Emile S. Greenhalgh

Carbon nanotube-based hierarchical composites: a review

The introduction of carbon nanotubes (CNTs) into conventional fibre-reinforced polymer composites creates a hierarchical reinforcement structure and can significantly improve composite performance. This paper reviews the progress to date towards the creation of fibre reinforced (hierarchical) nanocomposites and assesses the potential for a new generation of advanced multifunctional materials. Two alternative strategies for forming CNT-based hierarchical composites are contrasted, the dispersion of CNTs into the composite matrix and their direct attachment onto the primary fibre surface. The implications of each approach for composite processing and performance are discussed, along with a summary of the measured improvements in the mechanical, electrical and thermal properties of the resulting hierarchical composites.

Delamination Growth and Thresholds in a Carbon/Epoxy Composite Under Fatigue Loading

This paper presents a study on delamination growth in Mode I, Mode II and mixed mode under fatigue loading in an HTA/6376C composite. The computed slopes of the modified Paris plots were high. Therefore, threshold values of the strain energy release rate for delamination growth were determined. Low fatigue threshold values revealed a significant effect of fatigue loading. The largest effect was found for the ENF test (Mode II) for which the fatigue threshold value was only 10% of the critical strain energy release rate in static tests. Threshold values for MMB (mixed mode) and DCB (Mode I) tests were 15% and 23% of the static values, respectively. Fractographic evaluation revealed identical initial failure mechanisms in fatigue and static loading conditions for the ENF specimen. The ENF specimen failed by formation and coalescence of microcracks. The low fatigue threshold for the ENF specimen was explained by microscopical observations on the specimen edge. It was also shown that the fracture surfaces generated in static and fatigue DCB and MMB tests were similar.

Activation of structural carbon fibres for potential applications in multifunctional structural supercapacitors

The feasibility of modifying conventional structural carbon fibres via activation has been studied to create fibres, which can be used simultaneously as electrode and reinforcement in structural composite supercapacitors. Both physical and chemical activation, including using steam, carbon dioxide, acid and potassium hydroxide, were conducted and the resulting fibre properties compared. It was proven that the chemical activation using potassium hydroxide is an effective method to prepare activated structural carbon fibres that possess both good electrochemical and mechanical properties. The optimal activation conditions, such as the loading of activating agent and the burn-off of carbon fibres, was identified and delivered a 100-fold increase in specific surface area and 50-fold improvement in specific electrochemical capacitance without any degradation of the fibre mechanical properties. The activation process was successfully scaled-up, showing good uniformity and reproducibility. These activated structural carbon fibres are promising candidates as reinforcement/electrodes for multifunctional structural energy storage devices.

Failure of thick-skinned stiffener runout sections loaded in uniaxial compression

Abstract Recent efforts towards the development of the next generation of large civil and military transport aircraft within the European community have provided new impetus for investigating the potential use of composite material in the primary structure. One concern in this development is the vulnerability of co-cured stiffened structures to through-thickness stresses at the skin–stiffener interfaces particularly in stiffener runout regions. These regions are an inevitable consequence of the requirement to terminate stiffeners at cutouts, rib intersections or other structural features which interrupt the stiffener load path. In this respect, thicker-skinned components are more vulnerable than thin-skinned ones. This work presents an experimental and numerical study of the failure of thick-sectioned stiffener runout specimens loaded in uniaxial compression. The experiments revealed that failure was initiated at the edge of the runout and propagated across the skin–stiffener interface. High frictional forces at the edge of the runout were also deduced from a fractographic analysis and it is postulated that these forces may enhance the fracture toughness of the specimens. Finite element analysis using an efficient thick-shell element and the Virtual Crack Closure Technique was able to qualitatively predict the crack growth characteristics for each specimen.

