Krishnan Jayaraman

Quantification and analysis of Raman spectra of graphene materials

Graphene has received significant attention in recent years due to its outstanding electronic, mechanical, chemical and physical properties. Graphene materials can potentially be used in a variety of applications, such as functional nanocomposites, electrodes, flexible transparent devices and thin conductive films. This article focuses on the analysis of structural evolution and development of different of reduced graphene oxides (rGOs), and the results are compared with structural features of functionalised reduced graphene oxide and graphene. The aromatic disorder and irregularity of these materials influence their own properties; particularly, their electrical conductivity aspects were studied indirectly through Raman spectroscopy. The quantification of their Raman spectra and microstructural analysis were examined to assess the relationship between aromatic structures and electrical conduction mechanism. The results showed that aromaticity of GO changes under different chemical reduction treatments and hydroiodic acid reduction gave an electrical conductivity of 103.3xa0Sxa0cm−1 as highest amongst a number of rGOs produced. Moreover, the integrity of aromatic structure through different reduced graphene oxides changed quite significantly and the Raman results were able to correlate the electrical conductivity with their structural regularity.

Optimisation of hybridisation effect in graphene reinforced polymer nanocomposites

This article focuses on the optimisation of electrical and mechanical properties of hybrid blends of polyoxymethylene (POM) as primary thermoplastic matrix, polypyrrole (PPY) as secondary conducting polymer and graphene (G) as reinforcement. An initial Taguchi analysis was performed with a focus on improving electrical conductivity (σ) and tensile strength. A mixture analysis using ‘simplex’ statistical design was applied to develop an experimental subset that identified an optimal combination in weight-percentage. Both electrical and mechanical properties were improved by the addition of PPY and graphene particles due to hybridisation mechanism as well as double percolation threshold. The maximum electrical conductivity of 0.95 S cm−1 was achieved with POM reinforced with 3 wt.% of G and 2.5 wt.% of PPY loading. The mechanical properties were found to be increased with increase in addition of both G and PPY.

Vibration damping of flax fibre-reinforced polypropylene composites

This work investigates the effects of fibre content and fibre orientation on the damping of flax fibre-reinforced polypropylene composites. Laminates of various fibre contents were manufactured by a vacuum bagging process; their dynamic behaviour were then found from the vibration measurements of beam test specimens using an impulse hammer technique to frequencies of 1 kHz. The frequency response of a sample was measured and the response at resonance was used to estimate the natural frequency and loss factor. The single-degree-of-freedom circle-fit method and the Newton’s divided differences formula were used to estimate the natural frequencies as well as the loss factors. The damping estimates were also investigated using a “carpet” plot. Experiments were subsequently conducted on a range of samples with different fibre volume fractions and orientations. The results show significant variations in natural frequencies and loss factors according to the variations in fibre orientation. Composites containing 45°, 60° and 90° fibre orientation exhibit approximately the same natural frequencies. Composites with differing fibre orientations exhibit different loss factors for the various modes of vibration, and the maximum loss factor is obtained for the case of 45° fibre orientation, with the loss factor generally lying in the range of 2-7 %. It was found that the loss factor increases with increasing frequency and decreases slightly with increasing fibre content. These outcomes indicate that flax fibre-reinforced composite could be a commercially viable material for applications in which noise and vibration are significant issues and where a significant amount of damping is required.

Influence of Damping on the Bending and Twisting Modes of Flax Fibre-reinforced Polypropylene Composite

The effects of damping on the bending and twisting modes of flax fibre-reinforced polypropylene composites are investigated. The laminate was manufactured by a vacuum bagging process; its dynamic behaviour was then found from the vibration measurements of a beam test specimen using an impulse hammer technique to frequencies of 1 kHz. The frequency response of a sample was measured, and the bending and twisting responses at resonance were used to estimate the natural frequency and loss factor. The single-degree-of-freedom circle-fit method and Newton’s divided differences formula were used to estimate the natural frequencies as well as the loss factors. The damping estimates were also investigated using a “carpet” plot. The results show significant variations in loss factors depending on the type of mode. The loss factor generally lies in the range of 1.7-2.2 % for the bending modes, while 4.8 % on average for the twisting modes. Numerical estimates of the response, and in particular the natural frequencies, were made using a Mechanical APDL (ANSYS parametric design language) finite element model, with the beam being discretised into a number of shell elements. The natural frequencies from the finite element analysis show reasonably good agreement (errors < 5 %) with the measured natural frequencies.

Rotational moulding and mechanical characterisation of halloysite reinforced polyethylenes

Extensive experiments with rotationally moulded polyethylene halloysite nanocomposites have been conducted. Previous studies regarding the use of filler materials in rotational moulding often report problems with agglomerations or inward migration of the filler. Despite the previous advances in machine adaptations and mould configurations to apply internal pressure, this study has focused mainly on non-pressurized composite production. Halloysite is a natural, nano-size, mineral clay with different reactivities from the internal aluminol and external siloxane surfaces. Due to its morphology and chemical nature, halloysite is easier to process compared to other fillers and achieves good particle dispersion, making it a potential candidate for reinforcing rotationally moulded products. In this study, nanoparticle-reinforced composites containing halloysite with medium density or high density polyethylene were produced by rotational moulding. The influence of halloysite on the melt flow index and mechanical performance, tensile, flexural and impact properties, were investigated.

Experimentally quantified and computational anisotropic damage rules for flax fabric composites

Continuum damage mechanics models have been used for predicting failure in composites for several years now. However, their application to natural fibre composites is quite recent. In this work, an approach for evaluating damage processes in natural fibre composite has been developed by combining experimentally quantified damage rules with discrete representations of fabric geometry. This approach is demonstrated by predicting the tensile failure of both thermoplastic-based and thermoset-based flax fabric composites. Numerical models have been developed to estimate the damage rules from the fibre, matrix and interface properties incorporated into representative volume element (RVE) models of the composites. While both the experimentally quantified and numerically obtained sets of damage rules yield satisfactory predictions, the use of these numerical rules leads to predictions that are close to, and in some cases, better than those obtained from the quantified rules.