The dissertation will be defended at the public meeting of the Dissertation Defense Council of the Materials Engineering Scientific Field in the SRA-I Hall of Vilnius Gediminas Technical University at 10 a.m. on 16 May 2024.
The use of fiber-reinforced polymer (FRP) materials in various structural applications as an alternative to steel is gaining popularity due to their lightweight, electromagnetic transparency, and corrosion resistance. However, their mechanical performance under elevated ambient temperatures and humidity is known to degrade, and the extent of degradation needs to be quantified. This dissertation proposed a standardized testing layout and analytical model to quantify the flexural stiffness of the beam samples with composite reinforcement. The developed analytical model explicitly relates particular moment and curvature values, requiring neither iterative calculations nor load history. This feature enables comparative analysis and quantification of the flexural stiffness of repeatedly loaded composite elements with different reinforcement materials layouts. Thus, the proposed testing layout and analytical approach provide a uniform and systematic methodology to quantify and compare the mechanical performance of various composite reinforcement combinations subjected to repeated loading conditions. This study investigates the impact of unfavorable environmental effects on the mechanical performance of composite reinforcement. The research considers several typical composite reinforcement layouts, including embedded glass fiber-reinforced polymer bars, externally bonded carbon fiber (CF) sheets, and near-surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. The study has identified the efficiency of hybrid reinforcement using CFRP materials and steel bars. The proposed experimentally verified simplification of the analytical model describes the scientific novelty of this dissertation. It allows the model to solve the tension-stiffening problem independent of the loading conditions and quantify the flexural stiffness alteration under repeated loading conditions. The dissertation is structured into an introduction, three chapters, general conclusions, a list of the literature references, and a list of the Author’s publications on the dissertation topic. The first chapter reviews the literature on microstructure, physical properties, and constitutive models of FRP materials. The second chapter presents the proposed methodology for the stiffness analysis of composites with FRP reinforcement compositions subjected to monotonic, repeated mechanical, and combined loads. The third chapter presents several examples of the proposed procedure, considering the comparative analysis of the composite reinforcement subjected to monotonic and repeated mechanical loads and the elevated temperature (up to 60 °C) combined with repeated mechanical loads. It also investigates the degradation mechanisms of adhesive bonds between CF sheets and concrete. The list of the Author’s publications on the dissertation’s topic consists of eight publications (two with an Impact factor from ISI Web of Science Database) and six international conference presentations.
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