Effect of Hygrothermal Stress on the Failure of CFRP Composites

Composite Structures 133 (2015) 1024–1035

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Composite Structures

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Effects of hygrothermal stress on the failure of CFRP composites

http://dx.doi.org/10.1016/j.compstruct.2015.08.0160263-8223/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Fax: +44 (0)1752 586101.E-mail address: [email protected] (H.R. Le).

M. Meng, M.J. Rizvi, S.M. Grove, H.R. Le ⇑School of Marine Science and Engineering, Plymouth University, United Kingdom

a r t i c l e i n f o

Article history:Available online 6 August 2015

Keywords:CFRPMoisture diffusionFEAFailure mechanismInterlaminar shear

a b s t r a c t

This paper investigates the hygrothermal effects on the failure mechanisms in bending of carbon fibrereinforced polymer (CFRP) composites. Accelerated diffusion testing was carried out by immersion at50 �C constant temperature and 70 bar hydrostatic pressure to study the effects of fresh or sea water dif-fusion into pre-preg CFRP laminates. Consequently the composite laminates were tested in bending after1 and 3 months’ immersion. A three-dimensional finite element analysis (FEA) model was developed tocouple the moisture diffusion, hygrothermal expansion and bending. Optical and field emission scanningelectronic microscope (SEM) were employed to analyse the failure mechanisms of CFRP composites inbending after immersion. The study showed that the mechanical properties are significantly reducedafter short term immersion due to the edge effects, while the damage to the fibre/polymer interfacebecomes more significant to laminate degradation after longer-term immersion.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Due to their high stiffness and strength to weight ratios, carbonfibre reinforced polymer (CFRP) composites have been widelyrecognised as potential candidates for many key structures in off-shore applications, including renewable energy (wind and currentturbines) and naval structures (hulls, masts and propeller shafts)[1,2]. In such industries, the mechanical structure is designed tohave a service life of several decades. Therefore environmentalissues should be taken into account in the evaluation of theirsuitability.

In the marine environment, composites are subjected to mois-ture and hydrostatic pressure. These environmental conditionshave important effects on the polymer matrix in particular, whilethe fibres are typically not affected as much by moisture or pres-sure; swelling or contraction of the polymer matrix is resisted bythe fibre so that residual (hygrothermal) stress develops in com-posites [3]. The increase of moisture content may not only causea gradual reduction of the glass transition temperature, which isoften a critical selection factor [4], but also change the stress distri-bution in the composite laminate. Since the moisture diffusion istime dependant, the hygrothermal stress should be investigatedin the time domain. However, few publications of these topics havebeen found in the literature.

Moisture diffusion in isotropic material, such as pure polymer,is governed by Fick’s first and second laws [5]. However, many

previous publications, e.g. [6–9], have shown that moisture diffu-sion in polymer-based composites also follows Fick’s laws.According to these previous studies, temperature does not changethe saturated moisture content but accelerates the process of diffu-sion. Shen and Springer [6] pointed out that, for many polymercomposites, the temperature distribution approaches equilibriumabout one million times faster than the moisture concentration.Therefore, the short time scale fluctuations in temperature canbe neglected compared to the effects of the variation of moisturecontent.

Previous experimental observations [6,10] have demonstratedthat, for polymer composites, the expansion induced by moistureabsorption is generally a linear function of moisture content ifthe moisture content is less than 2%. This relation is normally usedto determinate the coefficient of hygrothermal expansion (CHE) ina unidirectional lamina, and Classical Laminate Theory (CLT) can beemployed to calculate the CHE at the laminate level [3]. Since themoisture distribution inside composites is non-uniform through-out any given ply, CLT is unlikely to predict the hygrothermalexpansion and the associated stresses for a laminate with a compli-cated lay-up. The effects of hygrothermal stress on the mechanicalproperties of laminated composites appear not to have been inves-tigated sufficiently.

The aim of the present work is to investigate the impact ofhygrothermal stress on the failure mechanisms of composite mate-rials in bending, by means of experimental and numericalapproaches. Fresh water (tap water) and sea water were used forthe diffusion test to investigate the effect of NaCl on the degrada-tion of composite properties. A hydrostatic apparatus was used to

Table 1Laminate configuration.

Laminate Lay-up Thickness (mm) Ply-thickness (mm)

UD [0]16 2.08 0.13UT [90]16 2.08 0.13CP [90/0]4s 1.92 0.12AP [±45]4s 1.92 0.12

UD: Unidirectional (longitudinal); UT: Unidirectional (Transverse); CP: Cross-Ply;AP: Angle-Ply.

M. Meng et al. / Composite Structures 133 (2015) 1024–1035 1025

provide constant 70 bar pressure (equivalent to a water depth of700 m). In order to accelerate the diffusion process, all the threechambers were placed in an oven at a constant temperature of50 �C. After soaking the samples for one and three months respec-tively, both the interlaminar shear strength and flexural strengthwere measured and compared with that of un-soaked samples. A3D FEA model was developed to simulate the moisture diffusion,hygrothermal expansion and the coupling of hygrothermal stressand bending. This paper uses the ‘forensic’ approach: the bendingtest was carried out until fracture and the measured critical loadwas input into the FEA model, then the FEA results of stress distri-bution were used to explain the failure mechanisms observed inthe experiment. The combination of FEA modelling and experimentshowed the change of hygrothermal stress after various immersionperiods and the variation in composite failure modes.

