - alley peak- inding algo i hm is hen applied o de ec sepa a e e en s. The unc-
ionali y o he en i e algo i hm is no desc ibed in his pape , as i has been explained in
de ail by Sha e i e al. [
15
]. Howe e , we modi ied his algo i hm o u he imp o e i s
pe o mance.
The RMS-based AE hi de ec ion algo i hm uses h ee unc ionali ies o ensu e he
de ec ion o possible AE hi s, namely wo h esholds: one o he RMS and he o he
o he o iginal AE signal, and he a io be ween he p esen peak and alley wi hin
he RMS en elope, which is hen used o alida e whe he he de ec ed AE hi is alid.
Sh
a e i e al. [15]
de i ed an exp ession o calcula ing a gi en peak/ alley a io depending
on he RMS en elope and he o iginal ime signal h esholds. Howe e , we ound ha his
a io depends on a numbe o ac o s, such as he size o he ime window o calcula ing
he RMS alue, he ype o senso , na u e o he AE signal, and he geome y being es ed.
Addi ionally, o a mo e e icien sepa a ion o he AE hi s, we implemen ed a wo-
le el h eshold ( ixed and loa ing) o he RMS signal wa e o m. In p ac ice, his h eshold
segmen s po en ial hi s in he i s phase using a p ede e mined ixed RMS h eshold, and
hen, depending on he RMS alue o he backg ound noise be o e he s a o he hi , he
maximum ampli ude o he RMS wa e o m is se as a new RMS h eshold, allowing mo e
e icien hi segmen a ion.

Polyme s 2023,15, 47 6 o 25
The p ocedu e abo e can be applied o wa e ansien s, which a e ob ained by he
con inuous eco ding o wa e o ms o a gi en measu ing channel o by using a ixed
ampli ude h eshold by he p ocedu e below:
1.
The AE ac i i y was eco ded a a ixed h eshold alue o 28 dB
AE
, while he du a ion
disc imina ion ime (DDT) and ea m ime (RAT) we e se o 100 µs;
No e: Acco ding o Vallen documen a ion, DDT is he pe iod in which no h eshold c ossing
mus occu o de e mine he end o a hi , whe eas RAT is he pe iod a e which he channel
is eady o gene a e a new hi da ase ;
2
The eco ded wa e ansien s we e subsequen ly subjec ed o an addi ional hi seg-
men a ion p ocess wi h he ollowing pa ame e s: wid h o he RMS window = 10
µ
s,
ixed alue o he RMS h eshold = 10 dB
AE
. The loa ing alue o he RMS h eshold
is gi en by:
ms loa ing = msmean +0.12[max( ms)− msmean](1)
whe e ms
mean
is he mean alue o he RMS be o e he s a o he AE hi (mean
alue om he i s 10 samples be o e he hi s a ), max( ms) is he maximum alue
o he RMS eco ding o he gi en AE hi , and peak/ alley a io = 0.25 (maximum)
o alid pai s exceeding he p incipal h eshold o 28 dBAE.
The AE signal pa ame e s [
36
] o ea u es ac as inpu a iables o he subsequen
clus e ing p ocess. These ea u es can be u he di ided in o di ec ly ex ac ed ea u es
and hose ha a e calcula ed du ing pos -p ocessing. Table 4lis s all he ea u es used in
his s udy and hei desc ip ions.
Table 4. Cha ac e isa ion o AE ea u es used in his s udy.
Fea u e Desc ip ion
Ampli ude (E)
La ges ol age peak o he gi en AE hi (in dB
AE
) (dB el. o 1
µ
V be o e
he inpu o he p eampli ie ).
Rise ime (E)
Time in e al (in
µ
s) be ween he i s h eshold c ossing and he eached
maximum ampli ude (in µs).
Du a ion (E)
Time in e al (in
µ
s) be ween he i s and las h eshold c ossings (in
µ
s).
Ene gy (E) In eg al o he squa ed AE signal o e ime (E=RDU( )2d ,D e e s o
he AE hi du a ion) (in aJ).
p—Peak equency (E) F equency co esponding o he maximum magni ude in he equency
spec um (in kHz).
c—F equency cen oid (E) Cen e o mass o he equency spec um (in kHz).
pw—Weigh ed peak equency (C) Squa e oo o he p oduc be ween he peak equency and equency
cen oid [37], namely pw =p p c(in kHz).
RA—RA alue (C)
p I ÷p VI—Pa ial powe (C)
Rise ime/peak ampli ude a io (in µs/dBAE) [38]. (RA is some imes
called he ise angle).
Non-dimensionalised a io be ween he powe in he equency in e al
I÷VI and he powe o he en i e equency spec um, ha is, wi hin he
〈50, 1100〉[kHz] equency in e al.
No e: I=h50,225), II =h225,400), III =h400,575), IV =h575, 750),
V=h750,925),
VI =h925,1100) [kHz].
No e: E—ex ac ed ea u e, C—calcula ed ea u e.
2.2. Failu e Mechanisms
2.2.1. Tensile Tes o Fib e Bundles and Compac Tension Tes o Bulk Resin
The AE signals o igina ing om ib e b eakage we e in es iga ed by conduc ing
ensile es s on he ib e bundles, which we e adequa ely adhe ed o he clamps using
a cyanoac yla e adhesi e. To elimina e he possible occu ence o ib e pull-ou in he
clamps, he pe iphe al pa s o he ib e bundle we e equipped wi h gua d senso s (DAKEL
MIDI), as shown in Figu e 4a. Be o e he ac ual e alua ion o he AE da a om he
ensile es esul s o he ca bon ib e bundles, i was necessa y o pe o m da a il e ing
Polyme s 2023,15, 47 7 o 25
because he ailu e o ca bon ib es, which a e no sepa a ed by a ma ix, has comple ely
di e en cha ac e is ics compa ed o con en ional composi es. Mo eo e , a ela i ely la ge
pe cen age o he AE ac i i y comes om he mu ual ic ion o he ib es, which has an
inc easing endency, especially when a la ge numbe o ib es ail a he same ime. The
b oken ib es hen begin o slide o e hose ha s ill ca y a ce ain load, wi h he na u e
o he de ec ed signal being e y simila o ha co esponding o he ib e ailu e. Hence,
i was necessa y o ind a sui able me hodology o sepa a ing he use ul signal (in ou
case, ib e ailu e) om he seconda y AE. Fo he gi en pu poses, an e alua ion algo i hm
in which he il e ing o AE hi s coupled wi h ib e b eakage is based on he de ec ion
o a dec ease in o ce o e a ime cou se was de eloped wi h MATLAB so wa e. The
in e al o in e es is de ined by he de ec ion o a nega i e g adien and he momen o
e u ning he o ce o i s o iginal alues, which includes he ex ension o bo h bo de s by
he o ce sampling pe iod (100 ms). Only hi s alling wi hin hese in e als a e subsequen ly
analysed. The only sho coming o he gi en app oach is ha i is no possible o il e
ou he seconda y AE emission i i occu s simul aneously wi h a ib e b eak, which is
associa ed wi h in e al epo ing o ce d ops.
