ENHANCED MECHANICAL PROPERTIES OF LAMINATES BY CARBON NANOTUBES
1. Introduction of Carbon Nanotubes (CNTs)
Since discovered in 1991 by Lijima [[i]], carbon nanotubes have attracted a lot of interests because of their superior properties, including their excellent mechanical performance, high electrical conductivity and thermal conductivity, which indicate their wide application prospect.
1.1 Structure of CNTs
Carbon nanotubes, in general, are classified as two broad categories, single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). SWNTs can be imaged as seamless rolls made by graphene. A three-dimension finite element model is constructed to describe SWNTs quantificationally. In this model, the way of grapheme wrapping is denoted by a chiral vector C, which consists of two integers, n and m. C can be indicated by (na1+ma2), where a1 and a2 are the unit cell vectors [[ii]]. The structure of SWNTs is defined as three patterns according to different values of n and m, including zigzag, armchair and chiral.
On the basic of SWNTs structure, two models of MWNTs were proposed by former researchers: Russian Dolls model and Parchment model . For Russian Dolls model, sheets of graphene are rolled up, forming a lot of concentric cylinders. As for the parchment model, one sheet of graphene is rolled, like a scroll of parchment.
1.2 Mechanical properties of CNTs
Carbon nanotubes possess high specific tensile strength, high stiffness and resilience. It was reported that the modulus of CNTs was up to 1.25TPa, which is larger in order of magnitude to metal. The remarkable tension strength, reaching up to 53 GPa as the maximum, makes it a good candidate for the application as the reinforcement of structural materials [[iii]]. Epoxy/single-wall carbon nanotube was prepared in L. Sun and his colleagues` experiment. The corresponding mechanical properties, including Young`s modulus, tensile strength, elongation and mode I critical stress intensity factor (KIC) were measured, as shown in Table 1 [[iv]]. It is evident that mechanical properties are improved by embedding carbon nanotubes: compared with pure epoxy, the Young`s modulus for composites reinforced by SWNTs increases by 20% approximately; the corresponding tensile strength rise from 64.1 MPa to 74.1 MPa around; as for the elongation, composites which reinforced by f-SWNTs get an increase about 25%. It can be seen that a significant improvement achieved due to addition of CNTs, although properties have not reach the theoretical level.
Table 1 Mechanical property of different experimental groups 
1.3 Applications of CNTs
Because of their superior properties, many potential applications are proposed for carbon nanotubes, such as high conductivity components, high strength composites and energy storage and energy conversion devices and so on [[v]].
Firstly, CNTs can be used to make good conducting composites because of their high aspect ratio and high conductivity. As shown in 2, a remarkable improvement is obtained by using MWNTs instead of carbon black: a relative high conductivity can be achieved by adding 0.1% MWNT, while the same conductivity level is attained at more than 1% concentration for carbon black.
Another typical application for CNTs is used to manufacture low density structural materials. Carbon nanotubes possess a high Young`s modulus, about 1TPa, therefore, they are one ideal candidates to reinforce composites. The ideal performance is more likely obtained if the challenge about the dispersion of nanotubes can be overcome .
Besides the two applications mentioned above, several other applications, like field emission, hydrogen storage and electrochemical devices are all potential areas.
1.4 Functionalisation of CNTs
In order to achieve better dispersion in the polymer matrix, an effective surface fuctionalisation is essential, which means introducing some special chemical functional groups to the bulk materials. According to former research, two different methods are used commonly: covalent functionalisation, providing the hydroxyl group (OH-CNTs) by acid treatment, and non-covalent functionalisation [[vii]].
2. Fabrication of Carbon/Epoxy Composites Embedded with CNT Nano-laminates
2.1 Processing techniques of CNTs introduction into composite hosts
Several methods are used to introduce carbon nanotubes into composites hosts. One common way is to make CNTs as dopant. This method was utilised to produce epoxy/cup-stack carbon nanotubes by T. Yokozeki et al [[viii]]. For this technique, three typical steps were involved. Firstly, epoxy and cup-stack carbon nanotubes (CSCNTs) were combined using the planetary mixer at 70℃. Then, CSCNTs were dispersed under the help of wet mill with zirconia beads at 70℃ for 45 min. After that, the epoxy system with CSCNTs was diluted by itself.
According to the previous description, it is obvious that the quality of products cannot be controlled easily. One main reason is that the procedure is not quantifiable. The second disadvantage of this method is time consuming in materials preparation. On the other hand, this method requires simple equipment and easy operations.
The second method is to make carbon nanotubes grow in situ on the fibres surface. As described in relative reports, the synthesis method involved a modified chemical vapour deposition (CVD) process [[ix]]. Firstly, an alumina cloth was soaked in a Fe (NO3)3·H2O solution for 5 min, then dried in ambient atmosphere. Next, the cloth covered the fibre surface before it was heated. In the heating process, catalyst nanoparticles could produce on the surface of the fibre. After that, hydrogen was introduced to reduce the catalyst. Finally, ethylene was introduced to make the CNTs start to grow.