2. Experimental methods

2.1. Material preparation

High strength carbon fibre/epoxy pre-preg (Cytec 977-2-12kHTS) was used in this study. This is a high temperature(180 �C) curing toughened epoxy resin with 212 �C glass transitiontemperature (Tg) which is formulated for autoclave moulding. Thearomatic epoxide-amine network [11] is constituted of bisphenolA diglycidyl ether (DGEBA) and diaminodiphenyl sulfone (DDS)which is shown schematically in Fig. 1. It can be seen fromFig. 1 thatthe DGEBA contains the hydroxyl radicals which are hydrophilic.

The pre-preg was laid up on a plane moulding tool and both thepre-preg plates and moulding tool were sealed in a vacuum bagbefore being put into the autoclave chamber which was under85 psi (0.6 MPa) pressure. Following the curing instructions in pro-duct technical data sheet [12], a heating ramp rate of 3 �C/min waschosen, followed by 2 h dwelling at 180 �C. The calculated fibrevolume fraction was Vf = 57.9% as described in [15].

Four typical lay-up sequences were chosen for the compositelaminates (Table 1). These four lay-ups are the simplest examplesof laminates which show a range of different laminate stacking:the unidirectional (UD) and unidirectional transverse (UT) lami-nates are fibre and matrix dominated which show the strongestand weakest mechanical properties, while the cross-ply (CP) andangle-ply (AP) laminates present intermediate properties. Thestudy of these four common lay-ups could provide a general viewof the interplay of fibre orientation and moisture diffusion. Thecomposite laminates were to be tested in bending following theISO standards [13,14] which required a nominal thickness of2 mm. Therefore all of the laminates in present work were made

Fig. 1. Schematics of crosslink network of 977-2 epoxy resin. The value n is in therange of 0–25.

up of 16 plies to satisfy the ISO standards. The two unidirectional(UD and UT) laminates were cut from the same composite plateswith different cutting orientations. The cutting pattern is shownin Fig. 2. The final thickness of the manufactured plates was notidentical. Possible reasons include manufacturing defects and mea-surement errors which have been discussed in the previous paper[15].

2.2. Accelerated water diffusion tests

In order to accelerate the water absorption, the chambers wereplaced in an oven at a constant temperature 50 �C. Three chamberswere used in the test, containing fresh water (tap water), sea waterand sea water at 70 bar hydrostatic pressure respectively. Fig. 3shows the hydrostatic chamber which was used in the diffusiontest. The hydrostatic chamber was made of stainless steel provid-ing approximate 20 litres of cylindrical space, and the pressurewas applied through the hose by a hydraulic pump. The specimenswere constrained and separated by breathing nylon cloth beforebeing immersed. The sea water was collected from the AtlanticOcean near Plymouth harbour, and the water was refreshed everymonth during the tests. The salinity of the sea water was variouswith seasons and depth, and the values were in range of 3.4–3.5% in weight. The salinity was similar with the open literaturesso that the chemical composition can be referred to the ASTMD1141 [16].

The composite laminates were immersed into the three cham-bers for the diffusion test. Following the ASTM D5229 [17], thespecimens were taken out at intervals to measure the moisturecontent and hygrothermal expansion. Before being immersed intothe water, all of the specimens were oven-dried at 70 �C for 48 h.The moisture content was measured by a weight scale with0.01 mg accuracy, while the dimension was measured by a verniercalliper with 0.01 mm nominal accuracy.

At least five specimens of each lay-up were immersed in eachchamber, and the mean values were calculated. The results areshown in Table 2. The equations used to calculate the apparentmoisture diffusivity (Dapp), longitudinal (DL) and transverse (DT)moisture diffusivities are shown in the Appendix A. Only UD andUT data were used to extract DT and DL for simplicity.

Because the longitudinal elastic modulus of carbon fibre ismuch higher than epoxy, the longitudinal hygrothermal expansionof the UD laminate is expected to be very small. Indeed, the mea-sured values of the CP and AP laminates were also smaller than theaccuracy of vernier calliper, and hence only the specimen with avery long width w (UT laminate) could provide measureableexpansion. The increase of hygrothermal expansion as a functionof moisture content is shown in Fig. 4. It can be seen from the fig-ure that the hygrothermal expansion of the ‘SP specimen’ showed asmaller value compared with ‘Tap specimen’.

2.3. Bending tests

The moisture content in composite laminates was saturatedafter 3-months of water immersion. In order to investigate the

Fig. 2. The cutting pattern of the composite laminates. UD and UT laminates were cut from one panel with perpendicular orientation, while CP and AP were from the other.

Fig. 3. The hydrostatic chamber used in the diffusion test.

1026 M. Meng et al. / Composite Structures 133 (2015) 1024–1035

environmental effects on the mechanical properties in timedomain, bending tests were carried out after 1-month and3-months of water immersion. The experiments were conductedusing 3-point bending according to the ISO standards [13,14].Typical loading force and displacement curves for the UD/UT/CPlaminates in both dry and 3-M SP conditions are plotted in Fig. 5.The zigzag aspect can be seen in the curves of UD and CP laminatesin dry condition; however the specimens showed a sudden breakafter immersion.