1
(a) (b)
Figu e 4.
In si u pho og aphs o he (
a
) ensile es o he ca bon ib e bundle, (
b
) BRCT specimen,
including he p esen uns able mac oc ack.
Figu e 5shows he ela ionship be ween he peak equency and o ce e sus displace-
men o he ep esen a i e specimen o he ca bon ib e bundle a e he il e ing p ocess
o he esul s om bo h AE senso s ( he blue highligh ed segmen s indica e he de ec ed
o ce dips). The esul s om he DWC 454 AE senso showed ha , pa icula ly in he
ini ial phase, AE signals wi h a peak equency abo e 500 kHz exis ed (see Figu e 5a). A e
eaching he maximum o ce, he ailu e o he ib es began o appea on a sligh ly wide
peak equency scale. This was mainly due o he supe posi ion o signals o igina ing om
bo h he ib e b eakage and he abo e-men ioned ic ion be ween he ib es in he bundle,
which occu s along he en i e leng h o he bundle.
A simila scena io was obse ed in he esul s ob ained om he Fujice a 1045S AE
senso , whe e he uppe limi o he peak equency in e al shi ed o 1000 kHz, whe eas
he lowe in e al s a ed a app oxima ely 300 kHz (see Figu e 5b). Fo he subsequen
clus e ing p ocess, we conside ed only he AE hi s om bo h senso s wi h a peak equency
highe han 400 kHz, which seemed like a easonable comp omise o bo h da ase s.
Rega ding he CT es s o he bulk esin specimens, he only mechanisms obse ed
we e ma ix mic oc acking and mac oc acking, which ha e he cha ac e is ic o b i le
ac u e. Because he ini ia ion p ocess o mic oc acks is p ac ically unde ec able, he
only mechanism co e ed by he AE ac i i y is he uns able p opaga ion o mac oc acks
associa ed wi h an o e all collapse o he s uc u al in eg i y (see Figu e 4b).
Polyme s 2023,15, 47 8 o 25
Polyme s 2022, 13, x FOR PEER REVIEW 8 o 25
o igina ing om bo h he ib e b eakage and he abo e-men ioned ic ion be ween he
ib es in he bundle, which occu s along he en i e leng h o he bundle.
A simila scena io was obse ed in he esul s ob ained om he Fujice a 1045S AE
senso , whe e he uppe limi o he peak equency in e al shi ed o 1000 kHz, whe eas
he lowe in e al s a ed a app oxima ely 300 kHz (see Figu e 5b). Fo he subsequen
clus e ing p ocess, we conside ed only he AE hi s om bo h senso s wi h a peak e-
quency highe han 400 kHz, which seemed like a easonable comp omise o bo h da-
ase s.
(a) (b)
Figu e 5. Peak equency and o ce e sus displacemen esul s o he ib e bundle ensile es . (a)
DWC 454 AE senso ; (b) Fujice a 1045S senso .
Rega ding he CT es s o he bulk esin specimens, he only mechanisms obse ed
we e ma ix mic oc acking and mac oc acking, which ha e he cha ac e is ic o b i le
ac u e. Because he ini ia ion p ocess o mic oc acks is p ac ically unde ec able, he only
mechanism co e ed by he AE ac i i y is he uns able p opaga ion o mac oc acks associ-
a ed wi h an o e all collapse o he s uc u al in eg i y (see Figu e 4b).
2.2.2. Compac Tension Tes o Unidi ec ional Specimens
The CT es s o he UCT specimens e ealed he p esence o se e al ailu e mecha-
nisms, namely ma ix mic oc acking, ib e/ma ix debonding, and ib e ailu e, as shown
in Figu e 6b. A e he ini ial phase o mic oc ack ini ia ion and coalescence, he p opaga-
ion o he mac oc ack began, which was cha ac e ised by a sudden d op in o ce. Mean-
while, he ib e-b idging phenomenon [39] (Figu e 6a) was obse ed in he CT es s o he
UCT specimens, indica ing ha his phenomenon signi ican ly con ibu es o ib e/ma ix
debonding and ib e ailu e, which ag ees wi h he esul s o Gu kin e al. [34].
Figu e 5.
Peak equency and o ce e sus displacemen esul s o he ib e bundle ensile es .
(a) DWC 454 AE senso ; (b) Fujice a 1045S senso .
2.2.2. Compac Tension Tes o Unidi ec ional Specimens
The CT es s o he UCT specimens e ealed he p esence o se e al ailu e mecha-
nisms, namely ma ix mic oc acking, ib e/ma ix debonding, and ib e ailu e, as shown in
Figu e 6b. A e he ini ial phase o mic oc ack ini ia ion and coalescence, he p opaga ion
o he mac oc ack began, which was cha ac e ised by a sudden d op in o ce. Mean-
while, he ib e-b idging phenomenon [39] (Figu e 6a) was obse ed in he CT es s o he
UCT specimens, indica ing ha his phenomenon signi ican ly con ibu es o ib e/ma ix
debonding and ib e ailu e, which ag ees wi h he esul s o Gu kin e al. [34].
Polyme s 2022, 13, x FOR PEER REVIEW 9 o 25
(a) (b)
Figu e 6. In si u pho og aph o he CT es o UCT specimens. (a) De ail o he ib e-b idging p ocess;
(b) SEM image o he ac u e su ace o a UCT ep esen a i e specimen.
2.2.3. Compac Tension and Tensile Tes o 0–90° C oss-Ply Specimens
The 0–90° c oss-ply specimens we e subjec ed o ensile and CT es s, and hei load-
ing cha ac e is ics we e compa ed. The CT es was comple ed o e a longe pe iod, du -
ing which he inal s age o he s uc u e iola ion was moni o ed. Meanwhile, he ensile
es was cha ac e ised by an ab up collapse, du ing which a ela i ely la ge amoun o
in o ma ion was los owing o he sudden elease o ene gy and he supe imposi ion o
he indi idual AE signals.
