Mechanical Property of Multiwall Carbon Nanotube Reinforced Polymer Composites

Abstract

This study investigates the multiwalled carbon nanotube as potential mechanical reinforcement in epoxy polymer. It is found that, by adding various concentrations of nanotube, both flow stress and fracture strain increased. Furthermore, the presences of the multiwalled carbon nanotubes are found to nucleate crystallization in the epoxy. This crystal growth is thought to enhance the strength of composite. The fracture surface analysis of the composite reinforced by carbon nanotube is used the scanning electron microscopy.

1. Introduction

The composite materials with epoxy-based matrix are used extensively as structural components in aircraft, aerospace and automobile industry. It results from their high-adhesion, low-weight, and good chemical resistance^1–3. In order to enhance the mechanical properties of epoxy-based composite, they can add many types fillers for example particles or short fibers. In the last decades, the multi-walled carbon nanotube (MWNT) is used as the filler in polymer matrix composite has attracted considerable interesting due its extraodinary mechanical, thermal and other functional properties^4,5. Epoxy composites with MWNTs reinforcements have been fabricated using different purification and dispersion processes^6–9. MWNTs tend to agglomerate during mixed together with polymers due to the strong van der Waals forces among them and inherently inert nature of MWNTs. Therefore, it becomes a very important role in controlling the mechanical and thermal characteristics of the nanocomposites to disperse MWNTs in polymer matrix. Efficient dispersion and alignment of MWNTs within a matrix is still a challenge and it could play the main role driving the diffusion of MWCNT as nano-reinforcements on industrial scale^10,11.

There many literatures reported that the percentage of MWNT additions to enhanced the mechanical property of composite. Several micromechanical models have been proposed to explain the mechanical properties of the nanocomposites^12–15. But there are very little published study on the influence of the strain rate on the mechanical properties and fracture mechanism of nanocomposite. During the manufacturing and using process, the structural components are usually subjected to the different strain rate effects. The main purpose of this study is investigated that the mechanical properties of epoxy composite samples reinforced by different percentage of multi-walled carbon nanotubes additions. The Material testing system (MTS) is used to exam the mechanical properties of nanocomposites samples with different strain rate. Furthermore, the carbon nanotube dispersion and the fracture surfaces are analyzed by performing scanning electronic microscope (SEM).

2. Material Preparation and Experimental Procedure

The polymer matrix used in this study is a commercially available low viscosity epoxy resin (NC826A), and NC826B hardener. The epoxy was supplied by Everwide Chemical Co., Ltd. Furthermore, multi-walled carbon nanotubes (MWNT) used in this study was supplied by NICHING Industrial Co. Ltd (see Figure 1 ) and they were grown by Catalytic Chemical Vapour-Deposition (CCVD) process. The average length and the diameter of MWNTs were 1 μm and 10 nm, respectively. The composite specimens are prepared with a MWNTs content of 0.1, 0.3 and 0.5 wt% in this study. The MWNT was mixed with methyl ethyl-ketone and then sonicated (Bandelin HD3200, 20 kHz) for 1 h. After sonication, the MWNTs were added into the epoxy resin. All mixture specimens were stirred in the convector to perform dispersion. Then all specimens were degassed in vacuum oven at 70°C for 40 min, and then cast in an aluminum mould (diameter 7 ± 0.2 mm and height 7 ± 0.2 mm) and cured at 120°C for 30 mins followed by 2 h at 130°C. Then the testing specimens are machining to 7 ± 0.1 mm (diameter and height).

Figure 1

SEM feature of MWNT

All the composite specimens and the neat resin specimen were mechanically polished to minimize the influence of surface flaws. The standard of mechanical testing is ASTM D3410. The compressive examinations were carried out at room temperature of 25°C by using Materials Testing System (model MTS 810 with the load cell of capacity 30 kN) and the strain rate of 10⁻³, 10⁻² and 10⁻¹ s⁻¹. The fracture surface analysis was performed by scanning electron microscopy (model FEI Quanta 400 F at 15 kV). The specimens were coated with a thin layer of gold prior to examination by SEM. The schematic illustration of the experimental procedure is shown in Figure 2 .

