An epoxy resin, cured with an anhydride, has been modified by the addition of silica nanoparticles.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. Atomic force and electron microscopy showed that the nanoparticles were well dispersed throughout the epoxy matrix. The glass transition temperature was unchanged by the addition of the nanoparticles, but both the modulus and toughness were increased. The measured modulus was compared to theoretical models, and good agreement was found. The fracture energy increased from 100 J/m 2 for the unmodified epoxy to 460 J/m 2 for the epoxy with 13 vol% of nanosilica. The fracture surfaces were inspected using scanning electron and atomic force microscopy, and the results were compared to various toughening mechanisms proposed in the literature. The toughening mechanisms of crack pinning, crack deflection and immobilised polymer were discounted. The microscopy showed evidence of debonding of the nanoparticles and subsequent plastic void growth. A theoretical model of plastic void growth was used to confirm that this mechanism was indeed most likely to be responsible for the increased toughness that was observed due to the addition of the nanoparticles. IntroductionEpoxy polymers are widely used for the matrices of fibre-reinforced composite materials and as adhesives. When cured, epoxies are amorphous and highly-crosslinked (i.e. thermosetting)polymers. This microstructure results in many useful properties for structural engineering applications, such as a high modulus and failure strength, low creep, and good performance at elevated temperatures.However, the structure of such thermosetting polymers also leads to a highly undesirable property in that they are relatively brittle materials, with a poor resistance to crack initiation and growth.Nevertheless, it has been well established for many years that the incorporation of a second microphase of a dispersed rubber, e.g. Hence rigid, inorganic particles have also been used, as these can increase the toughness without affecting the glass transition temperature of the epoxy. Here glass beads or ceramic (e.g. silica or alumina) particles with a diameter of between 4 and 100 μm are typically used, e.g. [9][10][11][12][13][14].However, these relatively large particles also significantly increase the viscosity of the resin, reducing the ease of processing. In addition, due to the size of these particles they are unsuitable for use with infusion processes for the production of fibre composites as they are strained out by the small gaps between the fibres.More recently, a new technology has emerged which holds promise for increasing the mechanical performance of such thermosetting polymers. This is via the addition of a nanophase structure in the polymer, where the nanophase consists of small rigid particles of silica [15][16][17][18]. Such nanoparticle-modified epoxies have been shown to not only increase further the toug...

The present paper considers the mechanical and fracture properties of four different epoxy polymers containing 0, 10 and 20 wt% of well-dispersed silica nanoparticles. Firstly, it was found that, for any given epoxy polymer, their Young's modulus steadily increased as the volume fraction, v f, of the silica nanoparticles was increased. Modelling studies showed that the measured moduli of the different silica-nanoparticle filled epoxy-polymers lay between upper-bound values set by the Halpin-Tsai and the Nielsen 'no-slip' models, and lower-bound values set by the Nielsen 'slip' model; with the last model being the more accurate at relatively high values of v f . Secondly, the presence of silica nanoparticles always led to an increase in the toughness of the epoxy polymer. However, to what extent a given epoxy polymer could be so toughened was related to structure/property relationships which were governed by (a) the values of glass transition temperature, T g , and molecular weight, M c , between cross-links of the epoxy polymer, and (b) the adhesion acting at the silicananoparticle/epoxy-polymer interface. Thirdly, the two toughening mechanisms which were operative in all the epoxy polymers containing silica nanoparticles were identified to be (a) localised shear-bands initiated by the stress concentrations around the periphery of the silica nanoparticles, and (b) debonding of the silica nanoparticles followed by subsequent plastic void-growth of the epoxy polymer. Finally, the toughening mechanisms have been quantitatively modelled and there was good agreement between the experimentally measured values and the predicted values of the fracture energy, G c , for all the epoxy polymers modified by the presence of silica nanoparticles. The modelling studies have emphasised the important roles of the stress versus strain behaviour of the epoxy polymer and the silica-nanoparticle/epoxy-polymer interfacial adhesion in influencing the extent of the two toughening mechanisms, and hence the overall fracture energy, G c , of the nanoparticle-filled polymers.

