Mechanical properties of carbon nanotubes based polymer composites

Accepted Manuscript Mechanical properties of carbon nanotubes based polymer composites M. Tarfaoui, K. Lafdi, A. El Moumen PII:

S1359-8368(16)30199-8

DOI:

10.1016/j.compositesb.2016.08.016

Reference:

JCOMB 4464

To appear in:

Composites Part B

Received Date: 1 April 2016 Revised Date:

8 June 2016

Accepted Date: 17 August 2016

Please cite this article as: Tarfaoui M, Lafdi K, El Moumen A, Mechanical properties of carbon nanotubes based polymer composites, Composites Part B (2016), doi: 10.1016/ j.compositesb.2016.08.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Figure : Composites with CNTs and their mechanical behavior.

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Mechanical properties of carbon nanotubes based polymer composites

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M. Tarfaoui 1, K. Lafdi 2, A. El Moumen 1,* ENSTA Bretagne, FRE CNRS 3744, IRDL, F-29200 Brest, France

University of Dayton Research Institute, Dayton, OH 45469-0168, United States

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*Corresponding author. E-mail address: [email protected]

Abstract:

The objective of this paper was to understand the effect of carbon nanotubes (CNT) additives on the elastic behaviors of textile based composites. The materials consist of three phases namely, carbon fibers fabric, Epoxy matrix and carbon nanotubes. Different volume

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fractions of CNTs were used (0% as reference, 0.5%, 1%, 2% and 4%). A set of mechanical tests as Open Hole Tension, shear Beam Test and Flatwise Tension tests were performed. A damage initiation and cracks propagation in composite specimens were controlled. The

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experimental results show an increase the mechanical performance of the composite up to 2% of CNT additives. However, beyond this value, the material strength shows a significant

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decay.

Keywords: A. Carbon-carbon composites; A. Polymer-matrix composites; B. Mechanical properties; A. Laminates; Carbon Nanotubes.

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1. Introduction Carbon nanotubes (CNTs) reinforced textile composites are a promising new class of composite materials finding their use in some military and aerospace applications.

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Consequently, the use of CNTs in polymers has attracted wide attention [1-2], because their excellent mechanical, electrical, thermal and structural properties. Especially, it has been addressed that CNTs have outstanding a great Young’s modulus, thermal and electrical

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conductivity [3]. For mechanical properties, previous studies, see for example [4-7], show that the Young’s modulus ranging from 600 to 1.4 TPa and tensile strength from 10 to 200 GPa. It

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should be mentioned that the measured properties of CNTs depending on the size and structure of nanotubes.

Studies on the mechanical properties of the composite based CNTs were carried out [810]. A review paper has been published on this subject [11]. Depending on the matrix class, a

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wide variety of composite materials based CNTs have been manufactured and characterized. For example, Peigney et al. [12] and Zhan et al. [13] have fabricated some specimens of CNTs reinforced ceramic resin and Milo et al. [14] and Qian et al. [15] have embedded CNTs

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in polymer matrix. Others works have interested to metallic composites containing aligned and non-aligned CNTs.

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The experimental measurement of elastic moduli of composites based CNTs shows that there is an increase in the composite modulus over the pristine matrix modulus. Qian et al. [15] have shown that the added of a few per cent of CNTs (about of 1wt.%) in a matrix material, the stiffness of a resulting composite can increase between 36% and 42% and the tensile strength by 25%. Schadeler et al. [16] found a 40% increase in the effective stiffness of Epoxy resin with 5 wt % of CNTs. The stress strain curve of Epoxy resin with CNTs has been observed by Zhu et al. [17] for 1 and 4 wt.%. For these weight fractions, the authors found an

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increase of 30-70 % in the elastic moduli. Composites based CNTs is one of the most promising applications have been studied. A review paper has been published on the subject [18]. Recently, in a series of papers, Baretta et al. [19-22] have been interested to modeling

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and characterization of nanobeams and nanorods materials and the CNTs based polymer using a new gradient elasticity model. It is noted that there is very little works related to CNTs reinforced laminate composites.

