Effect of cellulose nano fiber (CNF) on fatigue performance of carbon fiber fabric composites

Composites: Part A 76 (2015) 244–254

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Effect of cellulose nano fiber (CNF) on fatigue performance of carbon fiber fabric composites Yongzheng Shao ⇑, Tomoya Yashiro, Kazuya Okubo, Toru Fujii Department of Mech. Eng. and Systems, Doshisha University, Kyoto 610-0394, Japan

a r t i c l e

i n f o

Article history: Received 12 December 2014 Received in revised form 7 April 2015 Accepted 1 May 2015 Available online 9 June 2015 Keywords: A. Carbon fiber A. Polymer matrix composites (PMCs) B. Fatigue

a b s t r a c t The effect of cellulose nano fibers (CNF): micro-fibrillated cellulose and bacteria cellulose fibers were investigated on the fatigue life of carbon fiber (CF) fabric/epoxy (EP) composites. Epoxy used as the matrix was physically modified with CNF in advance before fabricating the laminates. The high cycle fatigue strength was significantly improved at 0.3 wt% CNF. There exists an appropriate CNF content which makes the fatigue life longest. An increase of adhesive strength between CF and matrix results due to physical modification with CNF. The adhesive strength much increases with increasing the CNF content. Almost no interfacial debonding occurs at 0.8 wt% CNF content when CF breakage takes place. On the other hand, some debonding occurs along CFs from the breaking point at 0.3 wt% CNF. Debonding is more significant in the case of no CNF addition to the matrix. An appropriate interfacial strength brought at 0.3 wt% CNF is the key of fatigue life extension. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The damage modes occurring in fiber reinforced polymer matrix composites (FR-PMC) are primarily matrix cracking including transverse cracking, interfacial debonding between fiber and polymer matrix, delamination between layers and fiber failure [1]. Woven fabric fiber reinforced PMC perform more balanced properties and exhibit excellent drapability reducing manufacturing cost. They also increase the resistance to impact damage. However, the internal damage progression under both monotonic and cyclic loadings is more complicated due to stress concentration at the cross-over points between warp and weft yarns. Many studies were conducted to significantly improve the mechanical properties of FR-PMC by arresting/trapping/delaying the growth of those damages due to an addition of nano fillers to the matrix, such as nano silica [2], nano rubber particles [3], nano clay [4], nano fibers including carbon nano tubes (CNT) [5], nano polyvinyl alcohol (PVA) fibers [6] and carbon nano fibers [7]. Many studies have been conducted to understand the effect of nano fillers. Manjunatha et al. [2] found that suppressed matrix cracking and reduced crack propagation rate in the matrix contributed to the enhanced fatigue strength of a GFRP composite modified by silica nanoparticles based on the matrix cracking and stiffness degradation observation ⇑ Corresponding author. Tel.: +81 774 65 6421; fax: +81 774 65 6432. E-mail address: [email protected] (Y. Shao). http://dx.doi.org/10.1016/j.compositesa.2015.05.033 1359-835X/Ó 2015 Elsevier Ltd. All rights reserved.

under transmitted light during testing. Grimmer and Dharan [8] found that the CNTs slowed the propagation rate of delamination due to CNTs bridging, fracture and their pull-out resulting in improvement of fatigue life by a factor of two to three by fatigue mode-I double cantilever beam (DCB) tests. Scanning electron microscopy (SEM) and scanning acoustic microscopy (SAM) on a CF/EP composite whose matrix contained some nano clay, revealed that delamination growth was delayed and the final failure was suppressed due to an improvement of CF/EP adhesion and formation of nano-clay induced dimples [4]. Internal damage progression in CF/EP composites by nano rubber particles modification was investigated by the acoustic emissions (AE) method, and residual strengths was examined as well. The improvement of fatigue strength for those composites was supposed due to an increase in the fatigue crack resistance to interlaminar delamination [3]. It was found that nano fillers in polymer matrix can delay the onset and propagation of matrix cracks and delamination, which must resultantly extend the fatigue life of FR-PMC. However, more studies are needed to more apparently understand the effect of nano fillers on the properties of matrix, interface and subsequent fatigue performance of FR-PMC. When surface-treated nano rubber particles (5–10 wt%) are dispersed into epoxy (EP) matrix, the interlaminar fracture toughness and fatigue performance can be significantly improved [9]. However, the observed Young’s modulus and heat resistance of the composites probably degrade. Nano fibers having high

