Sci Eng Compos Mater 2016; 23(6): 699–710

Yucheng Zhong and Sunil Chandrakant Joshi*

Improved impact response of hygrothermally

conditioned carbon/epoxy woven composites
Abstract: The effects of hygrothermal conditioning and Nomenclature
moisture on the impact resistance of carbon fiber/epoxy
composite laminates were investigated. Specimens were Di  rojectile deflection corresponding to incipient damage
P
fabricated from carbon fiber/epoxy woven prepreg mate- point, mm
rials. The fabricated specimens were either immersed in Dmax Maximum projectile deflection, mm
water at 80°C or subjected to hot/wet (at 80°C in water Dp Projectile deflection corresponding to the peak load point,
for 12 h) to cold/dry (at -30°C in a freezer for 12 h) cyclic mm
DZ Through-the-thickness diffusivity constant, mm2/s
hygrothermal conditions, which resulted in different
DI Damping index
moisture contents inside the laminates. It was found Ed Impact energy dissipated through damage creation and
that the absorbed moisture did not migrate out from propagation, J
composite materials at -30°C. Neither of the hygrother- Ed1 Damage generation energy before the peak load point, J
mal conditions in this study had detrimental effects on Ed2 Damage generation energy after the peak load point, J
Eel Impact energy absorbed through elastic deformation, J
the microstructure of the laminates. Low-velocity impact
Ep Total energy absorbed corresponding to the peak load point, J
testing was subsequently conducted on the conditioned Etotal Total impact energy or actual impact energy exerted on
specimens. When attacked by the same level of impact ­specimens, J
energy, laminates with different moisture levels experi- Fi Incipient damage load or load corresponding to the incipient
enced different levels of impact damage. Moisture signif- damage point, kN
icantly alleviated the extent of damage in carbon fiber/ Fp Peak contact force between the impactor and the
specimen, kN
epoxy woven laminates. The elastic response of the lami-
h Thickness of a plate, mm
nate under impact was improved after hygrothermal con- Mf Final moisture content of specimens, %
ditioning. The mechanism behind the improved impact Mm Effective equilibrium moisture content, %
resistance after absorbing moisture was proposed and M1 Moisture content at time t1, %
deliberated. M2 Moisture content at time t2, %
Wi Initial weight of laminates (measured after curing), g
Wf Final weight of specimens or weight before impact testing, g
Keywords: composites; damage characteristics; hygro-
thermal; impact resistance; moisture.

DOI 10.1515/secm-2014-0105
1 Introduction
Received April 8, 2014; accepted January 30, 2015; previously
­published online April 25, 2015 Laminated composites are generally fabricated by incor-
porating high-strength endless fibers (e.g., carbon fiber,
glass fiber, etc.) into a polymer matrix. They exhibit
light weight but high strength compared to some metals,
making them an ideal material for weight-critical applica-
tions. Because of their high strength/stiffness-to-weight
ratios, laminated composite materials are now widely
used in aerospace structures [1, 2], automobiles [3, 4],
*Corresponding author: Sunil Chandrakant Joshi, Division of sports equipment [5, 6], marine applications [7, 8], etc.
Aerospace Engineering, School of Mechanical and Aerospace Most polymer matrix composites (PMCs) are vulner-
Engineering, Nanyang Technological University, Singapore 639798,
able to hygrothermal environments. Much effort has been
e-mail: MSCJoshi@ntu.edu.sg
Yucheng Zhong: Division of Aerospace Engineering, School of
devoted to the investigation of the behavior of laminated
Mechanical and Aerospace Engineering, Nanyang Technological composites under combined hygrothermal environments.
University, Singapore 639798 Hygroscopic aging causes matrix swelling because of
700 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture