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In this work, we develop experimental techniques to estimate mechanical behavior of textile composites reinforced with CNTs, and their effect on elastic properties. First, different

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specimens with different volume fractions ranging from 0% to 4% were prepared Second, using Open Hole Tension (OHT), Shear Bear Tension (SBT) and Flatwise Tension Test (FTT), the elastic moduli of textile composites with and without CNTs are characterized. This is the main contribution of the paper. Indeed, while most of papers dealing only with the prediction of elastic properties and damage propagation of CNTs/polymer, CNTs/metallic

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or CNTs/ceramic composites. In the present work we investigate the case of textile composites using experimental techniques. We will focus in this paper to the mechanical performance of the composite without / with CNT through an analysis of the mechanical

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response in terms of the evolution of the maximum force and the equivalent stiffness of the material for the different cases considered. Special attention is given to the analysis of the

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development of damage in these materials for these tests.

2. Materials

In this study composites consist of three phase materials: Epon 862 Epoxy Resin (phase

1), in which fraction of CNTs was added between 0 to 4% by weight (phase 2). The composite resulting from mixing of phases 1 and 2 went through a is considered as second matrix reinforced with T300 6k carbon fibers fabric (phase 3). All panels fabricated with 53

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Harness satin weave fabric using infusion process. Each panel consisted of 12 layers of carbon fiber fabric (satin) interleaved with 12 layers of epoxy film with a target fiber volume of 50%. Panels were press cured according to manufacturer’s cure cycle under a pressure of

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200 MPa. After the panels had cooled, they were prepared for mechanical testing. Final panel thickness is nominally 4mm. The mechanical properties of these materials are shown in Table 1.

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The resin film between plies flows into the ply during fabrication process. We have used, ASTM D5766 for Open Hole Tension at room temperature, ASTM D7291 for interlaminar

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tension room temperature and hot-wet (180°F and 100% Humidity) and ASTM D2344 for interlaminar shear room temperature and hot-wet (180°F and 100% Humidity). Specimen dimensions are prepared for testing as follows in L x W x T format. Where L is the length, W the width and T is the thickness of the specimen.

Open Hole Tensile test (OHT): L=300mm, W=38mm, T=4.1mm

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Short Beam Shear (SBS): L=25,4mm, W=8.128mm, T=4.1mm

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Flatwise Tension (FWT): L=25,4mm, W=25.4mm, T=4.1mm

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An example of the manufactured panel and specimens for mechanical testing is presented

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on Figure 1.

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3. Tests procedure and experimental results 3.1. Open Hole Tension test (OHT) An experimental study was carried out to determine the open hole tension (OHT)

characteristics of carbon fiber-reinforced plastic (CFRP) with different carbon nanotubes volume fraction (CNT). Tests to failure and percentages of ultimate load were carried out and CNTs volume fraction effects were evaluated. An experimental test series was carried out to determine the ultimate strength and stiffness of a CFRP/CNT. 4

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The geometry of the tensile hole in plate specimen is shown in Figure 2. The specimen dimensions are the same as the standard tensile test, but the specimen has a 6.25mm hole drilled at the middle, to act as a stress concentrator. Strain was measured using an

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extensometer across a 220mm gauge length of the specimen. Specimens were tested at a displacement rate of 2mm/minute. Specimens are placed in the grips of Instron test machine and pulled until failure. Open Hole Tension ASTM D5766 is a mechanical test that measures

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the force required to break a composite specimens with hole. All specimens (ASTM D5766) have a constant rectangular cross section. Specimens that were bonded with Hysol 4800,

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conditioned at 180 F (82.22 °C) and 95% Humidity for two weeks, then dried for 2 weeks. Specimens with five volume fractions (0%, 0.5%, 1%, 2% and 4%) of CNTs have been tested. For each volume fraction, three specimens were considered and the mean values of the mechanical properties were recorded. Figure 3 shows a typical experimental loaddisplacement curves, of different specimens, obtained using Open Hole test for each volume

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fractions. Good reproducibility of tests is noted for the different CNT %. The general tendency is that the force level is increased by the addition of minor amount of CNTs which play the role of reinforcement and decrease in the case of 4% of CNTs. However, at higher

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CNTs%, the CNT tends to agglomerate and behave like an inclusion defect. Conversely, all curves of CNTs reinforced composites have the same tendency and show reproducible results.

