Modelling mechanical percolation in graphene-reinforced elastomer nanocomposites

Graphene is considered an ideal filler for the production of multifunctional nanocomposites; as a result, considerable efforts have been focused on the evaluation and modeling of its reinforcement characteristics. In this work, we modelled successfully the mechanical percolation phenomenon, observed on a thermoplastic elastomer (TPE) reinforced by graphene nanoplatelets (GNPs), by designing a new set of equations for filler contents below and above the percolation threshold volume fraction (Vp). The proposed micromechanical model is based on a combination of the well-established shear-lag theory and the rule-of-mixtures and was introduced to analyse the different stages and mechanisms of mechanical reinforcement. It was found that when the GNPs content is below Vp, reinforcement originates from the inherent ability of individual GNPs flakes to transfer stress efficiently. Furthermore, at higher filler contents and above Vp, the nanocomposite materials displayed accelerated stiffening due to the reduction of the distance between adjacent flakes. The model derived herein, was consistent with the experimental data and the reasons why the superlative properties of graphene cannot be fully utilized in this type of composites, were discussed in depth.Comment: 29 pages, 12 figure

The Ultrasonic Detection of Environmental Degradation in Adhesive Joints

There are many benefits to be gained when using adhesives compared with the use of more traditional joining techniques. Amongst these advantages can be listed the ability to join dissimilar materials, the uniform distribution of load over the area of the joint avoiding stress concentrations, the improvement in aesthetics and, potentially, a lower-weight for the component or structure. However several factors have retarded the more widespread use of adhesives. These principally are (i) the detrimental effect of moisture on the joint strength and (ii) the lack of a suitable non-destructive testing technique for detecting strength loss due to environmental attack. It is the latter problem that the present work addresses. The focus of this work has been to examine the bonding of aluminium alloy to aluminium alloy, using an epoxy-based adhesive

The Correlation of Ultrasonic Measurements with Toughness Changes During the Environmental Degradation of Adhesive Joints

Several factors have held back the more widespread use of adhesives. These principally are the detrimental effect of moisture on bond strength and also the lack of a suitable non-destructive testing technique for detecting strength loss due to environmental attack. It is the latter problem that this work attempts to answer. The focus of this work has been to look at the bonding of aluminium to aluminium using epoxy based adhesives, as would be used in the aerospace industry. Bonding of aluminium has been performed in the aerospace industry for many years, and there has been much work done to improve the durability of this type of joint. It has been seen that the improvement in corrosion resistance that can be achieved by treating aluminium prior to bonding has a significant effect on the durability of the bond produced. This is not surprising when it is often seen that a joint which has been exposed to a hot-wet environment will fail along the interface between the aluminium and epoxy, as opposed to through the adhesive when the joint has remained dry [1]. Therefore it is this interface region that is to be examined when searching for environmental attack. The most common form of pretreatment that is used when environmental attack is a concern is anodisation of the surface to be bonded. Anodising produces a thin oxide layer on the aluminium surface, typically 1 –3 μm thick. Joints that have been anodised are considerably more durable than joints that are not anodised, but they will still exhibit interfacial failure after exposure to hot-wet environments [1]. The problem for NDT techniques is that the oxide layer which we need to inspect is orders of magnitude smaller than the bounding layers; the aluminium being 1–5mm, and the adhesive being 0.1–0.5mm thick, as shown in Figure 1. Ultrasonics has appeared to be the most promising technique for inspecting for degradation of adhesive joints, and it is this technique on which we have concentrated our efforts [2–4]

Thoughts on two approaches for accounting for the scatter in fatigue delamination growth curves

This paper discusses two approaches that have been proposed to account for the data scatter observed in delamination growth tests under cyclic-fatigue loading and thereby enable an estimate of a worst-case delamination growth curve for use in the damage tolerance and durability assessment of composite and adhesively-bonded airframes. The two approaches discussed are: (a) the normalisation approach, whereby the energy release rate is divided by the resistance to delamination growth, GR(a), and (b) the Hartman-Schijve approach to delamination growth. It is shown that for the cases considered this normalisation approach can be used to yield curves that are similar to the ‘mean-3σ’, “worst-case”, i.e. upper-bound, curve obtained using the Hartman-Schijve equation. However, despite the reduction in the scatter that arises if this particular normalisation approach is adopted, there is still considerable scatter in the important “near-threshold” region. In this region the normalised curves are bounded above by the ‘mean-3σ’ curve obtained using the Hartman-Schijve equation. To address this issue, an alternative normalisation approach is then proposed. This alternative normalisation approach has the advantage of having reduced scatter in the near-threshold region but elsewhere is significantly more conservative than the Hartman-Schijve approach

