epoxy laminated composites

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The use of carbon nanofibers for thermal residual stress reduction in carbon fiber/epoxy laminated composites M.M. Shokrieh

a,* ,

A. Daneshvar a, S. Akbari a, M. Chitsazzadeh

b

a

Composites Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran b School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran 14395-515, Iran

A R T I C L E I N F O

A B S T R A C T

Article history:

Carbon nanofibers (CNFs) were incorporated into an epoxy matrix with three weight frac-

Received 27 October 2012

tions of 0.1%, 0.5% and 1% which were then reinforced with unidirectional carbon fibers

Accepted 10 March 2013

(CFs) to fabricate laminated composites with cross-ply configuration. Thermomechanical

Available online 26 March 2013

analysis and tensile tests of the specimens were carried out to characterize thermal and mechanical properties of CNF/epoxy composites and compare them with the behavior of the neat resin. Characterization showed that the coefficient of thermal expansion (CTE) of the epoxy matrix is significantly reduced by adding small amounts of CNF. CNFs also moderately increase the Young’s modulus of the epoxy. The slitting method was used for the measurement of residual stresses in cross-ply CF/epoxy and CNF-reinforced CF/epoxy laminates. It involves cutting a narrow slit progressively from one surface of the laminate, and measuring the released strains at the other surface. The results showed that the addition of 0.1%, 0.5% and 1 wt.% CNF leads to 4.4%, 18.8% and 25.1% reductions in residual stress, respectively. These findings confirm that CNFs possess excellent potential to be used as a thermal expansion compensator for the modification of the thermal behavior of the epoxy matrix and the reduction of the thermal residual stresses in the fiber/epoxy laminated composites.  2013 Elsevier Ltd. All rights reserved.

1.

Introduction

When a carbon fiber (CF)-reinforced polymer composite is cooled from its curing temperature to the surrounding temperature, the polymer matrix contracts remarkably due to its high coefficient of thermal expansion (CTE). On the other hand, CFs show negligible contraction because of their low CTE. This difference in thermal behavior between the CF and the matrix leads to induction of residual stresses. Residual stresses could fatally degrade the performance and strength of composite structures. Premature failure, delamination, warpage and matrix cracking are all negative effects of residual stresses [1–3]. Thus, it is important to

develop techniques to reduce these stresses. The most approaches used so far for this purpose are confined to modifying curing cycles [4–8]. In recent years, an increasing number of materials with negative thermal expansion have been discovered. One of the most important applications of these materials is to compensate for the undesirable effects of high thermal expansion of other materials [9]. Regarding this fact, the addition of nano-additives with negative CTE to a polymer is a potential approach to fabricate composites with an adjustable thermal expansion. Furthermore, the use of nano-additives with a negative thermal expansivity like carbon nanofibers (CNFs) and carbon nanotubes (CNTs) for the reinforcement of

* Corresponding author: Fax: +98 2177491206. E-mail address: [email protected] (M.M. Shokrieh). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.03.016

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polymers is a relatively new approach to reduce thermal residual stresses in fiber-reinforced polymer composites. The thermal residual stresses in fiber-reinforced composites are mainly functions of Young’s modulus and CTE of the composite constituents. Compared with polymer matrix, CNFs and CNTs have much higher Young’s modulus and much lower CTE. Thus, these nano-additives could be dispersed into the polymer to make its properties closer to the fiber ones [10–17], and consequently reduce the residual stresses of fiber-reinforced composites. The previous studies have shown that the enhancement of the dimensional stability in composite components could be accomplished by using the nano-additives [18–20]. But no previous work, to the best of the authors’ knowledge, has experimentally studied the effect of nano-additives on the residual stresses distribution in the laminated polymer composites. The objective of the present study is to examine the effect of CNFs on ply-level thermal residual stresses in the CF/epoxy laminated composites. The CTE of the polymer composites containing up to 1 wt.% CNF as well as the neat epoxy was measured using thermomechanical analysis (TMA). In order to quantitatively determine CNF effects on the residual stresses, the slitting method (or crack compliance method) [21,22] was used for the residual stress measurement in cross-ply CF/ epoxy and CNF/CF/epoxy laminates. A significant trend of reduction in residual stresses was found as the CNT weight fraction increased.