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To deduce a better view of the effect of CNTs on the elastic moduli of the textile composites, the maximum force and the specimen stiffness obtained at each volume fraction were determined. Histograms presented on the Figure 4 illustrate the evolution of the maximum force and the director coefficient as a function of CNTs volume fraction. It should be noted that the director coefficient represents the Young’s modulus of the specimens. The maximum value is obtained in the case of 1% of CNTs. However, mechanical properties

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decay was observed at 4% of CNTs volume fraction. This decay was noticeable due to the presence of porosities and CNTs aggregates. We have also tried to highlight the influence of the addition of carbon nanotubes on the

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mechanical response of the material in terms of maximum force and stiffness of the composite. Figure 5 shows the average maximum force versus CNT volume fraction.

IT seems that adding CNTs does not seem to have any influence on the maximum force,

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we can still note that for 2% this is slightly increased. A percentage of 0.5% leads to a drop of the effort, which can be interpreted by the fact that the CNTs are seen as defects.

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As shown in the Figure 6 that the slope of the curves, ie the elastic portion is lower for samples containing carbon nanotubes that the specimens without CNTs. In general, taking into account the standard deviation, the stiffness is almost constant for all specimens. However, the stiffness decreases when the percentage of the CNTs increases, which causes the decrease in mechanical performance.

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Table 2 and Table 3 give a summary of the results presented above. Thus it is possible to conclude that from an experimental point of view, in the case of

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Open Hole Tension tests, the contribution of the CNT is not considerable. 3.2. Short Beam Shear (SBS)

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The short-beam shear test has become a widely used method for characterizing the interlaminar failure resistance of fiber reinforced composites. This test method involves loading a beam under three-point bending with the dimensions such that an interlaminar shear failure is inducted. The simplicity of the test method makes it very popular materials screening tool.as pointed out in the title of ASTM Standard D-2344, this method measures the apparent interlaminar shear strength of composite materials.

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As the ASTM D 2344 standard prescribes, a specimens with L=25,4mm, W=8.128mm and T=4.1mm has been used. The schematic presentation of considered specimen dimensions and machine tests were given on Figure 7. The specimens were placed on the two 3 mm

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diameter supports. Not that, span between supports is fixed for all the tests. The carbon fiber of the specimen should be aligned with the longitudinal axis of the specimen.

The indenter has a nominal outer diameter of 6 mm and is made of steel. The specimen is

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placed on a horizontal shear test fixture, Figure 7b. The loading is then used to flex the specimen at a speed of 1.2 mm/min until total fracture. Displacement of the specimen was

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measured from the movement of the loading head through the use of displacement gauge. For reproducibility, tests were realized on 10 specimens for each volume fraction. Then 50 samples were tested. For this test, the evolution of experimental results as a function of the CNTs volume fraction is presented on Figure 8. The mean value of the considered cases is also determined and presented on Figures 9 and 10. It appears that, the curves of textile

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composites containing CNTs demonstrate a good correspondence compared to the case of baseline (0%-CNTs). Also the curves of same volume fractions have same tendency. Figure 10 shows that the adding of minor CNTs amounts less than 2% improves the mechanical

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properties of textile composites. However, at 4% of CNTs volume fraction there is a considerable drop of properties. The average value of maximum load and stiffness versus

