Ci a ion: Valencia, F.R.;
Cas illo-López, G.; Au ekoe xea, J.;
Lopez-A aiza, A. Fib e–Wood
Lamina e Biocomposi es: Seawa e
Imme sion E ec s on Flexu al and
Low Ene gy Impac P ope ies.
Polyme s 2022,14, 4038. h ps://
doi.o g/10.3390/polym14194038
Academic Edi o s: An onios
N. Papadopoulos, Juan
C. Suá ez-Be mejo and
Lop es o Valen ina
Recei ed: 19 Augus 2022
Accep ed: 20 Sep embe 2022
Published: 27 Sep embe 2022
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-
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Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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dis ibu ed unde he e ms and
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A ibu ion (CC BY) license (h ps://
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polyme s
A icle
Fib e–Wood Lamina e Biocomposi es: Seawa e Imme sion
E ec s on Flexu al and Low Ene gy Impac P ope ies
Fabue R. Valencia 1, Ge mán Cas illo-López 2, Jon Au ekoe xea 3and Albe o Lopez-A aiza 4,*
1Ins i u ü Mechanik und S a ik, Uni e si ä de Bundesweh München, 85577 Neubibe g, Ge many
2Ci il, Ma e ial and Manu ac u ing Enginee ing Depa men , Escuela de Ingenie ías Indus iales,
Uni e si y o Málaga, 29016 Málaga, Spain
3Mechanical and Indus ial Manu ac u ing Depa men , Mond agon Goi Eskola Poli eknikoa,
Mond agon Unibe si a ea, 20500 A asa e, Spain
4Facul y o Enginee ing in Bilbao, Uni e si y o he Basque Coun y (UPV/EHU), 48920 Po ugale e, Spain
*Co espondence: [email p o ec ed]; Tel.: +34-946-014-832
Abs ac :
The p esen pape explo es a new concep o a hyb id eco-composi e by subs i u ing he
na u al ib e plies wi h hin wood enee s. The new composi e, named Fib e–Wood Lamina e (FWL),
is inspi ed by ib e–me al lamina e ma e ials. The s udied FWL con igu a ion consis ed o a single
hin pinewood enee a each o he ou e laye s o a lax wo en ab ic ein o ced bio-epoxy composi e
manu ac u ed by in usion. Th ee-poin bending esul s showed ha wood enee gi es a highly
aniso opic na u e o he FWL. In he bes case, wi h he g ain o he wood a 0
◦
, he s i ness and he
s eng h inc eased by 28 and 41%, espec i ely, bu educed he s ain-a -b eak by 27% compa ed o
he lax ib e ein o ced bio-epoxy (FFRB). The pene a ion and pe o a ion ene gy h esholds and he
peak o ce o he FWL ob ained by alling weigh impac es s we e 32, 29, and 31% lowe han hose
o he FFRB, espec i ely. This weakening was due o using single wood enee s, so he challenge o
imp o ing impac p ope ies will be o explo e hicke FWLs wi h di e en s acking sequences and
o ien a ions. The e ec o imme sing he FWL in seawa e also showed conside able di e ences. The
epoxy ma ix illed he cellula s uc u e o he wood enee s, c ea ing a ba ie e ec and educing
he amoun o wa e abso bed by he lax ib es.
Keywo ds: lax ib e; lax/wood hyb id; low ene gy impac ; lexu al; seawa e ageing
1. In oduc ion
Fib e ein o ced polyme s (FRPs) a e becoming widely used in he manu ac u ing o
p oduc s whe e a high mechanical p ope y mus be accompanied by a low weigh . Conse-
quen ly, hese composi e ma e ials can be ound in indus ial sec o s such as ae onau ical,
au omo i e, wind powe , o na al [
1
,
2
]. The ib es a e usually glass o ca bon, while he
polyme is usually an epoxy, inyles e , o polyes e esin. Howe e , he e is a end owa d
he eplacing o syn he ic ib es wi h na u al ib es [
3
] and pe oleum-based he mose ing
esins wi h esins om enewable esou ces [4].
Na u al ib es such as lax, a an, bamboo, hemp, kena , o ju e ha e high speci ic
p ope ies such as s i ness, impac esis ance, and duc ili y. In addi ion, hey a e a ailable
in la ge amoun s, and a e enewable and biodeg adable. O he desi able p ope ies include
low cos , low densi y, acous ic and he mal insula ion, less equipmen ab asion, less skin
and espi a o y i i a ion, good ib a ion damping, and enhanced ene gy eco e y [
5
,
6
].
Among all he na u al ib es, lax is a p omising subs i u e o glass ib es o semi-s uc u al
and s uc u al applica ions [
7
,
8
], wi h a lowe en i onmen al impac [
9
]. Howe e , he
hyd ophilici y o he lax ib es esul s in high mois u e abso p ion and weak adhesion o
hyd ophobic ma ices [10].
Due o his hyd ophilici y, he use o na u al ib es in applica ions ha a e in con ac
wi h bo h eshwa e and seawa e , such as ma ine ci il enginee ing cons uc ions o mos
Polyme s 2022,14, 4038. h ps://doi.o g/10.3390/polym14194038 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2022,14, 4038 2 o 13
nau ical s uc u es, is limi ed [
11
–
16
]. In consequence, ex ensi e s udies ha e ca ied ou
hyb idising lax ib es wi h syn he ic ib es o achie e a be e combina ion o mechanical
p ope ies [
16
–
24
]. Ano he app oach o hyb idising composi es is using shee s ins ead
o ib es, and Fib e–Me al Lamina es (FML) ha e demons a ed good mechanical pe o -
mances o ma ine applica ions [
25
–
27
]. Gi en ha wood is a low-cos , eco- iendly, and
adi ional ma e ial used in ma ine cons uc ions [
28
], simila ly o FML, i can be in e-
g a ed as enee in o biocomposi es o ob ain a Fib e–Wood Lamina e (FWL) s uc u e.
Mo eo e , he posi ion o he wood in he ou e plies p o ides aes he ic p ope ies o he
lamina e, highly alued in he nau ical sec o . Acco ding o an ex ensi e li e a u e sea ch,
o da e, he e a e no any s udies on he lax ib e/wood enee hyb id ein o ced bio-epoxy
esins (FWL).
One o he mos c i ical loads du ing i s se ice li e o a ma ine s uc u e, such
as a ship hull, is low- eloci y impac s due o loa ing elemen s o manoeu e s in he
po [29–31]
. Some esea ch wo ks ha e been speci ically ocused on unde s anding he
impac damage esponse o lax ib e composi es [
16
–
23
], bu his ield is s ill a i s ea ly
s ages o he in es iga ion. Cuyne e al. [
32
] showed ha e en low ene gy impac e en s
(5 J) could educe he s i ness and s eng h o lax ib e ein o ced polyme s up o 10% and
20%, espec i ely. The e o e, hei impac beha iou mus be unde s ood o be used in
semi-s uc u al and s uc u al ma ine applica ions.
