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1

Destruction of high pressure vessels in pipeline structures

Rahimova Mahluqa S., Abbasov Sakit H., Ahmadov Alihikmat A.

Nafta-Gaz

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2023

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R. 79, nr 5

360--364

EN

The article indicates that engineering design criteria do not provide measures to prevent failures; this is evidenced by the occurrence of many accidents. Fracture prevention criteria should be derived from the principles of fracture mechanics, what should be developed further. However, the current concepts of fracture mechanics, when properly applied, provide an opportunity to ensure the reliability of the structure or organise the supervision of expensive structures to ensure their safe operation. These methods of preventing damage can be divided into two large groups: 1) checking for the formation of cracks and 2) monitoring their development. Both methods are based on similar principles; it would be easier to explain them with examples. To ensure the safe operation of the pressure vessel used in the reactor, the maximum allowable initial crack size should be known. The size of this crack should not expand to a critical point during the entire operation of the reactor. Knowing how the process of crack propagation proceeds and how the structure behaves during failure, it is possible to calculate the critical size of the defect and, based on this, calculate the maximum allowable size of the crack at the beginning of operation. Proper inspection of the new vessel will eliminate the possibility of shells that are larger than the original size. Checking for the presence of cracks, and determining their rate of growth during operation, presents significant difficulties. Therefore, checks should be avoided during operation. If the fracture and crack growth calculations, as well as the initial checks, are carried out correctly, then checks made during operation are an optional extra. However, in practice, such checks should still be performed. For vessels used in reactors, remote observation of crack growth using ultrasonic waves is a particularly useful method. If a crack is found, measures must be taken to either repair or replace the partially destroyed element.

PL

W artykule wskazano, że kryteria na etapie tworzenia projektu technicznego często nie uwzględniają środków zapobiegających awariom, o czym świadczą liczne wypadki przy pracy. Kryteria zapobiegania powstawaniu pęknięć powinny być wyprowadzane z zasad mechaniki powstawania pęknięć, co wymaga dalszego rozwoju. Jednak obecne koncepcje mechaniki powstawania pęknięć, przy ich właściwym stosowaniu, dają możliwość zapewnienia niezawodności konstrukcji lub zorganizowania nadzoru nad kosztownymi konstrukcjami, aby zapewnić ich bezpieczną eksploatację. Te metody zapobiegania uszkodzeniom można podzielić na dwie duże grupy: 1) kontrola pod kątem powstawania pęknięć, 2) monitorowanie ich wzrostu. Obie metody opierają się na podobnych zasadach i lepiej wyjaśnić je na przykładach. W celu zapewnienia bezpiecznej eksploatacji zbiornika ciśnieniowego używanego w reaktorze należy znać maksymalną dopuszczalną początkową wielkość pęknięcia. Wielkość takiego pęknięcia nie powinna wzrosnąć do wartości krytycznej przez cały czas pracy reaktora. Wiedząc, jak przebiega proces propagacji pęknięć i jak zachowuje się konstrukcja podczas uszkodzenia, można obliczyć krytyczną wielkość uszkodzenia i na tej podstawie obliczyć maksymalną dopuszczalną wielkość pęknięcia na początku eksploatacji. Prawidłowa kontrola nowego zbiornika wyeliminuje możliwość wystąpienia pęknięć większych niż o pierwotnym rozmiarze. Kontrole pod kątem obecności pęknięć i określenie tempa ich wzrostu podczas pracy wiążą się z dużymi trudnościami. Dlatego należy unikać wykonywania kontroli podczas pracy. Jeżeli obliczenia dotyczące pęknięć i ich wzrostu, jak również kontrole wstępne, zostały przeprowadzone prawidłowo, to kontrole podczas eksploatacji byłyby opcjonalnym dodatkiem. Jednak w praktyce takie kontrole i tak są przeprowadzane. W przypadku zbiorników używanych w reaktorach szczególnie przydatną metodą jest zdalna obserwacja wzrostu pęknięć za pomocą fal ultradźwiękowych. W przypadku stwierdzenia pęknięcia należy podjąć działania w celu naprawy lub wymiany częściowo zniszczonego elementu.

2

Mode I crack problems by coupled fractal finite element and meshfree method

Rao B. N., Rajesh K. N.