The SEM analysis o ep esen a i e samples om bo h ypes o es s e ealed he
occu ence o delamina ion, ib e/ma ix debonding, ma ix c acking, ib e ailu e, and
ib e pull-ou . The CPCT samples showed he p esence o egions wi h ensile and com-
p essi e loading cha ac e is ics (Figu e 7a). Comp essi e loading was mainly esponsible
o he delamina ion p ocess a he back pa o he ep esen a i e sample (Figu e 7b,c),
including ib e/ma ix debonding and ma ix mic oc acking (Figu e 7e), ollowed by ib e
ailu e (Figu e 7 ). In con as , he egion wi h ensile loading cha ac e is ics had no icea-
ble aces o ib e pull-ou and ib e b eakage (Figu e 7d), ollowed by egions showing
ib e/ma ix debonding and delamina ion. Simila obse a ions we e ound o he ensile
es o he CPT samples, and he same cha ac e is ics we e obse ed o he abo e-men-
ioned ailu e mechanisms.
(a) (b)
Figu e 6.
In si u pho og aph o he CT es o UCT specimens. (
a
) De ail o he ib e-b idging p ocess;
(b) SEM image o he ac u e su ace o a UCT ep esen a i e specimen.
2.2.3. Compac Tension and Tensile Tes o 0–90◦C oss-Ply Specimens
The 0–90
◦
c oss-ply specimens we e subjec ed o ensile and CT es s, and hei loading
cha ac e is ics we e compa ed. The CT es was comple ed o e a longe pe iod, du ing
Polyme s 2023,15, 47 9 o 25
which he inal s age o he s uc u e iola ion was moni o ed. Meanwhile, he ensile
es was cha ac e ised by an ab up collapse, du ing which a ela i ely la ge amoun o
in o ma ion was los owing o he sudden elease o ene gy and he supe imposi ion o he
indi idual AE signals.
The SEM analysis o ep esen a i e samples om bo h ypes o es s e ealed he
occu ence o delamina ion, ib e/ma ix debonding, ma ix c acking, ib e ailu e, and ib e
pull-ou . The CPCT samples showed he p esence o egions wi h ensile and comp essi e
loading cha ac e is ics (Figu e 7a). Comp essi e loading was mainly esponsible o he
delamina ion p ocess a he back pa o he ep esen a i e sample (Figu e 7b,c), including
ib e/ma ix debonding and ma ix mic oc acking (Figu e 7e), ollowed by ib e ailu e
(Figu e 7 ). In con as , he egion wi h ensile loading cha ac e is ics had no iceable aces
o ib e pull-ou and ib e b eakage (Figu e 7d), ollowed by egions showing ib e/ma ix
debonding and delamina ion. Simila obse a ions we e ound o he ensile es o
he CPT samples, and he same cha ac e is ics we e obse ed o he abo e-men ioned
ailu e mechanisms.
Polyme s 2022, 13, x FOR PEER REVIEW 9 o 25
(a) (b)
Figu e 6. In si u pho og aph o he CT es o UCT specimens. (a) De ail o he ib e-b idging p ocess;
(b) SEM image o he ac u e su ace o a UCT ep esen a i e specimen.
2.2.3. Compac Tension and Tensile Tes o 0–90° C oss-Ply Specimens
The 0–90° c oss-ply specimens we e subjec ed o ensile and CT es s, and hei load-
ing cha ac e is ics we e compa ed. The CT es was comple ed o e a longe pe iod, du -
ing which he inal s age o he s uc u e iola ion was moni o ed. Meanwhile, he ensile
es was cha ac e ised by an ab up collapse, du ing which a ela i ely la ge amoun o
in o ma ion was los owing o he sudden elease o ene gy and he supe imposi ion o
he indi idual AE signals.
The SEM analysis o ep esen a i e samples om bo h ypes o es s e ealed he
occu ence o delamina ion, ib e/ma ix debonding, ma ix c acking, ib e ailu e, and
ib e pull-ou . The CPCT samples showed he p esence o egions wi h ensile and com-
p essi e loading cha ac e is ics (Figu e 7a). Comp essi e loading was mainly esponsible
o he delamina ion p ocess a he back pa o he ep esen a i e sample (Figu e 7b,c),
including ib e/ma ix debonding and ma ix mic oc acking (Figu e 7e), ollowed by ib e
ailu e (Figu e 7 ). In con as , he egion wi h ensile loading cha ac e is ics had no icea-
ble aces o ib e pull-ou and ib e b eakage (Figu e 7d), ollowed by egions showing
ib e/ma ix debonding and delamina ion. Simila obse a ions we e ound o he ensile
es o he CPT samples, and he same cha ac e is ics we e obse ed o he abo e-men-
ioned ailu e mechanisms.
(a) (b)
Polyme s 2022, 13, x FOR PEER REVIEW 10 o 25
(c) (d)
(e) ( )
Figu e 7. SEM images o a CPCT ep esen a i e specimen: (a) gene al iew, (b) s uc u e collapse
due o comp essi e load, (c) ib e pull-ou , (d) delamina ion and ib e/ma ix debonding, (e) i-
b e/ma ix debonding and ma ix mic oc acking, ( ) ib e ailu e.
2.2.4. Tensile Tes o ± 45° C oss-ply Specimens
The SEM analysis o he CPTS specimens e ealed he p esence o delamina ion (Fig-
u e 8a), ma ix mic oc acking (Figu e 8b), and ib e/ma ix debonding.
(a) (b)
Figu e 8. SEM images o a CPTS ep esen a i e specimen: (a) delamina ed plies, (b) ma ix mi-
c oc acking and ib e/ma ix debonding.
Figu e 7. Con .
Polyme s 2023,15, 47 16 o 25
has o be no ed ha clus e one is ep esen ed in he g ea es abundance—see Figu e 17a,b.
Clus e wo hen ep esen s ib e/ma ix debonding, which was also con i med by he SEM
analysis o his ype o sample (Figu e 6b).