Figure 2

The schematic illustration of the experimental procedure

3. Results and Discussion

3.1 Stress-Strain Behaviour

Figure 3(a) shows the true stress-strain curves of the nanocomposite specimens deformed at temperatures of 25°C and strain rates of 10⁻³ s⁻¹, with different percentage of MWNT 0%, 0.1%, 0.3% and 0.5%, respectively. It can be observed that the flow stress increases with the percentage of MWNT. When the percentage of MWNT increases to 0.5%, it has lager flow stress. Furthermore, the fracture strain also increases with the percentage of MWNT. From the Figure 3(a) , it can be seen that the fracture strain increases to 0.54 at the percentage of 0.5% WMNT. It can be suggested that the MWNT can inhibit the fracture growth and increase the strength of nanocomposite. It can be certified by the SEM observation. With increasing strain rates to 10⁻² s⁻¹ and 10⁻¹ s⁻¹, Figures 3(b) and 3(c) , the flow curves exhibit significant work hardening behaviour. It is also seen that for a given strain rate, the flow stress decreases with increasing percentage of MWNT. Furthermore, at a constant percentage of MWNT, the flow stress also increases with increasing strain rate. Meantime, the fracture decreases with increasing strain rate.

Figure 3

Stress-strain curves of nanocomposite deformed at temperatures of 25°C, under different MWNT contents and strain rate of (a) 10⁻³ s⁻¹; (b) 10⁻² s⁻¹; (c) 10⁻¹ s⁻¹ procedure

3.2 Effects of Strain Rate

To investigate the effect of the strain rate the dynamic response of the nanocomposite specimens, Figures 4(a)–4(d) plot the true stress versus the logarithmic strain rate at true strains of 0.1 and 0.2 for different MWNT contents, respectively. It is shown that for both true strain values, the true stress increases linearly as the strain rate is increased from 10⁻³ to 10⁻¹ s⁻¹. However, the flow stress response of the nanocomposite specimens is particularly sensitive to the strain rate. The higher stress prompted by an increasing strain rate suggests that the deformation of the pure titanium specimens is dominated by a thermal activation mechanism ¹⁶ .

Figure 4

True stress as function of at true strains of 0.1 and 0.2 under different strain rate and the MWNT contents of (a) 0%; (b) 0.1%; (c) 0.3%; (d) 0.5%

The dependence of the flow stress on the strain rate can be calculated by using the following strain rate sensitivity parameter:

$$\beta =\partial \sigma /\partial \dot{\epsilon }=\left({\sigma }_2-{\sigma }_1\right)/\text{In}\left({\dot{\epsilon }}_2/{\dot{\epsilon }}_1\right),$$

(6)

where the compressive stresses σ₂ and σ₁ are obtained from tests conducted at average strain rates of ${\dot{\epsilon }}_2$ and ${\dot{\epsilon }}_1$, respectively. Figures 5(a)–5(d) present the variation of the strain rate sensitivity with the strain under the strain rate ranges of 10⁻³ to 10⁻¹ s⁻¹ for different MWNT contents, respectively. It is seen that for a given strain, the strain rate sensitivity decreases with increasing strain rate. In addition, it is observed that for a given strain rate range, the strain rate sensitivity decreases with increasing strain. It is thought that the decrease in the strain rate sensitivity at higher strain rates is the results of a thermal soften effect.

Figure 5

Variation of strain rate sensitivity with true strain as function of strain rate under different MWNT contents (a) 0%; (b) 0.1%; (c) 0.3%; (d) 0.5%

3.3 Fracture Behaviour Observations

Figures 6(a)–6(d) show the fracture morphologies of the specimens deformed at 25°C under strain rate of 10 ¹ s⁻¹ with MWNT content of 0%, 0.1%, 0.3% an 0.5%, respectively. It is found that catastrophic failure occurs at all tested conditions. From the pure epoxy specimens fractures ( Figure 6(a) ), it can be seen the smooth fracture surface. When the contents of MWNT increase, it is seen that the MWNT distributes in the composite matrix. However, it can be seen the MWNT can form a obstacle to prohibit the slip band. Thus, it can be reasonably recognized that the main strengthening mechanism for the nanocomposite specimens is the amount of MWNT contents in the specimens; therefore, the higher MWNT contents, the higher flow resistance and hardness. Furthermore, as the strain rate increases to 10⁻¹ s⁻¹ (see Figure 6(d) , 0.5% MWNT), it can be seen many slip bands are prohibited by MWNT, leading to a rapidly decrease in flow resistance. It is correspondence with the observation of stress-strain behaviour (Figures 3(a)–3(c)).

Figure 6

Fracture surface features at different MWNT contents (a) 0%; (b) 0.1%; (c) 0.3%; (d) 0.5%

4. Conclusions

Mechanical experiments on the nanocomposite with MWNT reinforcements have been conducted to investigate the influence of the strain rate and contents of MWNT on the deformation flow response and fracture analysis. The mechanical behaviour of the nanocomposite is sensitive to both strain rate and contents of MWNT. Fracture analysis show that catastrophic failure takes place in all tested condition specimens. The fracture observations show that the flow stress increases related to the inhibition of the slip band by MWNT.