Mechanical characterisation of carbon fibre–PEEK manufactured by laser-assisted automated-tape-placement and autoclave

Obtaining autoclave-level mechanical properties using in-situ consolidation of thermoplastic composites by Automated Tape Placement (ATP) is challenging. However, relatively recent availability of high quality ATP grade pre-preg material and tape heads equipped with more efficient heat sources (e.g. lasers) offers an opportunity to achieve improved mechanical properties and deposition rates. In the present study, carbon fibre–PEEK laminates, manufactured by laser-assisted ATP (LATP) and autoclave, are compared. Analysis of the through-thickness temperature distribution during LATP processing using thermocouples indicates that LATP cooling rates are extremely rapid and suggests full through-thickness melting of the pre-preg tape may not occur. Inadequate crystallinity, in conjunction with voids, compromised mechanical properties compared to autoclaved laminates but was beneficial in terms of the toughness of LATP laminates. Optimisation of pre-preg properties and processing parameters is required to realise the full potential of the LATP process in terms of mechanical properties, energy requirements, cost and deposition rates.     

The impact of the nature of nanofillers on the performance of wood polymer nanocomposites

Wood polymer nanocomposites have been prepared from aspen wood using melamine-urea-formaldehyde (MUF) and montmorillonite nanoclay. The nanoparticles were ground with a ball-mill and mixed with the prepolymer to form suspensions that were subsequently impregnated into the wood and in situ polymerized. The influence of the nature of nanofillers and interphase interactions between nanoparticles and MUF on the physical/mechanical properties of the resulting wood polymer nanocomposites was investigated, using SEM, TEM and Electron Probe Micro-Analysis (EPMA) methods. Significant improvements in wood properties, including surface hardness, modulus of elasticity, dimensional stability and water repellence, were obtained with the addition of hydrophobic nanoparticles into the wood. The improved properties could be ascribed to inherent properties as well as better interphase between MUF and nanoparticles, and their co-reinforcement on the wood. Ball-mill treatment favored the dispersion of the nanoparticles into the wood, but broke down functional groups on the hydrophobic nanoclay surface, which was detrimental for the bonding between the nanoparticles and the MUF matrix.     

On avoiding thermal degradation during welding of high-performance thermoplastic composites to thermoset composites

One of the major constraints in welding thermoplastic and thermoset composites is thermal degradation of the thermoset resin under the high temperatures required to achieve fusion bonding of the thermoplastic resin. This paper presents a procedure to successfully prevent thermal degradation of the thermoset resin during high-temperature welding of thermoplastic to thermoset composites. The procedure is based on reducing the heating time to fractions of a second during the welding process. In order to achieve such short heating times, which are much too short for commercial welding techniques such as resistance or induction welding, ultrasonic welding is used in this work. A particularly challenging scenario is analysed by considering welding of carbon-fibre reinforced poly-ether-ether-ketone, with a melting temperature of 340 °C, to carbon-fibre reinforced epoxy with a glass transition temperature of 157 °C.     

Forming induced wrinkling of composite laminates with mixed ply material properties; an experimental study

One disadvantage of multi-layer forming of unidirectional (UD) prepreg tape is the risk of out-of-plane wrinkling. This study aims to show how mixed ply material properties affect global wrinkling behaviour.An experimental study was performed using pre-stacked UD prepreg on a forming tool with varying cross sections. Parameters studied include local interply friction, effects of co-stacking and fibre stresses in critical fibre directions. Experimental evaluation was performed on out-of-plane defect height, type and location. The study shows that fibre stresses in some fibre directions were crucial for the shearing required to avoid wrinkling. The same fibre stresses may cause wrinkling if the lamina is stacked in a non-beneficial order. Changing the friction locally, or reducing the number of difficult combinations of fibre angles, improves the forming outcome slightly. However, in order to make a significant improvement, co-stacking or different fibre stacking is required.     