Polyme s 2022, 13, x FOR PEER REVIEW 16 o 25
ha e lowe ene gy alues (see Figu es 15b,c and 16b,c). Based on he cumula i e AE hi s
o indi idual clus e s (Figu e 17a,b), i is qui e e iden ha he clus e ou ac i i y, espe-
cially i s low- equency pa , occu s immedia ely a e he onse o mac oc ack p opaga-
ion. Ano he clea ly assignable sou ce is ib e ailu e, ep esen ed by clus e one (in bo h
da ase s). I has o be no ed ha clus e one is ep esen ed in he g ea es abundance—see
Figu e 17a,b. Clus e wo hen ep esen s ib e/ma ix debonding, which was also con-
i med by he SEM analysis o his ype o sample (Figu e 6b).
(a) (b) (c)
Figu e 15. Clus e ing esul s o a UCT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
(a) (b) (c)
Figu e 16. Clus e ing esul s o a UCT ep esen a i e sample using he spec al clus e ing algo-
i hm (DWC da ase ).
(a) (b)
Figu e 17. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —
UCT samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Figu e 15.
Clus e ing esul s o a UCT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
Polyme s 2022, 13, x FOR PEER REVIEW 16 o 25
ha e lowe ene gy alues (see Figu es 15b,c and 16b,c). Based on he cumula i e AE hi s
o indi idual clus e s (Figu e 17a,b), i is qui e e iden ha he clus e ou ac i i y, espe-
cially i s low- equency pa , occu s immedia ely a e he onse o mac oc ack p opaga-
ion. Ano he clea ly assignable sou ce is ib e ailu e, ep esen ed by clus e one (in bo h
da ase s). I has o be no ed ha clus e one is ep esen ed in he g ea es abundance—see
Figu e 17a,b. Clus e wo hen ep esen s ib e/ma ix debonding, which was also con-
i med by he SEM analysis o his ype o sample (Figu e 6b).
(a) (b) (c)
Figu e 15. Clus e ing esul s o a UCT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
(a) (b) (c)
Figu e 16. Clus e ing esul s o a UCT ep esen a i e sample using he spec al clus e ing algo-
i hm (DWC da ase ).
(a) (b)
Figu e 17. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —
UCT samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Figu e 16.
Clus e ing esul s o a UCT ep esen a i e sample using he spec al clus e ing algo i hm
(DWC da ase ).
Polyme s 2022, 13, x FOR PEER REVIEW 16 o 25
ha e lowe ene gy alues (see Figu es 15b,c and 16b,c). Based on he cumula i e AE hi s
o indi idual clus e s (Figu e 17a,b), i is qui e e iden ha he clus e ou ac i i y, espe-
cially i s low- equency pa , occu s immedia ely a e he onse o mac oc ack p opaga-
ion. Ano he clea ly assignable sou ce is ib e ailu e, ep esen ed by clus e one (in bo h
da ase s). I has o be no ed ha clus e one is ep esen ed in he g ea es abundance—see
Figu e 17a,b. Clus e wo hen ep esen s ib e/ma ix debonding, which was also con-
i med by he SEM analysis o his ype o sample (Figu e 6b).
(a) (b) (c)
Figu e 15. Clus e ing esul s o a UCT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
(a) (b) (c)
Figu e 16. Clus e ing esul s o a UCT ep esen a i e sample using he spec al clus e ing algo-
i hm (DWC da ase ).
(a) (b)
Figu e 17. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —
UCT samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Figu e 17.
Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —UCT
samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .

Polyme s 2023,15, 47 17 o 25
No e ha he Fujice a 1045S da ase shows a pa ial in e mingling o clus e s one and
wo, i.e., ib e ailu e and ib e/ma ix debonding, which is mos likely due o he la e
equency spec um compa ed o he DWC 454 AE senso in he gi en ange—see Figu e 3.
The esul s o he compac ension es o CPCT samples as well as he ensile es o
CPT and CPTS samples (Figu es 18–26) poin o he p esence o i e ailu e mechanisms,
namely ib e b eakage (clus e one), ib e/ma ix debonding (clus e wo), ib e pull-ou
(clus e h ee), ma ix c acking and delamina ion (clus e ou ), which a e di ec ly ela ed o
he es ed s acking sequence con igu a ion. The CT es o he CPCT specimens e ealed a
somewha di e en na u e o he ailu e e olu ion p ocess compa ed o he UCT specimens.
While he AE ac i i y o he UCT specimens was igge ed by he onse o mac oc ack
p opaga ion, he CPCT specimens exhibi ed conside able AE ac i i y be o e he collapse o
he s uc u e. In addi ion, bo h da ase s showed he p esence o ib e/ma ix debonding in
he ea ly phase, ollowed by ma ix c acking, and ib e pull-ou , especially in he case o
he Fujice a 1045S da ase (see Figu es 18,19 and 24).
Polyme s 2022, 13, x FOR PEER REVIEW 17 o 25
No e ha he Fujice a 1045S da ase shows a pa ial in e mingling o clus e s one and wo,
i.e., ib e ailu e and ib e/ma ix debonding, which is mos likely due o he la e e-
quency spec um compa ed o he DWC 454 AE senso in he gi en ange—see Figu e 3.
The esul s o he compac ension es o CPCT samples as well as he ensile es o
CPT and CPTS samples (Figu es 18÷26) poin o he p esence o i e ailu e mechanisms,
namely ib e b eakage (clus e one), ib e/ma ix debonding (clus e wo), ib e pull-ou
(clus e h ee), ma ix c acking and delamina ion (clus e ou ), which a e di ec ly ela ed
o he es ed s acking sequence con igu a ion. The CT es o he CPCT specimens e ealed
a somewha di e en na u e o he ailu e e olu ion p ocess compa ed o he UCT speci-
mens. While he AE ac i i y o he UCT specimens was igge ed by he onse o mac-
oc ack p opaga ion, he CPCT specimens exhibi ed conside able AE ac i i y be o e he
collapse o he s uc u e. In addi ion, bo h da ase s showed he p esence o ib e/ma ix
debonding in he ea ly phase, ollowed by ma ix c acking, and ib e pull-ou , especially
in he case o he Fujice a 1045S da ase (see Figu es 18, 19, and 24).
Fib e b eakage (clus e one) occu ed when a mo e signi ican s uc u al collapse was
p esen (see Figu es 18b and 19b). A ela i ely simila desc ip ion o he esul s can be
adop ed in he case o he ensile es o he CPT samples (Figu es 20, 21, and 25), whe e
he ib e/ma ix debonding mechanism and ma ix c acking we e e en mo e p edominan
han in he case o CPCT samples. The esul s o he CPTS samples o a ce ain ex en
eplica e hose o he CPT samples, bu wi h he di e ence ha he p edominan de ec ed
mechanism is ma ix c acking (see Figu es 22, 23, and 26).