moisture absorption, while thermal treatment might The findings and the understanding of various phenom-
promote additional curing of the matrix causing matrix ena are presented.
shrinkage [9]. Both fiber- and matrix-dominated properties
of PMCs have been reported to be degraded after hygro-
thermal conditioning [10, 11]. Wosu [12] studied the effects
of moisture and temperature on the dynamic compressive
2 Materials and methods
properties of graphite/epoxy composites by conducting
high strain rate penetration experiments. The decrease 2.1 Sample preparation
in the compressive strength, elastic modulus and energy
absorbed was observed when the composites were loaded Laminates were fabricated using a 12 K, carbon/epoxy,
under extreme temperature, moisture and combined mois- woven prepreg that had a standard resin content of 40% by
ture and temperature conditions. Chen [13] conducted a weight before curing. The prepregs were L-930HT (GT700)
compression test under a hygrothermal environment and (manufactured by J.D. Lincoln Inc., Costa Mesa, CA, USA).
found that this environment degraded the compression The woven pregregs were cut into laminae of 100 × 100 mm2
properties of both stitched and unstitched laminates (with after being de-frozen under the lab condition for 4–5 h.
a hole). Aniskevich [14] conducted mechanical testing on The laminae were then stacked into laminates consisting
moistened glass fiber-reinforced polyester composites and of eight layers according to a stacking sequence of [0/90]8.
found that specimens moistened up to the saturation level A roller was used to compact the plies during stacking
suffered from a decrease of 8% and 16.5% in both flexural in order to achieve void-free adhesion between adjacent
modulus and strength, respectively. However, the tensile layers. The laminated lay-ups were cured in a convection
modulus and strength of the material were not affected by oven at 130°C for 4 h. A pressure measuring 5 kPa was
the sorbed moisture. applied to the laminates during the curing by placing dead
The impact properties of polymer matrix composite weights on to the specimens. The average resin content of
materials have attracted special attention in both academic the cured laminates was found to be 38.87%.
field and the industry. The two components of a compos-
ite material, fibers and matrix resin, do not impart much
plastic deformation to the composites. Even a strike by a 2.2 Hygrothermal conditioning
relatively light impact load can result in permanent failure
of either the reinforcement or the matrix resin, endanger- As-fabricated specimens were either immersed in water at
ing the structural integrity of the composite parts. In many 80°C until saturation (isothermally conditioned group),
cases, these damages are initiated inside the structure and or exposed to a number of hygrothermal cycles (cyclically
therefore usually go unnoticed. Process parameters such conditioned group). The hygrothermal cycle had a dura-
as contact force, energy and projectile deflection, values tion of 12 h in water at 80°C and 12 h in a freezer at -30°C.
of which are normally gathered during an impact testing A general water bath (Figure 1A) which provides a uniform
event, are useful for the comparison of the impact prop- temperature profile was used for water immersion condi-
erties of laminated composites [15–17]. The influence of tioning. An analytical balance was used to measure the
hygrothermal conditioning on the capacity of PMCs to weight of the specimens periodically during conditioning.
resist impact loading, however, has not received enough To ensure the removal of superficial surface water every
investigation. It was, however, reported that moisture and time, the specimens were wiped dry using clean tissue
temperature affected the energy absorption process of the paper and exposed to an ambient lab environment for
laminate during an impact [18]. around 10 min.
The objective of this work is to identify the effects
of hygrothermal conditioning on the behavior of CFRP
(carbon fiber reinforced polymer) woven laminates under 2.3 Impact testing
impact loading. Laminates were fabricated using a com-
mercial CFRP prepreg material. Two hygrothermal con- After conditioning, the impact resistance properties of
ditions were selected for the aging of cured laminates. CFRP woven laminates were studied by an instrumented
Low-velocity impact testing was conducted under a low-velocity impact tester, Instron Dynatup 8250 drop
laboratory environment on hygrothermally conditioned tower impact tester (Figure 1B and C). The impactor
specimens. The microstructures and damage modes were height and weight applied during impact testing were
determined using a scanning electron microscope (SEM). 2.735 kg and 600 mm, respectively. The contact force,
 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture 701

A Laminates B C

Laminate

Figure 1: Key facilities used: (A) water bath; (B) clamping fixture of the impact tester; (C) impactor tup.

time, deflection, velocity and energy of each impact testing exceptionally slow with immersion time. The diffusion
event were all output using the data acquisition system. of moisture inside CFRP woven laminates at 80°C mani-
fested as typical Fickian behavior.
Mf in Table 1 was calculated as follows:
Wf -Wi
3 Results Mf = × 100%. (1)
Wi

3.1 Moisture absorption behavior DZ was calculated as follows according to ASTM D5229
[19]:
The hygrothermal conditioning scheme is summarized 2
2
in Table 1. The reference group represents specimens not  h   M -M 
DZ = π   2 1 , (2)
subjected to any prior hygrothermal conditioning. The 
 4 Mm   t - t 
2 1
weight changing history of two CFRP woven laminates 
immersed in water at 80°C is shown in Figure 2. Initially, where
the weight of the specimen increased linearly with t1/2.
M 2 -M 1
After the percentage increase in weight reached a certain = slope of the moisture absorption plot in the
point, the rate of weight increase with t1/2 reduced signifi- t2 - t1
cantly. Eventually, the weight of the specimens increased initial linear portion of the curve.