2.

Experimental

In this study, two-phase CNF/epoxy composites and threephase CNF/CF/epoxy composites were fabricated and characterized. CNFs were dispersed into the epoxy matrix with three weight ratios of 0.1%, 0.5% and 1% which were then reinforced with CFs. The slitting method was used for the measurement of the residual stresses in these laminates. A detailed description of these experiments is presented in the following sections.

2.1.

Materials

The diglycidyl ether of bisphenol-A epoxy resin, ML-506 (Mokarrar Engineering Materials, Tehran, Iran) and the curing agent, Aradur-830 (Huntsman, Sa¨ckingen, Germany) were used at 100:90 ratio for the fabrication of laminates. CNF used in this study (Grupo Antolin Ingenieria, Burgos, Spain) is synthesized by the floating catalyst method and characterized by an average diameter of 20–80 nm, length of 30 lm and density of 1.97 g/cm3. Unidirectional and cross-ply laminates were fabricated using unidirectional T300 CFs (Toray, Tokyo, Japan).

2.2.

Composite fabrication

The neat resin specimen was prepared by mixing pre-calculated amounts of ML-506 epoxy and Aradur-830 hardener. The mixture was stirred for 30 min at 2000 rpm with a two propelled stirrer. Then, the mixture was placed under vacuum for 30 min to remove the air bubbles.

CNF/epoxy composite specimens reinforced with three different contents of CNF were fabricated with the aid of sonication technique. First, resin was mixed with the desired CNF contents and stirred for 30 min at 2000 rpm. Then, in order to break the residual aggregates and obtain a homogeneous dispersed mixture of epoxy resin and CNFs, the mixtures containing 0.1, 0.5 and 1 wt.% CNFs were sonicated (Hielscher UP400S, Teltow, Germany) at 200 W with a probe of 14 mm diameter for 40, 60 and 80 min, respectively [23]. During the sonication, the mixture container was cooled in an ice-bath. Once the sonication was complete, the curing agent was added to the mixture and stirred for 20 min at 250 rpm. Then, the air bubbles were removed by degassing the solution in a vacuum chamber for 30 min. Finally, the bubble free mixtures of CNF/epoxy and neat resin were cast into the steel molds and cured for 6 h at 100 C followed by 6 h at 120 C. A mold-releasing agent was added to the mold surface to allow easy release of the cured specimens. The cured specimens were then allowed to cool slowly to the room temperature. Three-phase CNF/CF/epoxy composites were manufactured using the hand lay-up method. The CNF dispersed resin used in the manufacturing of CNF/CF/epoxy composites was prepared as identically as possible with the two-phase CNF/ epoxy composites. Subsequently, the curing agent was added to CNF/epoxy mixture and the multi-scale composite was fabricated by hand lay-up method. In addition, the same cure process as the two-phase composites was employed for the three-phase composites. The laminating process was carried out on a steel plate. The steel plate was waxed to ensure the cured laminate can be easily de-molded. A roller was used to remove the air entrapped during the hand lay-up process and to uniformly distribute the resin between all layers. The prepared laminated plate was cured using an oven. The volume fraction of the CF in final laminates was approximately 45% for all composite specimens.

2.3.

Mechanical and thermal characterization

The Young’s modulus of the fabricated CNF/epoxy specimens was obtained using tensile tests performed according to ASTM D638-Type I. In order to perform the tensile test, a universal testing apparatus (STM-150, Santam, Tehran, Iran) with a 50 kN load cell was used. The machine was run under the displacement control mode at a crosshead speed of 1.0 mm/ min. An extensometer of 50 mm gauge length was used for the strain measurements in these tests. Both ends of each specimen were clamped by the pneumatics grips of the testing machine with an inter-grip distance of 40.0 mm. Prior to the tensile test, all samples were mechanically polished to minimize the influence of surface flaws, especially the porosity. At least five samples were tested for each CNF content; the final property was the average result of the five tests. The neat epoxy and CNF/epoxy specimens prepared for tensile test are shown in Fig. 1. In order to determine the compliance coefficients used in the residual stress determination by the slitting method, the elastic constants of the unidirectional CNF/CF/epoxy composites are required. Static strength tests in the longitudinal and