In an a emp o p oduce sus ainable ma e ials wi h high mechanical and impac
pe o mances, lax/pinewood hyb id lamina es (FWL) a e e alua ed as ein o cemen s o a
bio-epoxy esin. Quasi-s a ic bending and low ene gy impac es s we e pe o med be o e
and a e seawa e imme sion, and he damage ex en was cha ac e ised by means o isual
inspec ion. A lax ib e ein o ced bio-epoxy (FFRB) composi e was used as e e ence.
2. Ma e ials and Me hods
2.1. Ma e ials and Manu ac u ing
The bio-epoxy used was Supe Sap 100/1000 En opy sys emsupplied by Fe e Dal-
mau, Ba celona, Spain. The mixing a io o he INF02 cu ing agen was 100:33 by weigh .
The lax ib e ein o cemen was supplied by Lineo Company as a bi-di ec ional (0
◦
/90
◦
)
wo en ab ic o 300 g/m
2
. The adia a pine ee wood was supplied by Made as Lobe a,
T apaga an, Bizkaia, Spain as enee s o 0.5 mm in hickness, close o he 0.75 mm o he
lax ab ic. Flax ab ics and wood enee s we e condi ioned a 50% ela i e humidi y,
sugges ed as he bes condi ions o ob aining balanced p ope ies wi h lax ib es [
33
],
and 23
◦
C o i e days be o e manu ac u ing. Two di e en ypes o 300 mm
×
300 mm
lamina es we e manu ac u ed by acuum in usion:
•
The lax ib e ein o ced bio-epoxy lamina es (FFRB) wi h i e plies, gi en a hickness
o 3.82 ±0.07 mm and a 28.8 ±0.12% ib e con en in olume.
•
Flax ib e/wood enee lamina es (FWL) o h ee lax ib e plies in he co e and a
single pinewood in each o he ou e laye s (Figu e 1a). The inal nominal hickness
was 3.2 ±0.03 mm, and he ib e con en was 30.6 ±0.12% in olume.
Polyme s 2022, 14, x FOR PEER REVIEW 3 o 13
(a) (b)
Figu e 1. (a) S acking con igu a ion o FWL; (b) schema ic diag am o acuum esin in usion p ocess
o FWL.
2.2. Seawa e Imme sion
Samples we e imme sed in he 20 m3 ank o he Resea ch Cen e in Expe imen al
Ma ine Biology and Bio echnology (PIE) (Basque Coun y, Spain) a he Can ab ian Sea.
The imme sion pe iod was om Ap il o Oc obe , he pe iod o highe ma ine bioac i i y,
longe han he wo mon hs epo ed in [15] o sa u a ing he FFRP composi es. The ank
seawa e was enewed con inuously wi h 300,000 L/day a an a e age empe a u e o 21
± 3 °C. The samples we e clamped in o a squa e ame, a oiding he con ac o he edges
wi h he seawa e and consequen ly he as sa u a ion due o he exposed ib e ends [13].
Fu he mo e, his se up ep oduces he ac ual si ua ion o he case o a ship hull. The ime
elapsed be ween he ex ac ion o he specimens om he seawa e ank and he mechan-
ical es s was always less han 48 h.
2.3. Th ee-Poin Bending Tes s
Flexu al es s we e conduc ed in acco dance wi h ISO 14125. A uni e sal es ma-
chine Se osis ME 405 (Se osis, Mad id, Spain) equipped wi h an HBM S9M/5 kN (in a
1 kN scale) was used. A 60 mm span and a c osshead o 2 mm/min we e used. Fi e sam-
ples o each lamina e we e es ed be o e and a e seawa e imme sion. The specimens
we e machined o hei nominal dimensions 15 mm × 90 mm. Rega ding he lax i-
b e/wood enee hyb id biocomposi es, he specimens we e machined bo h in he di ec-
ion o he wood g ain (FWL0) and in he pe pendicula di ec ion (FWL90).
2.4. Low-Veloci y Impac Tes s
Low- eloci y impac es s wi h di e en impac ene gies we e ca ied ou a oom
empe a u e on FFRB and FWL specimens. The d op-weigh machine, a F ac o is-Plus
om Ceas (Pianezza, I aly) was equipped wi h a 20 kN load cell a ached o a 20 mm
diame e hemisphe ical impac up. Combining di e en impac masses and alling
heigh s (2.045 o 3.045 kg and 0.05 o 1 m) enabled an ene gy (E0) ange be ween 1 and 30
J, co e ing a whole ange o beha iou s up o he pe o a ion o he samples. The an i-
ebound sys em o he machine a oided second impac s in ebound scena ios. The 70 mm
squa e pla es we e placed on a ing suppo ix u e wi h an inne and ou e diame e o
40 mm and 60 mm, espec i ely. Addi ionally, hey we e clamped by a pneuma ic de ice
consis ing o ano he annula ool.
Taking New on’s second law and he con ac o ce– ime da a (F( )) eco ded du ing
he impac e en s as he s a ing poin , displacemen – ime (δ( )) and ene gy– ime (E( ))
cu es we e calcula ed by in eg a ion [34]. Figu e 2 shows he mos ele an impac p op-
e ies o he composi es iden i ied om he F( ) and E( ) impac cu es. The damage en-
e gy h eshold (Ed), which co esponds o he i s in lec ion poin o he o ce– ime cu e
(Fd), di ides he impac e en s in o wo ca ego ies. Depending on whe he he inciden
ene gy is below o abo e Ed, impac e en s a e classi ied as subc i ical o supe c i ical.
The peak o ce (Fp) and dissipa ed ene gy (Edis) a e also ele an ea u es when cha ac e -
ising he impac beha iou o composi e ma e ials.
Figu e 1.
(
a
) S acking con igu a ion o FWL; (
b
) schema ic diag am o acuum esin in usion p ocess
o FWL.
Polyme s 2022,14, 4038 3 o 13
In using FWLs consis s o d illing se e al holes a he la e al side o he wood enee s
o acili a e he esin lux in he z-axis. In addi ion, a ne bleede o e he peel ply was
placed o acili a e he esin dis ibu ion in he x-y plane (Figu e 1b). Vacuum in usion was
ca ied ou a oom empe a u e, ollowed by a pos -cu e a 80
◦
C o 8 h. The quali y o he
FRRB and FWL lamina es was inspec ed as manu ac u ed by C-Scan. The equipmen used
consis s o an OmniScan MXU (Olympus, Tokyo, Japan) po able law de ec o , wo-axis
GLIDER
™
scanne , b oadband phase a ay p obe o 1 MHz, and speci ic wedges, showing
ha hey we e ee o delamina ion o d y zones. Void con en measu ed acco ding o
ASTM D2734-16 was, in all cases, lowe han 4%.