Computer Assisted Mechanics and Engineering Sciences

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2010

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Vol. 17, no. 2-3-4

113-135

EN

This paper presents a coupling technique for integrating the fractal finite element method (FFEM) with element-free Galerkin method (EFGM) for analyzing homogeneous, isotropic, and two-dimensional linear-elastic cracked structures subjected to Mode I loading condition. FFEM is adopted for discretization of domain close to the crack tip and EFGM is adopted in the rest of the domain. In the transition region interface elements are employed. The shape functions within interface elements which comprise both the element-free Galerkin and the finite element shape functions, satisfy the consistency condition thus ensuring convergence of the proposed coupled FFEM-EFGM. The proposed method combines the best features of FFEM and EFGM, in the sense that no structured mesh or special enriched basis functions are necessary and no post-processing (employing any path-independent integrals) is needed to determine fracture parameters such as stress-intensity factors (SIFs) and T-stress. The numerical results show that SIFs and T-stress obtained using the proposed method, are in excellent agreement with the reference solutions for the structural and crack geometries considered in the present study. Also a parametric study is carried out to examine the effects of the integration order, the similarity ratio, the number of transformation terms, and the crack-length to width ratio, on the quality of the numerical solutions.

3

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.

4

Damage mechanism in AlSi1MgMn alloy

Mrówka-Nowotnik G.

Archives of Materials Science and Engineering

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2008

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Vol. 29, nr 2

93-96

EN

Purpose: The main task of this work was to study the fracture mechanism in 6082 aluminium alloy. Design/methodology/approach: Microstructure and fractografic examination has been carried out on the samples in the peak aged condition after static tensile tests, crack resistance test and tensile test in the presence of sharp notch using an optical microscope - Nikon 300, scanning electron microscope HITACHI S-3400 (SEM) in a conventional back-scattered electron mode and JEOL - JEM 2100 ARP TEM/STEM electron microscope on polished sections etched in Keller solution. Findings: It has been found that, at room temperature, general cavitation (nucleaction of voids) occurs after appreciable strain. The nucleation of voids results from debonding along certain particle/matrix interfaces. Observations of microstructure revealed second fracture mechanism initiated by cracking of brittle intermetallic phases. Practical implications: All this knowledge . identification of the microstructural parameters for the different modes of void nucleation and cracking of intermetallic phases leading to fracture of the alloy . can be used to predict maximum ductility before fracture of tensile specimens. Originality/value: For deformation at room temperature different void populations have been defined: void nucleated by intermetallic particle fracture, by s particle/matrix decohesion and by �ż particle/matrix decohesion.

5

Structural causes of defects in a cast iron mill roll

Krawczyk J.

Archives of Foundry Engineering

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2008

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Vol. 8, iss. 2

93-98

EN

This paper describes an analysis of a defective microstructure of a mill roll. For this purpose, a piece of a broken roll was collected. That roll was made of mottled cast iron. Its microstructure consisted of modular graphite, transformed ledeburite and a matrix composed of bainite and martensite. Metallographic investigations were performed nearby the fracture in the roll. Observations were conducted on polished sections, first not etched, and then etched, which allowed us to trace carefully the propagation of the fracture. There was found a strict correlation between the microstructure of the roll and the progress occurring in the crack. It was ascertained that the basic reason for the damage to the roll was banded precipitations of ledeburitic cementite. In addition, cementite formed a continuous network. Another microstructure defects of that roll are also precipitations of secondary carbides on the boundaries of former grain of austenite as well as the occurrence of upper bainite in its matrix. The results obtained hereunder allow broadening the data base relative to the genesis of damages to mill rolls, which in future will permit one to design a proper microstructure of cast iron mill rolls. Proper microstructure of cast iron mill roll should be shaped at the stage of designing the chemical composition, conditions of crystallization or heat treatment if any.

6

Destruction mechanics: self-destruction

Cherepanov G. P., Esparragoza I. E.

International Journal of Applied Mechanics and Engineering

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2007

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Vol. 12, no 2

565-570

EN

Fracture meehanics studies the cracking of a material while destruction is the fraeturing/separation of a material into smali debris by very many cracks. In the latter case, a fraeture front or wave separates the intact material from that destructed into smali particles. As distinct from fracture mechanies, destruction mechanies does not concern about how and where each crack grows. Its main interest is to study the location and motion of the fracture front. The present paper is a brief presentation of destruction mechanics and, especially, self-destruction, or selfsustaining fracture in former terminology, as its most interesting subject. Destruction in this approach is treated like a specific phase transition of an intact material into a destructed one. The general condition of destruction is formulated in terms of energy. Some problems of destruction mechanies are briefly reviewed, including quasistatie problems of local destruction in stress concentration zones, stability and instability of destruction process, flame drilling, a theory of Tungus meteorite destruction, and dynamic problems such as the problems of underground explosion, collision of brttle bodies, and propagation of a plane fracture wave from a free boundary. The dynamic, explosive growth of the destruction zone occurring by itself, uncontrollably, is called the selfdestruction. Self- destruction starts from a small change in the boundary or loading playing the part of a trigger initiating a fracture wave that propagates at the speed of sound and destructs all material being at the metastable, overstressed condition. The destruction of Batavian tears is a well-known example of self-destruction. Coal bursts and rock bursts as self-destruction phenomena in deep rnines have been known for a long time. Some old but forgotten experiments on explosive self-destruction of glass specimens are described in the paper, as well as some calculations of fraeture waves and an estimate of the size of destructed particles.