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volume fractions is calculated and plotted, Figures 9 and 10. Moreover, it appears the important drop and degradation of the properties for 4% of CNTs. The same phenomenon of OHT test was observed for this test. The general tendency is that CNTs play the role of reinforcement is some cases, lower volume fractions, but also a defect in other cases. Table 4 and Table 5 summaries the results presented above. 3.3. Flatwise Tensile test (FWT)

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The Flatwise Tension (FWT) Test is used to measure the out-of-plane strength of composite laminates. Typically, in this test, solid tabs are bonded to opposite faces of a flat sample from a composite laminate and the sample is loaded in a direction normal to the plane

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of the laminate, Figure 11. In some cases, a thick specimen is manufactured and machined in order to produce a tapered section, ensuring that failure will occur in that region. Several different sample geometries have been used with the Flatwise Tension Test to characterize the

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out-of-plane strength of composite laminates [23-24]. The purpose of this section is to evaluate the contribution of CNT on out-of-plane strength of composite materials with

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nanotubes of carbon. ASTM D7291 is a more standard test used for Flatwise tensile test. The composite samples tested had a dimension of 25.4x25.4x4 mm. All of the specimens were bonded to ends tabs with metal material. Tensile specimens were bonded to grit blasted aluminum loading blocks with Loctite H4800 structural adhesive. The bonded assembly is loaded under tension in the thickness direction. Specimens are loaded under a constant

machine fixture.

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crosshead displacement of 0.1 mm/min. Figure 11 shows a picture of used materials and

Five tests were realized for each volume fraction. Typical load-displacement curves

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obtained for each case are listed on Figure 12. Regarding the reproducibility of the tests, we can say that there is a fairly good reproducibility especially for the linear part. It can be seen

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that the samples containing 2% CNT may allow a greater maximum force, 15kN. A significant drop in the maximum load is denoted by 4% CNTs, 8.5kN. We can also say that the failure mechanism is fragile for all samples; it is indicated by a sudden drop of the load. The stiffness of the various categories of specimens increases with increasing CNTs

volume fraction up to 2% which was a significant drop in this setting. The average maximum stiffness is recorded for 1% of CNT, Figure 13.

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Regarding the average maximum effort, we have recorded an increase with increasing CNT up to 2%, 15kN, then a fall to 8,8kN for 4%, Figure 14. It appears that the maximum effort is found in the case of specimens with 2% of CNTs.

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The maximum force has increased nearly 50% by the addition of 2% of CNTs. In order to determine the real effect of CNTs embedded textile composites, the director coefficient (stiffness) and the maximum force of each curve are determined, Figures 13 and 14. However,

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CNTs embedded textile composite samples, especially for 2% of volume fraction, failed at a much higher level than the case of samples without or containing much CNTs amounts.

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Compared to results of previous tests, important amount of CNTs degrade mechanical properties.

Table 6 and Table 7 summaries the results presented above.

From results of all tests, it can be seen that the reinforcement role is much reduced in the case of 4 % CNTs volume fraction. The same phenomenon was observed for stiffness and

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maximum force. This can be related to CNTs distributions and may be also the existence of porosity is some specimens. We conclude on the existence of CNT critical volume fraction

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threshold for which properties and mechanical performance are greatly reduced.

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4. Conclusions

In this paper, the effect CNTs added into traditional polymer mix based composites

studied. Small CNT concentration can alter considerably the mechanical behavior of composites. Considerable improvement has been obtained in the case of lower CNTs volume fractions. The critical volume fractions threshold was estimated to be between 0.5% and 2% of CNTs reinforced textile composites. The decrease and degradation of the mechanical behavior for 4% can be explained by the effect of CNTs distributions and the existence of an

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upper limit which starts from 2%. CNT dispersion and close porosity of the composite plays a central role for the ultimate mechanical properties. The degree of CNT dispersion and agglomeration was observed in the case 4% of CNTs volume fraction. At high CNT

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concentration, the nanocomposites viscosity had increases drastically and during the composite fabrication process, air bubble tend to get trapped leading to the formation of

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porosity which has a major effect on mechanical properties reduction.