2.2. Seawa e Imme sion
Samples we e imme sed in he 20 m
3
ank o he Resea ch Cen e in Expe imen al
Ma ine Biology and Bio echnology (PIE) (Basque Coun y, Spain) a he Can ab ian Sea.
The imme sion pe iod was om Ap il o Oc obe , he pe iod o highe ma ine bioac i i y,
longe han he wo mon hs epo ed in [
15
] o sa u a ing he FFRP composi es. The ank
seawa e was enewed con inuously wi h 300,000 L/day a an a e age empe a u e o
21 ±3◦C
. The samples we e clamped in o a squa e ame, a oiding he con ac o he
edges wi h he seawa e and consequen ly he as sa u a ion due o he exposed ib e
ends [
13
]. Fu he mo e, his se up ep oduces he ac ual si ua ion o he case o a ship hull.
The ime elapsed be ween he ex ac ion o he specimens om he seawa e ank and he
mechanical es s was always less han 48 h.
2.3. Th ee-Poin Bending Tes s
Flexu al es s we e conduc ed in acco dance wi h ISO 14125. A uni e sal es machine
Se osis ME 405 (Se osis, Mad id, Spain) equipped wi h an HBM S9M/5 kN (in a 1 kN
scale) was used. A 60 mm span and a c osshead o 2 mm/min we e used. Fi e samples
o each lamina e we e es ed be o e and a e seawa e imme sion. The specimens we e
machined o hei nominal dimensions 15 mm
×
90 mm. Rega ding he lax ib e/wood
enee hyb id biocomposi es, he specimens we e machined bo h in he di ec ion o he
wood g ain (FWL0) and in he pe pendicula di ec ion (FWL90).
2.4. Low-Veloci y Impac Tes s
Low- eloci y impac es s wi h di e en impac ene gies we e ca ied ou a oom
empe a u e on FFRB and FWL specimens. The d op-weigh machine, a F ac o is-Plus
om Ceas (Pianezza, I aly) was equipped wi h a 20 kN load cell a ached o a 20 mm
diame e hemisphe ical impac up. Combining di e en impac masses and alling heigh s
(2.045 o 3.045 kg and 0.05 o 1 m) enabled an ene gy (E
0
) ange be ween 1 and 30 J, co e ing
a whole ange o beha iou s up o he pe o a ion o he samples. The an i- ebound sys em
o he machine a oided second impac s in ebound scena ios. The 70 mm squa e pla es
we e placed on a ing suppo ix u e wi h an inne and ou e diame e o 40 mm and
60 mm
, espec i ely. Addi ionally, hey we e clamped by a pneuma ic de ice consis ing o
ano he annula ool.
Taking New on’s second law and he con ac o ce– ime da a (F( )) eco ded du ing he
impac e en s as he s a ing poin , displacemen – ime (
δ
( )) and ene gy– ime (E( )) cu es
we e calcula ed by in eg a ion [
34
]. Figu e 2shows he mos ele an impac p ope ies
o he composi es iden i ied om he F( ) and E( ) impac cu es. The damage ene gy
h eshold (Ed), which co esponds o he i s in lec ion poin o he o ce– ime cu e (Fd),
di ides he impac e en s in o wo ca ego ies. Depending on whe he he inciden ene gy
is below o abo e E
d
, impac e en s a e classi ied as subc i ical o supe c i ical. The peak
o ce (F
p
) and dissipa ed ene gy (E
dis
) a e also ele an ea u es when cha ac e ising he
impac beha iou o composi e ma e ials.
Polyme s 2022,14, 4038 4 o 13
Polyme s 2022, 14, x FOR PEER REVIEW 4 o 13
Figu e 2. Schema ic diag am o ypical o ce and ene gy s. ime impac cu es o composi es.
2.5. Scanning Elec on Mic oscopy
The ailu e su aces o lexu al o specimens we e used o illus a e he adhesion
be ween he esin and he lax ib e/pinewood lamina es. This analysis was pe o med
using a Jeol JSM-6400 (JEOL, Tokyo, Japan) Scanning Elec on Mic oscope (SEM). F ac u e
su aces o he composi e samples we e coa ed wi h gold and hen analysed using he
SEM ope a ed a 20 kV.
3. Resul s and Discussion
3.1. Seawa e Imme sion
All he biocomposi es expe ienced discolou a ion a e he long- e m imme sion,
showing ha he abso bed wa e molecules in he bio-epoxy ma ix accele a e he pho o-
oxida ion eac ions [35]. This was a e six mon hs o seawa e imme sion, enough o
sa u a ing he biocomposi es, as epo ed by [15]. Consequen ly, he FFRB hickened by
2.6%, whe eas he wood enee ac ed as a ba ie agains he wa e abso p ion o he lax
ib es, and he FWL did no show a ema kable inc ease in hickness.
3.2. Th ee-Poin Bending P ope ies
Table 1 shows he lexu al es esul s o bo h he FFRB and FWL be o e and a e
seawa e imme sion. Rega ding he FWL, specimens machined in he di ec ion o he
wood g ain (FWL0) and ac oss i (FWL90) we e es ed. The alues o he FFRB wi hou
imme sion a e simila o hose in he li e a u e [11,12]. Compa ing he esul s o he FWL0
wi h hose o he FFRB, he pine ee wood enee s inc eased lexu al modulus (27%) and
s eng h (40%), whe eas hey educed he elonga ion-a -b eak by 25%. In con as , FWL90
showed he lowes lexu al p ope ies due o he pe pendicula di ec ion o he wood
g ain. Thus, he highly aniso opic na u e o he enee can be bene icial i he g ain di-
ec ion is sui ably o ien ed pa allel o he main s ess di ec ion.
Table 1. Flexu al es esul s.
Ma e ial Be o e Seawa e Imme sion A e Seawa e Imme sion
E (GPa) σ (MPa) εb (%) E (GPa) σ (MPa) εb (%)
FFRB 6.8 ± 1.1 115 ± 5.0 3.3 ± 0.1 5.2 ± 0.6 101 ± 5.0 3,7 ± 0.1
FWL0 8.7 ± 0.5 162 ± 4.9 2.4 ± 0.1 7.7 ± 0.1 143 ± 1.8 2.2 ± 0.3
FWL90 4.1 ± 0.6 81 ± 4.0 3.6 ± 0.1 3.7 ± 0.5 72 ± 4.0 3.2 ± 0.1
E : Flexu al modulus; σ : lexu al s eng h; εb: elonga ion-a -b eak.
Figu e 3 shows he a ia ion in bending p ope ies o he h ee biocomposi es a e
six mon hs o imme sion in seawa e . The deg ada ion p ocess educed he s i ness o all
he biocomposi es, o a g ea e ex en in he FFRB (23.5%), ollowed by FWL0 (11.5%) and
Figu e 2. Schema ic diag am o ypical o ce and ene gy s. ime impac cu es o composi es.
