Papers by Author: Miroslav Vrána

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Authors: M. Rogante, Pavol Mikula, Miroslav Vrána
Abstract: Thermal neutrons are very useful probe in a nondestructive determination of internal stress/strains, due to their high penetration into most materials. In comparison with conventional X-ray techniques, real bulk information on both macro-and micro-strains in materials can be obtained by neutron diffraction (ND) techniques. Knowledge of the spatial and directional distribution of internal residual stresses (RS) is increasingly considered fundamental to determine their influence on properties of engineering materials and a consequent material behaviour. As the assesment of stresses is always related to the stress free material state, an accurate evaluation of the unstressed lattice parameters (e.g., the interplanar distance), in order to determine RS by ND is one of the key tasks. It helps to avoid inacceptable errors in the course of the real material strain and stress evaluation. The availability of carefully measured zero-strain standards is also essential to confirm the absence of systematic instrumental effects determining the diffraction profile at a chosen scattering angle. In this paper, the state of the art of the main analytical and experimental procedures currently established or adoptable to determine these critical parameters, particularly regarding industrial applications, is presented.
Authors: Pavol Mikula, Petr Lukáš, Miroslav Vrána
Abstract: The performance of an unconventional high-resolution neutron diffractometer using Bragg diffraction optics that can be efficiently employed in strain/stress diffractometers are reported. Presented results demonstrate their experimental abilities for powder diffraction, namely for residual strain/stress measurements. In addition to the macrostrain scanning capability, the device can also be used for microstrain/stress studies by suitable analysis of the diffraction profiles. Two examples of the strain measurements are presented.
Authors: Pavol Mikula, Jan Šaroun, Miroslav Vrána, Petr Lukáš, V. Wagner
Authors: Lubos Mraz, Leif Karlsson, Pavol Mikula, Miroslav Vrána
Abstract: It is well known that fatigue strength of welded joints does not depend on steel strength. Better fatigue strength of welded joints, e.g. longer life time of fatigue loaded weld structures, can be achieved with a smooth transition between the weld and the base material to minimize stress concentration. It has also been recognized that residual stresses play a critical role in the fatigue behaviour of welds. In the last decade an extensive research has been performed in order to increase the fatigue strength of high strength steel weldments. The martensite and bainite transformation start temperatures of weld metals have been shown to have a large effect on fatigue life time of high strength steel welds. This is of particular importance if the full potential of high strength steels is to be used in fatigue loaded constructions. A detailed investigation of the effect of phase transformation temperature on residual stress distribution in the vicinity high strength steel welds and its effect on fatigue life time has been performed. The transformation temperature of the weld metal was varied by changing the chemical composition of the filler material. Residual stress distributions have been measured by neutron as well as by X-ray diffraction and fatigue tests have been performed on the fillet welds. A strong effect of weld metal phase transformation temperature on residual stress level was observed. Fatigue strength increased approximately three times when an optimised low transformation temperature filler material was used in comparison to the application of conventional filler material.
Authors: D. Neov, Petr Šittner, Petr Lukáš, Václav Novák, Pavel Strunz, Miroslav Vrána, Pavol Mikula
Authors: Stefanus Harjo, Petr Lukáš, Yo Tomota, D. Neov, Miroslav Vrána, Pavol Mikula, M. Ono
Authors: Marc Seefeldt, Artur Walentek, Paul van Houtte, Miroslav Vrána, Petr Lukáš
Abstract: This paper reports about a combined neutron and X-ray diffraction study on the residual stresses in the ferrite matrix of cold-rolled fully pearlitic steel sheet. Neutron diffraction revealed compressive residual phase microstresses of about – 500 MPa in rolling direction. However, even in normal direction there are significant tensile residual microstresses, indicating that the morphology of the lamellar microstructure cannot be properly described as a “sandwich structure”. Neutron diffraction was also used during an in-situ tensile test to estimate the microstress level in the cementite phase. The combination of neutron and X-ray diffraction allows to separate, near the surface, the residual phase microstresses from the macrostresses. The latter are also important in rolling direction and imply some risk of undesirable shape changes after forming operations.
Authors: Gergely Farkas, Kristián Máthis, Petr Lukáš, Jan Pilch, Miroslav Vrána, Miloš Janeček, Zuzanka Trojanová
Abstract: Neutron diffraction method has been applied in the ex-situ investigation of the residual stresses in Mg-5wt.%Al-1 wt.%Sr (AJ51) magnesium alloy reinforced with short Saffil® fibers deformed in compression at room temperature. The residual stresses were measured in the axial and radial directions with respect to the load direction. It is shown that in the initial state the tensile stress is present in the matrix phase. The in-situ acoustic emission measurements were performed with the aim to reveal the main deformation mechanisms operating in the particular stages of the plastic deformation. Ex-situ neutron diffractions experiments showed that the tensile axial residual stress in the matrix increases with increasing plastic deformation while the radial residual stress decreases. In situ acoustic emission measurements indicate that the main deformation mechanisms are twinning and glide of bigger dislocation ensembles in the early stages of the compressive deformation while the fibers breakage was observed in the vicinity of the maximum stress.
Authors: Petr Lukáš, Jozef Zrník, M. Ceretti, D. Neov, Pavel Strunz, Miroslav Vrána, Pavol Mikula, J. Keuerleber
Authors: Yu. V. Taran, Jürgen Schreiber, Pavol Mikula, Petr Lukáš, D. Neov, Miroslav Vrána
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