Multifunctional structural supercapacitors for electrical energy storage applications

A novel concept of structural supercapacitors based on carbon fibre-reinforced composites has been introduced that can simultaneously act as a structural component and an electrical energy storing device. Supercapacitors consisting of woven carbon fibre mat electrodes; filter paper insulator and crosslinked poly(ethylene glycol) diglycidylether/diglycidylether of bisphenol-A polymer electrolytes were fabricated. Brunauer–Emmett–Teller surface area analysis and tensile tests were conducted on the as-received and activated carbon fibre reinforcements. Compression tests and ionic conductivity measurements were conducted on the polymer electrolytes while charge/discharge electrochemical tests and shear testing were done on the structural supercapacitors. This was to investigate the implications of increased diglycidylether of bisphenol-A loading in crosslinked poly(ethylene glycol) diglycidylether polymer electrolytes and carbon fibre activation on the multifunctionality of structural supercapacitors. The addition of diglycidylether of bisphenol-A increased the compressive stiffness, although the ionic conductivity was compromised. Specific capacitance of the structural supercapacitors was increased with the chemical activation of carbon fibre electrodes. Carbon fibre activation led to improved specific capacitance of the structural supercapacitors and the addition of diglycidylether of bisphenol-A increased the shear modulus, although the specific capacitance was compromised.

Damage propagation in composite structural elements-coupon experiments and analyses

Abstract An overview of experiments and analyses being performed at the coupon level within the GARTEUR (Group of Aeronautical Research in Europe) action group AG16: ‘Damage propagation in composite structural elements’ is given. Both basic delamination fracture experiments such as DCB, ENF, MMB, CLS and SEN and coupon tests with embedded artificial delaminations or impact damages are carried out. The experiments are analysed using different methods and energy release rate, stress-based failure criteria and damage models are evaluated.

Delamination growth in carbon-fibre composite structures

Abstract This paper describes a series of fractographic analyses to evaluate the effects of delaminations in carbon-fibre composite coupons and skin/stringer panels. In particular, it examines delamination growth and failure under compressive loading from simple, single-plane defects. The more complicated case of delamination growth and failure from impact damage is then examined. Finally, the similarities and differences between these forms of defect growth and failure are discussed.

Mechanical, electrical and microstructural characterisation of multifunctional structural power composites

Multifunctional composites which can fulfil more than one role within a system have attracted considerable interest. This work focusses on structural supercapacitors which simultaneously carry mechanical load whilst storing/delivering electrical energy. Critical mechanical properties (in-plane shear and in-plane compression performance) of two monofunctional and four multifunctional materials were characterised, which gave an insight into the relationships between these properties, the microstructures and fracture processes. The reinforcements included baseline T300 fabric, which was then either grafted or sized with carbon nanotubes, whilst the baseline matrix was MTM57, which was blended with ionic liquid and lithium salt (two concentrations) to imbue multifunctionality. The resulting composites exhibited a high degree of matrix heterogeneity, with the ionic liquid phase preferentially forming at the fibres, resulting in poor matrix-dominated properties. However, fibre-dominated properties were not depressed. Thus, it was demonstrated that these materials can now offer weight savings over conventional monofunctional systems when under modest loading.

Modeling the Lofting of Runway Debris by Aircraft Tires

Runway debris lofting by aircraft tires can lead to considerable damage to aircraft structures, yet there is limited understanding of the lofting mechanisms. The aim of this study is to develop accurate physically based models to understand and predict the stone lofting processes. The research entailed both experimental work and finite element modeling of a tire partially rolling over a stone. Parametric studies were conducted to characterize the influence of factors such as stone geometry and tire conditions in the lofting processes. To validate the finite element models, experimental studies were conducted using a modified drop weight impactor covered with rubber to simulate a tire vertically approaching aluminum balls and real stones. A high-speed video camera was used to observe the loft mechanisms and calculate the loft velocities, angles, and spin rates. A finite element model of the impactor demonstrated good agreement with the experimentally observed loft mechanisms.In general, lofting occurred either at high speed and low angles or vice versa, depending on the degree of interaction between the stone and the ground.

Carbon fibre-reinforced poly(ethylene glycol) diglycidylether based multifunctional structural supercapacitor composites for electrical energy storage applications

We have developed structural supercapacitors that can carry mechanical loads as well as can store electrochemical energy simultaneously. Structural supercapacitors are fabricated by impregnating carbon fibre mat electrodes and glass fibre mat separator with crosslinked poly(ethylene glycol) diglycidylether polymer electrolyte using the resin infusion under flexible tooling method. In this study, design parameters of the structural supercapacitors have been explored including the separators and the polymer electrolytes. The fabricated structural supercapacitors have been characterised using charge–discharge method and impedance spectroscopy to evaluate the electrochemical performance and in-plane shear properties to evaluate the mechanical performance. A structural supercapacitor, exhibiting a specific capacitance of 10.3 mF/cm3 and a shear modulus of 0.50 GPa simultaneously, have been fabricated.