2.5. Scanning Elec on Mic oscopy
The ailu e su aces o lexu al o specimens we e used o illus a e he adhesion
be ween he esin and he lax ib e/pinewood lamina es. This analysis was pe o med
using a Jeol JSM-6400 (JEOL, Tokyo, Japan) Scanning Elec on Mic oscope (SEM). F ac u e
su aces o he composi e samples we e coa ed wi h gold and hen analysed using he SEM
ope a ed a 20 kV.
3. Resul s and Discussion
3.1. Seawa e Imme sion
All he biocomposi es expe ienced discolou a ion a e he long- e m imme sion,
showing ha he abso bed wa e molecules in he bio-epoxy ma ix accele a e he pho o-
oxida ion eac ions [
35
]. This was a e six mon hs o seawa e imme sion, enough o
sa u a ing he biocomposi es, as epo ed by [
15
]. Consequen ly, he FFRB hickened by
2.6%, whe eas he wood enee ac ed as a ba ie agains he wa e abso p ion o he lax
ib es, and he FWL did no show a ema kable inc ease in hickness.
3.2. Th ee-Poin Bending P ope ies
Table 1shows he lexu al es esul s o bo h he FFRB and FWL be o e and a e
seawa e imme sion. Rega ding he FWL, specimens machined in he di ec ion o he
wood g ain (FWL0) and ac oss i (FWL90) we e es ed. The alues o he FFRB wi hou
imme sion a e simila o hose in he li e a u e [
11
,
12
]. Compa ing he esul s o he FWL0
wi h hose o he FFRB, he pine ee wood enee s inc eased lexu al modulus (27%) and
s eng h (40%), whe eas hey educed he elonga ion-a -b eak by 25%. In con as , FWL90
showed he lowes lexu al p ope ies due o he pe pendicula di ec ion o he wood g ain.
Thus, he highly aniso opic na u e o he enee can be bene icial i he g ain di ec ion is
sui ably o ien ed pa allel o he main s ess di ec ion.
Table 1. Flexu al es esul s.
Ma e ial Be o e Seawa e Imme sion A e Seawa e Imme sion
E (GPa) σ (MPa) εb(%) E (GPa) σ (MPa) εb(%)
FFRB 6.8 ±1.1 115 ±5.0 3.3 ±0.1 5.2 ±0.6 101 ±5.0 3,7 ±0.1
FWL0 8.7 ±0.5 162 ±4.9 2.4 ±0.1 7.7 ±0.1 143 ±1.8 2.2 ±0.3
FWL90 4.1 ±0.6 81 ±4.0 3.6 ±0.1 3.7 ±0.5 72 ±4.0 3.2 ±0.1
E : Flexu al modulus; σ : lexu al s eng h; εb: elonga ion-a -b eak.
Figu e 3shows he a ia ion in bending p ope ies o he h ee biocomposi es a e
six mon hs o imme sion in seawa e . The deg ada ion p ocess educed he s i ness o
all he biocomposi es, o a g ea e ex en in he FFRB (23.5%), ollowed by FWL0 (11.5%)
and FWL90 (9.8%). The los s eng h was p ac ically he same, abou 11%, o he h ee
biocomposi es. The highe di e gence was ound in elonga ion-a -b eak, since he FFRB
duc ili y inc eased by 12.1%, whe eas FWL0 and FWL90 dec eased by 8.3% and 11.1%,
Polyme s 2022,14, 4038 5 o 13
espec i ely. The e ec s o seawa e imme sion o he FFRB we e simila in end and
quan i a i ely o hose epo ed in he li e a u e [
11
,
12
]. Thus, he deg ada ion o lax ib es,
he epoxy ma ix, and he bond a he ib e/ma ix in e aces a e he imme sion epo ed
in hose s udies should be a he o igin o he a ia ions in he FFRB beha iou . Rega ding
he FWLs, wood enee s p o ec ed he inne lax ib es, educing hei wa e abso p ion
and he consequences in he lexu al p ope ies.
Polyme s 2022, 14, x FOR PEER REVIEW 5 o 13
FWL90 (9.8%). The los s eng h was p ac ically he same, abou 11%, o he h ee bio-
composi es. The highe di e gence was ound in elonga ion-a -b eak, since he FFRB duc-
ili y inc eased by 12.1%, whe eas FWL0 and FWL90 dec eased by 8.3% and 11.1%, e-
spec i ely. The e ec s o seawa e imme sion o he FFRB we e simila in end and quan-
i a i ely o hose epo ed in he li e a u e [11,12]. Thus, he deg ada ion o lax ib es,
he epoxy ma ix, and he bond a he ib e/ma ix in e aces a e he imme sion epo ed
in hose s udies should be a he o igin o he a ia ions in he FFRB beha iou . Rega ding
he FWLs, wood enee s p o ec ed he inne lax ib es, educing hei wa e abso p ion
and he consequences in he lexu al p ope ies.
F om a s uc u al design poin o iew, he esul s o he h ee-poin bending es s
show ha designe s mus especially conside he educ ion in s i ness when imme sing
he FFRB in seawa e . In he case o we FWLs, he loss o s i ness and s eng h is o equal
impo ance.
Figu e 3. Va ia ion in lexu al p ope ies a e seawa e imme sion.
3.3. SEM Analysis
F ac og aphy analysis o ailu e su aces a di e en magni ica ions helps o unde -
s and h ee-poin bending esul s. Be o e seawa e imme sion (Figu e 4a), he ailu e
mode o he FFRB is ypical o b i le ma e ials. The ellipse in Figu e 4a shows he de ach-
men o he lax ib e due o he low ib e–ma ix adhesion, usual in polyme ic composi es
based on lax ib es [11,12]. A e seawa e imme sion (Figu e 4b), he lax ib es g ow in
olume due o hei hyd ophilici y, leading o a s onge ib e–ma ix physical adhesion.
The specimen b oke by ib e pull-ou and ib e/ma ix debonding, indica ing ha he i-
b e/ma ix in e ace damage could be due o he highe s ain-a -b eak o he we FFRB.
(a) (b)
Figu e 3. Va ia ion in lexu al p ope ies a e seawa e imme sion.
F om a s uc u al design poin o iew, he esul s o he h ee-poin bending es s
show ha designe s mus especially conside he educ ion in s i ness when imme sing
he FFRB in seawa e . In he case o we FWLs, he loss o s i ness and s eng h is o
equal impo ance.
3.3. SEM Analysis
F ac og aphy analysis o ailu e su aces a di e en magni ica ions helps o unde -
s and h ee-poin bending esul s. Be o e seawa e imme sion (Figu e 4a), he ailu e mode
o he FFRB is ypical o b i le ma e ials. The ellipse in Figu e 4a shows he de achmen o
he lax ib e due o he low ib e–ma ix adhesion, usual in polyme ic composi es based on
lax ib es [
11
,
12
]. A e seawa e imme sion (Figu e 4b), he lax ib es g ow in olume
due o hei hyd ophilici y, leading o a s onge ib e–ma ix physical adhesion. The speci-
men b oke by ib e pull-ou and ib e/ma ix debonding, indica ing ha he ib e/ma ix
in e ace damage could be due o he highe s ain-a -b eak o he we FFRB.
