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Residual stress determination by neutron diffraction in low-carbon steel wires with accumulated shear deformation
Rogante, M. ; Mikula, Pavol ; Strunz, Pavel ; Zavdoveev, A.
Modern methods of severe plastic deformation (SPD) currently allow obtaining the ultrafine-grained (UFG) structure nearly in any material. In the manufacturing process of wire with UFG structure, a main restriction is the continuous drawing scheme in which this process occurs, thus it is important to consider the factors affecting the drawing efficiency. Despite the data on SPD steels such as strength, plasticity and conductivity, obtained by classical methodologies, further investigations are needed: a key information is the residual stresses (RS) status, and RS determination is an essential issue to improve knowledge of SPD effects. In this work, 15 wires samples made of low-alloyed quality structural steel Grade 08G2S GOST 1050 with accumulated shear deformation - as result of rolling with shear of the metal ingot and conventional wire drawing - have been investigated by neutron diffraction (ND). Results provide substantial data helping to evaluate the effect of shear deformation on RS of the considered steel, as well as additional support to complement the information already achieved by using the other characterization methodologies. Knowledge of the RS status can help developing a lowcarbon wire drawing technology with needed manufacturability and efficiency, playing a decisive role in the debugging of material selection and engineering design requirements.
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Lifetime Assessment of Particulate Ceramic Composite with Residual Stresses
Náhlík, Luboš ; Majer, Zdeněk ; Štegnerová, Kateřina ; Hutař, Pavel
A micro-crack propagation in particulate ceramic based composite was studied using finite element method (FEM). Subcritical crack growth (SCG) was numerically simulated under complex load conditions (mechanical loading and loading by internal residual stresses). The effect of residual stresses on the crack propagation was studied. Two-dimensional computational model of particulate ceramic composite with material properties corresponding to low temperature co-fired ceramics (LTCC) was developed. The results indicate that the presence of residual stresses significantly reduces values of stress intensity factor in the vicinity of composite surface and the direction of residual stresses around the particles contributes to the micro-crack deflection from the particles. The time to failure of the composite under mechanical loading was determined. Results obtained contribute to a better understanding of the role of residual stresses during micro-crack propagation in ceramic particulate composites.
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Effect of residual stresses on the fatigue lifetime of railway axle
Hutař, Pavel ; Pokorný, Pavel ; Poduška, Jan ; Fajkoš, R. ; Náhlík, Luboš
The operation of railway axles should fulfill at least two main demands: safety and low operation costs. A significant part of operation costs is given by the length of regular inspection intervals which should reveal potential fatigue cracks in railway axle. The detection of cracks is of a probabilistic nature, therefore their detection is not ensured in all cases. For the safe operation of trains, an existence of potential initial crack should be considered on the axle surface and residual fatigue lifetime should be conservatively determined for this case. Reliable procedure of residual fatigue lifetime estimation should take into account real axle geometry, material characteristics and loading of the railway axle. This paper shows methodology for determination of residual fatigue lifetime (RFL) based on the fracture mechanics approach, taking into account real spectrum of the loading cycles, existence of press-fitted wheels and surface residual stresses given by the thermo-mechanical surface treatment of the railway axle. It is demonstrated that the effect of the residual stresses is significant and should not be neglected in the numerical estimation of residual fatigue lifetime of the axle.
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3D Model of Crack Propagation in Particulate Ceramic Composite Containing Residual Stresses
Štegnerová, Kateřina ; Majer, Zdeněk ; Hutař, Pavel ; Náhlík, Luboš
A crack propagation and fracture behaviour of particulate ceramic composite were investigated. Influence of 3D shape of particles on the crack propagation was studied together with influence of the presence of residual stresses, which are developed inside the composite during manufacturing process. Finite element (FE) method was used for numerical simulation of propagating crack in the composite. Basic numerical models of low-temperature co-fired ceramics (LTCC) with alumina particles homogenously dispersed in the glass matrix were developed. Volume fraction of alumina phase was 20vol.%, which is typical amount for LTCC. The results show that existence of residual stresses retards the crack propagating under conditions of sub-critical crack growth (SCG). Presented results contribute to a better understanding of the role of residual stresses in particulate ceramic composites.