Acknowledgments

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This work was funded by DGA (Direction générale de l'armement - Ministry of Defense), MRIS project: Study of composites reinforced by carbon nanotubes (CNT). The Authors of this paper gratefully acknowledge the financial support of the DGA, France. Acknowledgments have also addressed to Pr. Bruno Mortaigne and Dr. Nickerson William

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Liste of tables

Carbon fiber

Epoxy matrix

CNT

E11 (GPa)

230

E11 (GPa)

2,72

E (GPa)

E22 (GPa)

15

E22 (GPa)

2,72

v

E33 (GPa)

15

E33 (GPa)

2,72

0,28

v12

0,3

v13

0,28

v13

0,3

v23

0,28

v23

0,3

15

G12 (GPa)

1,18

G13 (GPa)

15

G13 (GPa)

1,18

G23 (GPa)

15

G23 (GPa)

1,18

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G12 (GPa)

0,261

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v12

500

%CNT

0

Average Stiffness (kN/mm)

10,16

Standard deviation (kN/mm)

0,14

%CNT Average Fmax (kN) Standard deviation (kN)

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%CNT

Standard deviation (kN/mm)

4

9,94

9,876

9,71

9,37

0,30

0,39

0,43

0,38

0,5

1

2

4

67,30

62,10

66,72

69,45

66,14

1,12

0,96

1,24

3,15

1,19

0

0,5

1

2

4

5,37

5,52

5,65

5,29

4,57

0,2506

0,1547

0,1279

0,1793

0,3904

Average Stiffness versus CNTs volume fraction, SBS

%CNT

0

0,5

1

2

4

1,72

1,75

1,91

1,68

1,04

0,0953

0,0735

0,0792

0,0592

0,1030

Fmax (kN) Standard deviation (kN)

2

Maximum load versus % CNTs volume fraction, OHT

Stiffness (kN/mm)

Table 4.

1

0

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Table 3.

0,5

Stiffness versus CNTs volume fraction, OHT

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Table 2.

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Table 1. Material properties

Table 5.

Average Fmax versus % CNT, SBS

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0

0,5

1

2

4

Stiffness (kN/mm)

13,88

14,24

14,39

11,84

14,11

Standard deviation (kN/mm)

1,0526

1,2501

0,5212

0,8334

0,6794

Table 6.

%CNT

Stiffness versus % CNT, FWT

0

0,5

1

Fmax (kN)

10,58

10,01

12,50

Standard deviation (kN)

0,9076

0,9582

0,9435

2

4

15,30

8,81

1,2934

1,1069

Maximum load versus % CNT, FWT

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Table 7.

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%CNT

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Liste of figures

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Figure 1. Microscopic observation of a specimen, 4% CNT

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Figure 2. Manufactured panels and specimens for mechanical testing

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Figure 3. Manufactured specimens for Open Hole Tension using Instron.

(b) 0.5%

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(a) 0%

(c) 1%

(d) 2%

(e) 4%

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Figure 4. Tensile hole in plate test results of Carbon/Epoxy-NTC composites

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Figure 5. Maximum force vs. CNTs volume fraction, OHT

Figure 6. Stiffness vs. CNTs volume fraction, OHT

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(a)

(b)

(b) 0.5%

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(a) 0%

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Figure 7. Schematic and experimental shear test rig

(d) 2%

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(c) 1%

(e) 4%

Figure 8. Short Beam Shear test results of Carbon/Epoxy-NTC

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Figure 9. Stiffness vs. CNTs volume fraction, SBS

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Figure 10. Maximum force vs. CNTs volume fraction, SBS

Figure 11. Flatwise Tension configuration and dimensions using Instron machine

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(b) 0.5%

(d) 2%

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(c) 1%

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(a) 0%

(e) 4%

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Figure 12. Flatwise tension test results of Carbon/Epoxy-NTC composites

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Figure 13. Stiffness vs. CNTs volume fraction, FWT

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Figure 14. Maximum force vs. CNTs volume fraction, FWT