Polyme s 2022, 14, x FOR PEER REVIEW 5 o 13
FWL90 (9.8%). The los s eng h was p ac ically he same, abou 11%, o he h ee bio-
composi es. The highe di e gence was ound in elonga ion-a -b eak, since he FFRB duc-
ili y inc eased by 12.1%, whe eas FWL0 and FWL90 dec eased by 8.3% and 11.1%, e-
spec i ely. The e ec s o seawa e imme sion o he FFRB we e simila in end and quan-
i a i ely o hose epo ed in he li e a u e [11,12]. Thus, he deg ada ion o lax ib es,
he epoxy ma ix, and he bond a he ib e/ma ix in e aces a e he imme sion epo ed
in hose s udies should be a he o igin o he a ia ions in he FFRB beha iou . Rega ding
he FWLs, wood enee s p o ec ed he inne lax ib es, educing hei wa e abso p ion
and he consequences in he lexu al p ope ies.
F om a s uc u al design poin o iew, he esul s o he h ee-poin bending es s
show ha designe s mus especially conside he educ ion in s i ness when imme sing
he FFRB in seawa e . In he case o we FWLs, he loss o s i ness and s eng h is o equal
impo ance.
Figu e 3. Va ia ion in lexu al p ope ies a e seawa e imme sion.
3.3. SEM Analysis
F ac og aphy analysis o ailu e su aces a di e en magni ica ions helps o unde -
s and h ee-poin bending esul s. Be o e seawa e imme sion (Figu e 4a), he ailu e
mode o he FFRB is ypical o b i le ma e ials. The ellipse in Figu e 4a shows he de ach-
men o he lax ib e due o he low ib e–ma ix adhesion, usual in polyme ic composi es
based on lax ib es [11,12]. A e seawa e imme sion (Figu e 4b), he lax ib es g ow in
olume due o hei hyd ophilici y, leading o a s onge ib e–ma ix physical adhesion.
The specimen b oke by ib e pull-ou and ib e/ma ix debonding, indica ing ha he i-
b e/ma ix in e ace damage could be due o he highe s ain-a -b eak o he we FFRB.
(a) (b)
Figu e 4. SEM mic og aphs o he FFRB (×150) be o e (a) and a e (b) seawa e imme sion.
Polyme s 2022,14, 4038 6 o 13
Figu e 5shows he ac u e su aces o FWL0 be o e (a) and a e (b) seawa e imme -
sion, which a e also ep esen a i e o FWL90. The i s ele an conclusion is ha he epoxy
ma ix illed he cellula mic os uc u e o he wood du ing he in usion manu ac u ing
p ocess, ma ked wi h an ellipse in Figu e 5a. The e is a physical connec ion be ween epoxy
and wood cell walls, which can explain he ba ie e ec agains wa e abso p ion in he
FWLs. Rega ding he ac u e mic omechanisms, on he one hand, he cell walls o he
pinewood enee showed b i le ac u e a e imme sion (Figu e 5b). On he o he hand,
he ailu e o he lax ib e ya ns along he load di ec ion showed debonding, ib e pull-ou ,
and b i le ac u e. The epoxy ma ix showed a smoo h su ace in bo h cases, ypical o
b i le ac u e.
Polyme s 2022, 14, x FOR PEER REVIEW 6 o 13
Figu e 4. SEM mic og aphs o he FFRB (×150) be o e (a) and a e (b) seawa e imme sion.
Figu e 5 shows he ac u e su aces o FWL0 be o e (a) and a e (b) seawa e im-
me sion, which a e also ep esen a i e o FWL90. The i s ele an conclusion is ha he
epoxy ma ix illed he cellula mic os uc u e o he wood du ing he in usion manu ac-
u ing p ocess, ma ked wi h an ellipse in Figu e 5a. The e is a physical connec ion be-
ween epoxy and wood cell walls, which can explain he ba ie e ec agains wa e ab-
so p ion in he FWLs. Rega ding he ac u e mic omechanisms, on he one hand, he cell
walls o he pinewood enee showed b i le ac u e a e imme sion (Figu e 5.b). On he
o he hand, he ailu e o he lax ib e ya ns along he load di ec ion showed debonding,
ib e pull-ou , and b i le ac u e. The epoxy ma ix showed a smoo h su ace in bo h
cases, ypical o b i le ac u e.
(a) (b)
Figu e 5. F ac u e su ace mic og aphs o he FWL0 (×150) be o e (a) and a e (b) seawa e imme -
sion.
3.4. Low-Veloci y Impac Tes s
The ene gy plo ep esen s he e olu ion o he dissipa ed ene gy (E
dis
) e sus he
inciden impac ene gy (E
0
). The equal ene gy line (dashed line) is plo ed as a diagonal in
he ene gy plo and iden i ies he uppe limi o E
dis
. Figu e 6 shows he ene gy plo s o
he FFRB and FWL composi es. A d y condi ions (Figu e 6), bo h ma e ials ha e h ee
egions. A he lowe inciden ene gy le els, E
dis
inc eased quad a ically up o he equal
ene gy line, which de ines he pene a ion ene gy h eshold. This egion was composed
o wo beha iou s: subc i ical and supe c i ical be o e pene a ion. Howe e , he limi be-
ween hem (E
d
) was no iden i iable in his plo . The quan i a i e analysis o his i s
egion shows ha he FWL dissipa ed mo e ene gy han he FFRB. Ne e heless, i implies
a mo e se e e damage le el o he FWL [34] and a lowe pene a ion h eshold o 12.5 J,
whe eas FRRB s ill ebounded he impac up o 18.5 J. In he pene a ion egion, he sec-
ond one, composi es dissipa ed all he inciden ene gy, bu he impac o could no pene-
a e he sample. The ene gy ange in his egion was small o bo h ma e ials, since he
pe o a ion h eshold o he FWL and FRRB was 13.4 and 18.8 J, espec i ely. This pe o-
a ion ene gy ep esen s he maximum capaci y o he ma e ial o dissipa ing impac en-
e gy and e eals ha hyb idising he FRRB wi h he wood enee s is no ecommended
when he applica ion equi es o maximise he dissipa ion o impac ene gy. This e ec is
associa ed wi h he ac ha unde impac loading o a single hin enee , shea ailu e
pa allel o he g ain di ec ion o he wood plays a weakening ole [36].
Figu e 5.
F ac u e su ace mic og aphs o he FWL0 (
×
150) be o e (
a
) and a e (
b
) seawa e imme sion.