Table 1: Hygrothermal conditioning results of CFRP woven laminates.

Group Sample Wi (g) Wf (g) Mf (%) Time (h) Thickness (mm) Diffusivity
Dz (mm2/s)

Reference (unconditioned) group Ref 1 24.45333 – – – 1.768 –

Ref 2 23.91144 – – – 1.714 –
Ref 3 24.31863 – – – 1.730 –
Ref 4 24.30728 – – – 1.768 –
Average 24.24767 – – – 1.745 –
Cyclically conditioned group Cyc 1 24.63879 25.15477 2.09 522 1.843 –
Cyc 2 24.23862 24.80408 1.43 522 1.838 –
Cyc 3 24.11994 24.60285 2.00 522 1.787 –
Cyc 4 25.14306 25.63753 1.97 522 1.895 –
Average 24.53510 25.04981 1.87 522 1.841 –
Isothermally conditioned group Iso 1 24.77896 25.44373 2.68 522 1.850 1.777 × 10-6
Iso 2 24.34138 24.99595 2.69 522 1.845 1.527 × 10-6
Iso 3 24.66145 25.30232 2.60 522 1.786 1.032 × 10-6
Iso 4 24.37871 25.01305 2.60 522 1.785 1.075 × 10-6
Average 24.54013 25.18876 2.64 522 1.817 1.353 × 10-6
702 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture

and later polished by 800, 1200, 2400 grit SiC paper and
0.3 μm micro-polish alumina powder. The typical micro-
structure of the laminates from different hygrothermal
groups is shown in Figure 3.
The two constituents of a composite laminate, fiber
and matrix, have the coefficient of thermal expansion
(CTE) of different magnitudes. The epoxy resin has a
CTE greater than that of carbon fiber; as a consequence,
the laminate cannot expand or contract freely at chang-
ing temperatures, inducing thermal stresses at the fiber-
matrix interface and interlaminar region. The diffusivity
of moisture in the carbon fiber is negligible compared with
Figure 2: Weight change history of laminates immersed in water
at 80°C.
the epoxy resin. Stresses are also introduced at the fiber-
matrix interface and interlaminar region when the expan-
sion of the moisture-saturated epoxy resin is constrained
Research work on isothermal conditioning of compos- by the fibers. These moisture- and temperature-induced
ite materials can be easily found in the literature. However, stresses may cause the degradation of the microstruc-
cyclic hygrothermal conditions, which are inevitable envi- ture of the laminate, such as micro-cracks [23] and fiber
ronments for aerospace composite structures, received rel- debonding.
atively less attention. In this study, a hygrothermal cycle, The specimens of the isothermally conditioned group
which had a duration of 12 h in water at 80°C and 12 h in were put in 80°C water for 522 h and their average moisture
a dry freezer at -30°C, was designed to simulate certain in- content reached 2.64%. However, no surface micro-cracks
service hygrothermal conditions. CFRP woven laminates were found and their microstructure was analogous to
were exposed to a number of such hygrothermal cycles that of the unconditioned samples (Figure 3C) without
and their weight was measured regularly each time after delamination and fiber-matrix interface failure. The spec-
being taken out from the water bath or the freezer. Little imens of the cyclically conditioned group experienced
variation in weight was observed when the specimens both temperature cycling and possible volumetric expan-
were aged in the freezer, indicating that moisture resided sion associated with the water-to-ice transition. However,
inside CFRP laminates lost their transport capability at microstructure degradation was not found in the SEM
-30°C. The actual state of moisture inside composite mate- image of this group of specimen. Both Figure 3B and C
rials is still not fully understood and substantial disagree- show that no cracks at the fiber-matrix interface and the
ment needs to be resolved. Zhou and Lucas [20] proposed interlaminar region were found, indicating good environ-
that water molecules bind with epoxy resins through mental durability of the composite materials investigated.
hydrogen bonding, while Woo and Piggott [21] found that In summary, the current hygrothermal conditioning and
the moisture did not appear to be bound to polar groups time span of environmental aging did not have detrimen-
in the resin although water inside did not behave as free tal effects on the microstructure of the CFRP laminates.
water. Clustering of water molecules was found based on
experimental results. However, whether absorbed mois-
ture could form clusters that were large enough to allow 3.3 D
 amages of woven laminates under
the formation of ice crystals is not completely answered impact
[22]. In our study, it was found that the absorbed moisture
did not migrate out from the composite materials at -30°C. As concluded in the previous section, the hygrothermal
conditioning carried out in the current investigation did
not bring about any visible structural defects. The major
3.2 M
 icrostructure after hygrothermal difference between specimens of different groups was that
conditioning they had different moisture contents. The isothermally
conditioned group had the highest moisture content,
After hygrothermal conditioning, the microstructure of while the reference group had the lowest. After the hygro-
the laminates was characterized by observing their cross- thermal conditioning, specimens with different mois-
sections using a JOEL 5600LV SEM. Prior to SEM observa- ture saturation levels were tested under the same impact
tions, the laminates were sectioned by a diamond saw energy level.
 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture 703