(a) (b) (c)
Figu e 18. Clus e ing esul s o a CPCT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
(a) (b) (c)
Figu e 18.
Clus e ing esul s o a CPCT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
Polyme s 2022, 13, x FOR PEER REVIEW 17 o 25
No e ha he Fujice a 1045S da ase shows a pa ial in e mingling o clus e s one and wo,
i.e., ib e ailu e and ib e/ma ix debonding, which is mos likely due o he la e e-
quency spec um compa ed o he DWC 454 AE senso in he gi en ange—see Figu e 3.
The esul s o he compac ension es o CPCT samples as well as he ensile es o
CPT and CPTS samples (Figu es 18÷26) poin o he p esence o i e ailu e mechanisms,
namely ib e b eakage (clus e one), ib e/ma ix debonding (clus e wo), ib e pull-ou
(clus e h ee), ma ix c acking and delamina ion (clus e ou ), which a e di ec ly ela ed
o he es ed s acking sequence con igu a ion. The CT es o he CPCT specimens e ealed
a somewha di e en na u e o he ailu e e olu ion p ocess compa ed o he UCT speci-
mens. While he AE ac i i y o he UCT specimens was igge ed by he onse o mac-
oc ack p opaga ion, he CPCT specimens exhibi ed conside able AE ac i i y be o e he
collapse o he s uc u e. In addi ion, bo h da ase s showed he p esence o ib e/ma ix
debonding in he ea ly phase, ollowed by ma ix c acking, and ib e pull-ou , especially
in he case o he Fujice a 1045S da ase (see Figu es 18, 19, and 24).
Fib e b eakage (clus e one) occu ed when a mo e signi ican s uc u al collapse was
p esen (see Figu es 18b and 19b). A ela i ely simila desc ip ion o he esul s can be
adop ed in he case o he ensile es o he CPT samples (Figu es 20, 21, and 25), whe e
he ib e/ma ix debonding mechanism and ma ix c acking we e e en mo e p edominan
han in he case o CPCT samples. The esul s o he CPTS samples o a ce ain ex en
eplica e hose o he CPT samples, bu wi h he di e ence ha he p edominan de ec ed
mechanism is ma ix c acking (see Figu es 22, 23, and 26).
(a) (b) (c)
Figu e 18. Clus e ing esul s o a CPCT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
(a) (b) (c)
Figu e 19.
Clus e ing esul s o a CPCT ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
Polyme s 2023,15, 47 18 o 25
Polyme s 2022, 13, x FOR PEER REVIEW 18 o 25
Figu e 19. Clus e ing esul s o a CPCT ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b) (c)
Figu e 20. Clus e ing esul s o a CPT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
(a) (b) (c)
Figu e 21. Clus e ing esul s o a CPT ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b) (c)
Figu e 22. Clus e ing esul s o a CPTS ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
Figu e 20.
Clus e ing esul s o a CPT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
Polyme s 2022, 13, x FOR PEER REVIEW 18 o 25
Figu e 19. Clus e ing esul s o a CPCT ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b) (c)
Figu e 20. Clus e ing esul s o a CPT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
(a) (b) (c)
Figu e 21. Clus e ing esul s o a CPT ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b) (c)
Figu e 22. Clus e ing esul s o a CPTS ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
Figu e 21.
Clus e ing esul s o a CPT ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
Polyme s 2022, 13, x FOR PEER REVIEW 18 o 25
Figu e 19. Clus e ing esul s o a CPCT ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b) (c)
Figu e 20. Clus e ing esul s o a CPT ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
(a) (b) (c)
Figu e 21. Clus e ing esul s o a CPT ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b) (c)
Figu e 22. Clus e ing esul s o a CPTS ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
Figu e 22.
Clus e ing esul s o a CPTS ep esen a i e sample using a spec al clus e ing algo i hm
and seconda y il e ing using a pa ial powe a iable (Fujice a 1045S da ase ).
Polyme s 2023,15, 47 19 o 25
Polyme s 2022, 13, x FOR PEER REVIEW 19 o 25
(a) (b) (c)
Figu e 23. Clus e ing esul s o a CPTS ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b)
Figu e 24. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —
CPCT samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
(a) (b)
Figu e 25. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —CPT
samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Figu e 23.
Clus e ing esul s o a CPTS ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
Polyme s 2022, 13, x FOR PEER REVIEW 19 o 25
(a) (b) (c)
Figu e 23. Clus e ing esul s o a CPTS ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b)
Figu e 24. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —
CPCT samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
(a) (b)
Figu e 25. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —CPT
samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Figu e 24.
Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —CPCT
samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Polyme s 2022, 13, x FOR PEER REVIEW 19 o 25
(a) (b) (c)
Figu e 23. Clus e ing esul s o a CPTS ep esen a i e sample using a spec al clus e ing algo i hm
(DWC 454 da ase ).
(a) (b)
Figu e 24. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —
CPCT samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
(a) (b)
Figu e 25. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —CPT
samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Figu e 25.
Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —CPT
samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Polyme s 2023,15, 47 20 o 25
Polyme s 2022, 13, x FOR PEER REVIEW 20 o 25
(a) (b)
Figu e 26. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —
CPTS samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
b. Cha ac e isa ion o ailu e mechanisms
4.1. Delamina ion and Ma ix C acking
To he bes o ou knowledge, delamina ion is a syne gy o AE signals, which a e
cha ac e ised by a wide spec um o ene gy (up o 107 aJ) and ampli ude alues (up o 94
dBAE and occasionally mo e). Rega ding he peak equency, se e al au ho s ha e cha ac-
e ised his phenomenon as ha ing a low peak equency, o en below 100 kHz
[3,30,34,35]. This conclusion is pa ially con i med by he esul s o he BRCT samples
om bo h da ase s. Howe e , clus e ing esul s om bo h da ase s, which a e ela ed o
UCT, CPCT, CPT, and CPTS samples, clea ly demons a e ha he ma ix c acking/de-
lamina ion phenomenon spans om 50 o app oxima ely 200 kHz (see Figu e 27).
Figu e 27. Rep esen a i e AE signal o ma ix c acking (DWC 454 da ase ).