According to ISO 14130 [13], the interlaminar shear strengthcan be determined by the maximum loading Fmax and the specimendimension (width ‘w’ and height ‘h’),

Table 2Accelerated diffusion test results (50 �C).

aImmersion Mmax (%) Height (mm) Length (mm)

UD[0]16 Sea 0.81 2.06 204SP 0.89 2.06 204Tap 0.82 2.06 204

bUT[90]16 SP 0.88 2.06 15Tap 0.82 2.06 15

CP[90/0]4s Sea 0.89 1.95 286SP 0.92 1.95 286Tap 0.89 1.95 286

AP[±45]4s Sea 0.89 1.95 100SP 0.93 1.95 100Tap 0.89 1.95 100

a ‘Sea’: sea water immersion; ‘Tap’: tap water immersion; ‘SP’: sea water immersionb The UT laminate was immersed only in tap and SP conditions.

sappxz ¼ 34� Fmax

whð1Þ

At least five samples in each group were tested, and the appar-ent interlaminar shear strength sappxz (shown in Fig. 6), apparentflexural strength rapp

x and apparent flexural modulus Eappf (shown

in Table 3) were calculated. Please note that properties at 0-Month were measured prior to immersion so that they are inde-pendent of medium. So are the theoretical calculations.

Because the deflections of the UD/UT laminates were close tothe ‘large-deflection criterion’ (10%) and the deflection of CP lami-nate had exceeded the criterion, the flexural strength was then cal-culated by the ‘large-deflection correction’ [14],

ðr fmaxÞcor ¼

3FmaxL

2wh2 1þ 6Dmax

L

� �2

� 3Dmaxh

L2

� � !ð2Þ

The gradual degradation of interlaminar shear strength showeda similar trend with the work of Ryan et al. [18]whose specimenscontained the same epoxy system.

3. 3D FEA modelling of diffusion and bending

3.1. Moisture diffusion and hygrothermal expansion

It can be seen from Table 2 that the moisture diffusivity in thefibre direction is different from that in the transverse direction, soare the mechanical properties (elastic modulus, shear modulus andPoisson’s ratio). Based on this orthotropic assumption, a 3D FEA

Width (mm) Dapp (10�13 m2/s) DT (10�13 m2/s) DL (10�13 m2/s)

15 2.6 � �15 2.6 2.0 3.615 2.8 2.2 3.6

122 2.8 � �122 3.0 � �15 2.8 � �15 3.0 � �15 3.0 � �20 2.8 � �20 2.9 � �20 2.9 � �

with 70 bar hydrostatic pressure.

y = 0.3473x + 6E-05

y = 0.4412x - 0.0001

0.00%

0.05%

0.10%

0.15%

0.20%

0.25%

0.30%

0.35%

0.0% 0.2% 0.4% 0.6% 0.8% 1.0%

Hyg

roth

erm

al e

xpan

sion

Moisture content

SP

Tap

Fig. 4. Hygrothermal expansion of UT [90]16 laminate varying with moisturecontent.


model can be built using a rotated coordinate system to define thematerial properties of the off-axis plies,

x

y

� �¼ cosðhÞ � sinðhÞ

sinðhÞ cosðhÞ

� �X

Y

� �ð3Þ

where x and y are the original variables, X and Y are the transformedvariables in the rotated (h) coordinate system. The geometry andlay-up sequence of each group of laminates has been defined inTable 1.

In the experiments, the specimen was immersed into water, soall of the surfaces could be defined as being at the saturated mois-ture concentration. The saturated moisture concentration can becalculated by

cmax ¼ VqcMmax

18� 10�3V¼ qcMmax

18� 10�3 ð4Þ

where qc is the density of composite laminate, 18 � 10�3 is themolar mass of water with the unit kg/mol.

Substituting the saturated content Mmax = 0.9% from Table 2,and the density of CFRP composite qc = 1.6� 103 kg/m3, the saturatedmoisture concentration can be calculated as cmax = 800mol/m3.This value is necessary for FEA boundary condition.

Once the moisture distribution is solved, the associated straincan be calculated from the coefficient of hygrothermal expansion(b). Because the moisture absorption has no effect on the fibre,the ‘rule of mixture’ should be used [3], and the principal CHE val-ues at the lamina level can be calculated by

b1 ¼ EmE1

qcqm

bm

b2 ¼ ð1þ mmÞ qcqm

bm � b1m12

(ð5-1Þ

Fig. 5. Typical flexural force-extension curves of UD/UT/CP laminates.

qc ¼ qf Vfþ qmð1� Vf Þ ð5-2ÞFor many epoxy matrices, the bm value is of the order of 0.32

[19], which is used in the present FEA model. Classical LaminateTheory (CLT) can be used to predict hygrothermal expansion inthe off-axis laminates. Applying the 3D version of the transforma-tion matrix into the CHE vector gives

bk ¼ Teb

Te ¼

cos2 h sin2 h 0 0 0 coshsinhsin2 h cos2 h 0 0 0 �coshsinh0 0 1 0 0 00 0 0 cosh sinh 00 0 0 �sinh cosh 0

�2coshsinh 2coshsinh 0 0 0 cos2 h� sin2 h

26666666664

37777777775ð6Þ

Substituting Eq. (4) into the 3D version of CLT, the apparent CHEof laminate can be evaluated by Eq. (5) as described in [3],

b ¼ aNC ¼ aXnk¼1

Ckbktk ð7Þ

where a is the ‘a’ block of the ‘abbd’ matrix; NC is force per unitlength caused by free moisture expansion; Ck is the full 3D stiffnessmatrix of the kth ply; tk is the thickness of the kth ply. The 3D CLTformulae were solved by MATLAB [20].