A Carbon fiber B

Epoxy resin

Fiber/matrix
interface Interfacial area
connecting two layers

Figure 3: Microstructure of laminates of different groups: (A) unconditioned group, (B) cyclically conditioned group and (C) isothermally
conditioned group.

An impact load normal to a clamped laminate is damage site was observed under an SEM; the micro-
a bending load in nature. The largest stress and strain graphs of the laminates from each group are presented in
usually appear near the top and bottom surface of the Figures 5 and 6.
composite, and the general damage modes include matrix It was found that the specimen with highest mois-
cracking, delamination and fiber breakage. As shown ture content, Iso 2, had minimum impact damage (Figure
in Figure 4, when tested under the same impact energy 5A–D). There was no evident damage near the top surface
level, Ref. 1 had evident indentation, while no visible of the specimen (Figure 5A). Minor matrix cracking and
indentation was found at the top surface (surface that had delamination were observed near the bottom surface
direct contact with the impactor) of Iso 2. This apparently (Figure 5B). Thus, the major damage mode for Iso 2 was
means that the specimen with higher moisture contents delamination. More severe delamination and fiber break-
had lighter impact-induced damage. Severe fiber and tow age were observed in Cyc 1, as shown in Figure 5C. The
splitting was observed at the bottom surface of Ref 1, while epoxy resin in this composite material is a type of brittle
no splitting was found at the bottom surface of Iso 2. Only polymer. Therefore, large-scale matrix cracking was
minor delamination and matrix cracking were found at expected when severe delamination occurred (Figure 5C).
the bottom surface of Iso 2. The photographs of the three Compared with Iso 2, Cyc 1 experienced more severe
specimens with different moisture contents proved that delamination (Figure 5C), large-scale matrix crack-
the extent of impact damage decreased with moisture ing (Figure 6C) and evident fiber breakage (Figure 6D).
content. However, catastrophic failure was found in Ref 1 (Figure
A careful observation of the damage site of the bottom 5D), which was not hygrothermally conditioned and had
surface of these three laminates revealed that delamina- the lowest moisture content. The damage modes of Ref
tion and fiber breakage are the major modes of damage 1 included large-scale delamination (Figures 5D and 6A)
for CFRP woven laminates under impact. As the degree of and severe splitting (Figure 6B). Among all the three lami-
impact damage increased, the scale of delamination and nates, Ref 1 experienced the most severe delamination
volume of fiber breakage also increased. (Figure 5D) and fiber breakage (Figure 6B). In summary,
In order to examine the damage modes inside the moisture significantly affected the damage characteris-
laminate, the cross-section of the laminate near the tics of the laminates during impact testing. The degree of
704 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture

A B Ed 2 = Etotal -E p . (3)
Evident indentation
Lower values of Ed2 indicate that less impact damage
has been imparted to the test specimen by the impactor
after the peak load point.
The load vs. time, load vs. deflection and energy vs.
time curves of the three specimens selected from each
Matrix cracking Severe splitting
group are plotted in Figures 7–9, respectively. The relation-
ship between the moisture contents of these three speci-
C mens was Mf (Ref 1) < Mf (Cyc 1) < Mf (Iso 1).
D
Light indentation In Figure 7, the load vs. time curve of Iso 1 is more sym-
metrical than that of the other two specimens. The contact
force dropped suddenly for Ref 1 and Cyc 1, while the
instant sharp load drop of Ref 1 occurred earlier in time
than that of Cyc 1. The oscillations and sharp load drops
on the load vs. time and load vs. deflection curves are
Light splitting indications of the initiation and propagation of damages,
such as matrix cracking, fiber breakage and delamination.
E F Once certain damage occurred, the stiffness of the struc-
No visible
indentation
ture would decrease and become more compliant. Iso 1,
No splitting which had the highest moisture content, experienced the
lowest level of impact damage. Therefore, no sharp load
drop was observed on the load vs. time curve of Ref 1.
The area under the load vs. deflection curve repre-
sented the portion of impact energy that was stored in
the laminate, and therefore was a measure of the degree
of impact damage. It is evident that the area under the
Figure 4: Photographs of impact tested specimens from different load vs. deflection curve decreased as the moisture
groups, Mf (Ref 1) < Mf (Cyc 1) < Mf (Iso 2): (A) top surface of Ref 1; (B) content inside the material increased, which was con-
bottom surface of Ref 1; (C) top surface of Cyc 1; (D) bottom surface sistent with the conclusion that the extent of impact
of Cyc 1; (E) top surface of Iso 2; (F) bottom surface of Iso 2. damage decreased with the moisture content. No sharp
load drop occurred for Iso 1, while the sharp load drop of
impact damage decreased rapidly with moisture content, Cyc 1 occurred at larger projectile deflection when com-
which equally indicates that absorbed moisture alleviated pared with Ref 1. A sharp load drop on the load vs. time
the damage characteristics of CFRP woven laminates. and/or load vs. deflection curve should be attributed to
a significant damage created in the laminate. Damage
modes that could contribute such an evident drop in
3.4 Impact testing results contact force to the laminate are generally either severe
delamination or fiber breakage. However, if it were fiber
During an impact test, process parameters including breakage that caused the sharp drop in contact force, the
contact force, time, energy and deflection were output by three samples in Figure 8 should experience a sharp load
the data acquisition system to be used for the comparison drop at approximately the same projectile deflection.
and observation of the impact properties of testing struc- Therefore, it was the severe delamination that brought
tures. The characteristic parameters include Fi, Fp, Di, Dp, about the sharp drop in contact force in Cyc 1 and Ref 1.
Dmax, Ed, Eel, the definitions of which are also found in the Iso 1 did not exhibit such severe delamination, while Cyc
literature [16, 17]. Ed2 was also adopted in the discussion 1 suffered from severe delamination at a larger projectile
of impact properties in this study. Ed consists of two parts, deflection than Ref 1. This led to the important finding
namely, Ed up to the peak load point (Ed1) and Ed beyond that moisture could prevent or postpone the occurrence
the peak load point (Ed2). Under the assumption that after of delamination in a CFRP woven laminate.
the peak load point the most impact energy is devoted to The slope of the initial linear part on the load vs.
the formation of damage, Ed2 is calculated as deflection curve generally reflects the magnitude of the
 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture 705

A C

Top surface
Top surface
Delamination
Fiber breakage

Matrix cracking

B D

Top surface
Catastrophic failure
Delamination

Splitting

Matrix cracking

Fiber breakage

Figure 5: SEM graphs of the cross-sections of impact tested specimens with different moisture levels, Mf (Ref 1) < Mf (Cyc 1) < Mf (Iso 2): (A), (B) Iso
2 with the minimum level of impact damage; (C) Cyc 1 with more severe delamination and fiber breakage; (D) Ref 1 with catastrophic failure.

out-of-plane contact stiffness of the laminate. In Figure 8, laminates with different moisture contents had approxi-
the curves of all the three samples coincide with each mately the same out-of-plane stiffness. Moisture did not
other at the initial linear part. Therefore, before impact affect the out-of-plane stiffness of CFRP woven laminates.

A Large-scale delamination
B Splitting

C Matrix cracking D
Fiber breakage

Figure 6: Comparison of the damage features of Ref 1 and Cyc 1, Mf (Ref 1) < Mf (Cyc 1): (A) Ref 1 with large-scale catastrophic delamination;
(B) complete fiber splitting in Ref 1 after impact; (C) matrix cracking in Cyc 1; (D) fiber breakage in Cyc 1 after impact.
706 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture

