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1

An estimation of the high-pressure pipe residual life

Jovicic G., Nikolic R., Zivkovic M., Milovanovic D., Jovicic N., Maksimovic S., Djordjevic J.

Archives of Civil and Mechanical Engineering

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2013

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Vol. 13, no. 1

36--44

EN

The paper presents an estimation of the residual life of the power plant high-pressure pipe, which has been in exploitation for years. A crack was noticed in the pipe, thus it was necessary to estimate the pipe material residual life until its eventual failure. The combined methodology for residual life estimation, which consists of experimental and numerical investigations, was developed. The samples were taken directly from the real high pressure pipe and material properties were determined experimentally, both at room and elevated (operational) temperature. The experimental results also served for the verification of the developed numerical methodologies. The FEM and the X-FEM methods were used for the residual life numerical estimation of the high pressure pipe. The stress and strain fields, used for the estimate, were obtained by application of the Paris' law. The final verification of numerical results was realized by comparing the critical crack length to the experimentally obtained value.

2

Improvement of algorithm for numerical crack modelling

Jovicic G., Zivkovic M., Jovicic D., Milovanovic D.

Archives of Civil and Mechanical Engineering

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2010

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Vol. 10, no 3

19-35

EN

For numerical simulation of crack modelling in fracture mechanics the eXtended finite element method (Xfem) has been recently accepted as a new powerful and efficiency methodology. In the paper we present the details of implementation of the Xfem algorithm in our in-house finite elements based software. Also, in this study, we investigated the impact of the node enrichment variations on results of the developed numerical procedure. In this study, objective was to examine the properties of standard Xfem algorithm without using of Near Tip enriching functions in order to create possibilities for future application Xfem in the zone of plasticity. In order to evaluate the computational accuracy, numerical results for the Stress Intensity Factors are compared with both theoretical and conventional finite element data. Obtained numerical results have shown a good agreement with the benchmark solutions. For calculation of the Stress Intensity Factors (SIF), we used the J-Equivalent Domain Integral (J-EDI) Method. Computational geometry issues, associated with the representation of the crack and the enrichment of the finite element approximation, are discussed in detail.

PL

Rozszerzona metoda elementów skończonych eXtended (XFEM) jest ostatnio uznawana, jako skuteczne i efektywne narzędzie do modelowania numerycznego w mechanice powstawania pęknięć. W artykule przedstawiono szczególe zastosowania algorytmu XFEM we własnym oprogramowaniu bazującym na metodzie elementów skończonych. Celem badań było zweryfikowanie standardowego algorytmu XFEM w celu stworzenia możliwości przyszłej aplikacji tego algorytmu w obszarze plastycznym. W celu oceny dokładności obliczeń, wyniki liczbowe współczynników intensywności naprężeń zostały porównane z teoretycznymi i danymi z elementów skończonych. Uzyskane wyniki liczbowe wykazały dobrą zgodność ze wzorcami rozwiązań. Do wyliczenia wskaźników intensywności naprężeń użyta została metoda równoważnej całki J. Szczegółowo omówiono zagadnienia geometrii obliczeniowej związane z przedstawieniem pęknięcia i ulepszeniem aproksymacji w metodzie elementów skończonych.

3

FE analysis of tube forming process with experimental verification

Mandic V., Stefanovic M., Zivkovic M., Grujovic N., Misic B.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 18, nr 1-2