In the “growing in situ” way, the length of carbon nanotubes can be better controlled by varying the ethylene flow compared with the former method. In addition, the materials used in the experiment are available, like alumina, which is inexpensive. This method is more suitable in cases that the required properties are sensitive to the length of CNTs.
Another approach to introduce carbon nanotubes into composite hosts is to use ultrasonic agitation. It was utilised to improve the interlaminar shear property of modified glass fibre-reinforced polymer with different MWNTs according to L. Sun and his co-workers`s report [[x]]. In their experiment, the interlaminar shear strength (ILSS) was increased 8.16% via filling one type of MWNTs. A similar method was conducted by Y. Zhou et al [[xi]]. In their experiment, a high-intensity ultrasonic processor was the main equipment to mix their components, including the epon 862 epoxy resin, MWNTs and W curing agent .
Ultrasonic agitation is one of the most convenient and effective methods to dispersing CNTs. There is the equipment in our laboratory, so it is the best practicable means for us. To ensure CNTs disperse well, controlling the pulse frequency and operation time must be the main means.
2.2 Manufacturing methods of composite hosts
In general, there are several methods usually used to manufacture composite hosts such as autoclave and vacuum assisted resin transfer moulding (VARTM). Among them, autoclave is the most common way to produce composite laminates.
For laminates, actually there is no marked difference in the degree of cure between autoclave and VARTM. The selection criterion for these two methods is the determinant for the desired properties. Autoclave is usually utilised to get a high fibre dominant performance. The matrix dominated properties can be higher manufactured by RTM [[xiii]]. Some basic characteristics of RTM and autoclaving are illustrated in Table 2.
Table 2 Main characteristics of RTM and autoclaving 
According to the table above, it is evident that autoclaving process is widely used in high technology application fields, while RTM method has advantages in low cost and high production output.
Matrix cracking behaviours of laminates reinforced by CSCNTs were investigated by T. Yokozeki and his co-workers, in which autoclave method was utilised to manufacture samples . In the result analysis, the improvements on mechanical properties are satisfactory. As another example, E. J. Garcia et al did their experiment by using the same method to assemble laminates . In the short beam shear test, the laminates presented a significant enhancement of 69% compared with the unreinforced laminates. Cases mentioned previously indicate that autoclave is a reliable method to produce laminate composites.
3. Interlaminar Shear Strength (ILSS)
3.1 Testing methods
Generally, there are five methods are commonly used to measure interlaminar shear strength: short beam shear (SBS), compression shear test (CST), iosipescu shear test, inclined double notch shear test and five-point bending test. This review focuses on analysing the short beam shear (SBS) and the five-point bending test.
3.1.1 Short beam shear test (SBS)
SBS is an indirect method to measure ILSS, which is shown in 4 schematically. The main advantages of the SBS test are simplicity and economy in term of materials and equipment. However, the shortcomings of this method are numerous: it is strongly affected by the geometry of specimens, and it is precise only if pure interlaminar shear failure takes place [[xiv]].
According to the variation of the geometric configurations of the supports, short beam test are classified as six methods: knife-edge supports, large radius supports, medium radius supports, large diameter rotating roller supports, rotating rollers on swinging links and small diameter roller supports, shown in 5 in sequence.
3.1.2 Five-point bending test
Compared with three-point bending, five-point bending provides a smaller stress concentration in the middle zone. Additionally, in the particular middle section of the beam, there is pure shear stress, in which position the bending stress and the contact stress is negligible [[xvii]]. The normalized applied force distribution drawing of five-point bending test is shown in 6, and the shear stress distribution across the middle.
Because of the high shear stress and the small bending stress at the sensitive section provided by five-point bending testing, this test method is the most popular one to measure the interlaminar shear strength.
3.1.3 Other testing methods
Compression shear test (CST) is one of the most reliable methods to characterize ILSS seen from its principle. As shown in 9, the shear stress on the specimen is generated from two clamps directly. The advantage of CST is that this method can create pure interlaminar shear stress excluding the influence of other factors .
As another important testing method, iosipescu shear test has its feature for its specimen configuration (see 10). The most significant advantage of this method is to average the shear properties over an area [[xviii]].
3.2 Influencing factors for interlaminar shear strength (ILSS)
In the sample design, several correlative factors influencing the final interlaminar shear strength should be taken into account such as the patterns of lay-up, the dimensions of samples and the concentration of CNTs in samples.