However, after point a (Figure 9), the impact energy dis-

sipation rate of Ref 1 becomes lower than the other two
laminates. As shown in Figure 7, there was no sharp load
drop on the curve of Iso 1, while the instant sharp load
drop of Ref 1 occurred earlier in time than that of Cyc 1.
Compared to the other two laminates, Ref 1 suffered
from severe impact damage at an earlier time. Therefore,
the contact stiffness of Ref 1 suffered from a significant
decrease earlier than that of the other two specimens,
which consequently reduced the energy absorption ability
of the laminate. Moreover, the specimen with higher mois-
ture content had lower Ed and higher Eel, which proves
Figure 7: Typical load vs. time curves of woven laminates under that specimens with more moisture consumed more of the
low-velocity impact, Mf (Ref 1) < Mf (Cyc 1) < Mf (Iso 1).
impact energy through elastic deformation and the energy
accounting for damage generation reduced.
The impact testing results of the three groups of speci-
mens are plotted in Figures 10–13. As shown in Figure 10A,
Di of both cyclically conditioned and isothermally condi-
tioned groups was higher than that of the unconditioned
reference group. This indicates that for conditioned lami-
nates, the incipient damage or first significant damage
occurred at larger deflections (higher strain levels accord-
ingly) on an average scale. Thus, moisture absorbed
during conditioning played a positive role in the CFRP
woven laminates by improving their ability to sustain
higher deflections without damage. Projectile deflec-
tion at the peak load point increased monotonically with
Figure 8: Typical load vs. deflection curves of CFRP woven lami- average moisture content, proving that CFRP laminates
nates under low-velocity impact, Mf (Ref 1) < Mf (Cyc 1) < Mf (Iso 1). with higher moisture content could retain their resistance
to the impactor up to larger deflections. The increase in
Di and Dp proves that the elastic response of the laminate
This could be explained by the fact that the fibers in the
improved after absorbing moisture.
laminate carried the majority of the applied load and
As shown in Figure 10A, Dmax decreased with average
moisture did not affect the mechanical properties of the
Mf. This should be attributed to the difference in the
carbon fiber.
overall contact stiffness of the laminates with different
In Figure 9, there is no evident difference between the
moistures. As proved in Figure 8, before impact testing
initial parts of the energy vs. time curve of each specimen.
the laminates had approximately the same contact stiff-
ness. However, specimens with higher moisture content
suffered from lower impact damage and hence had a less
Eel

decrease in contact stiffness, which consequently resulted

in the higher overall contact stiffness of conditioned lami-
nates. Therefore, the intruding impactor was stopped
faster and traveled shorter distance (smaller projectile
Etotal

deflection accordingly).
Ed

As shown in Figure 10B, the average peak load of

the conditioned specimens increased with average mois-
ture content. As shown in Figure 8, when a sharp drop in
load occurred, the contact force no longer continued to
increase. This explained why Iso 1 had relatively larger
Figure 9: Typical energy vs. time curves of CFRP woven laminates Fp than the other two specimens. Cyc 1 experienced the
under low-velocity impact, Mf (Ref 1) < Mf (Cyc 1) < Mf (Iso 1). sharp load drop at a larger deflection, resulting in a larger
 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture 707

Figure 10: Relationship between average moisture content and projectile deflection, contact force of CFRP woven laminates during the
impact test: (A) Di, Dp and Dmax; (B) Fi and Fp.

Fp than that of Ref 1. Therefore, after absorbing moisture, kinetic energy of the impactor dissipated by CFRP woven
the sharp load drop was either prevented or postponed laminates through elastic deformation increased, and
to larger deflections, resulting in the increasing trend the rest of the impact energy which accounted for the
of average Fp with moisture content. The average Fi also damage induced to the laminate decreased. This is con-
exhibited a monotonic increasing trend with moisture sistent with the conclusion that moisture alleviated the
content. Despite some scatter in the test data, overall, the extent of impact damage in CFRP woven laminates. The
values of Fp and Fi showed a rising trend. The increase in
Fi was due to the higher Di of the conditioned CFRP lami-
nates. Before the incipient damage point, the contact force
generally increased with projectile deflection (Figure 8).
Therefore, higher Di would bring about higher Fi. The Fi
values of the CFRP woven laminates tested are plotted
against their respective Di in Figure 11. The test results
within the shown band prove that in general Fi is directly
proportional to Di.
As shown in Figure 12, the average elastic energy
Eel of each group of specimens increased evidently
with the average moisture content, while Ed showed an
opposite trend. For specimens of the isothermal group,
Eel increased by nearly 2 times, while Ed decreased by Figure 12: Relationship between average moisture content and Eel,
27.7%. After absorbing moisture, the portion of the Ed and Ed2.

