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  • Autor: M. Michalska x
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Abstract

The objective of this paper is to investigate the effects of scale of an auxetic cellular material sample on the evaluation of elastic properties. Size and boundary effects are studied in detail. This is achieved by conducting computer simulations of the auxetic structure under the typical loading exerted by the compression and simple shearing test performed by means of ABAQUS FEA. The material microstructure is discretized by the plane network of Timoshenko beam elements. The results of the studies give insight to the scale effects. Structures with designed properties can be potentially used for engineering applications.

Abstract

The Pechini sol-gel method was applied to obtain LiMn2–xTxO4 (T = Ni, Fe; x = 0.1 to 0.5) nanopowders. Crystal and electronic structures, chemical composition and magnetic properties of the materials were characterized by X-ray diffraction, XPS, SEM/EDX microscopy, prompt gamma-ray activation analysis (PGAA), Mössbauer spectroscopy and magnetic susceptibility, respectively. XRD measurements showed that the LiMn2–xNixO4 were single phase for x = 0.1 and 0.2. Three samples with higher Ni content contained some addition of a second phase. Analysis of the oxidation state of the dopants by XPS revealed ionic Ni2+ and Fe3+. Mössbauer spectroscopy also confirmed 3+ oxidation state of iron and its location in octahedral sites, which excluded the inverse spinel configuration. XPS examinations showed that Mn3+ ions dominated in the iron substituted series whereas the Mn4+ was dominant in the nickel series.

The objective of this paper is to present and compare the results of numerical solutions of contact problem for two types of seats subjected to typical sitting loadings. The first seat is made of a typical hyperelastic foam, the other is designed with an auxetic polyamid spring skeleton. Computer simulations of the seat structure under a typical static loading exerted by a human body are performed by means of ABAQUS FEA. The model provides an insight into deformation modes and stress field in relation to geometric and material parameters of the seat structure.The other type of seat, due to the fact of global auxecity and progressive springs characteristics reduces contact stress concentrations, giving an advantegous distribution of pressure and provides the sensation of physical comfort. The proper seat skeleton shape leads to an improvement of ergonomic quality.