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Fig. 1 – Neat epoxy and CNF/epoxy specimens used for the tensile test.

transverse directions were performed for the unidirectional CNF/CF/epoxy laminates as three-phase composites according to ASTM D3039. The dimensions of 0 unidirectional specimens were about 250 mm in length, 15 mm in width, and 1.2 mm in thickness. The dimensions of the 90 unidirectional specimens were about 175 mm in length, 25 mm in width, and 2.1 mm in thickness. Two 0 and 90 unidirectional samples are shown in Fig. 2. For each weight fraction of CNF, four specimens were tested. In order to investigate the CNFs effects on CTE of epoxy, CTE of the neat resin and CNF/epoxy specimens were studied by TMA. The CTE values were determined by measuring the inclination of the thermal strains versus temperature curve according to ASTM E831: a ¼ D L=L0 DT

ð1Þ

where DL is the change in the length of the composite specimen due to heating, L0 is the initial length of test specimen, and DT is temperature difference, over which the change in the specimen length is determined. TMA was carried out using a themomechanical analyzer (TMA-120, Seiko Instruments, Tokyo, Japan). The tested specimens had dimensions of 7 · 7 · 6 mm. The test was run up to 140 C at a constant heating rate of approximately 0.1 C/s. The effect of the weight fraction of CNF was assessed by measuring CTE of CNF/epoxy composites at three different weight fractions of 0.1%, 0.5%, and 1%.

2.4.

Residual stress measurement

The residual stress measurement of the composite laminates was carried out using the slitting method [21,22]. The slitting method is a well-established and reliable technique for measuring the residual stress in different engineering materials. This method represents a great capability for determining the stress level in surfaces and sub-surfaces of the stressed components. The conventional slitting method consists of cutting a narrow slit in successive increments from one surface of the specimen and measuring the released strains using a strain gauge bonded to the other surface [24,25].

Fig. 2 – Two unidirectional composite specimens used for longitudinal and transverse tensile tests.

Fig. 3 – Slitting method schematic [26].

2.4.1.

Principles of the slitting method

Fig. 3 shows the typical geometry of the slitting method with a back surface strain gauge. This geometry includes the specimen thickness t, the specimen length L, the specimen width B, the slit depth a, the slit width w and the strain gauge length l. The strain gauges bonded on the back surface of the stressed part directly opposite the slit are sensitive to all residual stress within the specimen thickness and are thus appropriate for the through-thickness measurements. The slit starts from one surface of the specimen and is extended in successive increments towards the other surface. For the configuration shown in Fig. 3, the slitting method will determine the unknown normal residual stress component perpendicular to the slit plane, ryy(x), using y-strain measured by the back surface strain gauge. Because of the spatial separation of the location of the strain measurement and the location of the calculated residual stress, the relationship between the residual stresses released along the slit face and the measured strains does not have a simple one-to-one form. In fact, this relationship has the following integral form [27]: Z ai eyy ðai Þ ¼ Cðx; ai Þryy ðxÞ dx ð2Þ 0

where eyy(ai) is the measured y-strain when the slit depth is ai. The Kernel function C(x,ai) is equal to the measured strain due to a unit stress at depth x within a slit of depth ai. In order to solve Eq. (2), an initial distribution for residual stress must be considered. A simple method for estimating the stress profile is pulse method [21,27]. In this method, a uniform stress for each increment of the slit depth is considered. In the layered materials like laminated composites, because of the discontinuity of the material properties across the layers boundaries, the stress profile is not continuous. An important advantage of the pulse method approximation is that it requires no initial assumption about the continuity of the residual stress distribution and thus can be used for laminated composites. A brief description of this method is presented below. Consider a residual stress profile acting on the faces of a slit of increasing depth as shown in Fig. 4(a). In the Pulse

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Therefore, the procedure of the residual stress determination involves computing the elements of the compliance matrix, measuring the released strains during the slitting and finally solving Eq. (8).

2.4.2.

