Applied Mechanics and Materials Vol. 821

Abstract: Ball screws and nuts represent the inseparable part of machinetool building. The thread lead accuracy ranges in hundredths of millimetres per 1 m. In addition to the undesirable phenomena caused by heat (dilatability) and, e. g., torsional and transverse oscillations, the bending stress of the ball screw is considerably undesirable too. This stress and at the same time behaviour are transferred to the bending stress of the ball screw nut. This has a very harmful influence on setting of the ball screws, when the machine tool is assembled. Using the analysis by means of the FEM method, the paper conducts a survey of deformation issue of the arrangement "machine – ball screw – ball screw nut" during the assembly stage as well as a survey of compensation possibilities and other technical measures determined to limit the undesirable deformations and stress of the ball screw originating already in the manufacturing stage of the machine tool.

Abstract: The paper deals with the issue of geometric precision measuring at large parts (especially at castings having big dimensions) in their manufacturing stage. The large parts of machinetool frames are required to have their geometric tolerances in thousandths of millimetres. The production or cooperation abilities of the particular company must be adapted to these demands and it is also necessary to adapt the company metrological equipment and measuring procedures to them. The standard measuring equipment cannot be used in most cases, because the particular parts are too large. For this reason, it is necessary to search such methods and procedures which enable to perform measuring with the relevant result. It could be considered to use e. g. 3D scanners. Unfortunately, their measuring precision has not reached the required tolerances up to now. For example, the HandyPROBE 3D scanner measures with the precision of 0,022 mm [1].

Abstract: Composite structures are now increasingly used for their properties in all areas of industrial production where high specific strength is demanded. They gradually replace metal parts and components not only because they are lighter, but above all for their comparable and in many ways even better mechanical properties. Knowledge of behavior of simple synergies between the fibres and the matrix allows the prediction of behavior of complex components and their application in practice. The subject of this article is a description of an experiment and numerical model, that compares the mechanical properties of carbon fiber composite with the values obtained using analytical models. Carbon composite samples were studied in laboratory conditions through Barrier test (ie. Crash test).

Abstract: An algorithm generating morphology of 2D and 3D stochastic Wang tiles is presented in thiscontribution. The algorithm is based on the discrete element method (DEM) and is therefore applicableprimarily for matrix-based microstructures with separate inclusions with emphasis on producingmaximally random dense packings. An open-source free DEM code YADE was used for all the computations.The method is illustrated with a 2D exemplary realization of Wang tiling and comparisonagainst Periodic Unit Cell representation in terms of spatial statistics is provided.

Abstract: Tungsten is currently considered as the most suitable plasma facing material for the first wall of a nuclear fusion reactor. First wall will be subjected to harsh conditions that will gradually deteriorate properties of the wall material. Some studies point out that fine-grained tungsten could be more resistant to the structure and property changes than coarse-grained tungsten. However, tailoring of tungsten microstructure is very laborious. Due to its high melting point, tungsten is very often processed mechanically and subsequently sintered into a compact body. In this study, preparation of ultrafine-grained tungsten by mechanical processing in a planetary ball mill was examined. Three types of tungsten samples were compared. One was made from coarse grained tungsten powder consolidated by SPS (spark plasma sintering). Other two samples were prepared from the powder processed in a planetary ball mill with and without addition of Y₂O₃. After ball milling, the powders were consolidated by SPS, i.e. fast sintering process that allows preserving fine-grained structure of the powder material. Properties of the samples such as hardness and thermal conductivity were examined and correlated with the processing history and microstructure.

Abstract: Acoustic emission (AE) is current trend of non-destructive monitoring methods. It suits perfectly for supporting of fatigue tests. The method is also applicable in monitoring of quasi-static tests. AE helps the engineers to understand the degradation process in the tested object and also keep track of the failures. This article offers an insight into the practical experience with acoustic emission. Tests of three different composite structures (wing, fuselage and hull panel) were chosen to illustrate the application of AE monitoring system during fatigue and quasi-static tests. These tests have shown that accuracy of localization is sufficient to identify damaged areas. Moreover, the system may offer an early warning of upcoming failure.

Abstract: In our work we tried to find a simple and reliable method to determine mechanical properties of a woven fabric with the main respect to the processing of these fabrics on a weaving machine. Some interesting results emerged by that way concerning for example the design of fabrics. We were using primarily the means of classical mechanics and we tried to avoid the use of computer resource demanding tools as much as possible. Also we were concerned by the applicability of results as large as possible since our weaving machines are processing a wide spectrum of fabrics.\parIn this paper, the principles of our approach are presented with some numerical examples. Few practical experiments were carried out at this moment, so only limited comparison with the reality could be done.

Abstract: Purpose of this study is investigation of energy absorption capability of the sandwich structures composed of combination of polystyrene and metal foam element and their suitability as new structure for design of protective helmets. Two types of the metal foams were experimentally tested and evaluated: Alporas (Shinko Wire Ltd., Japan) and Aluhab (Aluinvent Plc., Hungary). Samples of the sandwich structure are composed of two layers: bottom expanded polystyrene (EPS 200S) layer and upper metal foam layer which are glued together. Prepared samples are tested using a drop tower experiment to measure sample response (acceleration, reaction force) at different strain rates and energies. From acceleration/time history the Head Injury Criterion (HIC) is calculated as significant parameters in terms of protective helmets. Moreover, measured and derived characteristics are compared with pure EPS samples to obtain comparison of deformation behaviour between conventional structure for protective helmets and designed sandwich structures.

Abstract: In this work parametric modelling was utilized to design and produce two types of porous microarchitectures with auxetic compressive properties suitable for deformation energy mitigation applications such as blast and bullet protection. The samples were directly produced from acrylic material using a high resolution 3D printer and their compressive mechanical characteristics were tested. Two different structures exhibiting in-plane negative strain dependent Poisson’s ratio were selected for the analysis: i) two-dimensional inverted (re-entrant) honeycomb and ii) two-dimensional cut missing-rib. Stress-strain relationships were established from a set of quasi-static compression experiments where the strain fields were evaluated using digital image correlation applied to measure the full-field displacements on the samples' surface. From the displacement fields true strain – true stress curves were derived for each sample and relative elastic moduli were evaluated.

Abstract: Tests on miniature samples are increasingly used for the determination of mechanical properties of materials available in small volumes (non-destructive or semi-destructive approach). Small punch testing at constant deflection rate (SPT-CDR) of selected magnesium alloys and composites was performed at room temperature. Mechanical properties (yield strength, ultimate strength) were evaluated from SPT and correlated with results of uniaxial tensile tests (UTT). SPT characteristics were converted to uniaxial tensile properties by empirical formulas available in the literature. New formulas more appropriate for magnesium alloys were suggested.