4.2. Fib e/Ma ix Debonding
The epo ed ene gy and ampli ude alues o he ib e/ma ix debonding mecha-
nism a e somewha lowe han hose o delamina ion/ma ix c acking, ha is, <106 aJ and
<85 dBAE, espec i ely, wi hin he maximum limi ing alues, while mos AE hi s belonging
o ib e/ma ix debonding ha e ampli ude and ene gy alues below 70 dBAE and 105 aJ,
espec i ely. The published esul s om bo h da ase s also con i m ha bo h he ampli-
ude and ene gy alues a e closely ela ed o he loading cha ac e is ics and s acking se-
quence o he plies. A simila conclusion applies in he case o he peak equency, in
which he equency in e al is ela ed o he loading cha ac e is ics and s acking se-
quence o he plies. The 200-400 kHz peak equency ange we ound was he wides com-
pa ed o hose o o he s udies (see Figu e 28) [3,33,34].
Figu e 26.
Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —CPTS
samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
Fib e b eakage (clus e one) occu ed when a mo e signi ican s uc u al collapse was
p esen (see Figu es 18b and 19b). A ela i ely simila desc ip ion o he esul s can be
adop ed in he case o he ensile es o he CPT samples (Figu es 20,21 and 25), whe e
he ib e/ma ix debonding mechanism and ma ix c acking we e e en mo e p edominan
han in he case o CPCT samples. The esul s o he CPTS samples o a ce ain ex en
eplica e hose o he CPT samples, bu wi h he di e ence ha he p edominan de ec ed
mechanism is ma ix c acking (see Figu es 22,23 and 26).
4.1. Delamina ion and Ma ix C acking
To he bes o ou knowledge, delamina ion is a syne gy o AE signals, which a e cha -
ac e ised by a wide spec um o ene gy (up o 10
7
aJ) and ampli ude alues (up o 94 dB
AE
and occasionally mo e). Rega ding he peak equency, se e al au ho s ha e cha ac e ised
his phenomenon as ha ing a low peak equency, o en below 10
0 kHz [3,30,34,35]
. This
conclusion is pa ially con i med by he esul s o he BRCT samples om bo h da ase s.
Howe e , clus e ing esul s om bo h da ase s, which a e ela ed o UCT, CPCT, CPT, and
CPTS samples, clea ly demons a e ha he ma ix c acking/delamina ion phenomenon
spans om 50 o app oxima ely 200 kHz (see Figu e 27).
Polyme s 2022, 13, x FOR PEER REVIEW 20 o 25
(a) (b)
Figu e 26. Cumula i e sum o hi s o indi idual clus e s, including o ce e sus displacemen —
CPTS samples: (a) Fujice a 1045S da ase ; (b) DWC 454 da ase .
b. Cha ac e isa ion o ailu e mechanisms
4.1. Delamina ion and Ma ix C acking
To he bes o ou knowledge, delamina ion is a syne gy o AE signals, which a e
cha ac e ised by a wide spec um o ene gy (up o 107 aJ) and ampli ude alues (up o 94
dBAE and occasionally mo e). Rega ding he peak equency, se e al au ho s ha e cha ac-
e ised his phenomenon as ha ing a low peak equency, o en below 100 kHz
[3,30,34,35]. This conclusion is pa ially con i med by he esul s o he BRCT samples
om bo h da ase s. Howe e , clus e ing esul s om bo h da ase s, which a e ela ed o
UCT, CPCT, CPT, and CPTS samples, clea ly demons a e ha he ma ix c acking/de-
lamina ion phenomenon spans om 50 o app oxima ely 200 kHz (see Figu e 27).
Figu e 27. Rep esen a i e AE signal o ma ix c acking (DWC 454 da ase ).
4.2. Fib e/Ma ix Debonding
The epo ed ene gy and ampli ude alues o he ib e/ma ix debonding mecha-
nism a e somewha lowe han hose o delamina ion/ma ix c acking, ha is, <106 aJ and
<85 dBAE, espec i ely, wi hin he maximum limi ing alues, while mos AE hi s belonging
o ib e/ma ix debonding ha e ampli ude and ene gy alues below 70 dBAE and 105 aJ,
espec i ely. The published esul s om bo h da ase s also con i m ha bo h he ampli-
ude and ene gy alues a e closely ela ed o he loading cha ac e is ics and s acking se-
quence o he plies. A simila conclusion applies in he case o he peak equency, in
which he equency in e al is ela ed o he loading cha ac e is ics and s acking se-
quence o he plies. The 200-400 kHz peak equency ange we ound was he wides com-
pa ed o hose o o he s udies (see Figu e 28) [3,33,34].
Figu e 27. Rep esen a i e AE signal o ma ix c acking (DWC 454 da ase ).
4.2. Fib e/Ma ix Debonding
The epo ed ene gy and ampli ude alues o he ib e/ma ix debonding mechanism
a e somewha lowe han hose o delamina ion/ma ix c acking, ha is, <10
6
aJ and <85
Polyme s 2023,15, 47 21 o 25
dB
AE
, espec i ely, wi hin he maximum limi ing alues, while mos AE hi s belonging
o ib e/ma ix debonding ha e ampli ude and ene gy alues below 70 dB
AE
and 10
5
aJ,
espec i ely. The published esul s om bo h da ase s also con i m ha bo h he ampli ude
and ene gy alues a e closely ela ed o he loading cha ac e is ics and s acking sequence
o he plies. A simila conclusion applies in he case o he peak equency, in which he
equency in e al is ela ed o he loading cha ac e is ics and s acking sequence o he
plies. The 200-400 kHz peak equency ange we ound was he wides compa ed o hose
o o he s udies (see Figu e 28) [3,33,34].
Polyme s 2022, 13, x FOR PEER REVIEW 21 o 25
Figu e 28. Rep esen a i e AE signal o ib e/ma ix debonding (DWC 454 da ase ).
4.3. Fib e Failu e
AE hi s associa ed wi h he ib e ailu e mechanism we e cha ac e ised by ene gies
lowe han 104 aJ and ampli udes up o 80 dBAE. As in o he cases, bo h a iables end o
be somewha lowe in he case o he Fujice a 1045S da ase . Howe e , a majo di e ence
was ound in he case o he peak equency, whe e he DWC 454 da ase e ealed a 400–
600 kHz equency ange (see Figu e 29), whe eas he Fujice a 1045S da ase ex ended he
uppe bound o 1000 kHz. This obse a ion was con i med by he subsequen il e ing o
clus e h ee in he Fujice a 1045S da ase . By compa ing he achie ed esul s wi h hose
in Table 1, we can s a e ha he lowe limi is in acco dance wi h he gi en alues.