Fig. 4 – (a) An unknown residual stress distribution on slit faces; (b) approximation of unknown residual stress using series of uniform strip loads.

method, the stress profile is estimated by a series of strip or pulse loads over each increment of the slit, as shown in Fig. 4(b). Therefore, in the pulse method the residual stress is estimated by the following equation: rðxj Þ ¼

n X rj Uj ðxÞ

ð3Þ

j¼1

where rj corresponds to the stress value in the jth increment. n is the total number of the increments. The pulse functions are defined as follows:  1 aj1 6 x 6 aj Uj ðxÞ ¼ ð4Þ 0 x < aj1 ; x > aj Substituting Eq. (3) in Eq. (2) results in: Z ai Z ai n n X X eðai Þ ¼ Cðx; ai Þ rj Uj ðxÞ dx ¼ rj Cðx; ai Þ Uj ðxÞ dx 0

j¼1

j¼1

n X rj Cij ¼

0

ð5Þ

j¼1

Details of slitting experiment

Residual stress measurement was performed by the slitting method on CNF/CF/epoxy composite specimens with [04/ 904]s lay-up. For each weight fraction of CNF, four samples were tested. The prepared specimens for slitting experiment had a length of 60 mm, a width of 20 mm and a thickness of 4.8 mm. The reliability of the slitting method when used for the through-thickness measurement of the residual stresses depends strongly on the introduction of the completely equal depth increments. For this purpose, the slitting experiments were carried out using a computer numerically controlled (CNC) milling machine (PC Mill 155, EMCO Maier, Hallein, Austria), which allows an accurate measurement of the slit depth. Fig. 5 illustrates the support condition of the composite specimens during the slitting experiment. The specimens were clamped from one side away from slit and gauge, so the other side could deform freely and recorded strains are correct. A circular cutter with 0.2 mm thick and 23 mm in diameter was used. The rotation speed of the cutter was 5000 rpm. At the surface of each specimen, slitting started in the longitudinal direction (fiber-direction) of the first layer. Fig. 5 shows the relative position of the strain gauge and cutter in the slitting experiment. In order to measure the relaxed strains associated with incremental slitting, a single element strain gauge (UBFLA03, TML, Tokyo, Japan) with a gauge length of 0.3 mm was used. The reason that this small gauge was selected was to minimize the effect of averaging of the strain over the gauge length and to increase the precision of the strain readings at the desired location. The strain gauges were bonded with CN adhesive (TML, Tokyo, Japan), following the manufacturer’s instructions. Prior to bonding strain gauges, the surface of the specimens was

Therefore, Cij or the elements of the compliance matrix are expressed by the following equation: Z ai Z aj Cij ¼ Cðx; ai Þ Uj ðxÞdx ¼ Cðx; ai Þ dx ð6Þ 0

aj1

Comparing with Eq. (2) indicates that a specific element of the compliance matrix, Cij, represents the measured strain at the strain gauge location for a slit of depth ai when the residual normal stress distribution at the domain aj-1 6 x 6 aj is equal to the unit load: Cij ¼ eða ¼ ai ; rðxÞ ¼ Uj ðxÞÞ

ð7Þ

Considering the superposition principle, the compliance coefficients could be determined by applying unit loads to the slit plane. In this work, the elements of the compliance matrix were calculated using finite element simulations. Considering Eq. (5), the relationship between the measured strain data and the stresses within each depth increment can be expressed as a matrix equation: ½Cfrg ¼ feg

ð8Þ

Fig. 5 – Experimental setup of slitting test.

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degreased with acetone. Enough care should be taken to ensure that the strain gauge is bonded directly opposite the slit. This is because this area experiences the largest deformation on the surface and the amplitude of the released strains declines rapidly from this point. It is also very important to ensure that an accurate alignment exists between the strain gauge and the slit. Finally, the strain gauge was connected to a computerized data acquisition system using a three-wire temperature-compensating circuit. The strain measurement error is the major error source in the residual stress measurement by the slitting method. The measurement noise originates from the thermallyinduced strains, machining-induced strains and instrumentation errors. On the other hand, the reduction of the calculation steps will considerably increase the effect of error in the measured strains on the calculated stresses. As a result, the use of great quantity of strain data will lead to highly unstable results [28–30]. In order to minimize the strain measurement errors, it is necessary to limit the number of depth increments. In this study, the number of depth increments was chosen as 8. The slitting was carried out in successive depth increments of 0.3 mm to the final depth of 2.4 mm, which is equal to the half thickness of the specimens. Thus, the depth increment was equal to the thickness of each layer of the laminate. After each slitting step, the cutter was withdrawn from the specimen and stopped. Strain measurement was performed 3 min after each slitting step. This time allowed stabilization of any temperature fluctuations resulted from the slitting process.