The FEA solution gives the distribution of moisture concentra-tion in 3D. Integration should be carried out to obtain the moisturecontent, and then hygrothermal expansion can be calculated bymultiplying the coefficient of hygrothermal expansion with mois-ture content. Considering the inverse form of Eq. (4) in an infiniteelement, the expansion term can be expressed as

eCHE ¼ 18� 10�3cVqcV

b ¼ 18� 10�3cbqc

ð8Þ

Eq. (6) can be used to specify the coupling relation in the FEAmodel since the expansion term relates the moisture concentrationto mechanical expansion (eCHE). It should be noted that both themoisture concentration and the expansion are variable in timeand space domains.

The elastic properties are then introduced to calculate thehygrothermal stresses. The mechanical properties and diffusionproperties used in the FEA model are shown in Table 4.

0

20

40

60

80

100

120

Sea SP Tap Sea SP Tap Sea SP Tap

0M

1M

3M

CP UD AP

Fig. 6. Measured ILSS (MPa) of UD/CP/AP laminates before (0 M) and after moisturediffusion. 0-M: dry condition; 1-M: 1-month immersion; 3-M: 3-month immersion.

Table 3Bending test results and their Standard Deviations (SDs).

Immersion rappx ðMPaÞb Eappf ðGPaÞ

0-Ma 3-M CLTc 0-M 3-M

UD Sea 1598 ± 56 1696 ± 41 139 120 ± 3 121 ± 4SP 1780 ± 122 121 ± 4Tap 1688 ± 137 122 ± 2

UT SP 117 ± 5 99 ± 6 8.8 8.4 ± 0.3 9.1 ± 0.1Tap 102 ± 4 9.0 ± 0.2

CP Sea 1416 ± 53 1441 ± 40 62 56 ± 2.2 58 ± 0.4SP 1398 ± 88 58 ± 1.6Tap 1400 ± 74 57 ± 0.8

a 0-M: dry condition; 3-M: 3-month immersion.b The values of UD and CP laminates were calculated with the ‘large-deflection correction’.c Calculated by Classical Laminate Theory (CLT) with the elastic properties shown in Table 4.

Fig. 7. Illustration of the bending model.

Table 5Geometries and boundary conditions in different groups of coupons.

UD CP AP

Longbeam

Shortbeam

Longbeam

Shortbeam

Shortbeam

Length (mm) 100 20 100 20 20Width (mm) 15 10 15 10 10Height (mm) 2.08 2.08 1.92 1.92 1.92Span (mm) 80 10 79 10 10Force (N) 853 2933 574 2223 1395

r1 = r2 (mm) 2cmax (mol/m3) 800


3.2. Hygrothermal stress coupled with bending

Due to the development of hygrothermal stresses after waterabsorption, the flexural stresses re-distribute when the compositelaminates are subjected to bending. Therefore, the hygrothermalexpansion was introduced as the initial strain in the mechanicalmodel, and the diffusion/expansion were solved simultaneously.Since the diffusion (as well as the expansion) is time dependent,the mechanical model was solved in time domain, although theapplied load was static.

The geometries used in the FEA models were the same as thetest condition, and the maximum static loads evaluated in dry con-dition were applied in the mechanical model. The flexuralstrengths were investigated by long beam method [14], while theinterlaminar shear stresses were investigated by short beammethod [13]. Fig. 7 shows the schematics of FEA model.

Reis’ study [21] on the 3-point bending modelling reported thecompressive failure of longitudinal fibre at the loading point whenthe contact was considered in the FEA model. The authors’ previouspaper [15] presented similar failure mechanism using sinusoidaldistributions instead of contact condition shown in Fig. 7, in orderto minimise the stress concentration at the loading and supportfixtures. The setup of the mechanical models and the mesh qualitycontrol have been presented previously [15].

Table 5 shows the dimensions and loading forces for differentgroups of coupons. The thickness of specimen was adjusted tothe 0.13 mm for UD and 0.12 for CP/AP laminates in order to com-pensate the consistence of each ply. Once the FEA models were setup, the moisture distribution and the induced hygrothermalexpansion inside laminate were investigated. The process of diffu-sion is very slow so that the moisture distribution varies in spatialand time domains. Additionally, the computing time (or DOFs)increased exponentially when the coupling term was introduced.Therefore, only 90 days of immersion were investigated. The FEAmodels were solved by COMSOL Multiphysics [22].

Table 4Material properties for moisture diffusion modelling.

Longitudinal modulus E1 (GPa) 139Transverse modulus E2 = E3 (GPa) 8.8In-plane shear modulus G12 = G13 (GPa) 4.7Transverse shear modulus G23 (GPa) 3.0In-plane Poisson’s ratio m12 = m13 0.26Transverse Poisson’s ratio m23 0.48Longitudinal diffusivity D1 (m2/s) 3.6 � 10�13

Transverse diffusivity D2 = D3 (m2/s) 2.2 � 10�13

Longitudinal CHE b1 0Transverse CHE b2 = b3 0.49

4. Results and discussions

4.1. Moisture diffusion and hygrothermal expansion

After 3-months accelerated water absorption, all of the speci-mens had become saturated. Table 2 has given the saturated mois-ture content of each group of specimens and their apparentmoisture diffusivities. With the different geometries of UD andUT laminates, the longitudinal DL and transverse DT moisture diffu-sivities were calculated by Eq. (A4), shown in the Appendix A. It isshown that the moisture diffusion along fibre orientation (DL) isabout 60% faster than transverse direction (DT).