Figu e 29. Rep esen a i e AE signal o ib e ailu e (DWC 454 da ase ).
4.4. Fib e Pull-Ou
Along wi h ib e b eakage, ib e pull-ou equi ed he addi ional il e ing o clus e
h ee da a belonging o he Fujice a 1045S da ase . This mechanism can be easily iden i ied
in he case o he DWC 454 da ase , which is pa o clus e one, excep o he esul s in
he case o FCB samples. In addi ion, conside ing he il e ed da a om he Fujice a 1045S
da ase , he AE hi s belonging o ib e pull-ou we e mainly cha ac e ised by peak e-
quencies highe han 700 kHz (see Figu e 30), ampli udes below 60 dBAE, and ene gies no
exceeding 103 aJ. The u ilisa ion o Fujice a 1045S in simila measu emen s may he e o e
in ol e he es ima ion o he pa ial powe o he wa e ansien o dis inguish ib e b eak-
age and ib e pull-ou . The achie ed esul s in e ms o peak equency a e undoub edly
highe han hose gi en in he s udies lis ed in Table 1.
Figu e 28. Rep esen a i e AE signal o ib e/ma ix debonding (DWC 454 da ase ).
4.3. Fib e Failu e
AE hi s associa ed wi h he ib e ailu e mechanism we e cha ac e ised by ene gies
lowe han 10
4
aJ and ampli udes up o 80 dB
AE
. As in o he cases, bo h a iables end o
be somewha lowe in he case o he Fujice a 1045S da ase . Howe e , a majo di e ence
was ound in he case o he peak equency, whe e he DWC 454 da ase e ealed a 400–600
kHz equency ange (see Figu e 29), whe eas he Fujice a 1045S da ase ex ended he uppe
bound o 1000 kHz. This obse a ion was con i med by he subsequen il e ing o clus e
h ee in he Fujice a 1045S da ase . By compa ing he achie ed esul s wi h hose in Table 1,
we can s a e ha he lowe limi is in acco dance wi h he gi en alues.
Polyme s 2022, 13, x FOR PEER REVIEW 21 o 25
Figu e 28. Rep esen a i e AE signal o ib e/ma ix debonding (DWC 454 da ase ).
4.3. Fib e Failu e
AE hi s associa ed wi h he ib e ailu e mechanism we e cha ac e ised by ene gies
lowe han 104 aJ and ampli udes up o 80 dBAE. As in o he cases, bo h a iables end o
be somewha lowe in he case o he Fujice a 1045S da ase . Howe e , a majo di e ence
was ound in he case o he peak equency, whe e he DWC 454 da ase e ealed a 400–
600 kHz equency ange (see Figu e 29), whe eas he Fujice a 1045S da ase ex ended he
uppe bound o 1000 kHz. This obse a ion was con i med by he subsequen il e ing o
clus e h ee in he Fujice a 1045S da ase . By compa ing he achie ed esul s wi h hose
in Table 1, we can s a e ha he lowe limi is in acco dance wi h he gi en alues.
Figu e 29. Rep esen a i e AE signal o ib e ailu e (DWC 454 da ase ).
4.4. Fib e Pull-Ou
Along wi h ib e b eakage, ib e pull-ou equi ed he addi ional il e ing o clus e
h ee da a belonging o he Fujice a 1045S da ase . This mechanism can be easily iden i ied
in he case o he DWC 454 da ase , which is pa o clus e one, excep o he esul s in
he case o FCB samples. In addi ion, conside ing he il e ed da a om he Fujice a 1045S
da ase , he AE hi s belonging o ib e pull-ou we e mainly cha ac e ised by peak e-
quencies highe han 700 kHz (see Figu e 30), ampli udes below 60 dBAE, and ene gies no
exceeding 103 aJ. The u ilisa ion o Fujice a 1045S in simila measu emen s may he e o e
in ol e he es ima ion o he pa ial powe o he wa e ansien o dis inguish ib e b eak-
age and ib e pull-ou . The achie ed esul s in e ms o peak equency a e undoub edly
highe han hose gi en in he s udies lis ed in Table 1.
Figu e 29. Rep esen a i e AE signal o ib e ailu e (DWC 454 da ase ).

Polyme s 2023,15, 47 22 o 25
4.4. Fib e Pull-Ou
Along wi h ib e b eakage, ib e pull-ou equi ed he addi ional il e ing o clus e
h ee da a belonging o he Fujice a 1045S da ase . This mechanism can be easily iden i ied
in he case o he DWC 454 da ase , which is pa o clus e one, excep o he esul s
in he case o FCB samples. In addi ion, conside ing he il e ed da a om he Fujice a
1045S da ase , he AE hi s belonging o ib e pull-ou we e mainly cha ac e ised by peak
equencies highe han 700 kHz (see Figu e 30), ampli udes below 60 dB
AE
, and ene -
gies no exceeding 10
3
aJ. The u ilisa ion o Fujice a 1045S in simila measu emen s may
he e o e in ol e he es ima ion o he pa ial powe o he wa e ansien o dis inguish
ib e b eakage and ib e pull-ou . The achie ed esul s in e ms o peak equency a e
undoub edly highe han hose gi en in he s udies lis ed in Table 1.
Polyme s 2022, 13, x FOR PEER REVIEW 22 o 25
Figu e 30. Rep esen a i e AE signal o ib e pull-ou (DWC 454 da ase ).
Table 5 summa ises he cha ac e is ics o he epo ed ailu e mechanisms, which e-
sul ed om clus e ing analysis o he Fujice a 1045S and DWC 454 da ase s.
Table 5. Cha ac e isa ion o epo ed ailu e mechanisms.
Failu e Mechanism A [dBAE] E [aJ] Peak F equency [kHz]
Delamina ion/Ma ix c acking 40÷94 (occas. 100) <107 50÷200
Fib e/Ma ix debonding 40÷70 (occas. 85) <105 (occas. 106) 200÷400
Fib e ailu e <80 <104 400÷600(1000)
Fib e pull-ou <60 <103 >700
5. Conclusions
The iden i ica ion o indi idual ailu e mechanisms, including hei de ailed cha ac-
e isa ion, has been he subjec o a numbe o publica ions. Howe e , many au ho s ha e
only deal wi h speci ic mechanisms om he abo e scale, which p o ided an incen i e
o u he esea ch in his a ea by ou eam. To ob ain a mo e gene al cha ac e is ic o he
indi idual ailu e modes, we used wo di e en wideband senso s. This solu ion has he
ad an age o mu ually e i ying he esul s om bo h da ase s.