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strength, Young’s modulus and Tg (glass transition temperature) are considered more effective reinforcement than nano-particles for PMC due to their high aspect ratio. CNT has superior mechanical properties, considered as one of excellent nano fillers to improve the durability of PMC [10,11]. 1 wt% multi-wall CNT incorporation can achieve 250% extension of fatigue life of a GF/EP composite [5]. Unfortunately, the cost and carcinogenicity limit its application. Furthermore, the issues are still challenging such as surface treatment for better CNT/polymer matrix adhesion and the dispersion of CNT into the matrix system. Cellulose nano fibers (CNFs): BC (bacterial cellulose) secreted extracellularly by certain types of bacteria, or MFC (micro fibrillated cellulose) extracted from plant pulp, possess high strength and high stiffness combined with lower density than 1.5 g/cm3, biodegradability, renewability and good compatibility with resin [12,13]. It is prospective to use CNF as a modifier of polymer matrix. In our previous works, CNFs having the web-like network structure have already been used to improve the interfacial adhesion in a bamboo polymer composite [14] and the interlaminar fracture toughness. The great possibility of fatigue life extension for CF/EP composites was also shown [15,16]. In this study, the effect of MFC and BC on the mechanical properties, especially fatigue performance of CF/EP composites is investigated.

2. Experimental method 2.1. Materials Plain-woven (PW) carbon fiber fabric (Mitsubishi Rayon TR3110MS) and carbon fiber tow (Mitsubishi Rayon TR50S 12L) were used as reinforcement for woven fabric and unidirectional (UD) composites respectively. The balanced plain fabric has a thickness of 0.23 mm, an area density of 200 g/m2, the same number of ends and picks per inch of 12.5. It is woven by tow TR30S 3L with a tensile strength of 4.12 GPa, tensile modulus of 234 GPa, and a density of 1.79 g/cm3. TR50S 12L has a tensile strength of 4.9 GPa, tensile modulus of 240 GPa, and a density of 1.82 g/cm3. Epoxy resin (Epikote 828) and modified aliphatic polyamine (JER cure-113) were used as matrix and curing agent (both were supplied by Japan Epoxy Resins Co. Ltd.). MFC (Celish KY110G, water slurry containing 10 wt% fibers, Daicel Chemical Industries, Ltd.) and BC (Nata de Coco, Fujicco Co., Ltd) were used as physical modifiers of epoxy matrix. TEM and SEM images of MFC and BC with a net structure are shown in Fig. 1. The diameter was 50–200 nm. The mechanical properties are found elsewhere [17].

2.2. Preparation of CF/EP composites MFC is always supplied as water slurry since MFC is very hydrophilic. The solvent exchange method with ethanol was used to remove the water in MFC slurry. First, MFC slurry was mixed with ethanol 10 times as great as water in the slurry by a homogenizer, and then filtered by a vacuum pump. H2O molecules were replaced by ethanol molecules due to its stronger hydro bonding to MFC. This process was repeated twice to confirm no water existence in the ethanol based MFC slurry. Then, it was put and mixed in the epoxy for 30 min by a process homogenizer. The weight fraction of MFC was set 0, 0.3 and 0.8 wt% of the epoxy matrix. Finally, the mixture was heated at 85 °C in a vacuum oven for 3 days to remove ethanol as much as possible. For BC, nata de coco was mixed with water and smashed by a homogenizer. Then the mixture was filtered by vacuum pump to remove water and acetic acid. In order to remove the contained acetic acid completely, this process was repeated for 5 times. Then, solvent exchange with ethanol, and following BC/epoxy mixture preparation were conducted as same as the MFC/EP mixture preparing process. The weight fraction of BC was also set 0, 0.3 and 0.8 wt% of the epoxy matrix. After adding 33 wt% curing agent into the above EP/CNF mixture, it was degassed for 30 min to remove voids. Plain-woven carbon fabrics (8 layers) were laminated by the hand-lay-up method using the above epoxy resin. The CF volume fraction is about 50%. Then, the laminates were hot pressed at 120 °C for 2 h to form PW CF/EP composites. Parallel sided specimens (200  25  2 mm with a gage length of 100 mm) were prepared for tensile and fatigue test. Aluminum alloy tabs with a thickness of 2 mm were glued on both edges. UD CFRP specimens were filament wound at room temperature using the degassed EP/CNF mixture and then cured at 120 °C for 2 h. The CF volume fraction was also about 50%.

