University of Architecture, Civil Engineering and Geodesy

About: University of Architecture, Civil Engineering and Geodesy is a education organization based out in Sofia, Bulgaria. It is known for research contribution in the topics: Finite element method & Beam (structure). The organization has 3808 authors who have published 3822 publications receiving 30736 citations. The organization is also known as: Sofia Polytechnic.

A posteriori error estimates for Fredholm integral equations of the first kind

Abstract: We consider an adaptive finite element method for the solution of a Fredholm integral equation of the first kind and derive a posteriori error estimates both in the Tikhonov functional and in the regularized solution of this functional. We apply nonlinear results obtained in Beilina et al., (Journal of Mathematical Sciences, 167, 279–325, 2010), Beilina and Klibanov, (Inverse Problems, 26, 045012, 2010), Beilina et al., (Journal of Mathematical Sciences, 172, 449–476, 2011), Beilina and Klibanov, ( Inverse Problems, 26, 125009, 2010), Klibanov et al., (Inverse and Ill-Posed Problems), 19, 83–105, 2011) for the case of the linear bounded operator. We formulate an adaptive algorithm and present experimental verification of our adaptive technique on the backscattered data measured in microtomography.

Mechanical Behavior of Composite Sandwich Structures Subjected to Low Velocity Impact -Experimental Testing and Finite Element Modeling

Abstract: Sandwich materials with a cellularfoam core are increasingly used in engineering applications. The basic advantage of these materials is their high strength-weight ratio. However, sandwich structures are notoriously sensitive to failure by the application of highly localised external lateral loads. Therefore the proper design requires an understanding of the response of these materials to localised loads. The purpose of this work was the evaluation of the low velocity localised impact behaviour of sandwich structures made of glass-fibre reinforced composite laminate face sheets and polyvinylchloride (PVC) foam core. The impact tests were performed with a drop-weight rig. Beam and panel sandwich specimens were impacted using steel cylindrical and spherical impactors, respectively. During the testing, the specimens were supported by a stiff steel continuous substrate in order to avoid global bending. The load and deceleration histories were recorded during impact. A finite element model was developed to simulate the impact response. Results obtained by this model exhibit good correlation with the experimental data. It is shown that the model can be used to successfully predict the elastic-plastic response of sandwich composite structures subjected to low velocity localised impact.

High Temperature Deformation Behavior of In-Situ Synthesized Titanium-Based Composite Reinforced with Ultra-Fine TiB Whiskers.

Abstract: A TiB/Ti-6Al-4V composite reinforced with ultra-fine TiB whiskers (UF-TiB) was prepared by the powder metallurgy method. High temperature compression tests were carried out to study the hot deformation behavior of the UF-TiB/Ti-6Al-4V composite. The compressive deformation was performed in the temperature range of 900–1200 °C and the strain rate range of 0.001–10 s−1. The results showed that stable flow occurred at the condition of 900–1200 °C/0.001–0.01 s−1. The optimum working condition was 900 °C/0.001 s−1, with the deformation mechanism of dynamic recrystallization (DRX). Instable flow occurred when the strain rate was higher than 0.01 s−1, where the failure modes included adiabatic shear deformation, whisker breakage and whisker/matrix debonding. The deformability of the UF-TiB/Ti-6Al-4V composite was much better than the traditional casted and the pressed + sintered TiB/Ti-6Al-4V composites, which are typically reinforced with coarse-grained TiB whiskers. The high deformability was primarily attributed to the ultra-fine reinforcements, which could coordinate the deformation more effectively. In addition, a fine matrix microstructure also had a positive effect on deformability because the fine matrix microstructure could improve the grain boundary sliding.

Detection of stress concentration regions in cyclic loading by the heat monitoring method

Abstract: We present a method for determining stress concentration regions in structures under multiply repeated cyclic loads. To determine the stress concentration regions, we propose to stabilize the structure temperature characteristics before the cyclic loading and then, in the process of cyclic loading, measure the structure temperature field by thermal imaging equipment with subsequent analysis and processing of the obtained thermograms. The stress concentrations regions in the structures correspond to anomalous regions of thermograms, where the temperature excess is more than 0.3° C, and hence the stress concentration regions are clearly seen in the thermograms.