Figure 13: Relationship between average moisture content and

Figure 11: Relationship between Fi and Di of CFRP woven laminates. contact time.
708 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture

increase in Eel further supported the conclusion that the Moreover, Figure 5 also provides three degrees of impact
elastic response of CFRP woven laminates improved after damage of the CFRP woven laminate investigated in this
absorbing moisture. It was also noted that Ed2 decreased study. When the laminate experienced only moderate
with Mf. For unconditioned laminates, after the peak load impact damage such as that of Iso 2 (Figure 5A), the major
point the impactor still had certain speed which was rela- damage modes included light delamination and moder-
tively higher when compared with the case of conditioned ate matrix cracking. This type of damage is schematically
laminates. Therefore, Ed2 of the unconditioned group was depicted in Figure 14B. If the impactor strikes the lami-
larger than the other two groups. This also explained the nate with higher velocity, more severe impact damage
fact that Dmax of the reference group was larger of the two is expected. More severe delamination and large-scale
conditioned groups. matrix cracking and fiber breakage will occur (Figure 5C).
For the drop weight impact testing of composite mate- As the impact energy increased, catastrophic delamina-
rials, contact time between the impactor and the laminate tion and severe fiber splitting also occurred. This process
is also a parameter that generally reflects the behavior of is depicted in Figure 14C. The impact damage evolution
the laminate under an impact load. Figure 13 shows the process of the CFRP woven laminates shown in Figure 14
variation of contact time with respect to the average mois- proves that the first significant damage was delamina-
ture content of each group, which clearly shows that spec- tion and the catastrophic failure of the laminate usually
imens with higher moisture content had shorter contact occurred after severe delamination was initiated. For
time. The impactor stroke the laminate with approximately laminated structures, once large-scale delamination was
the same speed (same impact energy). If the laminate had initiated, the laminate lost its structural integrity and
relatively higher overall stiffness (lower compliance), the hence the mechanical properties along the through-the-
speed of the impactor would be reduced to zero faster and thickness direction.
shorter contact time would be expected. In this study, it
is assumed that initially laminates of different moisture
contents had approximately the same stiffness (Figure 8). A
However, laminates with less moisture content experi-
enced more severe impact damage, which reduced their
contact stiffness consequently. Therefore, specimens with
less moisture content had longer average contact time.
Thus, the experimental results showed that the mois-
ture absorbed by CFRP woven laminates during hygrother- B
mal conditioning alleviated the degree of impact-induced
damage. Another important finding from the data col-
lected during the impact test was that moisture delayed
the occurrence of delamination. Laminates with higher
moisture contents could deform to larger projectile deflec-
tions without damage. The moisture also altered the
Matrix cracking Delamination
energy dissipation mechanism of a CFRP woven laminate.
The kinetic energy of the impactor consumed by elastic C Fiber breakage Large-scale delamination
deformation increased with moisture content, while less
impact energy was left for damage generation resulting in
the alleviated damage state in the laminate. Although the
fibers in the laminate carried the majority of the load, it
was moisture that improved the impact resistance of the
CFRP woven laminate.

4 Mechanism behind improvement Figure 14: Schematic representation of the evolution of impact
damage in a CFRP woven laminate. (A) Laminate before impact;
(B) moderate impact damage consisted of delamination and matrix
In Figure 5, the damage characteristics of the three cracking; (C) severe impact damage with fiber breakage, splitting
laminates with different moisture levels are depicted. and large-scale delamination.
 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture 709

Moisture improved the elastic deformation ability of conditioned laminate with 2.69% of moisture expe-
the epoxy resin within this prepreg material by weaken- rienced only minor delamination and light matrix
ing Van der Waals forces and by reducing the number of cracking near the bottom surface.
hydrogen bonds. The mobility of the various mers (or the 4. For laminated structures, its resistance to low-velocity
chain segments) of the epoxy molecules was therefore impact depends largely on its capability of resisting
promoted. Consequently, the ductility of the epoxy resin interlaminar failure. Once large-scale delamination
improved [24]. A matrix with higher ductility would benefit was initiated, the laminate lost its structural integ-
the laminate by effectively preventing or postponing the rity and hence the mechanical properties along the
occurrence of interlaminar failure and matrix cracking. through-the-thickness direction, resulting in an
Therefore, after absorbing moisture the first significant instantaneous drop in contact force.
failure occurred at larger projectile deflections (higher Di). 5. It was proposed that absorbed moisture promoted
The rapid decrease in load which was attributed to severe the mobility of the chain segment of the epoxy mol-
delamination was also delayed (Figures 7 and 8). This ecules and increased the ductility of the epoxy resin.
explains the important finding that Iso 1 which had the The epoxy matrix with higher ductility benefited the
highest moisture content did not experience such a sharp laminate by effectively preventing or postponing the
load drop while Cyc 1 experienced a sharp load drop at a occurrence of delamination and matrix cracking.
larger projectile deflection compared with Ref 1. Moisture The elastic response of the laminate was therefore
extended the elastic limit of the epoxy resin, which in turn improved. The portion of the kinetic energy of the
delayed the initiation of matrix cracking and delamination impactor dissipated by the laminate through elastic
in the laminate and maintained the structural integrity of deformation was increased by nearly 2 times after
CFRP woven laminates. The elastic response of the lami- absorbing 2.64% wt. of moisture. This explains the
nate was therefore improved, which explains the higher alleviated impact damage in conditioned CFRP woven
Eel exhibited by conditioned laminates. laminates.

