The mechanism of evolution of polycyclic aromatic hydrocarbons (PAHs) into carbonaceous particles in combustion, atmosphere, and interstellar space has been the subject of intense debate. Recently, ...there has been emerging evidence supporting resonantly-stabilized radicals as key players in PAH growth. In this work, we build on this hypothesis and propose that, beyond a critical size, PAH reactivity can be assimilated to that of radicals. We found that odd-C-numbered PAHs embedding 5-membered rings rapidly lose a hydrogen atom to form resonantly-stabilized radicals in combustion conditions, while even-C-numbered PAHs react as open-shell rather than closed-shell molecules independently of temperature, as usually assumed. Acenes were used as molecular models of large even-C-numbered PAHs. The construction of a kinetic model including these findings allows to interpret experimental soot oxidation data otherwise irreconcilable with existing chemical kinetic mechanisms.
We discuss mechanical performance of fibre-reinforced cementitious composites under exposure to four aggressive environments, namely alkaline, saline, sulphuric acid and distilled water immersion. A ...standard commercial Portland cement based matrix is considered alongside its lightweight modification wherein quarzitic sand is partially replaced by recycled rubber crumbs. Also, virgin polypropylene fibres are contrasted to PP+PET blended fibres where the PET fraction is obtained from recycling food packaging waste. Performance is assessed in bending as well as in compression. We find that recycled based specimens perform surprisingly well and that exposure to the aggressive environments mainly affects the matrix and it is not necessarily more detrimental to the lightweight partially recycled phase. A one-way analysis of variance (ANOVA) confirms the statistical significance of the results, which fully support the idea that the adoption of a substantial recycled fraction in construction materials still allows for high performance and durability standards.
Stroh’s sextic formalism represents the equilibrium equations of anisotropic elasticity in a particularly attractive form, that is most suitable for studying interface-dominated multilayered solids, ...composite materials and time-harmonic problems. Taking advantage of the fact that the Stroh formalism really amounts to the canonical form of the equations in the Hamiltonian sense, the case of Biot’s reversible (i.e. no fluid dissipation) poroelasticity is here addressed, in the absence of a fluid pressure gradient. This framework is the same as thermoelasticity of perfect conductors. Two Hamiltonian formulations are developed: the first describes both the solid and the fluid phases and it exhibits, besides energy conservation, momentum conservation, as a result of pressure uniformity (perfectly drained conditions). The second is restricted to the solid skeleton and perfectly parallels anisotropic elasticity, where the Stroh matrices refer to the effective stress tensor. The case of weak fluid–solid coupling is also considered and it produces a perturbation from anisotropic elasticity with the same structure as incompressibility, although in an “opposing” manner. This comparison suggests that the incompressibility limit introduced by Biot should be revised. The energy conservation integral and the edge impedance matrix are also illustrated.
•The Stroh-like form of the equations of reversible poroelasticity is derived.•Accounting for both solid and fluid, energy and momentum conservation hold.•Accounting for the solid alone, the structure of anisotropic elasticity is retrieved.•Biot’s incompressible limit does not lend incompressible elasticity.•Instead, weak solid–fluid coupling provides a correction similar to incompressibility.
When investigating nonlinear wave propagation in slender hyperelastic rods, the usual stance is to construct a reduced kinematics and then derive a system of coupled nonlinear PDEs for the unknown ...functions. To make further analytical progress, the linear Love hypothesis, that connects longitudinal and transversal strain, is often reverted to. The viability of this assumption, that was originally proposed within the framework of linear elasticity, remains uncertain. In this paper, a refined Love hypothesis is derived in the weakly nonlinear regime by slow-time perturbation of the motion equations. For the sake of illustration, the simplest two-modal setting is adopted. This refined Love assumption is not equivalent, not even in principle, to that derived by Porubov and Samsonov (1993) by accommodating for the free boundary conditions at the rod mantle. Besides, the perturbation process lends a uni-dimensional model equation which parallels that obtained by Ostrovskii and Sutin (1977) with the help of the linear Love hypothesis, with yet different coefficients in the dispersive term. The corresponding longitudinal motion is compared numerically against the solution of the bimodal nonlinear system and the transversal motion is contrasted with the linear Love hypothesis. For both motions, excellent agreement is found and the quality of the approximation extends to a wide range of values for the small parameter. Finally, within this setting, the corresponding unimodal Lagrangian is also derived, and it remains accurate regardless of the first correction terms to the linear Love hypothesis.
•A refined Love hypothesis is derived from a bimodal kinematics by slow-time perturbation.•Fulfilment of the boundary conditions is not a feature of the asymptotic model.•The corresponding model equation is the Boussinesq equation.•Comparison with numerical integration of the original nonlinear system is excellent.
Several beam and plate models have been recently developed in the literature to accommodate for size-dependence. These are usually obtained starting from a generalized continuum theory (such as the ...couple-stress, strain-gradient or non-local theory or their modifications) and then deducing the governing equations through Hamilton’s principle and ingenuous kinematical assumptions. This approach, originated by Kirchhoff, usually fails to reproduce the dispersion features of the equivalent 3D theory. Besides, it produces a variety of models, in dependence of the different assumptions, such as Kirchhoff’s or Mindlin’s. In contrast, in this paper we adopt asymptotic reduction: moving from the couple-stress linear theory of elasticity with micro-inertia, we deduce new models for elongation and flexural deformation of microstructured plates. The resulting models are consistent, in the sense that they reproduce the dispersion features of the corresponding 3D body. Also, models are unique, for they may only differ by the order of the approximation. We find that microstructure especially affects inertia terms, which can be hardly captured by a-priori kinematical assumptions. For static flexural deformations, our results match those already obtained assuming plane cross-sections within the modified couple-stress theory. In fact, we show that couple-stress, reduced couple-stress and strain gradient theories all lead to equivalent results. Higher order models are also given, that describe the near first-cut-off behaviour and account for thickness deformations in the spirit of Timoshenko.