3.4. Low-Veloci y Impac Tes s
The ene gy plo ep esen s he e olu ion o he dissipa ed ene gy (E
dis
) e sus he
inciden impac ene gy (E
0
). The equal ene gy line (dashed line) is plo ed as a diagonal
in he ene gy plo and iden i ies he uppe limi o E
dis
. Figu e 6shows he ene gy plo s
o he FFRB and FWL composi es. A d y condi ions (Figu e 6), bo h ma e ials ha e h ee
egions. A he lowe inciden ene gy le els, E
dis
inc eased quad a ically up o he equal
ene gy line, which de ines he pene a ion ene gy h eshold. This egion was composed
o wo beha iou s: subc i ical and supe c i ical be o e pene a ion. Howe e , he limi
be ween hem (E
d
) was no iden i iable in his plo . The quan i a i e analysis o his i s
egion shows ha he FWL dissipa ed mo e ene gy han he FFRB. Ne e heless, i implies
a mo e se e e damage le el o he FWL [
34
] and a lowe pene a ion h eshold o
12.5 J,
whe eas FRRB s ill ebounded he impac up o 18.5 J. In he pene a ion egion, he second
one, composi es dissipa ed all he inciden ene gy, bu he impac o could no pene a e he
sample. The ene gy ange in his egion was small o bo h ma e ials, since he pe o a ion
h eshold o he FWL and FRRB was 13.4 and 18.8 J, espec i ely. This pe o a ion ene gy
ep esen s he maximum capaci y o he ma e ial o dissipa ing impac ene gy and e eals
ha hyb idising he FRRB wi h he wood enee s is no ecommended when he applica ion
equi es o maximise he dissipa ion o impac ene gy. This e ec is associa ed wi h he
ac ha unde impac loading o a single hin enee , shea ailu e pa allel o he g ain
di ec ion o he wood plays a weakening ole [36].
Ene gy plo s o he we FRRB and FWL composi es (Figu e 7), as o he d y compos-
i es, show he h ee egions, bu pene a ion and pe o a ion h esholds a e highe . The
FRRB c i ical ene gies inc ease o 22.3 and 24.8 J, espec i ely, while o he FWL, 14.6 J is
needed o pene a ing and 15.1 J o pe o a ing. The o igin o his imp o emen could be
a ibu ed o a ia ions in he ma ix and ein o ced beha iou when imme sed in seawa e .
The abso bed wa e by he lax ib es and he wood enee so ens hem [
11
,
16
,
20
], and
Polyme s 2022,14, 4038 7 o 13
e en i i is seconda y, he ma ix [14,19], leading o highe s ain be o e ailu e. Addi ion-
ally, lax ib es swell wi h abso bed wa e molecules, which could ill he gaps be ween
he ib e/ma ix in e ace [
11
] and be ween he ib es in he bundle [
19
], inc easing he
in e acial s eng h. The ac ha he pene a ion and pe o a ion h esholds o he we
FWL inc eased less han in he FFRB case seems o be ela ed o he ba ie e ec ha can
play he wood enee , educing he amoun o wa e abso bed by he lax ib es.
Polyme s 2022, 14, x FOR PEER REVIEW 7 o 13
Figu e 6. Ene gy plo s o he d y FRRB and FWL composi es.
Ene gy plo s o he we FRRB and FWL composi es (Figu e 7), as o he d y compo-
si es, show he h ee egions, bu pene a ion and pe o a ion h esholds a e highe . The
FRRB c i ical ene gies inc ease o 22.3 and 24.8 J, espec i ely, while o he FWL, 14.6 J is
needed o pene a ing and 15.1 J o pe o a ing. The o igin o his imp o emen could
be a ibu ed o a ia ions in he ma ix and ein o ced beha iou when imme sed in sea-
wa e . The abso bed wa e by he lax ib es and he wood enee so ens hem [11,16,20],
and e en i i is seconda y, he ma ix [14,19], leading o highe s ain be o e ailu e. Ad-
di ionally, lax ib es swell wi h abso bed wa e molecules, which could ill he gaps be-
ween he ib e/ma ix in e ace [11] and be ween he ib es in he bundle [19], inc easing
he in e acial s eng h. The ac ha he pene a ion and pe o a ion h esholds o he we
FWL inc eased less han in he FFRB case seems o be ela ed o he ba ie e ec ha can
play he wood enee , educing he amoun o wa e abso bed by he lax ib es.
(a) (b)
Figu e 7. Ene gy plo s o he we FRRB (a) and FWL composi es (b).
The isual inspec ion o he damage on he impac ed and back aces suppo s he
conclusions o he ene gy plo s ela ed o he pene a ion and pe o a ion h esholds. As
can be seen in Figu e 8a, he d y and we FFRBs we e no pe o a ed a 10 J. On he im-
pac ed ace o bo h composi es, he hemisphe ic impac o le a pe manen den . In he
d y FFRB sample, he impac o induced concen ic ings o damage, while on he we
FFRB i did no . On he back ace o he d y FFRB, ib e b eakage-domina ed mic omech-
anisms [37] gene a ed a c oss-shaped c acking, wi h he c acks pa allel o he wa p and
we di ec ions o he ab ic. Ins ead, on he we sample (Figu e 8b), only plas ic de o -
ma ion de eloped a he den . The e o e, i is demons a ed ha he highe ene gy dissi-
pa ed by he d y FFRB was a he cos o mo e damage. The images o he samples im-
pac ed a 20 J (Figu e 8c) also show a signi ican modi ica ion o he ailu e induced by he
wa e abso p ion. Whe eas he d y FFRB was comple ely pe o a ed, wi h ma ix and ib e
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Edis [J]
E0[J]
D y FFRB
D y FWL
0
5
10
15
20
25
30
0 5 10 15 20 25 30
E
dis
[J]
E
0
[J]
We FFRB
D y FFRB
0
5
10
15
20
25
30
0 5 10 15 20 25 30
E
dis
[J]
E
0
[J]
We FWL
D y FWL
Figu e 6. Ene gy plo s o he d y FRRB and FWL composi es.
Polyme s 2022, 14, x FOR PEER REVIEW 7 o 13
Figu e 6. Ene gy plo s o he d y FRRB and FWL composi es.
Ene gy plo s o he we FRRB and FWL composi es (Figu e 7), as o he d y compo-
si es, show he h ee egions, bu pene a ion and pe o a ion h esholds a e highe . The
FRRB c i ical ene gies inc ease o 22.3 and 24.8 J, espec i ely, while o he FWL, 14.6 J is
needed o pene a ing and 15.1 J o pe o a ing. The o igin o his imp o emen could
be a ibu ed o a ia ions in he ma ix and ein o ced beha iou when imme sed in sea-
wa e . The abso bed wa e by he lax ib es and he wood enee so ens hem [11,16,20],
and e en i i is seconda y, he ma ix [14,19], leading o highe s ain be o e ailu e. Ad-
di ionally, lax ib es swell wi h abso bed wa e molecules, which could ill he gaps be-
ween he ib e/ma ix in e ace [11] and be ween he ib es in he bundle [19], inc easing
he in e acial s eng h. The ac ha he pene a ion and pe o a ion h esholds o he we
FWL inc eased less han in he FFRB case seems o be ela ed o he ba ie e ec ha can
play he wood enee , educing he amoun o wa e abso bed by he lax ib es.
