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Varbinka Valeva

Institute of Mechanics, Bulgarian Academy of Science (BG)

Recent publications
1.  Mróz Z., Yanakieva A., Valeva V., Ivanova J., Analytical pullout analysis for carbon nanotube-cement composites under static loading, Comptes rendus de l'Académie bulgare des Sciences, ISSN: 1310-1331, Vol.66, No.3, pp.431-438, 2013

Abstract:
The paper presents analytical pullout analysis for Carbon nanotube-cement composite based on Shear-lag assumptions. The composites under study are cement matrix composites reinforced by means of unidirectional Carbon nanotubes (CNT). The interface is assumed to be a material line with constitutive behaviour characterized by a relation between sliding stress and relative sliding displacement. Frictional sliding at the interface region is described by three different interface models: (i) Constant-τ model; (ii) Linear slip-hardening model and, (iii) Linear slip-softening model. The pullout analysis is performed considering static loading. The numerical examples are performed for concrete geometrical and material characteristics for CNT/cement composite. The obtained results are illustrated by figures and discussed.

Keywords:
Carbon nanotube cement reinforced composites, Interface sliding models, Pullout analysis, Shear-lag model, Static loading

Affiliations:
Mróz Z. - IPPT PAN
Yanakieva A. - Institute of Mechanics, Bulgarian Academy of Science (BG)
Valeva V. - Institute of Mechanics, Bulgarian Academy of Science (BG)
Ivanova J. - Institute of Mechanics, Bulgarian Academy of Science (BG)
2.  Gambin B., Ivanova J., Valeva V., Nikolova G., Precracking and interfacial delamination in a bi-material structure: Static and dynamic loadings, ACTA MECHANICA SINICA, ISSN: 0567-7718, DOI: 10.1007/s10409-011-0414-3, Vol.27, No.1, pp.80-89, 2011

Abstract:
The behavior of a precracked bi-material structure interface under given static and dynamic axial loading is an interest object in the present paper. Firstly, it is shown that the shear-lag model is a proper tool to analyze a delamination process in a precracked bi-material structure undergoing static loading. Secondly, the “shear-lag model” is applied to the structure under dynamic loading. To solve the problem for an interface delamination of the structure and to determine the debond length along the interface, our own 2D boundary element method (BEM) code is proposed in the case of static loading, and the shear-lag model together with the Laplace transforms and half-analytical calculations are used in the case of dynamic loading. The interface layer is assumed as a very thin plate compared with the other two. The parametric (geometric and elastic) analysis of the debond length and interface shear stress is done. The results from the 2D BEM code proved the validity of analytical solutions to the shear-lag model. In the dynamic case, the influence of loading characteristics, i.e., frequencies and amplitude fluctuations on the shear stress and the value of debond length for an interval of time, is discussed. The analysis of the obtained results is illustrated by an example of the modern ceramic-metal composite, namely cermet, and depicted in figures.

Keywords:
Debond length, Precracked bi-material structure, Shear-lag model, Delamination, BEM

Affiliations:
Gambin B. - IPPT PAN
Ivanova J. - Institute of Mechanics, Bulgarian Academy of Science (BG)
Valeva V. - Institute of Mechanics, Bulgarian Academy of Science (BG)
Nikolova G. - Institute of Mechanics, Bulgarian Academy of Science (BG)
3.  Valeva V., Ivanova J., Gambin B., BEM and Shear lag method for interface problem of bi-material structure under static loading, JOURNAL OF THEORETICAL AND APPLIED MECHANICS, ISSN: 1429-2955, Vol.41, No.9, pp.17-29, 2011

Abstract:
The behaviour of the interface of a pre-cracked bi-material ceramic-metal structure under static axial loading is an object of interest in the present paper. To solve the problem for interface delamination of the structure and to determine the debond length along the interface, a 2D BEM code was created and applied. The interface plate is assumed as a very thin plate comparing with the others two. The parametric (geometric and elastic) analysis of the debond length and interface shear stress is done. First, the obtained numerical results are compared with analytical ones from 1D. Shear lag analysis of the considered structure. The respective comparison is illustrated in figures and shows a good agreement. The comparison between the calculated using 2D BEM code elastic-brittle debond lengths with Song's experimental data for the bi-material structure Zinc/Steel as well as with respective results from FEM simulation shows good coincidence.

Keywords:
BEM, Shear lag analysis, bi-material structure, debond length

Affiliations:
Valeva V. - Institute of Mechanics, Bulgarian Academy of Science (BG)
Ivanova J. - Institute of Mechanics, Bulgarian Academy of Science (BG)
Gambin B. - IPPT PAN
4.  Ivanova J., Valeva V., Mróz Z., Interphase model for a multilayer structures applied in heat conduction problems, JOURNAL OF THEORETICAL AND APPLIED MECHANICS, ISSN: 1429-2955, Vol.34, pp.15-30, 2005
5.  Valeva V., Ivanova J., Mróz Z., Parametric analysis of the decay rate of end effects in heat conduction for multi-layered semi-infinite sandwich structures, Journal of Theoretical and Applied Mechanics, BULGARIAN ACADEMY OF SCIENCES, ISSN: 1313-9665, Vol.32, No.4, pp.39-52, 2002

Abstract:
This article is concerned with the effect of imperfect interfaces on the decay of end effect for heat conduction in non-symmetric sandwich strip with - layers. The constituent layers are isotropic and the interface conditions between the layers are assumed to be either LC-type or HC-type at vanishing temperature at the bottom and top side of the strip. The characteristic decay length in all considered cases for three-layered sandwich strip is numerically calculated and represented by the smallest real and positive root of a corresponding transcendental equation. The results of parametric analysis of decay rate depending on volume fraction and heat conductivity of constituents provided for three-layered sandwich strip are illustrated in figures and discussed. Some optimal parameters for the considered structure at which the maximal value of the decay rate is reached are proposed.

Keywords:
decay rate of end effects, heat condition, semi-infinite sandwich structure

Affiliations:
Valeva V. - IPPT PAN
Ivanova J. - Institute of Mechanics, Bulgarian Academy of Science (BG)
Mróz Z. - IPPT PAN

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