Unconventional linear flexoelectricity in two-dimensional materials

We predict a large in-plane polarization response to bending in a broad class of trigonal two-dimensional crystals. We define and compute the relevant flexoelectric coefficients from first principles as linear-response properties of the undistorted layer, by using the primitive crystal cell. The ensuing response (evaluated for SnS$_{2}$, silicene, phosphorene and RhI$_{3}$ monolayers and for an h-BN bilayer) is up to one order of magnitude larger than the out-of-plane components in the same material. We illustrate the implications of our findings by calculating the spontaneous axial polarization in nanotubes of arbitrary geometry, and the longitudinal electric fields induced by a flexural phonon

Direct and converse flexoelectricity in two-dimensional materials

Building on recent developments in electronic-structure methods, we define and calculate the flexoelectric response of two-dimensional (2D) materials fully from first principles. In particular, we show that the open-circuit voltage response to a flexural deformation is a fundamental linear-response property of the crystal that can be calculated within the primitive unit cell of the flat configuration. Applications to graphene, silicene, phosphorene, BN and transition-metal dichalcogenide monolayers reveal that two distinct contributions exist, respectively of purely electronic and lattice-mediated nature. Within the former, we identify a key $metric$ term, consisting in the quadrupolar moment of the unperturbed charge density. We propose a simple continuum model to connect our findings with the available experimental measurements of the converse flexoelectric effect.Comment: To appear in Phys. Rev. Let

Design and analysis of a composite beam for infrastructure applications - Part I: preliminary investigation in bending

The objective of this study is to contribute to the development of a composite beam for use in civil engineering systems. Based on the limitations in existing concepts, a new beam design is proposed and its behaviour studied. Using the classical beam theory, the Timoshenko beam theory, the Timoshenko plate theory, as well as the transformed section approach, borrowed from reinforced concrete, a simplified analytical approach, which could be used in design, is developed to conduct first and second order analysis of the proposed beam in order to achieve a rational sizing of its section before a rigorous testing regime is carried out. Finally, to validate the analytical model and gain confidence in the design, the analytical and experimental results are compared to a rigorous non linear finite element solution. It was found that the analytical model agreed relatively well with the experiments and the FE analyses, giving confidence in the validity of the underlying assumptions

Design and analysis of a composite beam for infrastructure applications - Part II: preliminary investigation in shear and torsion

Bending behaviour was dealt with in the preceding prequel, its associated failure modes identified, and a simplified theoretical approach was proposed for design purposes. However, this approach would not be complete without a simplified method for estimating the shear resistance of the beam and its torsional response

New fibre-reinforced polymer box beam: investigation of static behaviour

This thesis discusses the development of a new type of fibre-reinforced polymer (FRP) beam for use in civil engineering systems. After a detailed evaluation of the advantages and disadvantages of current FRP beam technology, a different approach is proposed which combines traditional laminates with a novel casting technique. To pre-dimension the beam, the classical beam theory is adapted to allow for FRP materials. The resulting formulae were used to determine critical parameters, such as laminate thickness and location in the cross-section, and core dimensions, and to identify failure modes. Based on the results of this analytical study, a detailed testing program was developed. In addition to classical tests, such as bending, shear, and lateral torsion, the performance of the beam was also examined under particular loading regimes specifically designed to induce local failure modes, such as buckling of the web and bearing failure of the section under concentrated loads. The experimental results revealed very good agreement with the analytical predictions. These results were corroborated by a detailed non-linear finite-element analysis, including core cracking and laminate damage. This analysis, in particular, highlighted the synergy between bending and shear behaviour of the beam. This study has revealed that this new type of FRP beam behaves in a predictable manner. Furthermore, the experimental results verified that the cross-section, which combines traditional laminates with cast polymer concrete, did not suffer from many of the disadvantages identified in current FRP beams. The cracking of the polymer concrete under shear, however, does cause the beam to fail prior to the laminates reaching their ultimate shear stress

Design and analysis of a composite beam for infrastructure applications - Part III: experimental results and nonlinear FE analysis

Using the analytical approaches developed, the cross section of the new fibre composite beam described in the prequels to this paper is designed in order to avert secondary failure modes. A series of specimens have been built and put through a thorough testing regime to establish the performance of the beam. To gain confidence in the analytical models and achieve further understanding of the beam behaviour, a rigorous nonlinear finite element analysis is also presented. It was found that the analytical model agreed relatively well with the experiment and the FE analysis, thus validating the underlying assumptions

Transporte termoeléctrico en puntos cuánticos acoplados a contactos normales y superconductores

[abstract not available

Larval growth and allometry in the cabbage butterfly Pieris brassicae (Lepidoptera: Pieridae)

By adopting a longitudinal study design and through geometric morphometrics methods, we investigated individual and ontogenetic variation in size, shape and timing during larval development of the cabbage butterfly Pieris brassicae under laboratory conditions. We found that ontogenetic size progression departs modestly, but significantly, from growth at a constant rate and that size at hatching contributes considerably to determine the size of the individual at all subsequent stages. As for the shape, ontogenetic allometry is much more conspicuous than static allometry, the latter in many cases being close to isometry. Analysis of developmental timing revealed a stage of apparently more effective developmental control at stage 3, supported by both the relatively small variance in cumulative developmental time up to stage 3 and by the pattern of correlation between duration of single stages. While presenting detailed quantitative aspects of growth in P.\ua0brassicae, in particular with respect to individual variation, this study and the associated dataset can provide a basis for further explorations of the post-embryonic development in this insect and contribute to the ongoing investigations on growth regulation and control in insects

Size and shape regulation during larval growth in the lepidopteran Pieris brassicae

By adopting a longitudinal study design and through geometric morphometrics methods, we investigated size and shape regulation in the head capsule during the larval development of the cabbage butterfly Pieris brassicae under laboratory conditions. We found evidence of size regulation by compensatory growth, although not equally effective in all larval stages. Size compensation is not attained through the regulation of developmental timing, but rather through the modulation of per-time growth rate. As for the shape, neither the variance of the symmetric component of shape, nor the level of fluctuating asymmetry show any evidence of increase across stages, either at the population or individual level, which is interpreted as a mark of ontogenetic shape regulation. In addition, also the geometry of individual asymmetry is basically conserved across stages. While providing specific documentation on the ontogeny of size and shape variation in this insect, this study may contribute to a more general understanding of developmental regulation and its influence on phenotypic evolution