Browsing by Author "Stankov V."

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    Bismuth–Titanium–Silicate–Oxide Glass Ceramics for Various Dielectric Applications
    (2025-12-01) Slavov S.; Stankov V.
    Ceramics based on bismuth titanate with added SiO2 and Nd2O3 were synthesized from the Bi2O3–TiO2–SiO2–Nd2O3 system through rapid melt quenching followed by controlled cooling. By adjusting the initial compositions and applying heat treatments between 1450 °C and 1100 °C, either homogeneous crystalline products or multiphase glass–ceramics were obtained. The identified crystalline phases included Bi12TiO20 and Bi4Ti3O12, coexisting with amorphous networks enriched in silicon, bismuth, titanium, and aluminum oxides. In previous investigations, the materials were characterized using X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy, which collectively confirmed the presence of both ordered and disordered structural domains within the bulk samples. Electrical properties were evaluated through measurements of conductivity (4 × 10−9 S/m to 30 S/m), dielectric constant (real part from 10 to 5 × 103 and imaginary part from 5 to 5 × 104), and dielectric loss (0.02 to ~100) over the frequency range 1 Hz–1 MHz. These results provide a foundation for rational control of phase evolution in this quaternary oxide system and highlight strategies for tailoring the functional properties of glass–ceramic materials for dielectric applications. The aim of the present study is to investigate the relationship between phase composition, structural features, and dielectric behavior in cast Bi–Ti–Si–Nd glass–ceramics. Particular attention is given to the influence of the amorphous network containing SiO2 as a traditional glass former, as well as the formation of amorphous crosslinking Si–O–Ti structures acting as non-traditional glass formers.
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    MODELLING CONFINED ZONE EFFECT IN REINFORCED CONCRETE CORBEL STRENGTHENED BY CFRP
    (2025-11-02) Stankov V.; Ivanova I.; Assih J.; Dontchev D.
    Confined zone effect, which is primarily observed in reinforced concrete (RC) columns subjected to compression, is crucial for the load-bearing capacity of (RC) structures. This occurs when, despite an increase in the load, the resulting deformations are nearly zero. The same effect can be observed in structures strengthened by different types of fabrics (carbon, glass, Kevlar, etc.). It is well known that experimental studies are a high-resources and time-consuming process. Therefore, a combination of experimental study and non-linear finite element (FE) simulation strategy is considered. The structure investigated in this paper is an RC corbel strengthened by bonding carbon fibre reinforced polymer (CFRP). Two RC corbel structures were developed, one with strengthening by three layers of CFRP and one without strengthening. The experimental results were used to validate the FE simulation. The model was successfully validated and provides opportunities for future parametric investigations. The aim of this study is to model he confined zone effect in reinforced concrete corbels strengthened by CFRP. Modelling confined zone effect is a significant challenge for engineers. This study shows a new approach to FE simulation. The resulting model successfully simulates the mechanical behaviour of the structure.
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    Numerical simulation of shear strength in a short reinforced concrete corbel strengthened with composite material compared with experimental results
    (2018-01-01) Ivanova I.; Assih J.; Stankov V.; Dontchev D.
    This paper describes a numerical analysis carried out to assess the shear strength of strengthened short reinforced concrete corbel by using Carbon Fiber Reinforced Fabrics (CFRF). To extend the life and shear strength of the reinforced concrete structures often used technique for external bonding of composite materials. This study provides a comparison of curves obtained with numerical model and experimental curves of corbels. Results show that in general, ultimate load, crack patterns and deformation capacity were satisfactorily reproduced. The Finite element model results are successful compared and validated by experimental results.