The present paper investigates the effect of adding silica nanoparticles to an anhydride-cured epoxy polymer in bulk and when used as the matrix of carbon-and glass-fibre reinforced composites. The formation of 'hybrid' epoxy polymers, containing both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber microparticles, is also discussed. The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. The fracture energy of the bulk epoxy polymer was increased from 77 to 212 J/m 2 by the presence of 20 wt.% of silica nanoparticles. The observed toughening mechanisms that were operative were (a) plastic shear-yield bands, and (b) debonding of the matrix from the silica nanoparticles, followed by plastic void-growth of the epoxy. The largest increases in toughness observed were for the 'hybrid' materials. Here a maximum fracture energy of 965 J/m 2 was measured for a 'hybrid' epoxy polymer containing 9 wt.% and 15 wt.% of the rubber microparticles and silica nanoparticles, respectively. Most noteworthy was the observation that these increases in the toughness of the bulk polymers were found to be transferred to the fibre composites. Indeed, the interlaminar fracture energies for the fibre-composites materials were increased even further by a fibre-bridging toughening mechanism. The present work also extends an existing model to predict the toughening effect of the nanoparticles in a thermoset polymer. There was excellent agreement between the predictions and the experimental data for the epoxy containing the silica nanoparticles, and for epoxy polymers containing micrometre-sized glass particles. The latter, relatively large, glass particles were investigated to establish whether a 'nanoeffect', with respect to increasing the toughness of the epoxy bulk polymers, did indeed exist.

Search for additional heavy neutral Higgs and gauge bosons in the ditau final state produced in 36 fb −1 of pp collisions at √ s = 13 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.ch Abstract: A search for heavy neutral Higgs bosons and Z bosons is performed using a data sample corresponding to an integrated luminosity of 36.1 fb −1 from proton-proton collisions at √ s = 13 TeV recorded by the ATLAS detector at the LHC during 2015 and 2016. The heavy resonance is assumed to decay to τ + τ − with at least one tau lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2-2.25 TeV for Higgs bosons and 0.2-4.0 TeV for Z bosons. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in benchmark scenarios. In the context of the hMSSM scenario, the data exclude tan β > 1.0 for m A = 0.25 TeV and tan β > 42 for m A = 1.5 TeV at the 95% confidence level. For the Sequential Standard Model, Z SSM with m Z < 2.42 TeV is excluded at 95% confidence level, while Z NU with m Z < 2.25 TeV is excluded for the non-universal G(221) model that exhibits enhanced couplings to third-generation fermions. 6 Background estimation 10 6.1 Jet background estimate in the τ had τ had channel 10 6.1. The ATLAS collaboration 37-1 - JHEP01(2018)0551 IntroductionThe discovery of a scalar particle [1, 2] at the Large Hadron Collider (LHC) [3] has provided important insight into the mechanism of electroweak symmetry breaking. Experimental studies of the new particle [4][5][6][7][8] demonstrate consistency with the Standard Model (SM) Higgs boson [9][10][11][12][13][14]. However, it remains possible that the discovered particle is part of an extended scalar sector, a scenario that is predicted by a number of theoretical arguments [15,16]. The Minimal Supersymmetric Standard Model (MSSM) [15,17,18] is the simplest extension of the SM that includes supersymmetry. The MSSM requires two Higgs doublets of opposite hypercharge. Assuming that CP symmetry is conserved, this results in one CPodd (A) and two CP-even (h, H) neutral Higgs bosons and two charged Higgs bosons (H ± ). At tree level, the properties of the Higgs sector in the MSSM depend on only two non-SM parameters, which can be chosen to be the mass of the CP-odd Higgs boson, m A , and the ratio of the vacuum expectation values of the two Higgs doublets, tan β. Beyond tree level, a number of additional parameters affect the Higgs sector, the choice of which defines various MSSM benchmark scenarios. In the m mod+ h scenario [19], the top-squark mixing parameter is chosen such that the mass of the lightest CP-even Higgs boson, m h , is close to the measured mass of the Higgs boson that was discovered at the LHC. A different approach is employed in the hMSSM scenario [20,21] in which the measured value of m h can be used, with certain assumptions, to predict the remaining masses and couplings of the MSSM Higgs bosons without explicit reference to the sof...

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