418 research outputs found

    evaluation of the effects of the numerical modelling choices on the simulation of a tensile test on cfrp composite

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    Abstract The goal of the present work is to define a method to build a FE model which is able to reproduce an experimental tensile test on CFRP specimen with different stacking sequences (UD and balanced). The defined method assesses the material numerical parameters by means of a simulation that replicates, as a virtual test, the experimental tensile one, and in the future, it will be possible to exploit the data obtained to create a reliable model for the simulation of low velocity impacts. Analyses have been performed using the non-linear solver ABAQUS Explicit. The current work further studies how to model damage and the effect of modifications of the numerical parameters on the results. Indeed, the numerical simulation of composite materials is very sensitive to the numerical choices made. Moreover, from the literature and experiments, the mechanical properties of composites are very variable and hence the evaluation of the model response to such modifications is of particular interest

    Optical study of the vibrational and dielectric properties of BiMnO3

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    BiMnO3 (BMO), ferromagnetic (FM) below Tc = 100 K, was believed to be also ferroelectric (FE) due to a non-centro-symmetric C2 structure, until diffraction data indicated that its space group is the centro-symmetric C2/c. Here we present infrared phonon spectra of BMO, taken on a mosaic of single crystals, which are consistent with C2/c at any T > 10 K, as well as room-temperature Raman data which strongly support this conclusion. We also find that the infrared intensity of several phonons increases steadily for decreasing T, causing the relative permittivity of BMO to vary from 18.5 at 300 K to 45 at 10 K. At variance with FE materials of displacive type, no appreciable softening has been found in the infrared phonons. Both their frequencies and intensities, moreover, appear insensitive to the FM transition at Tc

    Synthesis and characterization of multiferroic BiMn7_7O12_{12}

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    We report on the high pressure synthesis of BiMn7_7O12_{12}, a manganite displaying a "quadruple perovskite" structure. Structural characterization of single crystal samples shows a distorted and asymmetrical coordination around the Bi atom, due to presence of the 6s26s^{2} lone pair, resulting in non-centrosymmetric space group Im, leading to a permanent electrical dipole moment and ferroelectric properties. On the other hand, magnetic characterization reveals antiferromagnetic transitions, in agreement with the isostructural compounds, thus evidencing two intrinsic properties that make BiMn7_7O12_{12} a promising multiferroic material.Comment: 4 pages, 3 figure

    Commensurate structural modulation in the charge- and orbitally-ordered phase of the quadruple perovskite (NaMn3_3)Mn4_4O12_{12}

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    By means of synchrotron x-ray and electron diffraction, we studied the structural changes at the charge order transition TCOT_{CO}=176 K in the mixed-valence quadruple perovskite (NaMn3_3)Mn4_4O12_{12}. Below TCOT_{CO} we find satellite peaks indicating a commensurate structural modulation with the same propagation vector q =(1/2,0,-1/2) of the CE magnetic order that appears at low temperature, similarly to the case of simple perovskites like La0.5_{0.5}Ca0.5_{0.5}MnO3_3. In the present case, the modulated structure together with the observation of a large entropy change at TCOT_{CO} gives evidence of a rare case of full Mn3+^{3+}/Mn4+^{4+} charge and orbital order consistent with the Goodenough-Kanamori model.Comment: Accepted for publication in Phys. Rev. B Rapid Communication

    Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMn3_3Mn4_4O12_{12}

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    By means of neutron powder diffraction, we investigated the effect of the polar Bi3+^{3+} ion on the magnetic ordering of the Mn3+^{3+} ions in BiMn3_3Mn4_4O12_{12}, the counterpart with \textit{quadruple} perovskite structure of the \textit{simple} perovskite BiMnO3_3. The data are consistent with a \textit{noncentrosymmetric} spacegroup ImIm which contrasts the \textit{centrosymmetric} one I2/mI2/m previously reported for the isovalent and isomorphic compound LaMn3_3Mn4_4O12_{12}, which gives evidence of a Bi3+^{3+}-induced polarization of the lattice. At low temperature, the two Mn3+^{3+} sublattices of the A′A' and BB sites order antiferromagnetically (AFM) in an independent manner at 25 and 55 K, similarly to the case of LaMn3_3Mn4_4O12_{12}. However, both magnetic structures of BiMn3_3Mn4_4O12_{12} radically differ from those of LaMn3_3Mn4_4O12_{12}. In BiMn3_3Mn4_4O12_{12} the moments MA′\textbf{M}_{A'} of the A′A' sites form an anti-body AFM structure, whilst the moments \textbf{M}B_{B} of the BB sites result from a large and \textit{uniform} modulation ±MB,b\pm \textbf{M}_{B,b} along the b-axis of the moments \textbf{M}B,ac_{B,ac} in the acac-plane. The modulation is strikingly correlated with the displacements of the Mn3+^{3+} ions induced by the Bi3+^{3+} ions. Our analysis unveils a strong magnetoelastic coupling between the internal strain created by the Bi3+^{3+} ions and the moment of the Mn3+^{3+} ions in the BB sites. This is ascribed to the high symmetry of the oxygen sites and to the absence of oxygen defects, two characteristics of quadruple perovskites not found in simple ones, which prevent the release of the Bi3+^{3+}-induced strain through distortions or disorder. This demonstrates the possibility of a large magnetoelectric coupling in proper ferroelectrics and suggests a novel concept of internal strain engineering for multiferroics design.Comment: 9 pages, 7 figures, 5 table
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