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Abstract: Standardi in dokumenti za določitev hitrosti širjenja razpoke predvidevajo uporabo standardnih preizkušancev, na katerih je mogoče meriti dolžino razpoke od nekaj deset do nekaj sto milimetrov prirastka. Problem postaja težaven pri zvarnih spojih, kadar se merijo prirastki razpoke na skupni razdaljile nekaj milimetrov. Za to so poglavitni razlogi naslednji: utrujanje poteka z relativno nizko amplitudo faktorja intenzitete napetosti v področju utrujenostnega praga, mikro strukturna in trdnostna heterogenost zvarnega spoja, zaostale napetosti kot posledica varjenja, med utrujanjem se način odpiranja razpoke lokalno spreminja iz enoosno nateznega na ravninsko strižno in nasprotno. Omenjeni dejavniki se kažejo z izrazito neurejeno sliko hitrosti širjenja utrujenostne razpoke. V prispevku je opisan postopek za predobdelavo eksperimentalno izmerjenih vrednosti (prirastek razpoke, št. nihajev), ki omogoča zanesljivejšo določitev hitrosti širjenja utrujenostne razpoke v zvarnem spoju za majhne prirastke do zloma, ko faktor intenzitete napetosti preseže lomno žilavost.
Keywords: zvarni spoji, utrujenostna razpoka, faktor intenzivnosti napetosti, prag
Published in DKUM: 01.06.2012; Views: 2207; Downloads: 118
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Abstract: High-alloyed Cr-Ni-based two-phase stainless steel (SS) cast alloys are commonly used in nuclear power plants. The mechanical equipment in these facilities can contribute to a reduction in its resistance to stable crack growth as a result of extended operating times and high temperatures. The toughness of these materials strongly depends on their delta (▫$/delta$▫) ferrite content, which spinodally decomposes into two phases with different ratios of Cr and Ni at a relatively low (slightly above 300 °C) temperature. This temperature is similar to the operating temperature of the vital parts, for example, the coolant system. The formation of two phases with the same crystalstructure but different lattice parameters causes internal elastic stresses that result in a hardness increase and an impact-toughness decrease. The result is an increased risk of crack formation in the stress-concentration zones such as the critical regions of different welded joints (e.g. "L, T, K and X" shapes). The values of the critical stress intensity factor change according to its position along the crack contour. Therefore, the aim of our study was to assess the influence of the materials' changes on the crack extension and the decrease of the primary pipeline's bearing capacity by taking account of the increased temperature and time of operation for the given loading conditions. The SINTAP (European Structural Integrity Assessment Procedure) was used for this assessment.
Keywords: welding, welded joints, pipelines, fracture toughness testing, structure integrity assessment procedure
Published in DKUM: 01.06.2012; Views: 2032; Downloads: 95
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Abstract: The effect of residual stress on the fatigue crack growth was investigated forcyclic tension. Three-point bend specimens were used with through thicknessnotches at the center line of the welded joint. The experiments were performed for different load amplitudes and load ratios under conditions of small-scale yielding. The influence of the residual stresses on fatigue crack growth was estimated by experimentally observed fatigue crack growth rate and measurement of residual stresses at the surface of specimen. This paper describes an approach taken in two models to estimate the residual stress effect on fatigue crack propagation and to predict remaining service life of awelded structure.
Keywords: fracture mechnaics, fatigue crack, crack growth, residual stresses, fatigue crack growh rate, X-ray
Published in DKUM: 01.06.2012; Views: 2029; Downloads: 95
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Abstract: The paper by I. D. Landau et al. presents an analysis of two structures of coupled van der Pol oscillators done by using the Krylov-Bogoliubov (K-B) averaging method. They devise the K-B method to effectively compute self-excited and externally driven oscillations, respectively on systems presumably subjected to control, where amplitudes and phases are slowly varying functions of time. After they present feedback structures of single and coupled generalized van der Pol equations, respectively, they perform the K-B analysis of these equations, which results in explanation of various regimes of self-excited and externally driven oscillations including the analysis of the combustion instability model. Although the proposed K-B methodmay be considered as a step forward in modeling self-oscillating systems, this discussion wants to show, that problems solved by the K-B method can be treated also in an alternative way. Based on the paper [1], it is obvious that coupled van der Pol oscillators, driven by an external excitation with slowly varying frequency or amplitude, can be analyzed by using Extended Lindstedt- Poincare (EL-P) method with multiple time scales.
Keywords: van der Pol oscillators, external excitation, multiple time scales
Published in DKUM: 31.05.2012; Views: 1623; Downloads: 71
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Abstract: A mathematical model for the calculation of thermal conductivity of nanofluids containing nanoparticles and nanotubes on the basis of statistical mechanics is presented. This appears to be the first such attempt published in the scientific literature. We focus on the influence of the liquid layer structure around nanoparticles as the reason for enhancement of the thermal conductivity of nanofluids. Analytical results of both the current analysis and statistical mechanics are compared with experimental data reported in the scientific literature, which show good agreement.
Keywords: statistical thermodynamics, mathematical models, thermal conductivity, nanofluids, transport properties, statistical nano-mechanics
Published in DKUM: 31.05.2012; Views: 2173; Downloads: 109
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