3.

Results and discussion

3.1. Influence of CNFs on the Young’s modulus of CNF/ epoxy composites Fig. 7 shows the averaged tensile test results for all three types of composites as well as the neat resin. The Young’s modulus for each specimen was extracted from the average of the slope of the stress versus strain curves in the linear region. It is observed in Fig. 7 that the dispersion of CNF into the epoxy has a moderate effect on the Young’s modulus of the two-phase composites. At 1% loading, the Young’s modulus of CNF/epoxy composite was 11% larger than that of the neat resin. To investigate the dispersion state of CNFs in the matrix, scanning electron microscopy (SEM) was performed using Merlin field emission electron microscope (Carl Zeiss, Jena, Germany). The specimens were coated by a thin layer of gold prior to the observation. Fig. 8(a–d) depict the dispersion state of CNFs in the neat epoxy and CNF/epoxy composites contained 0.1, 0.5 and 1 wt.% CNFs, respectively. The fracture surface of the neat epoxy is smooth. However, by addition of CNF, the fracture surface roughness increases and the CNF broken stem can be seen. It might be noteworthy that in Fig. 8(d) the CNFs are buried under gold coat so they cannot be observed easily. Fig. 8(a–c) indicate good dispersion of CNFs; thus it can be assumed that CNF played its role as a nano-additive.

3.2. Influence of CNFs on the CTE of CNF/epoxy composites Fig. 9 represents the CTE values at the room temperature for the CNF/epoxy composite specimens at three different weight

Finite element modeling

The calculation of compliance coefficients was carried out using finite element simulations (ANSYS software). The mesh was constructed using 8-node 3D layered elements, Solid46, and was refined towards the slit location to accommodate the strong variations of the stress and strain quantities, as shown in Fig. 6. The elements were equally spaced in the z-direction. Considering real setup condition of the tested specimens, one side of the model was completely clamped. Experimentally determined values for the elastic constants of CNF/CF/epoxy composites were used in the finite element model. Each step of the slitting process was simulated by removing the elements at the slit area, applying loads on the elements forming boundary of the slit, and then calculating the corresponding strain at each gauge location. The simulated gauge strain was determined from the displacement of the nodes on the gauge borders as well as the gauge length [31].

4

Young's modulus (GPa)

2.4.3.

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3.5

3

2.5

2

0

0.1 0.5 CNF weight fraction (%)

1

Fig. 7 – Young’s modulus comparisons for CNF/epoxy composites with three different weight fractions of CNFs.

Fig. 6 – Side view of finite element model.

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Fig. 8 – Fracture surface of (a) neat epoxy (b) 0.1 wt.% (c) 0.5 wt.% and (d) 1 wt.% CNF.

70 65

-6 o

CTE (10 / C)

60 55 50 45 40 35 30

0

0.1

0.5

1

CNF Weight fraction (%)

Fig. 9 – CTE comparisons for CNF/epoxy composites with three different weight fractions of CNFs.

fractions as well as the neat epoxy. It is observed that the CTE values of all the composites significantly decreases with an increase in the weight fraction of CNF, which is readily expected because the CNFs have a much lower CTE than that of the neat epoxy. For CNF weight fractions of 0.1%, 0.5%, and 1%, composite CTE decreased by 6.3%, 26.0%, and 32.5%, respectively. Results of the characterization tests showed that adding CNFs to the epoxy has negligible impact on the matrix Young’s modulus. On the contrary, CNFs have a significant