Because the fibres do not absorb the moisture, the saturation ismainly dependent on the matrix and the fibre volume fraction. Ithas been known that the saturation and diffusivity vary for differ-ent polymer system. The typical saturation of polymer is epoxy1.5% [12], vinyl ester 1.5% [23], polyester 1.5% [24], polyimide4.4% [25] and PEEK 0.5% [26]. The diffusion test results of Ryanet al. [18] gave a similar nominal saturation with the present work.

Fig. 8 shows the water absorption in UD, UT, CP and AP lami-nates vs. square root (time). It can be seen that, within each lay-up, the saturated moisture content and the moisture diffusivity

Fig. 8. Moisture diffusion in UD/UT/CP/AP laminates immersed at 50 �C sea water (‘Sea’), tap water (‘Tap’) and sea water with 70 bar hydrostatic pressure (‘SP’).


do not show noticeable differences between the sea water and tapwater medium.

However the higher pressure induces a larger saturation in allfour lay-ups. In the present case of 70 bar hydrostatic pressure,the saturation is 5% larger than that at ambient pressure. The dif-fusion slope ðM2 �M1Þ=ð

ffiffiffiffit2

p � ffiffiffiffit1

p Þ in the high pressure case isalso larger. However, by substituting the diffusion slope and satu-ration into Eq. (A2), it is found that the moisture diffusivity in thehigh pressure environment is the same as the normal pressure, asshown in Table 2.

The CP/AP laminates were cut from one composite plate, whilethe UD/UT laminates were cut from another plate. Therefore, theCP/AP laminates show similar results, as well as the UD/UT lami-nates. The calculated moisture diffusivities of the four lay-upsare shown in Table 2.

Although the longitudinal and transverse diffusivities showapparently different values, as shown in Table 2, the FEA modellingpresented a smooth distribution of moisture concentrationthroughout the whole laminate with both complex lay-up (suchas AP and CP laminates) and simple lay-up (such as UD and UTlaminates). This is because the moisture diffusion is time depen-dent and the process is very slow. Fig. 9 shows the FEA result formoisture distribution within an AP short beam laminate in slice-view after one month water immersion indicating different depthof water penetration in longitudinal and transverse. It can be seenthat the moisture concentration distributed smoothly in the slice-section of xz-plane and yz-plane. Specifically, the moisture distri-bution through-thickness was extracted to analyse the effect ofthe longitudinal and transverse diffusivities, as shown in Fig. 10.

Fig. 10 shows the moisture distribution on the mid-line of the APlaminate in the time domain, which indicates smooth distributionthrough-thickness regardless of the ply-orientation at differenttimes. It can also be seen that the saturation occurs after 90 daysof immersion.

The saturated hygrothermal expansion measured in the UT lam-inate showed a value of about 0.4% in tap water immersion (nor-malised to Mmax = 0.9%), and a lower value (0.32%) in sea waterwith 70 bar hydrostatic pressure (SP), as shown in Fig. 1. One pos-sible reason is that the hydrostatic pressure reduced thehygrothermal expansion. However, no measureable expansionwas observed in UD/CP/AP laminates.

Substituting the saturated concentration, CHE and compositedensity in Eq. (6), a reference transverse strain in an infinite unidi-rectional plate can be estimated to be

eT ¼ 18� 10�3cmaxb3

qc¼ 18� 10�3 � 800� 0:49

1:6� 103 ¼ 0:44%:

In order to compare with this reference value, the hygrothermalstrains of three types of laminates were extracted from the FEAsimulations. Since the specimen was under free expansion inducedby hygrothermal effects, the apparent normal strains can beextracted from the average normal displacements on the surfaces,

ex ¼ uxl2

� ��1

ey ¼ uyw2

� ��1

ez ¼ uzh2

� ��1

8>><>>: ð9Þ

Fig. 9. Moisture distribution according to FEA in AP short beam laminate after 30 days’ water immersion (dimension unit: mm). The slice plot shows a smooth distribution ofmoisture concentration regardless of the ply orientations.

Fig. 10. FEA moisture distribution of AP short beam laminate along the mid-lineafter 6/30/60/90 days’ water immersion respectively. The moisture diffusedsmoothly inside the laminate, and converged to saturation.

0.0%

0.1%

0.2%

0.3%

0.4%

0.5%

0.6%

0.7%

0 2 4 6 8 10

Hyg

roth

erm

al e

xpan

sion

Sqrt(day)

UD

UT

CP

AP

Fig. 11. FEA out-of-plane hygrothermal expansion of UD/UT/CP/AP short beamlaminates.


where ex, ey and ez are the apparent normal strains; ux, uy and uz arethe average normal displacements of yz, xz and xy surfaces;. l

2,w2 and

h2 are the half-length, half-width and half-height of specimenrespectively.