Traditionally, there are three main patterns for lay-up, namely unidirectional (UD), cross-ply (CP) and quasi-isotropic (QI). This literature review focuses on the last pattern. For the quasi-isotropic pattern, one typical stack sequence is (-45o/0o/45o/90o) 4s. The mechanical properties for different lay-up patterns were researched by P. Feraboli and K. Tedward, and the experimental results are shown in Table 3 [[xx]]. Although the property differences between these three groups are not significant, in the view of the isotropy of other mechanical properties, for instance wallops from multi-directions, the stack sequence of quasi-isotropic is the pattern which is most widely adopted.
Table 3 Experimental data about shear stress of specimens in different lay-ups 
Considering the dimension factor, the shear stress distributions are varying with the dimensions of samples. The value of width/thickness (b/h) is an important parameter that must be considered carefully in the short beam shear testing. The analyses of shear stress distributions across the beam width for particular composites such as carbon fibre reinforced plastic and glass fibre reinforced plastic are taken by K. T. Kedward, and the illustration is shown in 11.
In addition to width/thickness ratio, the span-to-depth ratio has a significant effect on the shear fracturing experiments as well, because this ratio determinates the fracture mechanism of samples. When the span-to-depth ratio (L/d) is less than 5, an interlaminar shear fracture starts first, however, a tensile fracture plays a dominant role when the value is larger than 16, and a mixed mechanism occurs when the ratio locates between 5 and 16 [[xxii]]. According to many experimental experiences, to measure the ILSS accurately, the designed span-to-depth ratio is about 4.
Due to the embedment of carbon nanotubes in specimens, the content of CNTs is another influence factor for ILSS. CNTs is the reinforcement in the composite system, however, the enhancement to the mechanical properties is not proportional to its concentration. The CNTs concentration influence on mechanical properties is deliberated by Y. Zhou et al, the experiment results are shown in Table 4.
Table 4 Mechanical properties of CNTs/epoxy composite with different CNTs concentrations [[xxiii]]
Although the interlaminar shear strength was not measured in this experiment, but it is evident that for mechanical properties, the concentration of CNTs is not the more the better, because CNTs may agglomerate in the epoxy matrix. This phenomenon is shown in 12, in which there are both individual CNT and agglomerated CNTs in the 0.3 wt% system. The relationship between the concentration of CNTs and ILSS is illustrated in 13. According to this diagram, it is obvious that the highest concentration for CNTs is approximate 0.5 wt% in order to improve interlaminar shear strength. Additionally, based on 13, compared to the system without dispersant, the composite with dispersant behaves a higher ILS strength, because well dispersion can take full advantage of carbon nanotubes.
3.3 Mode Ⅰ and Mode Ⅱ fracture toughness
There are three modes of fracture toughness, and each of them has their cracking mechanism, illustrated in 14.
Mode Ⅰ and mode Ⅱ are involved in laminate fracture. For mode Ⅰ the applied force is perpendicular to the fracture plane. Nowadays, double cantilever beam (DCB) test is the most widely used method to measure the mode Ⅰ interlaminar critical strain energy release rate, as shown in 15.
For mode Ⅱ, the fracture occurs in the direction of the shear stress. So far, the test method for mode Ⅱ is not standardised. There are several candidates to measure the mode Ⅱ energy release rate, GⅡ, including end-notch flexure (ENF), and the four-point end-notched flexure (4ENF). The schematic plots of the two methods are illustrated in 16.
The disadvantage of the end-notched flexure (ENF) is that the crack growth is not stable and there is only one data point can be obtained in per sample [[xxvii]]. Compared to ENF, the four-point bend end-notched flexure test has a significant improvement in these two aspects. As a result, 4ENF is the more reliable option to measure the interlaminar fracture toughness for mode Ⅱ.
4. Investigating the Load Transfer Efficiency in Carbon Nanotubes Reinforced Nanocomposites
The load transfer from matrix to the CNTs is an important influencing factor for reinforcing effect. The load transfer efficiency in CNTs/epoxy system is dependent on the CNT aspect ratios (l/d), CNT volume fractions (vf) and the matrix properties.
A series of analyses about axial stress and interfacial shear stress transfer in SWNTs/epoxy system were taken by A. Haque and A. Ramasetty, which involves the influencing factors mentioned previously [[xxix]]. The distribution of normalized axial stress along the CNTs length is illustrated in 17. It is obvious that the axial stress increases from the ends of CNTs and climbs up to the top at the centre of CNTs. Another piece of important information observed in 16 is that the axial stress increasing rate becomes higher and the saturation plateau becomes longer with the increased aspect ratios. This phenomenon implies us that the CNT aspect ratio should be equivalent to 1000. As for the interfacial shear stress, a contrary trend is presented in the same report, as shown in 18. According to the results, there is almost no shear stress along CNTs when the aspect ratios reach 1000 around.
Where f is the axial stress in the SWNTs, and is the saturated stress [[xxx]]
The ratio of Leff/L indicates the efficiency of the load transfer. The ratio more close to one, the more efficient the transfer is. According to the trend chart illustrated in 20, The ratio increases with the aspect ratio of CNTs.
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