effect on the matrix CTE. The previous studies have widely discussed the reasons for modification of thermal and mechanical properties of polymer by addition of nano-additives such as CNF or CNT [10–17]. In summary, the significant reduction of CTE of polymer reinforced with nano-additives can be attributed to a large interfacial area between the nano-additives and the matrix, a strong interface bonding and a good impregnation of the nano-additives with the matrix. According to the CF data sheet, CFs have a Young’s modulus of 230 GPa and a CTE of 0.41 · 106/C. The neat epoxy has a Young’s modulus of 3.13 GPa and a CTE of 62.46 · 106/C. The incorporation of 1 wt.% CNF into the epoxy increases its Young’s modulus to 3.46 GPa and reduces its CTE to 42.17 · 106/C. Therefore, 1 wt.% CNF loading results in 0.14% and 32.3% reduction in the Young’s modulus and CTE mismatch between the fiber and matrix, respectively. As a result, any reduction in the residual stresses in CNF/CF/ epoxy composites could be attributed to the reduction of the matrix CTE.

3.3. Influence of CNFs on the Young’s modulus of CNF/CF/ epoxy composites The shapes of the stress–strain curves for the three-phase composites were almost linear. Fig. 10 shows the averaged test results of Young’s modulus for all types of CNF/CF/epoxy composites. It was found that the longitudinal Young’s

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120

Longitudinal Young's modulus (Gpa)

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Table 1 – Average recorded strains (le) for CNF/CF/epoxy composite specimens with different contents of CNFs.

110

Slit depth (mm)

100

90

80

0

0.1 0.5 CNF weight fraction (%)

1

Fig. 10 – Longitudinal Young’s modulus comparisons for unidirectional CNF/CF/epoxy composites with three different weight fractions of CNFs.

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4

CNF (wt.%) 0

0.1

0.5

1

0 56 148 244 360 512 666 837 1040

0 53 139 235 344 495 646 816 1028

0 44 114 196 289 415 541 686 867

0 38 106 188 272 395 526 679 877

30

Residual Stress (MPa)

Transverse Young's modulus (GPa)

10 9 8 7

20 10 0

5

0

0.1 0.5 CNF weight fraction (%)

1

Fig. 11 – Transverse Young’s modulus comparisons for unidirectional CNF/CF/epoxy composites with three different weight fractions of CNFs.

modulus is not affected seriously by the CNF dispersion. On the contrary, the transverse Young’s modulus is somewhat affected by CNF dispersion (Fig. 11). Three phase composites with 0.1%, 0.5% and 1 wt.% CNF showed increase in transverse modulus by about 1.9%, 6.9%, and 11.4% respectively. The inconsistent behavior of the Young’s modulus in the longitudinal and transverse directions is due to the role of the CFs and matrix. In the longitudinal direction, the Young’s modulus of composite depends essentially on the stiffness of fibers which tolerate the load, while in the transverse direction the fibers have actually no reinforcing effect and the modulus value depends on the matrix stiffness. In such a case, by addition of CNFs, it was shown that the Young’s modulus of CNF/epoxy composite increased. The changes in the transverse Young’s modulus of CNF/CF/epoxy composites are very close to those of the CNF/epoxy composites.

3.4.

Residual stress results for CNF/CF/epoxy composites

The slitting experiment was repeated for four coupons of each composite specimen. The average strains measured during the slitting experiments for each weight fraction of CNFs are reported in Table 1. As shown in this table, the value of the

0.5 wt% CNF 1 wt% CNF

-20 -30

6

0 wt% CNF 0.1 wt% CNF

-10

1

2

3

4 5 Layer No.

6

7

8

Fig. 12 – Residual stress distribution in different layers of [04/ 904]s CNF/CF/epoxy composites for different contents of CNFs.

measured strain increased as the slit depth increased. Because of the symmetry, the final slit depth of the laminates was considered to be equal to the half of the specimen thickness. The residual stress vs. depth profiles for each specimen was calculated following the procedure specified in Section 2.4.1. The stress results are shown in Fig. 12. As shown in this figure, the stresses are tensile through the 0 layers and compressive through the 90 layers. It is observed that the residual stress in almost all layers decreases by increasing the CNF content. In the third layer, the amount of residual stress increases. Generally, the residual stress has more reduction in the layers that have more residual stress. The maximum tensile and compressive residual stress occurs at 4th and 5th layers respectively. Comparing the values of residual stresses at 4th and 5th layers of 1 wt.% CNF-reinforced CF/ epoxy composite with CF/epoxy composite indicates that the residual stress in these layers decreases by about 38%. In addition to the reduction of the residual stress in most layers, the increase in the CNF content leads to the more uniformity in the residual stress distribution. The uniformity of the residual stress distribution can reduce the stress concentration across the layers boundaries. The change in the residual stresses in different layers of CNF/CF/epoxy composites due to addition of CNFs is reported in Table 2. An average decrease of about 4.4%, 18.8% and