5 Conclusions References
The hygrothermal conditioning experiment was con- [1] Argüelles A, Viña J, Canteli AF, Bonhomme J. Int. J. Damage
ducted by exposing a commercial carbon fiber/epoxy Mech. 2011, 20, 963–978.
composite material to two different hygrothermal envi- [2] Simsiriwong J, Sullivan RW. Int. J. Veh. Noise. Vib. 2010, 6,
149–162.
ronments. The low-velocity impact test was conducted
[3] Aoki Y, Kim H, Ben G. Int. J. Crash 2009, 14, 469–476.
on hygrothermally conditioned CFRP woven laminates. [4] Katnam KB, Crocombe AD, Sugiman H, Khoramishad H,
Important findings include the following: ­Ashcroft IA. Int. J. Damage Mech. 2011, 20, 1217–1226.
1. Weight-change curves of the laminates revealed that [5] Betzler NF, Slater C, Strangwood M, Monk SA, Otto SR,
the transport of moisture in this material followed ­Wallace ES. Sports Eng. 2011, 14, 27–37.
[6] Chou PHC, Ding D, Chen WH. J. Reinf. Plast. Compos. 2000, 19,
Fickian behavior. At -30°C, the absorbed moisture lost
848–862.
its capability of migrating out from the laminate. [7] Motley MR, Liu Z, Young YL. Compos. Struct. 2009, 90,
2. Both cyclic hygrothermal conditioning and isother- 304–313.
mal water immersion conditioning did not exhibit [8] Öndürücü A. Int. J. Damage Mech. 2012, 21, 153–170.
detrimental effects on the microstructure of the CFRP [9] Wang Y, Hahn TH. Compos. Sci. Technol. 2007, 67,
laminates. No cracks were found at the fiber-matrix 92–101.
[10] Abdel-Magid B, Ziaee S, Gass K, Schneider M. Compos. Struct.
interface and the interlaminar region. The major dif-
2005, 71, 320–326.
ference between laminates from the different groups [11] Chung K, Yoshioka K, Seferis JC. Polym. Compos. 2002, 23,
lay in their moisture contents. 141–152.
3. The moisture trapped within significantly allevi- [12] Wosu SN, Hui D, Daniel L. Composites Part B 2011, 43,
ated the extent of damage to CFRP woven laminates 841–855.
[13] Chen G, Cheng X, Li Z, Kou C. J. Reinf. Plast. Compos. 2004, 23,
attacked by low-velocity impact. When tested under
1663–1671.
the same impact energy level, the unconditioned [14] Aniskevich K, Aniskevich A, Arnautov A, Jansons J. Compos.
laminate suffered from catastrophic delamination Struct. 2012, 94, 2914–2919.
and severe fiber splitting, while the isothermally [15] Ali M, Joshi SC. Int. J. Damage Mech. 2012, 21, 1106–1127.
710 Y. Zhong and S.C. Joshi: Improved impact response of woven composites after absorbing moisture

[16] Feraboli P. J. Aircraft 2006, 43, 1710–1718. [20] Zhou J, Lucas JP. Polymer 1999, 40, 5505–5512.
[17] Feraboli P, Kedward KT. AIAA J. 2004, 42, 2143–2152. [21] Woo M, Piggott MR. J. Compos. Tech. Res. 1987, 9,
[18] Karasek ML, Strait LH, Amateau MF, Runt JP. J. Compos. Tech. 101–107.
Res. 1995, 17, 3–10. [22] Moy P, Karasz FE. Polym. Eng. Sci. 1980, 20, 315–319.
[19] ASTM D5229/D5229M-12. Standard Test Method for Moisture [23] Garnich MR, Dalgarno RW, Kenik DJ. J. Compos. Mater.
Absorption Properties and Equilibrium Conditioning of Polymer 2011, 45, 2783–2795.
Matrix Composite Materials. ASTM: West Conshohocken, PA. [24] Zhong Y, Joshi SC. J. Compos. Mater. 2015, 49, 829–841.