Fig. 11 shows the expansion curves of UD/UT/CP/AP short beamlaminates as a function of square root of time. It can be seen thatthe curves of UD/UT laminates are overlapped, so are the CP/APlaminates. It is noted that the maximum hygrothermal expansionof UD/UT laminates is in line with the reference value (0.44%)but CP/AP laminates have significantly higher values.

The CLT calculation of laminate CHE can explain this increase ofexpansion in CP/AP laminates. Applying Eqs. (4) and (5) to CP lam-inate, the apparent CHE can be calculated giving bx = by = 0.04,bz = 0.70. The predicted out-of-plane CHE value (bz) is in line withthe maximum CHE extracted from FEA simulations shown inFig. 11.

4.2. Hygrothermal stresses and edge effect

In the FEAmodel, the concentration on all surfaces was constantduring the entire diffusion process. The hygrothermal stresseswere induced in ‘free edge’ region at the very beginning, and thenthe stresses propagated inside the specimen following the mois-ture diffusion. Fig. 12 shows the through-thickness distributionof interlaminar shear stress (s13) within the AP short beam lami-nate after one month’s diffusion. It can be seen that the interlam-inar shear stress induced by hygrothermal expansion could be ashigh as the 20% of the interlaminar shear strength (as shown inFig. 6). This high value of stress might induce the stress re-distribution when the laminate is subjected to mechanical loading.However, this induced interlaminar shear stress only appeared atthe interfaces of plies and decayed rapidly inside the laminateand finally converged to zero in the centre. The coupling of thehygrothermal stress with bending will be discussed in below.

Such a strong ‘free edge’ effect can also be found in the CP lam-inate. Fig. 13 shows the surface plot of interlaminar shear stress s13in the CP short beam laminate. Compared with the interlaminar

Fig. 12. FEA result for interlaminar shear stress s13 induced by hygrothermal effectin AP short beam laminate after one month’s diffusion.

Fig. 14. FEA through-thickness distribution of r2 at central point (x = 10 mm,y = 5 mm) of CP short beam laminate in the time domain. The transverse stressshifts into compression.


shear strength measured in dry condition, this induced stress s13was so high that it could not be neglected in the context of fractureor fatigue initiation. Additionally, this induced shear stress s13would reduce the measured interlaminar shear strength signifi-cantly, as shown in Fig. 6.

In the previous section, the ‘free edge’ effect induced byhygrothermal expansion was observed in 3D FEA model. It showedthe induced interlaminar shear stress s13 is of the order of 20 MPain CP/AP laminate. Transverse normal stresses are also as high as:30 MPa (r2 or r3) in CP/AP laminates and 10 MPa (r2 or r3) in UD/UT laminates. And this edge effect decayed rapidly inside the lam-inate within a couple of ply-thickness.

The FEA model introduced this hygrothermal expansion as theinitial strain. When the bending condition was applied, this effectwould be coupled with the bending stress leading to stress re-distribution. Due to the small deformation in FEA model, the

Fig. 13. FEA result for interlaminar shear stress s13 induced by hygrothermal e

diffusion and expansion could be considered as a one-way couplingproblem: the effect of structural deformation on moisture diffusionwas not considered in the FEA model. Therefore, the time depen-dent diffusion was the same as the case without bending. On theother hand, the hygrothermal stresses shown in the case of ‘freeexpansion’ were coupled with the bending stresses. Fig. 14 showsthe time dependent distribution of transverse normal stress in CPshort beam laminate. With the increase of moisture content inthe laminate, the hygrothermal stress showed an increasing trendwith the concentration.

Fig. 15 shows the through-thickness distribution of interlami-nar shear stress (s13) of the AP short beam laminate. Comparedwith the dry condition (shown in [15]), the coupling had no effecton the maximum value of s13. However, the stress was asymmetricabout the mid-plane, and the positions of peaks shifted to the bot-tom side.

xpansion in CP short beam laminate after one month’s water absorption.

Fig. 15. FEA through-thickness distribution of s13 after one month’s diffusion in APshort beam laminate.


Fig. 16 gives a typical failure image of AP laminate from a bend-ing test. The sample was immersed for one month in tap water.Compared with the dry condition, more cracks were found onthe bottom side and the cracks propagated a longer distance insidethe laminate. However, the interlaminar shear strength of the APlaminate showed a small variation due to the water immersion.

Fig. 6 has shown the measured ILSS after three kinds of waterimmersion. The samples were taken out of the chambers for bend-ing test after 1 month or 3 months’ immersion. The AP laminateshowed a consistent value of ILSS for all the immersion conditions,and no degradation of this property was found. However, itshowed quite a different failure mode from the dry condition,due to the hygrothermal expansion. Withstanding the same levelof bending load, the AP laminate presented much larger cracksafter water immersion, and more cracks appeared on the bottomside, as shown in Fig. 16. This is because the peak values of inter-laminar shear stress shift to the bottom side, which has beenshown in the FEA results in Fig. 15.

The ILSS of the CP laminate showed a sharp reduction (20%)after 1 month immersion and a slight increase after 3 monthsimmersion. However, regardless of the three kinds of medium,the ILSS of CP laminate remained at the same level with respectto immersion time (0/1/3 month), which is similar to the UD andAP laminates.