2.3. Mechanical tests 2.3.1. Static tests Tensile strength and Young’s modulus of PW CF/EP composites were measured according to ASTM D3039-08 at a cross-head speed, 1 mm/min. At least 5 specimens of each kind of CF/EP composite have been tested. Mode I fracture toughness [18,19], GIC for unmodified and modified (0.3% and 0.8% CNF) epoxy matrixes was measured using coupon specimens with a notch in their one side to reveal how much physical modification increased the resin toughness due to a small amount of nano fibers addition. All tests were conducted under the laboratory condition.

BC

MFC

200nm

245

2 µm

Fig. 1. TEM image of MFC and SEM image of BC fibers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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2.3.2. Fatigue tests Tension–tension fatigue tests were conducted at a stress-ratio, R = 0.1 and frequency f = 5 Hz according to ASTM D3479 at room temperature while specimens were cooled by a fan. The maximum cyclic stress of 600, 550 and 500 MPa was applied. Each fatigue test was run to 2 million cycles unless final failure happened before 2 million cycles. Both static tensile and fatigue cyclic loads were applied along the wrap direction. 3. Results and discussion 3.1. Static tensile properties of CF/EP composites The effect of CNF addition on the static tensile properties of CF/EP composites is shown in Fig. 2. The tensile strength of CF/EP composites containing 0.3 wt% MFC as well as BC slightly increases about 5.8% at most compared to the unmodified composite. The improvement of tensile strength is also slight when 0.8 wt% MFC or BC was added. Some scatter is observed which might due to the unstable dispersion quality of CNF with increasing its ratio. The effect of CNF type is unobvious. Young’s modulus of CF/EP composites with different content of MFC or BC almost unchanged. The addition of nano fibers only has a slight effect on the improvement of tensile properties of FR-PMC as reported by other works [6,20,21] since carbon fibers dominantly govern the static tensile properties. A slight improvement of tensile properties especially for fabrics composites can be ascribed to an increase in the resistance to transverse crack growth in the weft bundles and matrix cracks due to nano fibers which will be discussed later. Therefore, the total damage accumulation decreased. Consequently, the strain at failure increased as well as the ultimate tensile strength [22]. More extensive matrix deformation was found in CF/EP composites modified with CNF than the unmodified composite as shown in Fig. 3 (circled regions) [15]. However, all specimens macroscopically failed in the transversely straight behavior and pull-out behavior of carbon fibers from matrix was not obvious microscopically (Fig. 3). It shows less significant influence of fillers on the failure mode as similar behavior discussed in the previous work [22]. 3.2. Tension–tension fatigue lives of CF/EP composites

900

Tensile strength

80

800

Young's modulus

70

700 600 500 400 300

60 50 40 30

200

20

100

10

0

Young's modulus, GPa

Tensile strength, MPa

Tension–tension fatigue lives at different maximum cyclic stress levels (S–N curves) are shown in Fig. 4. It is obvious that the fatigue life of CF/EP composites significantly increases due to physical modification of epoxy matrix with nano fibers: MFC and BC. Fatigue life extension due to CNF modification becomes more

0

Fig. 2. Tensile properties of CF/EP composite containing different ratio of CNF. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

significant at high cycle fatigue. When epoxy matrix contains 0.3 wt% of CNF, the fatigue life is 10–30 times greater than that of unmodified composite. However, the increase in fatigue life for 0.8 wt% CNF is not as great as that for 0.3 wt% CNF. Excessive CNF does not well improve the long time endurance of CF/EP composites. An appropriate CNF fraction may exist between 0.3 and 0.8 wt%, enhancing the fatigue life extension. However, it should be emphasized that a very few amount of CNF fraction less than 1 wt% is enough to get the significant effect on fatigue life extension in comparison with other particle fillers such as rubber and silica. A relative large scatter in CF/EP/BC composites might due to the unstable dispersion quality of BC in matrix resin because BC are thinner and possess higher water holding capacity than MFC [23,24]. Internal fatigue damage accumulates in the fabric composite with increasing the loading cycles, such as fiber debonding, transverse cracks, meta-delamination at the cross-over points between warp and weft, interlaminar delamination and CF breakage. As a result, observed Young’s moduli decrease for all specimens as the number of loading cycle increases. The observed moduli of all CF/EP composites in tension were in-situ measured by an extensometer (25 mm gauge length) during fatigue. Typical modulus decay evolution described by Eq. (1) is plotted in Fig. 5-top where the maximum cyclic stress is 550 and 500 MPa to quantify the internal damage accumulation.