On the controlled electrochemical preparation of R4N+ graphite intercalation compounds and their host structural deformation effects

AbstractWe present electrochemical studies of tetraalkylammonium (R4N+) reduction chemistry at Highly Orientated Pyrolytic Graphite (HOPG) and glassy carbon (GC) electrodes. We show that by electrochemically controlled intercalation and formation of a graphite intercalation complex (GIC) into layered HOPG, the irreversible reduction of the tetraalkylammonium cation can be prevented and subsequent de-intercalation of the GIC via the use of potentiostatic control is achievable. R4N+ cations with varying alkyl chain lengths (methyl, ethyl and butyl) have been shown to exhibit excellent charge recovery effects during charge/discharge studies. Finally the effects of electrode expansion on the degree of recovered charge have been investigated and the observed effects of R4N+ intercalation on the graphite cathode have been probed by scanning electron microscopy (SEM) and X-ray diffraction (XRD)

Toughening mechanisms in novel nano-silica epoxy polymers

A crosslinked epoxy polymer has been modified by the addition of nano-silica particles. The particles were introduced via a sol-gel technique which gave a very well dispersed phase of nanosilica particles which were about 20 nm in diameter. The glass transition temperature was unchanged by the addition of the nano-particles, but both the modulus and toughness were increased. The fracture energy increased from 100 J/m2 for the unmodified epoxy to 460 J/m2 for the epoxy with 13 vol% of nano-silica. The microscopy studies showed evidence of debonding of the nano-particles and subsequent plastic void growth of the epoxy polymer. A theoretical model of plastic void growth was used to confirm this mechanism

A novel route for tethering graphene with iron oxide and its magnetic field alignment in polymer nanocomposites

We present a new route for tethering graphene nanoplatelets (GNPs) with Fe3O4 nanoparticles to enable their alignment in an epoxy using a weak magnetic field. The GNPs are first stabilised in water using poly(vinylpyrrolidone) (PVP) and Fe3O4 nanoparticles are then attached via coprecipitation. The resultant Fe3O4/PVP-GNPs nanohybrids are superparamagnetic and can be aligned in an epoxy resin, before gelation, by applying a weak magnetic field as low as 0.009 T. A theoretical model describing the alignment process is presented. The resulting nanocomposites exhibit anisotropic properties with significantly improved electrical conductivities (three orders of magnitude) in the alignment direction and dramatically increased fracture energy (about 530%) when the nanohybrids are aligned transverse to the crack growth direction, compared with the unmodified epoxy. Compared with the randomly-oriented nanocomposites, these aligned nanocomposites show approximately 50% increase in toughness transverse to the alignment direction and a seven-fold increase in electrical conductivity in the alignment direction

The effect of deletion of the orphan G – protein coupled receptor (GPCR) gene MrgE on pain-like behaviours in mice

<p>Abstract</p> <p>Background</p> <p>The orphan GPCR <it>MrgE </it>is one of an extended family of GPCRs that are expressed in dorsal root ganglia (DRG). Based on these expression patterns it has been suggested that GPCRs like <it>MrgE </it>may play a role in nociception however, to date, no direct supporting evidence has emerged. We generated mutant mice lacking <it>MrgE </it>and examined the effects of deletion of this gene in three pain behavioural models. The effect of <it>MrgE </it>gene deletion on expression of <it>Mrgs </it>and genes involved in sensory neurone function was also investigated.</p> <p>Results</p> <p>The absence of <it>MrgE </it>had no effect on the development of pain responses to a noxious chemical stimulus or an acute thermal stimulus. However, in contrast, the development but not the maintenance of neuropathic pain was affected by deletion of <it>MrgE</it>. The expression of <it>Mrg </it>genes was not significantly affected in the <it>MrgE </it>knockout (KO) mice with the sole exception of <it>MrgF</it>. In addition, the expression of 77 of 84 genes involved in sensory neuron development and function was also unaffected by deletion of <it>MrgE</it>. Of the 7 genes affected by <it>MrgE </it>deletion, 4 have previously been implicated in nociception.</p> <p>Conclusion</p> <p>The data suggests that <it>MrgE </it>may play a role in selective pain behavioural responses in mice.</p