In proposed study the effect of the composition and structure of polyurethane/ground rubber (PUR/GR) composites on the mechanical behavior was evaluated. The structure-properties relationship was studied for PUR matrices with different polarity, porosity and mechanical properties. The morphology of PUR/GR composites was expressed by two structural parameters-interspace filling and interspace volume. Experimental data of mechanical testing were fitted by powerlaw functions containing mentioned structural parameters whereas exponents of these parameters reflect differences in mechanical properties and chemical composition of PUR matrices. Obtained equations describe the mechanical behavior with respect to the structure and morphology of porous PUR/GR composites consisting of one filler and different PUR matrices. Proposed study suggests new approach to the description of the relationship between mechanical behavior and structure of porous composite materials.

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The cohesive strength of 3, 5, and 11 grain boundaries (GBs) in clean and hydrogen-segregated fcc nickel was systematically studied as a function of the superimposed transverse biaxial stresses using ab initio methods. The obtained results for H-free GBs revealed a quite different response of the coherent twinning boundary 3 to the applied transverse stresses in comparison to the other GB types. While the cohesive strength of 5 and 11 GBs increased with increasing level of tensile transverse stresses, the strength of 3 GB remained constant for any applied levels of transverse stresses. In the case of GBs with segregated hydrogen, the cohesive strength of 3 was distinctly reduced for all levels of transverse stresses, while the strength reduction of 5 and 11 GBs was significant only for a nearly isotropic (hydrostatic) triaxial loading. This extraordinary response explains a high susceptibility of 3 GBs to crack initiation, as recently reported in an experimental study. Moreover, a highly triaxial stress at the fronts of microcracks initiated at 3 boundaries caused a strength reduction of adjacent high-energy grain boundaries which thus became preferential sites for further crack propagation.

Abstract. NiTi shape memory alloy has been widely used in many engineering applications (e.g. mechatronic devices, medical tools, seismic sensors etc.). In this work, the Young's moduli of twinned NiTi martensite in selected crystallographic directions are calculated from first principles and compared with previous data for perfect (untwinned) NiTi structure. The obtained results show that the selected twinning mode does not have a significant influence on the Young's moduli and, thus, their reduction observed in some experiments is probably caused by changes in martensitic microstructure (arrangement of twinning variants).

Mechanical and fracture properties of unidirectional composites reinforced with R-glass fibres and utilizing various commercially available polysiloxane resins as matrix precursors were investigated. As the matrix becomes more brittle after the pyrolysis the impact toughness and flexural strength of the composites fall. On the other hand, the shear modulus rises after the pyrolysis as the matrix becomes stiffer in shear. The appearance of fracture surfaces generated during the flexural strength at room temperature (RT) and elevated temperatures is discussed.