•A new detailed kinetic model for surrogate fuels is proposed.•The proposed model includes detailed chemistry for surrogate fuels, PAHs and NOx.•The proposed model has been validated against ...comprehensive experimental data.
There is an increasing demand for kinetic models of surrogate components to predict the combustion and emissions of real fuels. In this paper, a new surrogate fuel mechanism, C3MechV3.3, is proposed by the Computational Chemistry Consortium (C3). This mechanism is constructed based on a C0 – C4 core mechanism, with important species of interest in complex fuel surrogates such as the hexane isomers, n-heptane, iso-octane, nC8 – nC12 linear alkanes as well as polycyclic aromatic hydrocarbons (PAHs) and NOx as pollutants. This kinetic model consists of the latest chemistry subsets developed by the different partners in the context of the C3 effort. The proposed model was tested against a comprehensive set of experimental data for various fuels and blends over a wide range of temperatures, pressures, dilutions and equivalence ratios. Overall, the model shows good predictions for most of the experimental data. In particular, the focus of the validation is on natural gas/n-alkanes, primary reference fuel (PRF) and toluene primary reference fuel (TPRF) mixtures. Due to the large size of C3MechV3.3, a mechanism processing tool was developed to abstract species and reactions to generate any particular surrogate fuel and multi- fuel mechanisms of smaller size which can be used for preliminary mechanism reduction.
We characterize poly(3-hydroxybutyrate-
-3-hydroxyhexanoate) (PHBH) scaffolds for tissue repair and regeneration, manufactured by three-dimensional fused filament fabrication (FFF). PHBH belongs to ...the class of polyhydroxyalkanoates with interesting biodegradable and biocompatible capabilities, especially attractive for tissue engineering. Equally, FFF stands as a promising manufacturing technology for the production of custom-designed scaffolds. We address thermal, rheological and cytotoxicity properties of PHBH, placing special emphasis on the mechanical response of the printed material in a wide deformation range. Indeed, effective mechanical properties are assessed in both the linear and nonlinear regime. To warrant uniqueness of the material parameters, these are measured directly through digital image correlation, both in tension and compression, while experimental data fitting of finite-element analyses is only adopted for the determination of the second invariant coefficient in the nonlinear regime. Mechanical data are clearly porosity dependent, and they are given for both the cubic and the honeycomb infill pattern. Local strain spikes due to the presence of defects are observed and measured: those falling in the range 70-100% lead to macro-crack development and, ultimately, to failure. Results suggest the significant potential attached to FFF printing of PHBH for customizable medical devices which are biocompatible and mechanically resilient.
Background:
Lack of interphase compatibility between the fabric and the matrix significantly impairs the load-bearing capacity of textile reinforced mortar (TRM). In this study, we consider the ...application of two inorganic surface coatings for enhancing the interphase bond properties.
Methods:
Either of two silica-based coatings, namely nano- and micro-silica, were applied to alkali-resistant glass (ARG) and to hybrid carbon–ARG woven fabric. Mechanical performance of TRM reinforced with the uncoated and the coated fabric was compared in uniaxial tensile tests.
Results:
Mechanical testing provides evidence of a remarkable enhancement in terms of ultimate strength and deformability for the coated specimens. This effect can be ascribed to the improved hydrophilicity of the fibers’ surface and to the activation of pozzolanic reaction at the interphase. In addition, penetration of nano- and microparticles in the bundle of the textile yarns reduces the occurrence of telescopic failure.
This paper combines qualitative information from the Eurosystem Bank Lending Survey with micro-data on loans for the participating Italian banks to assess the role of supply and demand factors in ...lending to enterprises developments, with a focus on the 2007–2009 financial crisis. Both demand and supply have played a relevant role, in the whole sample period and during the crisis. A counterfactual exercise shows that the effect of supply factors on the growth of lending was strongest after the Lehman collapse. On average, over the crisis period the negative effect on the annualized quarter-on-quarter growth rate of the panel banks’ lending to enterprises can be estimated in a range of 2.3–3.1 percentage points, depending on the specification. About one fourth of the total supply effect can be attributed to costs related to the banks’ balance sheet position, the rest to their perception of credit risk.
In the recent literature stance, purely nonlocal theory of elasticity is recognized to lead to ill-posed problems. Yet, we show that, for a beam, a meaningful energy bounded solution of the purely ...nonlocal theory may still be defined as the limit solution of the two-phase nonlocal theory. For this, we consider the problem of free vibrations of a flexural beam under the two-phase theory of nonlocal elasticity with an exponential kernel, in the presence of rotational inertia. After recasting the integro-differential governing equation and the boundary conditions into purely differential form, a singularly perturbed problem is met that is associated with a pair of end boundary layers. A multi-parametric asymptotic solution in terms of size-effect and local fraction is presented for the eigenfrequencies as well as for the eigenforms for a variety of boundary conditions. It is found that, for simply supported end, the weakest boundary layer is formed and, surprisingly, rotational inertia affects the eigenfrequencies only in the classical sense. Conversely, clamped and free end conditions bring a strong boundary layer and eigenfrequencies are heavily affected by rotational inertia, even for the lowest mode, in a manner opposite to that brought by nonlocality. Remarkably, all asymptotic solutions admit a well defined and energy bounded limit as the local fraction vanishes and the purely nonlocal model is retrieved. Therefore, we may define this limiting case as the proper solution of the purely nonlocal model for a beam. Finally, numerical results support the accuracy of the proposed asymptotic approach.
PDF