DE ¼

EðNÞ  Eð0Þ Eð0Þ

ð1Þ

where E(N) is the quasi-static modulus defined as the slope of stress–strain curve at a cycle number N. E(0) is the initial modulus at the first cycle of fatigue test. It was found that CNF modified CF/EP composites showed a slower decrease of stiffness as the number of fatigue cycles increased until final failure than the unmodified composite at the different maximum cyclic stress, indicating that less fatigue damage occurred and accumulated at the same fatigue cycle. At an early stage of fatigue, stiffness significantly degrades due to an occurrence of matrix cracks, mainly transverse cracks in weft yarns and meta-delamination. Stiffness of CNF modified composites drops slowly than the unmodified composite at this stage. Both CF/EP/MFC and CF/EP/BC composites show almost the same stiffness decrease rate regardless of nano filler content ratio at both 500 and 550 MPa of maximum cyclic stress. Only some samples’ results are given in Fig. 5. Sometimes 0.8 wt% CNF modified composites perform a slower decrease rate than 0.3 wt% CNF modified composites. The stiffness degradation slows down when transverse cracks saturate in weft direction, transverse to load, known as the characteristic damage state (CDS) [25]. At maximum cyclic stress of 550 MPa, after CDS, 0.8 wt% CNF modified composites show faster decrease of stiffness than 0.3 wt% CNF modified composites. CNF modified composite failed at a higher stiffness decay ratio than unmodified composite indicating a higher damage accumulation capability. At maximum cyclic stress of 500 MPa, stiffness decay of unmodified composite slows down after 30,000 cycles which is earlier than that of CNF modified composites regardless of the ratio or type of CNF Namely, unmodified CF/EP composite displays an earlier saturation of transverse crack and onset of delamination than modified composites. Moreover, at this state, fatigue damage causes a higher stiffness reduction in unmodified composite than CNF modified composites. It indicates that more transverse cracks accumulate in unmodified composite before CDS of transverse crack. CF/EP composites containing CNF show a slower drop of stiffness than that of unmodified composite after CDS. 0.8 wt% CNF modified composites decrease their stiffness faster compared to 0.3 wt% CNF modified composites. Initiation and accumulation

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MFC 0.3wt%

BC 0.3wt%

Unmodified

MFC 0.8wt%

BC 0.8wt%

650 630 610 590 570 550 530 510 490 470 450 102

a

Run-out

Unmodified 0.3wt% MFC 0.8wt% MFC 103

104

105

106

Maximum cyclic stress, MPa

Maximum cyclic stress, MPa

Fig. 3. SEM images of the fracture surface of CF/EP composites after tensile test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

107

650 630 610 590 570 550 530 510 Unmodified 490 0.3wt% BC 470 0.8wt% BC 450 2 10 103 104

Number of cycles to failure, Nf

b

Run-out

105

106

107

Number of cycles to failure, Nf

Modulus decay ratio (%)

Fig. 4. Tension–tension fatigue properties of CF/EP composite with different content of CNF (a. CF/EP/MFC composites; b. CF/EP/BC composites). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20

Maximum cyclic stress=550MPa

0

50000

100000

150000

200000

250000

300000

Unmodified 0.3wt%-MFC 0.3wt%-BC 0.8wt%-MFC 0.8wt%-BC

350000

400000

450000

500000

Modulus decay ratio (%)