Fig. 16. Typical failure image of AP short beam laminate in ILSS test (side v

The ILSS of the UD laminate showed a gradual decrease from1 month to 3 months water absorption, but no significant differ-ence among the three kinds of medium was observed. It was foundthat, compared with the interlaminar failure in dry condition, theUD laminate failed by the plastic deformation after water absorp-tion. Fig. 17 shows a typical failure image of UD short beam lami-nate. The laminate shown in Fig. 17b was immersed in sea waterfor 1 month before being tested in bending. A large number ofmicro cracks were found in the optical microscope image.

4.3. Flexural stress and modulus

According to the previous 3D FEA study [15], the UD laminatefailed by compression in bending condition, and the flexuralstrength was the same as the laminate compressive strength inuniaxial compression. If the thermal residual stress is taken intoaccount, the composite laminate was subjected to initial compres-sion in the dry condition. When the composite laminates wereimmersed, the hygrothermal expansion should relax the thermalresidual stress. As a consequence, the UD laminate showed a rela-tive higher flexural strength in bending after moisture absorption.Fig. 18 shows the normalised flexural strength of CP/UD/UT lami-nates in three kinds of water immersions. The flexural strengthof UD laminate showed a 5–10% increase (of the order of100 MPa) compared to that in dry condition.

Although many researchers have investigated the degradationdue to the moisture ingress, most of them have focused on eithertension or compression. Some researchers had reported that thetensile strength showed gradual decrease with increasing immer-sion time [4,18,27]. In bending, composite laminates are subjectedto tension, compression and shear, which is quite different fromthe uniaxial tension and compression. Therefore, the bending testrepresents more general condition, and the change of stress distri-bution contributed significantly to the failure mechanisms.

On the other hand, the flexural strength of UT laminate showeda dramatic decrease after moisture diffusion, while the CP laminateretained the flexural strength of the dry condition. Since the failuremechanism of UT laminate was matrix dominated, the flexuralstrength was strongly dependent on the bonded interface betweenmatrix and fibre.

Fig. 19 shows the normalised flexural moduli of CP/UD/UT lam-inates in three kinds of water immersions. It can be seen that theflexural modulus of CP/UD laminates had a small fluctuation aftermoisture absorption, however, the UT laminate showed an oppo-site trend to its strength. Compared to the decrease in flexuralstrength shown in Fig. 16, the UT laminate became stiffer after

iew). (a) in dry condition; (b) after one month’s tap water immersion.

Fig. 17. Typical failure image of UD short beam laminate in ILSS bending test (side view): (a) in dry condition, (b) after 1-month’s water immersion. The delamination wasuncompleted in wet condition (b).

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

CP UD UT

Sea

SP

Tap

Fig. 18. Flexural strength of CP/UD/UT laminates after 3-month’s water absorption.The values were normalised by the measurement in dry condition.


moisture absorption. One possible reason is because the epoxybecame stiffer when the water molecules diffuse inside the longmolecular chain of polymer. The chemical structure of epoxy resinwas shown in Fig. 1. Due to the competition of sulfone bridge, theelectron density of nitrogen–carbon bond between DDS andDGEBA is relatively low which leads to water resistance of DDSunit [28]. According to reference [29], the propagation rate con-stant for hydrocarbon oxidation in propanol unit is higher thanthe one in isopropylidene unit, which means that the hydroxylradical in DGEBA exhibits relatively more hydrophilic. As aconsequence, the water molecular was mainly absorbed by the

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

CP UD UT

Sea

SP

Tap

Fig. 19. Flexural modulus of CP/UD/UT laminates after 3-month’s water absorption.The values were normalised by the measurement in dry condition.

sub-branch (OH-) of DGEBA which had no effect on strength ofthe cross-link network but might slightly enhance the stiffnessof the resin. Therefore, the decrease of strength observed in UTlaminate might be caused by the degradation of fibre/matrixinterface which will be discussed in below.

Scanning electronic microscopy (SEM) was used to examine theinterface of fibre/epoxy at the fracture surface. The fracture debriswas taken from UT laminate, and dry/tap/sea conditions wereexamined. The samples were coated with gold/paladium beforebeing examined in SEM. Figs. 20–22 show the particular fracturesurfaces of the three conditions. There are two magnifications ineach figure, 500 times and 4000 times.

Fig. 20 illustrates the transverse fracture surface within a drycondition UT sample. The epoxy was still attached to the carbonfibre, so that the fractured polymer showed a tough wave-likemorphology. Without water ingression, the epoxy provided ade-quate adhesion to the carbon fibre, and the failure mode tendedto be the tensile fracture of epoxy rather than the debonding offibre/epoxy interphase.

Figs. 21 and 22 show similar characters of transverse fracturesurface of tap water and sea water conditions. At a lower magnifi-cation, it can be seen in Fig. 21 that the epoxy became porous;however the wave-like morphology was still observed which indi-cates that the failure mode was the tensile fracture of epoxy. At ahigher magnification in Fig. 22, the carbon fibre showed sections

Fig. 20. SEM image of dry sample with a local magnification.

Fig. 21. SEM image of tap water condition.

Fig. 22. SEM image of sea water condition.


of bare surface which indicates that the adhesion of epoxy on car-bon fibre had deteriorated after water absorption.

It should be noted that the SEM could only examine a local areaunder a relative high magnification. It was found that, in sea watercondition, the number of bare fibres was higher than the tap watercondition. Therefore, it is reasonable to believe that the degrada-tion in sea water is more severe in long term exposure.