(a) (b)
Figu e 7. Ene gy plo s o he we FRRB (a) and FWL composi es (b).
The isual inspec ion o he damage on he impac ed and back aces suppo s he
conclusions o he ene gy plo s ela ed o he pene a ion and pe o a ion h esholds. As
can be seen in Figu e 8a, he d y and we FFRBs we e no pe o a ed a 10 J. On he im-
pac ed ace o bo h composi es, he hemisphe ic impac o le a pe manen den . In he
d y FFRB sample, he impac o induced concen ic ings o damage, while on he we
FFRB i did no . On he back ace o he d y FFRB, ib e b eakage-domina ed mic omech-
anisms [37] gene a ed a c oss-shaped c acking, wi h he c acks pa allel o he wa p and
we di ec ions o he ab ic. Ins ead, on he we sample (Figu e 8b), only plas ic de o -
ma ion de eloped a he den . The e o e, i is demons a ed ha he highe ene gy dissi-
pa ed by he d y FFRB was a he cos o mo e damage. The images o he samples im-
pac ed a 20 J (Figu e 8c) also show a signi ican modi ica ion o he ailu e induced by he
wa e abso p ion. Whe eas he d y FFRB was comple ely pe o a ed, wi h ma ix and ib e
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Edis [J]
E0[J]
D y FFRB
D y FWL
0
5
10
15
20
25
30
0 5 10 15 20 25 30
E
dis
[J]
E
0
[J]
We FFRB
D y FFRB
0
5
10
15
20
25
30
0 5 10 15 20 25 30
E
dis
[J]
E
0
[J]
We FWL
D y FWL
Figu e 7. Ene gy plo s o he we FRRB (a) and FWL composi es (b).
The isual inspec ion o he damage on he impac ed and back aces suppo s he
conclusions o he ene gy plo s ela ed o he pene a ion and pe o a ion h esholds. As
can be seen in Figu e 8a, he d y and we FFRBs we e no pe o a ed a 10 J. On he impac ed
ace o bo h composi es, he hemisphe ic impac o le a pe manen den . In he d y FFRB
sample, he impac o induced concen ic ings o damage, while on he we FFRB i did
no . On he back ace o he d y FFRB, ib e b eakage-domina ed mic omechanisms [
37
]
gene a ed a c oss-shaped c acking, wi h he c acks pa allel o he wa p and we di ec ions
o he ab ic. Ins ead, on he we sample (Figu e 8b), only plas ic de o ma ion de eloped a
he den . The e o e, i is demons a ed ha he highe ene gy dissipa ed by he d y FFRB
was a he cos o mo e damage. The images o he samples impac ed a 20 J (Figu e 8c) also
show a signi ican modi ica ion o he ailu e induced by he wa e abso p ion. Whe eas
he d y FFRB was comple ely pe o a ed, wi h ma ix and ib e ac u e along he p incipal
ib e di ec ions and lexu al bending ailu es o he quad an s c ea ed, he we FFRB was
s ill in he pene a ion ene gy egion (Figu e 8d). Thus, he plas icisa ion and lax ib e
swelling e ec s o wa e abso p ion imp o e he maximum ene gy dissipa ion capaci y o
he FFRB composi es.
Polyme s 2022,14, 4038 8 o 13
Polyme s 2022, 14, x FOR PEER REVIEW 8 o 13
ac u e along he p incipal ib e di ec ions and lexu al bending ailu es o he quad an s
c ea ed, he we FFRB was s ill in he pene a ion ene gy egion (Figu e 8d). Thus, he
plas icisa ion and lax ib e swelling e ec s o wa e abso p ion imp o e he maximum
ene gy dissipa ion capaci y o he FFRB composi es.
(a) (b)
(c) (d)
Figu e 8. Rep esen a i e pos -impac images o he on and back aces o d y FFRB a 10 J (a), we
FFRB a 10 J (b), d y FFRB a 20 J (c), and we FFRB a 20 J (d).
The images o impac ed FWL also help o unde s and he esul s o he ene gy plo .
In he 10 J impac es s, he FWL did no pe o a e, nei he in i s d y no we condi ion. A
he on ace o he d y FWL impac ed a 10 J (Figu e 9a), he wood enee was ac u ed
a he con ou o he den . The back ace damage consis ed o a c oss-shaped c acking
mechanism, wi h a longe c ack leng h pa allel o he wood g ain han in he pe pendic-
ula di ec ion. The damage mechanisms in he we FML (Figu e 9b) we e less se e e, as
in he on ace he wood enee was no ac u ed ye , and in he back he c oss-shaped
c acking was in i s ea lies s age. The images om he FWL impac ed a 15 J show ha he
d y FWL was pe o a ed (Figu e 9c), wi h almos no damage ab oad he impac o con ac
zone, whe eas a dep h den , wi hou pe o a ion, can be seen in he we FWL (Figu e 9d).
As in he FFRB composi e, in he FWL, he abso bed wa e educed i s damage and was
a he o igin o he mode a e ene gy dissipa ion imp o emen .
(a) (b)
(c) (d)
Figu e 8.
Rep esen a i e pos -impac images o he on and back aces o d y FFRB a 10 J (
a
), we
FFRB a 10 J (b), d y FFRB a 20 J (c), and we FFRB a 20 J (d).
The images o impac ed FWL also help o unde s and he esul s o he ene gy plo . In
he 10 J impac es s, he FWL did no pe o a e, nei he in i s d y no we condi ion. A he
on ace o he d y FWL impac ed a 10 J (Figu e 9a), he wood enee was ac u ed a he
con ou o he den . The back ace damage consis ed o a c oss-shaped c acking mechanism,
wi h a longe c ack leng h pa allel o he wood g ain han in he pe pendicula di ec ion.
The damage mechanisms in he we FML (Figu e 9b) we e less se e e, as in he on ace
he wood enee was no ac u ed ye , and in he back he c oss-shaped c acking was in
i s ea lies s age. The images om he FWL impac ed a 15 J show ha he d y FWL was
pe o a ed (Figu e 9c), wi h almos no damage ab oad he impac o con ac zone, whe eas
a dep h den , wi hou pe o a ion, can be seen in he we FWL (Figu e 9d). As in he FFRB
composi e, in he FWL, he abso bed wa e educed i s damage and was a he o igin o he
mode a e ene gy dissipa ion imp o emen .
Polyme s 2022, 14, x FOR PEER REVIEW 8 o 13
ac u e along he p incipal ib e di ec ions and lexu al bending ailu es o he quad an s
c ea ed, he we FFRB was s ill in he pene a ion ene gy egion (Figu e 8d). Thus, he
plas icisa ion and lax ib e swelling e ec s o wa e abso p ion imp o e he maximum
ene gy dissipa ion capaci y o he FFRB composi es.