Number of cycles 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20

Maximum cyclic stress=500MPa

0

100000

200000

300000

400000

500000

600000

Unmodified 0.3wt%-MFC 0.3wt%-BC 0.8wt%-MFC 0.8wt%-BC

700000

800000

900000

1000000

Number of cycles Fig. 5. Typical stiffness degradation of CF/EP/CNF composites under the maximum cyclic stress of 550 and 500 MPa. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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of fatigue damage in CF/EP composites incorporated with CNF is delayed compared to unmodified composite. Correspondingly, CF/EP fatigue specimens containing different ratio of CNF exhibit different fracture modes after tested at maximum stress of 550 MPa (Fig. 6a–c and j–k). A comprehensive interlaminar delamination is found in unmodified CF/EP composite caused by early occurrence and fast growth of delamination indicating a low interfacial adhesion (Fig. 6a). CF/EP/MFC0.3 or CF/EP/BC0.3 specimens fail at a lower degree of interlaminar delamination compared to unmodified composite (Fig. 6b and j). On the other hand, CF/EP/MFC0.8 or CF/EP/BC0.8 composites (Fig. 6c and k) show cracks propagate transversely rapidly accompanying with some CF bundles breakage. It has a brittle behavior with sudden failure of all layers, without absorbing much energy. Moreover, almost no delamination is found around failure area. It indicates that interfacial adhesion is enhanced by addition of CNF resulting in an increase of resistance to propagation of interlaminar delamination. The fractography can often reflect the detailed information about the fracture mechanism. The delamination region of inside CF layer (white circled regions in Fig. 6a, b and j) was further observed by SEM (Fig. 6d–f). Instead of a smooth surface at interface of unmodified composite, a cohesive failure feature is found in CNF modified composite. It is a more energy absorption mode and reflects a stronger interfacial bonding in modified composites. Furthermore, single cellulose fiber and cellulose microfibrils are found on fracture surfaces (Fig. 6f, h and i). Bridging effect of single cellulose fiber or cellulose microfibrils, MFC debonding, void nucleation around cellulose fiber breakage points, pull-out of cellulose microfibrils from EP accompanying with cellulose fibers bridging, matrix deformation and crack deflection around CNF are considered to be the major reinforcing mechanism causing energy

dissipation at a micrometer scale during cyclic loading. It has been confirmed that energy absorption mechanisms such as filler debonding, plastic void growth, matrix deformation and fiber bridging can toughen the matrix to resist the cracks propagation [26]. 3.3. Delamination growth rate under cyclic loading It has been noted that the final fatigue failure are related to the growth rate of interlaminar delamination. It was found that propagation of interlaminar delamination under the cyclic loading was well prohibited by addition of CNF (Fig. 6). Mode-I DCB test under cyclic loading at a displacement-ratio, R = dmax/dmin = 0.1 and frequency f = 3 Hz was conducted at room temperature to investigate delamination growth rate under the effect of CNF. The dimension and geometry of DCB specimens is shown in Fig. 7a. The length of delamination cracks was determined based on the compliance-crack length relationship found in the static DCB test (ASTM D5528-01). The delamination growth rate versus the energy release rate range (da/dN–DG) is plotted in Fig. 7b and c. DGth represents the threshold value for crack propagation, namely, crack does not propagate when DG is lower than this value. It was found that da/dN of CF/EP composites decreased by a factor of one with incorporation of only a small amount of CNF under the same DG. It indicates that the capability of resistance to the growth of delamination is enhanced significantly by the addition of CNF under cyclic loading. DGth value of modified CF/EP composites increases by 1.6–1.7 times compared to unmodified composite despite the type of CNF. It provides as an evidence of resistance to initiation of delamination in composite under the effect of CNF. At middle linear region, the slope of da/dNDG curve is almost same regardless of CNF content or types.

Delamination a

b

c

MFC 0.3wt%

Unmodified

MFC 0.8wt%

d

e

f

g

h

i

j

BC 0.3wt%

k

BC 0.8wt%

Fig. 6. Macro fracture modes (a–c and j–k) and SEM images observed at delaminated area (d–i) after failure of CF/EP/CNF composites under the maximum cyclic stress of 550 MPa. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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a

10-4

unmodified

10-5

0.3wt%-MFC 0.8wt%-MFC

10-6 ΔGth

100

c

10-2

10-3

10-4

Decrease of crack growth rate

10-3

Crack growth rate da/dN (mm/cycle)

b

10-2

Decrease of crack growth rate

Crack growth rate da/dN (mm/cycle)

Crack growth direction

unmodified

10-5

0.3wt%-BC 0.8wt%-BC

10-6

1000

100

ΔGth

1000

Energy release ratio range ΔG (J/m2)

Energy release rate range ΔG (J/m2)