303--306

EN

Purpose: of this paper: The paper presents some results of extensive investigation of steel seam-welded tubes forming process, by applying combined treatment of material narrowing and expansion. The objective of executed numerical FE and experimental investigations was to determine the optimal technology for production of tubular product at simple tools and at the standard forming processing machines, without defects and with demanded dimensional accuracy. Design/methodology/approach: Tube forming was done in one phase, in tool with two shaping spherical parts, that enables the expansion of the specimen’s central zone and the filling of the die due to narrowing of the specimen ends, when certain conditions are achieved and with appropriate combination of influential process parameters (outer diameter, height and tube’s wall thickness, as well as friction conditions). Series of physical and numerical FE experiments was performed. Findings: Optimal dimensions of tubular product, with required process stability, were obtained this way. Also, results of experiments pointed out that the best process stability and die filling is achieved with specimens whose surfaces had previously been chemically treated. Results of numerical FE simulations of process are quite verified by experiments. Practical implications: Obtained results have practical significance in solving similar processing problems. It also enables to investigate and broaden the knowledge on stability of these kinds of processes, beyond the scope of experimental investigations. Tubular product that is subjected to numerical-experimental investigation in this paper is prepared by narrowing it at both ends thus excluding the possibility to apply holder inside the tube. Originality/value: Proposed method offers possibility for production of tubular products at simple forming machines and tools, without complex and expensive hydroforming equipment.

4

Evaluation of lubricants for pressing of aluminium pieces

Adamovic D., Stefanovic M., Lazic V., Zivkovic M.

Tribologia

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2005

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nr 5

9-30

EN

In the process of ironing, the tribological factors, namely the realised friction forces, play a significant role. The stress-strain state of plastically formed piece, the possibility for successful forming and force necessary for execution of forming depend on the size and distribution of contact stresses. Considering the fact that the metal forming is done in conditions of high contact pressures, absence of lubricants would, in such conditions, lead to direct contact of formed material and the tool, i.e. the micro-welding or sticking of the softer material onto the harder tool would occur, and due to that the severe disruption of forming conditions would take place. The main tribological factors in metal forming by pressing are: forming speed, strain ratio (depends on load and semi-angle of the die cone), state of materials in contact (surfaces topography, physical and chemical characteristics of materials) and type of lubricant. By combining them, one can carry out mixed, i.e. limit friction, and thus minimise the contact between the tool and working piece (sheet metal). Classical methods for estimation of lubricants (Almen-Wieland test, Falex test, Timken-test, four-ball test, etc) are not the most convenient for estimation of lubricants in ironing. Much more convenient tests are those that properly model the real working conditions in such forming. In this paper, the obtained results for the evaluation of various lubricants used in pressing will be presented on the adequate tribo-model of sheet metal test-piece sliding between pairs with skewed contact surfaces.

PL

W procesie ciągnienia czynniki tribologiczne, takie jak np. siły tarcia, odgrywają znacząca rolę. Stan naprężeń i odkształceń formowanego plastycznie przedmiotu, możliwość pomyślnej obróbki oraz niezbędna siła docisku zależą od wartości i rozkładu naprężeń kontaktowych. Ponieważ obróbka plastyczna wykonywana jest w warunkach wysokich nacisków, brak smaru mógłby doprowadzić do bezpośredniego styku materiału obrabianego i narzędzia. To z kolei wywołuje mikrozgrzewanie lub przywieranie materiału bardziej miękkiego do twardszej powierzchni narzędzi. Prowadziłoby to do poważnych zaburzeń warunków obróbki. Do najważniejszych czynników tribologicznych w procesie obróbki plastycznej za pomocą ciągnienia należą: prędkość ciągnienia, stosunek odkształceń (zależy od obciążenia oraz kąta ciągadła), stan powierzchni materiału i narzędzi (topografia, właściwości fizykochemiczne) oraz rodzaj smaru. Poprzez zmianę ich kombinacji można np. ograniczać tarcie i w ten sposób wpływać na zmniejszenie się kontaktu narzędzia z materiałem. Klasyczne metody oceny smarów (aparaty Almen-Wieland, Falex, Timken, czterokulowy itd.) nie są wskazane do testowania smarów przeznaczonych do ciągnienia. Należy poszukać takich testów, które w o wiele większym stopniu odwzorują (zamodelują) rzeczywiste warunki pracy. W pracy tej do badania smarów przeznaczonych do ciągnienia, użyto specjalnego, modelowego stanowiska tribologicznego, w którym obrabiany materiał testowy przesuwany jest pomiędzy parą ciągadeł z ukośną powierzchnią roboczą.