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Table 2 – Change in residual stress (%) due to adding CNFs in different layers of [04/904]s CNF/CF/epoxy composites compared with CF/epoxy composite. Layer no.

1 2 3 4 5 6 7 8 Average

CNF (wt.%) 0.1

0.5

1

4.3 10.6 18.9 10.2 12.8 2.2 7.0 11.6 4.4

21.7 26.5 13.5 25.4 25.6 14.9 23.8 26.1 18.8

32.6 17.7 20.3 38.1 38.5 29.9 30.8 33.3 25.1

The experimental results indicate that the reduction of the CTE mismatch between the polymer matrix and fiber reinforcements could effectively reduce the thermal residual stresses in laminated polymer composites, and increase the dimensional stability of the composite components. It is expected that the introduction of the higher weight fractions of CNFs into matrix will lead to more reduction in the residual stresses. However, attaining acceptable quality of the dispersion of the CNFs in higher weight fractions could create serious practical challenges. This is because when the content of CNFs reaches a certain limit, the possibility of CNFs aggregation significantly increases, and the CNFs dispersion becomes inhomogeneous in the matrix.

4.

Fig. 13 – Room temperature shape of CF/epoxy laminate with unsymmetric [02/902] layup.

25.1% in residual stress was observed with an addition of 0.1, 0.5 and 1 wt.% of CNF respectively. The classical lamination theory predicts that the unsymmetric laminates have a saddle shape at room-temperature [32]. The experimental observation confirms this prediction, Fig. 13. This figure indicates that a neat [02/902] CF/epoxy laminate is warped or curved after cooling down from curing temperature to the ambient temperature. The buildup of thermal residual stresses is the main reason of this phenomenon. The effect of CNF loading on the shape of the rectangular composite laminates with unsymmetric layup following the cure is shown in Fig. 14. As shown in Fig. 14, with increase in the weight fraction of CNF in the matrix, the warpage of the unsymmetric laminates decreases noticeably. This finding is consistent with the earlier reports of the enhanced dimensional stability of the composite components through the reinforcement with nano-additives with negative thermal expansivity [18–20].

Conclusions

The use of a stiff and low CTE nano-additive as a thermalexpansion compensator for the reinforcement of the compliant and high CTE polymer matrix can decrease the mismatch in the CTE between the fiber and matrix phases and facilitate the fabrication of fiber-reinforced polymer composites with the reduced thermal residual stresses. CNFs are the ideal candidate for this purpose, because they have much higher Young’s modulus and much lower CTE compared with the conventional polymers. TMA measurements of CNF/epoxy composites indicated that a CTE reduction of about 32.5% could be attained with 1 wt.% CNF loading. Also, the characterization tests indicated that the dispersion of CNF results in the moderate enhancement of the Young’s modulus of the epoxy matrix. The residual stresses of three-phase CNF/CF/epoxy composites were measured using the slitting method. The results of the slitting experiments showed that the addition of 0.1, 0.5 and 1 wt.% CNF leads to 4.4, 18.8 and 25.1% reduction in the residual stresses respectively. Further reduction in residual stresses is anticipated with an increase in the CNF loading. These results indicate that the nano-additives with a negative thermal expansivity provide a new route toward the solution of the issue of thermal residual stresses in fiber-reinforced composites. The warpage and curvature of the unsymmetric laminates is a direct consequence of buildup of thermal residual stresses during the curing process. Fabrication of unsymmetric laminates showed that the CNFs could effectively reduce their warpage.

R E F E R E N C E S

Fig. 14 – Effect of CNF loading on the warpage and curvature of CNF/CF/epoxy laminates with unsymmetric [02/902] layup.

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