The expe imen al sec ion included he ealisa ion o ensile o compac ension es s
on bulk esin and ca bon ib e bundle specimens, o which only one damage mechanism
could be expec ed, ollowed by ensile/compac ension es s on uni/bidi ec ional ply spec-
imens, whe e mul iple ailu e mechanisms we e assumed o occu .
The measu ed da ase s we e hen indi idually subjec ed o clus e analysis, which
in ol ed a spec al clus e ing app oach u ilising an unsupe ised k-means clus e ing al-
go i hm. The ea u e space included besides he con en ional AE signal pa ame e s such
as ampli ude, ene gy, ise ime, o equency cha ac e is ics, non-dimensionalised AE sig-
nal powe in p eselec ed equency in e als.
The clus e ed da a om bo h da ase s we e hen compa ed o each o he and o he
SEM analysis esul s. This combina ion p o ed o be e y e ec i e, especially in he case
o cha ac e ising ma ix c acking/delamina ion, o ib e/ma ix debonding ailu e mecha-
nisms. Fo a mo e de ailed cha ac e isa ion o ib e ailu e o ib e pull-ou mechanisms
in he case o he Fujice a 1045S da ase , a me hodology based on seconda y il e ing using
he non-dimensionalised AE signal powe in p eselec ed equency in e als was p o-
posed.
Finally, he p esen ed me hod o e s a obus me hodology o he classi ica ion o
AE signals in e ms o hei pa ame e s, which bene i s om he use o wo di e en wide-
band senso s. The indispu able ad an age o he a o emen ioned me hod is also i s ull
applicabili y in indus ial p ac ice, which, howe e , en ails he necessi y o use wice he
numbe o senso s compa ed o con en ional echnology.
Figu e 30. Rep esen a i e AE signal o ib e pull-ou (DWC 454 da ase ).
Table 5summa ises he cha ac e is ics o he epo ed ailu e mechanisms, which
esul ed om clus e ing analysis o he Fujice a 1045S and DWC 454 da ase s.
Table 5. Cha ac e isa ion o epo ed ailu e mechanisms.
Failu e Mechanism A [dBAE] E [aJ] Peak F equency [kHz]
Delamina ion/Ma ix c acking 40÷94 (occas. 100) <10750÷200
Fib e/Ma ix debonding 40÷70 (occas. 85) <105(occas. 106)200÷400
Fib e ailu e <80 <104400÷600(1000)
Fib e pull-ou <60 <103>700
5. Conclusions
The iden i ica ion o indi idual ailu e mechanisms, including hei de ailed cha ac e -
isa ion, has been he subjec o a numbe o publica ions. Howe e , many au ho s ha e
only deal wi h speci ic mechanisms om he abo e scale, which p o ided an incen i e
o u he esea ch in his a ea by ou eam. To ob ain a mo e gene al cha ac e is ic o he
indi idual ailu e modes, we used wo di e en wideband senso s. This solu ion has he
ad an age o mu ually e i ying he esul s om bo h da ase s.
The expe imen al sec ion included he ealisa ion o ensile o compac ension es s
on bulk esin and ca bon ib e bundle specimens, o which only one damage mechanism
could be expec ed, ollowed by ensile/compac ension es s on uni/bidi ec ional ply
specimens, whe e mul iple ailu e mechanisms we e assumed o occu .
The measu ed da ase s we e hen indi idually subjec ed o clus e analysis, which
in ol ed a spec al clus e ing app oach u ilising an unsupe ised k-means clus e ing
algo i hm. The ea u e space included besides he con en ional AE signal pa ame e s such
Polyme s 2023,15, 47 23 o 25
as ampli ude, ene gy, ise ime, o equency cha ac e is ics, non-dimensionalised AE signal
powe in p eselec ed equency in e als.
The clus e ed da a om bo h da ase s we e hen compa ed o each o he and o he
SEM analysis esul s. This combina ion p o ed o be e y e ec i e, especially in he case o
cha ac e ising ma ix c acking/delamina ion, o ib e/ma ix debonding ailu e mecha-
nisms. Fo a mo e de ailed cha ac e isa ion o ib e ailu e o ib e pull-ou mechanisms in
he case o he Fujice a 1045S da ase , a me hodology based on seconda y il e ing using he
non-dimensionalised AE signal powe in p eselec ed equency in e als was p oposed.
Finally, he p esen ed me hod o e s a obus me hodology o he classi ica ion o
AE signals in e ms o hei pa ame e s, which bene i s om he use o wo di e en
wideband senso s. The indispu able ad an age o he a o emen ioned me hod is also i s
ull applicabili y in indus ial p ac ice, which, howe e , en ails he necessi y o use wice
he numbe o senso s compa ed o con en ional echnology.
Au ho Con ibu ions:
M.Š. and P.Š.; me hodology, so wa e, da a cu a ion, w i ing, M.P.; w i ing
—o iginal d a p epa a ion, unding acquisi ion, A.V.; in es iga ion, M.B.; isualiza ion, F.G.; unding
acquisi ion. All au ho s ha e ead and ag eed o he published e sion o he manusc ip .
Funding:
This wo k was suppo ed by he p ojec Inno a i e and Addi i e Manu ac u ing Tech-
nology —New Technological Solu ions o he 3D P in ing o Me als and Composi e Ma e ials
(CZ.02.1.01/0.0/0.0/17_049/0008407) and by he Eu opean Regional De elopmen Fund in he Re-
sea ch Cen e o Ad anced Mecha onic Sys ems p ojec , CZ.02.1.01/0.0/0.0/16_019/0000867 wi hin
he Ope a ional P og amme Resea ch, De elopmen , and Educa ion and he p ojec SP2022/60 Ap-
plied Resea ch in he A ea o Machines and P ocess Con ol suppo ed by he Minis y o Educa ion,
You h, and Spo s.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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Disclaime /Publishe ’s No e:
The s a emen s, opinions and da a con ained in all publica ions a e solely hose o he indi idual
au ho (s) and con ibu o (s) and no o MDPI and/o he edi o (s). MDPI and/o he edi o (s) disclaim esponsibili y o any inju y o
people o p ope y esul ing om any ideas, me hods, ins uc ions o p oduc s e e ed o in he con en .