Fig. 7. Fatigue DCB test: (a) is the dimension of fatigue DCB specimen; (b and c) are crack growth rate of CF/EP/MFC and CF/EP/BC composites respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Furthermore, the fracture surfaces of DCB specimens after fatigue test at propagation areas were observed by SEM (Fig. 8). It is clear that the fracture mode of unmodified CF/EP composite is interfacial failure that delamination propagate along weak CF/EP interface accompanying with a small amount of CF breakage. The smooth surfaces of CFs further confirm the weak CF/EP adhesion. On the other hand, the dominant failure behavior is matrix cohesive failure in CF/EP/CNF0.3 composites, where many resin containing CNF remain on the fracture surface. More residual resin containing nano fillers are found on the fracture surface of CF/EP/CNF0.8 composite reflecting a stronger CF/matrix adhesion. CNF toughens the EP matrix and causes cracks to deflect accompanying with nano fiber bridging and propagate along tougher matrix (Fig. 8b). In this case, more energy is absorbed during damage progression. It can be considered that the effect of CNF on resistance to initiation and propagation of delamination contributes to improvement of fatigue life of CF/EP composites.

a

3.4. Fatigue damage progression The fatigue damage progression of carbon fabric composite who failed at high fatigue cycles can be described as four stages [3,9]. At fatigue early stage (Stage r), the internal damage was dominant by transverse cracks at weft bundles and matrix cracks. When the transverse cracks approach to the warp bundles, they can be arrested, and subsequently initiates the meta-delamination due to stress concentration at crack tips (Stage s). The density of transverse crack increases as the number of fatigue cycles increases and finally reaches a saturated state (Stage t). The observed stiffness of composite reduces significantly before CDS of transverse crack, and then slows down after CDS due to the slight influence of delamination. Further, interlaminar delamination occurs at final stage due to the inhomogeneous stress redistribution between CF layers and processes to cause final failure of specimens (Stage u).

Crack growth direction

b

Unmodified

Observed delaminaon surface

Cellulose nano fiber SEM image of fracture surface

MFC 0.3wt%

BC 0.3wt%

MFC 0.8wt%

BC 0.8wt%

Fig. 8. Fracture surface of CF/EP/CNF specimens after fatigue DCB test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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The occurrence of transverse crack at Stage r can be considered as playing an important role in fatigue damage progression because it causes stiffness loss significantly. The longitudinal section of fatigue specimens after 106 cycles under the low maximum cyclic stress of 350 MPa was carefully grinded and observed by SEM to investigate the accumulated damage inside (Fig. 9). Many transverse cracks, subsequent meta and interlaminar delamination occur in unmodified composites even at a low applied stress. On the other hand, less of transverse cracks and almost no meta or interlaminar delamination are found in modified composites regardless of CNF content. It confirms that nano fillers have provided an effect on resistance to occurrence of matrix cracks including transverse cracks and subsequently delay the initiation of meta and interlaminar delamination because of the improved CF/EP adhesion. 3.5. Resistance to progression of matrix crack including transverse crack From the typical fatigue stiffness decay evolution (Fig. 5), a lower degree of fatigue damage was accumulated in CNF modified composites at the fatigue early stage (Stage r). It indicates that CNF addition enhances the resistance capability of initiation and growth of matrix crack including transverse crack. First, mode-I fracture toughness of bulk EP containing CNF was investigated to characterize the capability of modified matrix to resist the initiation and propagation of matrix cracks. A tensile loading was

Unmodified

Loading direction

a

applied in this test. The specimen dimension is shown in Fig. 10a. The pre-crack with was carefully prepared by a fresh razor blade at tip of the notch made by a diamond cut. It is found GIC value of EP/MFC0.3 and EP/MFC0.8 increases 48.4% and 87% compared to pure EP respectively. GIC values of EP/BC0.3 and EP/BC0.8 increase 54.5% and 122.4% respectively. Furthermore, the rougher fracture surface of modified EP specimens in the form of deflected flow lines and patch pattern is observed compared to unmodified epoxy, associating with high energy dissipation by bridging effect of CNF and deflection of matrix cracks (Fig. 10). It reveals that resistance to initiation and propagation of matrix crack could be enhanced by addition of CNF under static loading. In addition, 3 points cyclic bending load was applied on single notched UD CF/EP/MFC specimens (90° compared to carbon fiber) to investigate the ability to resist the propagation of cracks in transverse bundles. Fatigue test was conducted at a displacement-ratio, R = dmax/dmin = 0.1 and frequency f = 7 Hz at room temperature. Dimension of specimens is shown in Fig. 11. The length of notch made a diamond cut was 1 mm. A pre-crack at tip of notch made by a fresh razor blade was several micro-meters. After failure, the fatigue fracture surface was observed by SEM (Fig. 11). The dominant fracture mode of unmodified composite is interfacial failure where the CF surface is smooth and the matrix deformation is insignificant. It is a lower energy dissipation mode when cracks initiate and propagate along the weak CF/EP interface. On the fracture surface of CF/EP/MFC0.3 composite, the residual resin adhered on CF surface and residual CF fracture

