Materials Structure & Micromechanics of Fracture V

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Authors: Luboš Náhlík, Pavel Hutař, Zdeněk Knésl
Abstract: The objective of this paper is to investigate the transverse fatigue cracking of layered structures. To this end a fatigue crack propagation rate for a crack growing perpendicularly through the interfaces between individual layers of the structure is estimated. The suggested procedure takes into account the influence of different material properties of the layers on the stress field around the fatigue crack and the change of the stress singularity exponent in the special case of a crack touching the material interface. The assumptions of linear elastic fracture mechanics and elastic behaviour of the composite are considered. It is shown that the existence of the interfaces can have a negative influence on the residual fatigue life of layered structures.
Authors: Luboš Náhlík, Lucie Šestáková, Pavel Hutař
Abstract: The objective of the paper is to investigate the direction of a further crack propagation from the interface between two elastic materials. The angle of crack propagation changes when the crack passes the interface. The suggested procedure makes it possible to estimate an angle of propagation under which the crack will propagate into the second material. The assumptions of linear elastic fracture mechanics and elastic behavior of the body with interfaces are considered. The finite element method was used for numerical calculations. The results obtained might contribute to a better understanding of the failure of materials with interfaces (e.g. layered composites, materials with protective coatings) and to a more reliable estimation of the service life of such structures.
Authors: Šárka Houdková, František Zahálka, Michaela Kašparová
Abstract: The tribological properties of parts surface, namely their wear resistance and friction properties, are in many cases determining for their proper function. To improve surface properties, it is possible to create hard, wear resistant coatings by thermal spray technologies. Using these versatile coatings it is possible to increase parts lifetime, reliability and safety. The thermally sprayed cermet composite coatings show, thanks to their specific properties, excellent resistance to abrasive and erosive wear, as well as corrosion resistance. To predict the behavior, lifetime and application area of thermally sprayed cermet coatings it is necessary to completely understand the relationships between technology, process parameters, microstructure and properties of the coatings. The finding of these relationships and use this understanding to develop deposits with improved wear resistance for coating of various applications is the main aim of the presented work. It was done by studying the coatings microstructure and mechanical properties. Four different tests of wear resistance were done to study the mechanism of surface degradation, to confirm the results of mechanical testing and to predict the lifetime of coated parts - the abrasive wear performance of the coatings was assessed using a dry/sand rubber wheel test according to ASTM G-65, wet slurry abrasion test according to ASTM G-75, pin-on-disc test according to ASTM G-99 and erosion wear resistance for three impact angles. On the basis of obtained data the new possibilities of coatings application was determined, tested and implemented.
Authors: Hamid Sarraf, Ludmila Škarpová
Abstract: The effect of anodic surface treatment on the polyacrylonitrile (PAN)-based carbon fibers surface properties and the mechanical behavior of the resulting carbon fiber-polymer composites has been studied in terms of the contact angle measurements of fibers and the fracture toughness of composites. Results from contact angle measurements revealed that the angle of electrolyte solution largely decreases with increasing current densities of treatments up to 0.4-0.5 A m-2. The results obtained from the evolution of KIC with flexure of the composites as a function of electric current density shown that the KIC of the composite continually increases with increased current densities of the treatments up to 0.5 A m-2, and a maximum strength value is found about 294 MPa cm1/2 at the anodic treatment of 0.5 A m-2. It can be concluded that the anodic surface treatment is largely influenced in the fiber surface nature and the mechanical interfacial properties between the carbon fiber and epoxy resin matrix of the resulting composites, i.e., the fracture toughness. We suggest that good wetting plays an important role in improving the degree of adhesion at interfaces between fibers and matrices of the resulting composites.
Authors: Galina Gosmanová, Ivo Kraus, Michal Kolega, Věra Vrtílková
Abstract: The results of X-ray diffraction analysis of macroscopic stresses σ and crystallite size D in oxide layers are presented in this contribution. The oxide layers were formed on tubular specimens of Zircaloy 4 and Zr1Nb alloys which were simultaneously oxidized for various times under temperature transient conditions (oxidation in water at 360 °C with a short-time shock in steam at 500 °C). A qualitative relation was outlined between the residual stresses in oxide layers and corrosion kinetics of the alloys under investigation.
Authors: Lubomír Grmela, Pavel Tománek, Pavel Škarvada
Abstract: The local spatial distribution of photoluminescence due to the creation of hot luminescence centers was measured in the optical near-field by Scanning near-field optical microscope at emission peaks of materials (λ =595nm), which is due to the luminescence of Mn2+ in ZnS. The excitation bandgap of ZnS forms exitons, and these excitons get the center of Mn2+ through nonradiation dominates, by means of transition of 4T1 – 6A1 luminescence. This spectrum is evidence that Mn2+ has been incorporated into the ZnS nanoparticles. In comparison with the bulk ZnS:Mn phosphors these nanoparticles have clearly higher luminescent efficiency with its luminescent decay time at least 4 orders of magnitude slower. It means that the oscillator intensity of luminescent centers in ZnS:Mn nanocrystal enhances at least 4 orders of magnitude than that in corresponding bulk ZnS:Mn.
Authors: Vĕra Rothová, Jiří Buršík, Milan Svoboda, Jiří Čermák
Abstract: In the temperature range 600–1000 °C, the effect of material purity on self-diffusion along grain boundaries has been studied in both the pure (Puratronic 99.9945%) and the technical (99.5%) nickel. The penetration profiles were measured by the serial sectioning method using the 63Ni radiotracer. The extensive electron backscatter diffraction (EBSD) analysis was performed on the same samples in order to reveal possible differences in microstructure induced by the impurity content. The obtained microstructure characteristics were further interpreted in terms of the coincidence site lattice (CSL) model.
Authors: Ivo Stloukal, Jiří Čermák
Abstract: Diffusion of 65Zn in Mg-based alloy AZ91 with short Saffil fibres was studied by sectioning method in the temperature interval 648 – 728 K. The results were compared with those obtained in the measurements made with the same materials without the reinforcement. This enabled to assess the influence of the interfaces between the matrix and the fibres and to evaluate the diffusion characteristics of interface self-diffusion.
Authors: Regina Mikulíková, Kateřina Kolářová, Václav Švorčík, Barbora Dvořánková, Tomáš Sopuch
Abstract: The properties of polyethylene doped with Ca2+ salt of oxidized cellulose was studied by different techniques. FTIR spectroscopy was used for the determination of crystalline phase in polymer film, surface wettability was determined by standard goniometry and surface morphology was examined by SEM microscopy. Adhesion of mouse 3T3 fibroblasts on the doped polymer was studied in vitro. It was found that the polyethylene doped with the cellulose derivative can be sterilized in boiling water. The number and homogeneity of adhering cells were shown to depend on the surface wettability and morphology.
Authors: Petr Slepička, J. Heitz, Jakub Siegel, M. Špírková, Václav Švorčík
Abstract: This work is focused on laser modification of polymers with different angles of incidence. Periodic surface structures generated by linearly polarized F2 laser light (157 nm) on polyethyleneterephthalate (PET) were studied. Atomic force microscopy was used to study the topological changes induced by the laser irradiation. The laser irradiation induces the formation of periodic ripple structures, the width of and the height being angle dependent. The periodic structures were formed with the laser fluence 4.70 mJ/cm2. The laser modified PET foils were coated with a 50 nm thick gold layer by sputtering. After Au deposition on the laser PET foils with ripple structure, the roughness of surface decreases in comparison to PET with ripples without Au coating.

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