7

Effect of temperature on tensile fracture mechanisms of a Ni-base superalloy

Sajjadi S. A., Zebarjad S. M.

Archives of Materials Science and Engineering

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2007

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Vol. 28, nr 1

34-40

EN

Purpose: The Ni-base superalloy GTD-111 is used in manufacturing of the first stage blades of powerful gas turbines (over 125MW). The alloy posses appropriate microstructure and high temperature properties through precipitation hardening heat treatment. Among the properties, tensile properties of the alloy have strong influence on stability and life of the blades. Design/methodology/approach: Tensile tests over a wide range of temperatures from 25 to 950°C with a constant strain rate of 10 -4s -1 were performed to study the tensile fracture mechanisms of the cast and heat treated superalloy. Scanning electron microscopy was used to provide structural and fractography evidence of the superalloy GTD-111at different temperatures. Findings: The fractography results of the tensile tested specimens were in good agreement with the variation in alloy ductility. Many fractography features such as: transgranular and intergranular fracture with fine dimples, cleavage facets and a combination of them were observed in the specimens tested at different temperatures. Research limitations/implications: Because fatigue is an important fracture mechanism at the service condition of the alloy it is suggested for future research to work on the simultaneous effects of tension and fatigue on the fracture mechanisms although, tensile properties alone are important for the alloy. Originality/value: It was found that different fracture mechanisms operated in different temperature ranges for example, while transgranular dimple fracture was dominant at 650°C, the dominant fracture mechanism at room temperature was intergranular.

8

Study of fracture mechanisms of a Ni-Base superalloy at different temperatures

Sajjadi S. A., Zebarjad S. M.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 18, nr 1-2

227--230

EN

Purpose: The Ni-base superalloy GTD-111 gains its appropriate microstructure and high temperature strength through precipitation hardening mechanism. Because of their service condition, tensile properties of the alloy have strong influence on stability and life of the blades. Design/methodology/approach: Tensile fracture mechanisms of the cast and heat treated superalloy were studied over a wide range of temperatures from 25 to 950°C with a constant strain rate of 10-4s-1. The present paper provides structural and fractography evidence by means of scanning electron microscopy at different temperatures for the superalloy GTD-111. Findings: The variation in alloy ductility was found to correlate well with the fractography results of the tensile tested specimens. Transgranular and intergranular fracture with fine dimples, cleavage facets and a combination of them were shown in the fractographs. Research limitations/implications: Although tensile properties alone are important for the alloy, it is suggested for future research to work on the simultaneous effects of tension and fatigue on the fracture mechanisms. Originality/value: It was cleared that different fracture mechanisms operate in different temperature ranges; while transgranular dimple fracture was dominant at 650°C, the dominant fracture mechanism at room temperature was intergranular.

9

Irwin criterion for dielectric with strong electrostriction

Kurlandzka Z.

Journal of Technical Physics

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2001

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Vol. 42, no 3

287-303

EN

A fracture criterion, being an extension of the Irwin criterion for the case of elastic dielectric with strong electrostriction, is derived. The paper is a generalization of the previous paper by the author [3], where electromagnetic interactions resulting from the Lorentz force according to the Dixon-Eringen model where taken into account. Here nonlinear electromagnetic interactions are supplemented with terms, which include the additional material constants responsible for strong electrostriction.

10

A cracked piezoelectric material under generalized plane electromechanical impact

Wang B., Noda N.

Archives of Mechanics

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2000

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Vol. 52, nr 6

933-948

EN

Solution of the generalized plane deformation problem of a piezoelectric material strip with a crack is proposed. Laplace and Fourier transforms are used to reduce the problem to the solution of singular integral equations in the Laplace transform plane. Laplace inversion yields the results in the time domain. This analysis yields six independent stress and three electric displacement components. The model is general enough to account for arbitrary polarization direction, under transient or steady state load, for any mechanical or electrical mode of cracking. Numerical solutions for a piezoelectric material strip under electromechanical impact are illustrated. The influences of strip thickness and crack position on time-dependent crack tip fields are investigated. The results show that the transient electric displacement loads can increase or reduce the stress intensity factors at different time, dependent on the applied electric displacement load direction.