Delamination

Fague specimen MFC 0.3wt%

MFC 0.8wt%

BC 0.3wt%

BC 0.8wt%

Fig. 9. SEM images of longitudinal section of CF/EP specimens with addition of different MFC or BC ratio after 106 cycles under the maximum cyclic stress of 350 MPa. (a) The observation position on the fatigue specimen. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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b Critical energy release rate GIC, kJ/m2

a

T=0.3mm

3 2.5 2 1.5 1

MFC

0.5 0 0.0

BC 0.2

0.4

0.6

0.8

1.0

Nano cellulose fibers content, wt%

Unmodified

0.8wt%MFC

0.8wt%BC

Fig. 10. Single edge notched tensile specimens (a), fracture toughness (b), and fracture surface of pure epoxy and 0.8 wt% CNF filled epoxy. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

fragments are found. It consumes more energy during crack propagation because of crack deflection and CF breakage. It acts as an evidence that CF/EP0.3 performs stronger adhesion than CF/unmodified EP. The fracture surface of CF/EP/MFC0.8 specimens display more residual resin and broken CFs fragment, indicating the existence of much stronger CF/EP adhesion. It reveals that MFC incorporation can provide a positive effect on the resistance to propagation of cracks in transverse bundles under fatigue loading. 3.6. Effect of carbon fiber/epoxy adhesion Based on above observation, the CF/matrix adhesion was largely enhanced when matrix was modified by CNF. It was further investigated by single fiber fragmentation test (SFFT) (Fig. 12, top). SFFT

has been used to investigate the interfacial shear strength (IFSS) between fiber and polymer comprehensively [27,28]. Herein, the dimensions of SFFT specimens were about 0.3 mm thick, 7 mm wide and 20 mm gauge length. The fragment length was measured at saturated state under an optical microscope. The IFSS was estimated by the Kelly–Tyson model [29]. It is found that the IFSS between CF and matrix of CF/EP/MFC0.3 and CF/EP/MFC0.8 composite increased by 19.8% and 48.4% respectively. CF/EP adhesion increases by 25.2% and 76.9% when EP contains 0.3 wt% and 0.8 wt% BC respectively. The results show that the addition of CNF has a positive effect on improvement of CF/matrix adhesion. CF/matrix adhesion gradually increases with increasing CNF weight fraction. Furthermore, the micro damage behavior was observed by optical microscope (Fig. 12, bottom). The dominant fracture mode is interfacial debonding (red circled region)

Crack growth direction

Unmodified

a

W=15mm

Observed area

MFC 0.3wt%

MFC 0.8wt%

Fig. 11. Fracture surface of the notched specimens of UD CF/EP composites with various MFC content after 3 points transverse fatigue bending test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Interfacial shear strength (IFSS), MPa

120 100 80 60 40 20 0

Unmodified

0.3wt% MFC

0.3wt% BC

0.8wt% MFC

0.8wt% BC

Debonding

Debonding Fiber breakage

Debonding Fiber breakage

Fiber breakage

Fiber breakage

Fiber breakage

Almost no debonding

Unmodified

0.3wt% MFC

0.3 wt% BC

0.8wt% MFC

0.8 wt% BC

Fig. 12. IFSS between CF and EP modified by different ratio of CNF (top), and typical micro damage modes (bottom) according to single fiber fragmentation test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

occurring after broken of CF and a slight matrix crack also happens at CF broken region in all the specimens. Interfacial debonding degree shows differently in the different specimens. The interfacial debonding degree reduces with increasing the CNF content. It further confirms that CF/EP adhesion is improved by CNF modification. On the other hand, almost no interfacial debonding occurs at 0.8 wt% CNF content when CF breakage takes place. It acts as an evidence that the existence of much stronger CF/matrix adhesion. From macro fracture mode of CF/EP composites after fatigue (Fig. 6), fracture of CF/EP/CNF0.8 specimens behaved brittlely. It