5. Conclusions

Composite structures exposed in marine environment are sub-jected to many aspects, among which this paper has investigatedthe effects of water immersions on the CFRP composites. A robust3D FEA model has been developed to analyse the hygrothermaleffects. The experimental results showed a good agreement withthe FEA solutions, which has also been validated by CLT calcula-tion. Some findings are concluded according to the study:

(a) The moisture diffusivity in the tap/sea/SP water immersionsshowed negligible difference at the same temperature.Although the longitudinal moisture diffusivity presented a

much higher value than the transverse diffusivity (60% inthis study), the moisture diffusion in composite laminatesshowed a smooth distribution through thickness regardlessof ply orientation.

(b) Hygrothermal stresses could be induced at the very begin-ning of diffusion, and these stresses mainly appeared atthe edge region (edge effect) which means that the laminatelay-up becomes a critical issue for the exposed surfaces. Forinterlaminar shear stress, the induced hygrothermal stresscould be as high as 20% of the strength. Therefore, in thedesign of marine composites, it is desirable to avoid compli-cated lay-ups at the connection region of composite joints ornotches in order to improve the fatigue properties.

(c) The water absorption had no significant effects on thestrength of DGEBA-DDS cross-link system (i.e. 977-2 epoxyresin). However, the composite strength was reduced signif-icantly due to the degradation of fibre/matrix interface, andit was observed that the degradation of the interface in seawater was more significant than in fresh water.

(d) The SEM analysis has shown a variety of matrix fracturemorphologies and the degradation of fibre/matrix interfaceincreases the risk of interface debonding in CFRP compositesin simulated marine environment.

Acknowledgments

The authors would like to thank Professor Long-yuan Li for hisadvice on FEA modelling, Dr Richard Cullen for his kind help withcomposites manufacturing, Terry Richards for his support of themechanical tests, and the financial support of the School ofMarine Science and Engineering, Plymouth University.

Appendix A

According to the previous study [3], the moisture diffusivity ofcomposites can be expressed as an exponent function of the envi-ronmental temperature (known as an Arrhenius relation),

D ¼ D0 exp�Ea

RT

� �ðA1Þ

where R = 8.31 J mol�1 K�1 is the gas constant; D0 is a material con-stant; and Ea is the activation energy for diffusion.

Since the carbon fibre is assumed not to absorb moisture, thecomposite’s moisture diffusivity is mainly dependent on the poly-mer. A previous study [30] provides the reference parameters ofEq. (A1) for 977-2 epoxy: D0 = 2 � 10�9 m2 s�1, Ea/R = 2910 K�1.Therefore, the moisture diffusivity of HTS/977-2 can be roughlyestimated of the order of 3 � 10�13 m2/s in the present case (50 �C). According to ASTM D5229 [17], the reference time period foreach measurement is established by 0.02 h2/D, giving an approxi-mate interval of 5 days.

Fick’s laws were applied to calculate the apparent moisture dif-fusivity (Dapp) once the saturated moisture content (Mmax) is esti-mated [17],

Dapp ¼ p h4Mmax

� �2 M2 �M1ffiffiffiffit2


p� �2

ðA2:1Þ

M ¼ Mmax 1� exp �7:3Dappt

h2

� �0:75 !" #

ðA2:2Þ

where h is the specimen thickness;M is the specimenmoisture con-tent (%); t is the absorption time; ðM2 �M1Þ=ð

ffiffiffiffit2


p Þ is the slopeof moisture absorption plot in the initial linear portion of the curve.

Fig. 23. Orthotropic moisture diffusivity assumption and the two geometries forthe calculation of longitudinal and transverse diffusivities.


Like the orthotropic elastic properties in laminated composites,the previous study had reported that diffusion properties also showorthotropic [31]. Fig. 23 is an illustration of the longitudinal andtransverse moisture diffusivities in both micro and macro scales.

If the specimen dimension is finite (which is happening in pre-sent case), the longitudinal and transverse moisture diffusivitiesshould be used to compensate the edge correction [31],ffiffiffiffiffiffiffiffiffiDapp

p¼ wþ h

w

ffiffiffiffiffiffiDT

pþ h

l

ffiffiffiffiffiffiDL

pðA3Þ

The apparent diffusivity Dapp is determined by Eq. (A2), while DL

and DT can be determined by two different geometric samples,

DT ¼ l1ffiffiffiffiffiffiffiffiffiDapp

1

q� l2

ffiffiffiffiffiffiffiffiffiDapp

2

q 2� w1þh

w1� l1 � w2þh

w2� l2

�2

DL ¼ l1h

ffiffiffiffiffiffiffiffiffiDapp

1

q� w1þh

w1


p h i28>><>>: ðA4:1Þ

With a very long length l and short width w (as in the UD lam-inate), Eq. (A3) is reduced toffiffiffiffiffiffiffiffiffiDapp

1

q¼ w1 þ h

w1


pðA4:2Þ

With a very long width w and short length l (as in the UT lam-inate), Eq. (A3) is reduced toffiffiffiffiffiffiffiffiffiDapp

2

q¼


pþ hl2

ffiffiffiffiffiffiDL

p: ðA4:3Þ

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Effect of Hygrothermal Stress on the Failure of CFRP Composites