Original scientific paper The subject of this paper is testing of complex geometrical structures, i.e. stress analysis of a pipe branch model of A6 third pipeline at Hydropower Plant Perućica, Nikšić. Analysis of pipe branch model stress distribution was carried out by using finite element method and experimental methods. Finite element method defined 3D pipe branch model. Numerical calculation defined critical locations on model, i.e. locations with the greatest stress concentration. Strain gauges were placed on identified critical locations. Measured stress values confirmed values obtained by using numerical calculation. Experimental measurements were also taken by using new digital image correlation (DIC) method. DIC method is an optical non-contact method that can provide full 3D stress field and enables better understanding of complex structures such as pipe branches. The aim of this paper was to determine stress concentration factor for the pipe branch model by using numerical and experimental analysis.Keywords: finite element method; pipe branch model; strain gauges; stress concentration factor; 3D DIC method Numeričko i eksperimentalno određivanje faktora koncentracije naprezanja na modelu cijevne račve Izvorni znanstveni članak Predmet ovog rada je ispitivanje struktura složene geometrije, odnosno analiza naprezanja modela cijevne račve A6 trećeg cjevovoda u hidroelektrani Perućica, Nikšić. Analiza stanja naprezanja modela cijevne račve provedena je numeričkom metodom konačnih elemenata i eksperimentalnim metodama. Metodom konačnih elemenata definiran je 3D model cijevne račve. Numeričkim putem određena su kritična mjesta na modelu, odnosno mjesta najveće koncentracije naprezanja. Mjerne trake su pozicionirane na identificirane kritične zone. Izmjerene vrijednosti naprezanja metodom mjernih traka potvrdile su vrijednosti dobivene metodom konačnih elemenata. Mjerenja su provedena i uporabom 3D optičke beskontaktne metode koja se bazira na digitalnoj korelaciji slika. Tom metodom se može dobiti cjelokupno 3D polje naprezanja, odnosno ona omogućava bolje razumijevanje kompleksnih struktura kao što su cijevne račve. Cilj ovog rada je bio određivanje faktora koncentracije narezanja na modelu cijevne račve uporabom numeričkog i eksperimentalnog pristupa.Ključne riječi: faktor koncentracije naprezanja; konačni elementi; mjerne trake; model cijevne račve; 3D DIC metoda

This paper describes pipeline stress analysis, primarilybranch junctions, as a structural element in hydro-power plants. Pipelines are exposed to internal pressure,which is present under working conditions. Analysis of stresses in the pipeline of the hydropower plant is based on analytical, numerical, and experimental methods. In this paper, we will define the critical elements of the pipeline. After that, we will determine critical areas in the branch junction, under experimental conditions, where strain gauges should be installed. The obtained resultsshow that a boiler formula can be efficiently applied in the stress analysis. Also, a correlation between the internal pressure and the maximum circumferential stresses in the elastic zone is given. In the final sections of the paper, the limit value of the internal pressure as a load for which stress in the zone of plasticity appears and the safety factor of the branch junction in the exploitation conditions are determined. The contribution of this work is the unification and deepening of the topic related to the problem of the testing ofhydro-power structural elements.

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