(a) (b)
(c) (d)
Figu e 8. Rep esen a i e pos -impac images o he on and back aces o d y FFRB a 10 J (a), we
FFRB a 10 J (b), d y FFRB a 20 J (c), and we FFRB a 20 J (d).
The images o impac ed FWL also help o unde s and he esul s o he ene gy plo .
In he 10 J impac es s, he FWL did no pe o a e, nei he in i s d y no we condi ion. A
he on ace o he d y FWL impac ed a 10 J (Figu e 9a), he wood enee was ac u ed
a he con ou o he den . The back ace damage consis ed o a c oss-shaped c acking
mechanism, wi h a longe c ack leng h pa allel o he wood g ain han in he pe pendic-
ula di ec ion. The damage mechanisms in he we FML (Figu e 9b) we e less se e e, as
in he on ace he wood enee was no ac u ed ye , and in he back he c oss-shaped
c acking was in i s ea lies s age. The images om he FWL impac ed a 15 J show ha he
d y FWL was pe o a ed (Figu e 9c), wi h almos no damage ab oad he impac o con ac
zone, whe eas a dep h den , wi hou pe o a ion, can be seen in he we FWL (Figu e 9d).
As in he FFRB composi e, in he FWL, he abso bed wa e educed i s damage and was
a he o igin o he mode a e ene gy dissipa ion imp o emen .
(a) (b)
(c) (d)
Figu e 9.
Rep esen a i e pos -impac images o he on and back aces o d y FWL a 10 J (
a
), we
FWL a 10 J (b), d y FWL a 15 J (c), and we FWL a 15 J (d).
The peak o ce e sus he inciden ene gy cu es, such as hose o he FFRB and FWL
composi es in Figu e 10, inc ease acco ding o a powe law and each a pla eau associa ed
wi h he maximum allowable impac load o he ma e ial [
34
]. The i s conclusion is ha
Polyme s 2022,14, 4038 9 o 13
he hyb idisa ion wi h a wood enee does no imp o e he maximal o ce, as he FFRB
eached a alue o 2798
±
69 N, whe eas he FWL only wi hs ood 2232
±
86 N. The second
conclusion is ha wa e abso p ion did no imp o e he peak o ce alues. Indeed, he
maximum load o he we FFRB was p ac ically iden ical (2911
±
11 N) and sligh ly lowe
o he we FWL (1986 ±31 N).
Polyme s 2022, 14, x FOR PEER REVIEW 9 o 13
Figu e 9. Rep esen a i e pos -impac images o he on and back aces o d y FWL a 10 J (a), we
FWL a 10 J (b), d y FWL a 15 J (c), and we FWL a 15 J (d).
The peak o ce e sus he inciden ene gy cu es, such as hose o he FFRB and
FWL composi es in Figu e 10, inc ease acco ding o a powe law and each a pla eau as-
socia ed wi h he maximum allowable impac load o he ma e ial [34]. The i s conclu-
sion is ha he hyb idisa ion wi h a wood enee does no imp o e he maximal o ce, as
he FFRB eached a alue o 2798 ± 69 N, whe eas he FWL only wi hs ood 2232 ± 86 N.
The second conclusion is ha wa e abso p ion did no imp o e he peak o ce alues.
Indeed, he maximum load o he we FFRB was p ac ically iden ical (2911 ± 11 N) and
sligh ly lowe o he we FWL (1986 ± 31 N).
(a) (b)
(c)
Figu e 10. Peak o ce plo s o he d y FRRB and FWL (a), we FFRB (b), and we FWL (c).
Fo ce– ime and ene gy– ime impac cu es (Figu e 11) add in o ma ion o unde -
s and he di e ence in beha iou shown by he FFB and FWL in d y and we condi ions.
Rega ding he supe c i ical impac s be o e pene a ion (10 J), he d y and we FFRBs
showed almos he same cu es (Figu e 11a), wi h he damage h eshold app oxima ely
loca ed a 1620 N, co esponding o 1.3 J. Following he damage h eshold, he o ce s a-
bilised in a pla eau and inc eased up o he peak o ce. Finally, when he unloading pa h
s a ed, he con ac o ce and ene gy eached hei maximum alue, and he impac o e-
bounded, as can be deduced om he ene gy educ ion. The 10 J impac cu es o he d y
and we FWLs (Figu e 11b) we e simila bu qui e di e en o hose o he FFRB. The in-
lexion poin o damage h eshold o he we FWL was sligh ly lowe , loca ed a 1100 N
and 0.7 J, whe eas he damage h eshold o he d y FWL was a 1480 N and 1.15 J. As
wi h he pene a ion and pe o a ion h esholds, he FWL’s damage h eshold was lowe
han he FFRB’s. The o ce inc eased con inuously a e he damage h eshold up o he
0
500
1000
1500
2000
2500
3000
0 5 10 15 20 25 30
F
p
[N]
E
0
[J]
D y FFRB
D y FWL
0
500
1000
1500
2000
2500
3000
0 5 10 15 20 25 30
F
p
[N]
E
0
[J]
We FFRB
D y FFRB
0
500
1000
1500
2000
2500
3000
0 5 10 15 20 25 30
F
p
[N]
E
0
[J]
We FWL
D y FWL
Figu e 10. Peak o ce plo s o he d y FRRB and FWL (a), we FFRB (b), and we FWL (c).
Fo ce– ime and ene gy– ime impac cu es (Figu e 11) add in o ma ion o unde s and
he di e ence in beha iou shown by he FFB and FWL in d y and we condi ions. Re-
ga ding he supe c i ical impac s be o e pene a ion (10 J), he d y and we FFRBs showed
almos he same cu es (Figu e 11a), wi h he damage h eshold app oxima ely loca ed a
1620 N, co esponding o 1.3 J. Following he damage h eshold, he o ce s abilised in a
pla eau and inc eased up o he peak o ce. Finally, when he unloading pa h s a ed, he
con ac o ce and ene gy eached hei maximum alue, and he impac o ebounded, as
can be deduced om he ene gy educ ion. The 10 J impac cu es o he d y and we
FWLs (Figu e 11b) we e simila bu qui e di e en o hose o he FFRB. The in lexion
poin o damage h eshold o he we FWL was sligh ly lowe , loca ed a 1100 N and
0.7 J, whe eas he damage h eshold o he d y FWL was a 1480 N and 1.15 J. As wi h
he pene a ion and pe o a ion h esholds, he FWL’s damage h eshold was lowe han
he FFRB’s. The o ce inc eased con inuously a e he damage h eshold up o he peak
o ce. The main di e ence conce ning he FFRB was in he pos -peak egion, as damage
de eloped a an almos cons an o ce le el in he FWL and ebound did no s a a he
peak o ce.
