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This work focuses on the numerical modelling of confined concretes when subjected to multiaxial loading accounting for a nonlinear material response. Particularly, an improved return mapping procedure is here presented to cope with the issue of singularities (corners) in the yield surface, able to catch locally the optimal return point on the active yield surface. The algorithm is applied to the Menetry and Willam's yield surface, with a plastic potential as suggested by Grassl and the softening law proposed by Cervenka suitable for concrete materials. The model capabilities to reproduce the confined behaviour of concrete, as well as its post-peak behaviour, and to predict perfect plasticity or softening are here discussed. The proposed approach can be applied to any elastoplastic material with singular yield surface along the hydrostatic axis., Comment: 17 pages, 13 figures

We introduce the first oxygen tolerant ultra-low volume (as low as 5 μL total reaction volume) photoinduced copper-RDRP of a wide range of hydrophobic, hydrophilic and semi-fluorinated monomers including lauryl and hexyl acrylate, poly(ethylene glycol methyl ether acrylate) and trifluoroethyl (meth)acrylate. In the absence of any external deoxygenation, well-defined homopolymers can be obtained with low dispersity values, high end-group fidelity and near-quantitative conversions. Block copolymers can be efficiently synthesized in a facile manner and the compatibility of the system to larger scale polymerizations (up to 0.5 L) is also demonstrated by judiciously optimizing the reaction conditions. Importantly, the online monitoring of oxygen consumption was also conducted through an oxygen probe and the role of each component is identified and discussed.\ud

The current Special Issue entitled “Advances in Structural Mechanics Modeled with FEM” aims to collect several numerical investigations and analyses focused on the use of the Finite Element Method (FEM) [...]

Within the framework of numerical algorithms for the three-dimensional random packing of granular materials this work presents an innovative formulation for polydispersed ellipsoidal particles, including an overlapping detection algorithm for an optimized simulation of the mesostructure of geomaterials, particularly concrete. Granular composite cement-based materials can be so reconstructed with adequate precision in terms of grain size distribution. Specifically, the algorithm performance towards the assumed inclusion shape (ellipsoidal or spherical) and degree of regularity (round or irregular) is here discussed. Examples on real grading curves prove that this approach is effective. The advantages of the proposed method for computational mechanics purposes are also disclosed when properly interfaced with visualization CAD (Computer Aided Design) tools.

The increasing attention to numerical issues related to material modeling is still a strong incentive to develop sound mechanical models that can explain material behavior up to failure. A procedure to handle a robust geometric meso-scale reconstruction of concrete materials is here proposed, based on X-ray Computed Tomography (CT-scan or X-ray CT). This study applies X-ray CT on ordinary concrete made with limestone aggregates. In this case the technique allows to acquire the overall inner geometry and distribution of the aggregates and also voids, thanks to the difference in material density of the components. Solid models have been generated with such technique and discretized in space to be numerically studied via the Finite Element (FE) method. The numerical results are compared with uniaxial compression tests on the same scanned specimens. For the numerical analyses a specific non-associated elasto-plastic constitutive behavior, coupled with damage, is developed for the cement matrix, whereas the coarse aggregates are treated as elastic. The mechanical characteristics of the components are gathered through a specific experimental campaign. The study confirms that a predictive simulation of damage triggering and evolution in concrete under generic 3D stress states requires the characterization of the continuum at a meso-scale level. Comparisons between numerical and experimental results proves the soundness of the proposed constitutive description to evaluate the brittle behaviour of cementitious materials and to satisfactorily simulate damage triggering under generic 3D stress states.

The paper aims to investigate the natural frequencies of sandwich plates by means of a Finite Element (FE) formulation based on the Reissner-Mindlin Zig-zag (RMZ) theory. The structures are made of a damaged isotropic soft-core and two external stiffer orthotropic face-sheets. These skins are strengthened at the nanoscale level by randomly oriented Carbon nanotubes (CNTs) and are reinforced at the microscale stage by oriented straight fibers. These reinforcing phases are included in a polymer matrix and a three-phase approach based on the Eshelby-Mori-Tanaka scheme and on the Halpin-Tsai approach, which is developed to compute the overall mechanical properties of the composite material. A non-uniform distribution of the reinforcing fibers is assumed along the thickness of the skin and is modeled analytically by means of peculiar expressions given as a function of the thickness coordinate. Several parametric analyses are carried out to investigate the mechanical behavior of these multi-layered structures depending on the damage features, through-the-thickness distribution of the straight fibers, stacking sequence, and mass fraction of the constituents. Some final remarks are presented to provide useful observations and design criteria.

Predicting plastic deformation and localization band around a wellbore drilled through a porous rock is a challenging task that could have important implications for the prediction of instability and sand production. This study focuses on analyzing stress field, plastic zones, and localized deformations around a horizontal borehole drilled at a great depth through a highly porous rock formation. Several laboratory studies demonstrate that, depending on the loading path, highly porous rocks are susceptible to different failure mechanisms, but most of these mechanisms are mainly due to shear-induced dilation and shear-enhanced compaction. Plasticity models, in conjunction with bifurcation analysis, represent an extremely useful framework for describing such detailed constitutive responses. This paper presents a new elastoplastic constitutive model characterized by two yield surfaces intersecting smoothly, that is able to capture the different failure modes. The model is validated against experimental data for several different porous rocks, and it is then used to determine the stress and strain distributions around a horizontal wellbore using nonlinear finite element analysis. Particular interest is devoted to predicting the condition for the formation of a localized band of intense deformation, elucidating the factors that either prevent or enhance the band initiation. Results of simulations show the key role played by the elastoplastic constitutive model and the effects of the mud pressure, the in-situ stress condition and geometric imperfections in the development and propagation of plastic zone, as well as in the initiation of localization zone.

A preliminary study on the increased thermal capacity in thermal energy storage concrete solids with inclusions of phase change materials is presented here. Particularly, the change in the ...

Regioselective α,α-difunctionalization adjacent to a ketone is a significant synthetic challenge. Here, we present a solution to this problem through the transition-metal-free coupling of esters with geminal bis(boron) compounds. This forms an α,α-bis(enolate) equivalent which can be trapped with electrophiles including alkyl halides and fluorinating agents. This presents an efficient, convergent synthetic strategy for the synthesis of unsymmetrical blocked ketones.

The role of PP fibres on concrete at high temperature is numerically evaluated.Porosity variations in concrete at elevated temperatures are accounted for.An addictive decomposition for porosity is presented.An appropriate hygro-thermal FE code is developed to such a scope. In recent years experimental investigations have demonstrated that the risk of spalling for a concrete material under fire conditions is drastically reduced by using a low dosage of polypropylene fibres in the mix design. This effect can be explained by considering the increase in voids connectivity within the cement paste and in concrete porosity due to polypropylene evaporation above certain temperatures.In this work the complex mechanism of polypropylene contribution on concrete behaviour under thermal conditions has been numerically evaluated through a coupled hygro-thermal-mechanical finite element formulation.

A three-stage study on the behaviour of storage plants employing concrete with upgraded thermo-mechanical characteristics is here developed. The first stage defines the experimental campaign on a mixing at improved conductivity, via the SolTeCa experimental system, with review of the storage elements geometry, location of thermocouples and cycling procedures. The experimental results, obtained by ENEA via a comparison with appropriately performed numerical calculations, are interpreted during the second stage. Finally, a first design of a new equipment for the thermal cycling of storage elements up to 400 °C is proposed, based on Joule-effect heating. The numerical results are reported, in order to understand the thermal dynamics as well as the induced thermo-mechanical effects on concrete elements. © 2017 Elsevier Ltd