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Computational Modelling of Welding and Heat Treatment Process of Steel with Application of Elastic-Viscoplastic Material Model
Jarý, Milan ; Daněk, Ladislav (referee) ; Dymáček, Petr (referee) ; Junek, Lubomír (advisor)
This dissertation thesis deals with the improvement of computational approaches for prediction of residual stresses in welded joints of welded structures in order to ensure greater compliance of the calculated results with the real conditions of welding and heat treatment. The improvement of computational approaches is based on application of elastic-viscoplastic material models which are able (compared with elastic-plastic material models) to take into account the viscoplastic processes ongoing during welding and heat treatment. This leads to more accurate calculated results which enter into further assessment of limit states and directly decide on the safety and lifetime of welded structures. Performed computational and experimental works, confronted with results published in the world, confirm the influence and benefit of application of elastic-viscoplastic material models in the frame of welding and heat treatment numerical analyses. Therefore elastic-viscoplastic material model is further applied in solution of practical project solved by IAM Brno. Solution of this project, whose aim is the development of repair of dissimilar metal welds (without post-weld heat treatment) in Dukovany and Temelin nuclear power plants using "Weld overlay method", has confirmed that application of elastic-viscoplastic material model leads to more accurate calculated results. For this reason the elastic-viscoplastic computational approach will be included into all future tasks of IAM Brno.
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Propagation of inclined cracks to the interface of ceramic laminates
Novotná, Lenka ; Trunec, Martin (referee) ; Chlup, Zdeněk (advisor)
Composite materials with laminated structure provide advantages which are utilised during component design. Low density, temperature and chemical stability are the profitable properties predetermining application of ceramic laminates. The main obstacle for wide spread of ceramic materials is their inherent brittleness. Therefore, in this thesis, the crack propagation in ceramics laminates has been extensively studied. Laminated structures with various volume fractions of components (alumina and zirconia) were prepared by electrophoretic deposition. Evaluation of crack propagation through the interface and determination of basic mechanical properties was conducted on the basis of extensive literature search. Crack deflection originated in both presence of internal stresses and differences in elastic modulus during the crack interface passing was monitored. A special type of specimen geometry was employed with the aim to set arbitrary angle between crack and interface. It was experimentally found that the degree of crack deflection is dependent on entering angle and volume fraction of components. Higher crack deflection was already found in the bulk of the test piece comparing to the test piece surface. The 3D fracture surface reconstruction generated using laser confocal microscopy was used in this detailed crack propagation study. Further basic elastic and strength characteristics of laminates were determined and compared to those obtained from monolithic materials. The validity of the mix rule for elastic characteristics was confirmed by comparing of elastics modulus. The most reliable method for elastic modulus determination was marked the dynamic resonance method due to low scatter and consistency in measurement. The flexural strength of all laminates tends to be close to the flexural strength of the weakest component. Therefore the mix rule is not applicable for flexural strength estimation on the contrary of elastic characteristics. The change of component volume fraction leads only to change of flexural strength scatter. Thanks to gained knowledge about crack propagation and basic characteristic determination will be possible to design ceramic laminates more efficiently for given needs of application.
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Influence of residual stresses in ceramic laminates on the crack propagation
Máša, Bohuslav ; Zouhar, Michal ; Náhlík, Luboš
Příspěvek je zaměřen na lomové chování vrstevnatých keramických kompozitů – Al2O3/ZrO2. Byly uvažovány rozdílné tloušťky vrstev. Reziduální napětí získané v průběhu slinovacího procesu bylo zjištěno a předepsáno v modelech. Byly předpokládány podmínky k nasazení lineární elastické lomové mechaniky. Složky napětí a přetvoření v blízkosti kořene trhliny byly získány numerickým modelováním metodou konečných prvků implementovanou v komerčním software Ansys. K určení součinitele intenzity napětí byly použity analytické vztahy. Článek může přispět k lepšímu porozumění propagace trhliny mezi jednotlivými vrstvami.
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Influence of bimaterial interface on fatigue crack closure
Náhlík, Luboš
The article deals with the behaviour of a fatigue crack propagating across a bi-material interface. A stability criterion for the crack touching the bi-material interface, taking into account the residual stresses closing the crack faces is formulated. The linear elastic fracture mechanics is assumed to apply and the finite element method is used in the calculations. The criterion proposed is applied to determine the fatigue threshold stress for crack propagation across the interface. It is shown that the threshold values for crack propagation are influenced by the residual stresses closing the crack and by the specific combination of the elastic constants of the materials used. Two different geometries and two different loading ratios are considered. It is shown that the threshold values for crack propagation are influenced by the residual stresses closing the crack and by the specific combination of the elastic constants of the materials used.
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