Anomalous transfer phenomenon of carbon nanotube in the blend of polyethylene and polycarbonate

A contradict interphase transfer of multi-walled carbon nanotubes (MWCNT) is detected in the immiscible polymer pair of polyethylene (PE) and polycarbonate (PC). When laminated sheets composed of PE with MWCNTs and PC are annealed in the molten state of both polymers, MWCNTs are found to move from PE to PC. This transfer phenomenon is originated from the difference in the interfacial tension with the aid of Brownian motion. On the contrary, MWCNTs prefer to reside in the PE phase in the blend of PE, PC and MWCNTs, even when MWCNTs are first dispersed in PC. This result indicates that MWCNTs transfer from PC to PE. The opposite direction of the transfer is attributed to the PE molecules being adsorbed on the surface of MWCNTs, which are generated during mixing.     

Resin infusion analysis of nanoclay filled glass fiber laminates

This paper focuses on the resin flow characteristics of nanoclay filled glass fiber laminates processed by Vacuum Assisted Resin Infusion Molding (VARIM). Laminates with varying quantities of nanoclays (0–5 wt.%) were prepared and the effect of these nanoclays on the epoxy resin flow characteristics was studied. It was found that the flow rate of resin continuously decreased as nanoclay content continuously increased. The reduction in the flow rate was attributed to the rate of change of curing and the subsequent change in viscosity of the nanoclay filled resin. Analysis of infusion process by Darcy’s law show that the permeability of the fiber decreased in the nanoclay filled resin system. Nanoclay filled laminates show improved static and dynamic mechanical properties than that of unfilled resin composites.     

Micro-infiltration of three-dimensional porous networks with carbon nanotube-based nanocomposite for material design

Epoxy composite beams reinforced with a complex three-dimensional (3D) skeleton structure of nanocomposite microfibers were fabricated via micro-infiltration of 3D porous microfluidic networks with carbon nanotube nanocomposites. The effectiveness of this manufacturing approach to design composites microstructures was systematically studied by using different epoxy resins. The temperature-dependent mechanical properties of these multifunctional beams showed different features which cannot be obtained for those of their individual components bulks. The microfibers 3D pattern was adapted to offer better performance under flexural solicitation by the positioning most of the reinforcing microfibers at higher stress regions. This led to an increase of 49% in flexural modulus of a reinforced-epoxy beam in comparison to that of the epoxy bulk. The flexibility of this method enables the utilization of different thermosetting materials and nanofillers in order to design multifunctional composites for a wide variety of applications such as structural composites and components for micro-electromechanical systems.     

A quantitative study of the morphology of montmorillonite filled thermosets based on a tailor made homogenisation model

Thermoset/montmorillonite nanocomposites were fabricated and their elastic modulus was measured using experimental modal analysis. The morphology of the nanocomposite was considered as a distribution of several components: exfoliated clay platelets, intercalated clay layers, primary particles and clay agglomerates. A novel homogenisation model, which involves a five-phase sequence based on the Halpin–Tsai equations, was developed to calculate the elastic modulus of the nanocomposites. This model was then used to quantify the morphology of the nanocomposites by back calculating the exfoliation, intercalation and agglomeration fractions from the measured values of the elastic modulus. Additionally, this approach led to quantify the efficiency of the fabrication process, which proved to be optimal for 2.5% clay content.     

Design of composite materials with improved impact properties

Composites have been widely used in applications where there is a risk of impact, due to the excellent properties these materials display for absorbing impact energy. However, composites during impact situations typically generate an enormous number of small pieces, due to the energy absorption mechanism of these materials, a mechanism which does not include plastic deformation. This can prove dangerous in sports competitions, where the small fragments of the original structure may harm competitors.This study was designed to explore the possibility of incorporating a material which, whilst maintaining a high level of energy absorption without any plastic deformation mechanism, was able to maintain its original form, or at least significantly reduce the number of pieces generated after impact.The addition of a polyamide layer, NOMEX^®, to a monolithic fabric laminate was investigated in this paper. The process of fabrication is described and the different properties of the material under consideration: interlaminar fracture toughness energy (G_IC), indentation (id) and delamination after impact (A_i) and compression after impact (σ_CAI), were measured and compared with those of the original monolithic fabric.