a

can be noted that CFs in warp bundles did not resist transverse cracks efficiently. In order to investigate the resistance capability of warp CF breakage when they encounter with transverse cracks (Fig. 13b), the single edge notched specimens of UD CF/EP composites (0° compared to carbon fiber) containing various ratio of CNF under 3 points bending load was conducted to investigate the critical energy release rate (GIC). Dimension of specimens is displayed in Fig. 13a. The pre-crack was made by a fresh razor blade. It was found that GIC value was almost unchanged at 0.3 wt% CNF (Fig. 13c). The crack tends to propagate perpendicularly to CF direction (0°) accompanying with a large scale of CF/EP interfacial

Load direction

b

Warp bundle (0 degree)

Transverse bundle (90 degree) W=10mm

UD carbon fiber

c

Fiber breakage

Transverse crack

Meta-delamination

26

GIC, KJ/m2

22

18 MFC

14

BC 10 0.0

0.2

0.4

0.6

0.8

1.0

Nano cellulose fibers content, wt% Fig. 13. The notched 3 points bending test of UD CF/EP composites (0°) with various CNF content. (a) The dimension of specimen; (b) crack propagation direction; (c) GIC values. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Single carbon fiber/fiber bundle Debonding

Debonding Crack

Crack

Unmodified

Fiber breakage Crack

Modified with 0.3wt% CNF

Modified with 0.8wt% CNF

Fig. 14. A model to describe the effect of IFSS on warp CFs fracture behavior under the effect of CNF. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

debonding along 0° [30]. However, GIC value of CF/EP/MFC0.8 and CF/EP/BC0.8 composite decrease by 15% compared to unmodified composite. CF/EP interfacial debonding along 0° is hard to take place due to stronger CF/EP adhesion in this case. It indicates that CFs in warp bundle of CF/EP/CNF0.8 specimens are easier to break when transverse cracks approaching to warp bundles. With increasing the CF/EP adhesion, the efficiency of stress reduction caused by CF/EP interfacial debonding decreases, resulting in occurrence of brittle fracture behavior around interface. A simple model can be considered to describe the fracture mechanism of CF/EP/CNF composites (Fig. 14). Transverse cracks in weft bundles occurred early and initiated meta-delamination easily due to weak CF/EP adhesion in unmodified CF/EP composite. It did not cause the CFs in warp bundles to break due to the reduction of stress concentration at tips by meta-delamination. The following early occurrence and fast growth of interlaminar delamination resulted in a short fatigue life of unmodified composite. With addition of CNF, a slower growth of matrix cracks including transverse cracks compared to unmodified composite was achieved. After transverse cracks propagating to warp bundle, they were arrested and stress concentration at tips was reduced by formation of meta-delamination when content of CNF was 0.3 wt%, without triggering the fracture of CFs in warp bundles. A delay of initiation and growth of meta-delamination was also obtained in modified composites. Subsequently, resistance to propagation of interlaminar delamination due to improved CF/matrix adhesion was achieved, and finally fatigue performance of composite was improved. However, when weight fraction of CNF increased to 0.8 wt%, the achieved much stronger CF/matrix made the weft/warp interfacial debonding hard to occur and the stress concentration at tips of transverse cracks became serious. It subsequently caused CFs in warp bundles to break easily before the occurrence of comprehensive meta-delamination and resulted in final failure of specimens fast. It is worth noting that a suitable weight fraction of CNF exists for achieving the optimum fatigue life of FR-PMC due to an appropriate CF/matrix interfacial adhesion.

4. Summary The effect of modification of CNF with different ratio on the mechanical properties of CF/EP composites was investigated. Incorporation of CNF just performed a slight effect on improvement of static tensile properties of CF/EP composites. The fatigue life of 0.3 wt% CNF modified composites extended significantly by 10–30 times at different applied stress compared to unmodified composite. With addition of CNF, a resistance to initiation and propagation of matrix crack and subsequent delamination was found according to corresponding mechanical test. It contributed to improvement of fatigue life finally. However, the CF/matrix adhesion became much stronger when the CNF content increased to 0.8 wt%, which caused CFs in warp bundles easier to break due to ineffective stress reduction and subsequently reduced the fatigue life of CF/EP composites in comparison with 0.3 wt% CNF

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