Papers by Author: Ladislav Pešek

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Authors: Peter Burik, Ladislav Pešek
Abstract: The macroscopic mechanical properties of steel are highly dependent upon microstructure, morphology, and distribution of each phase present. Nanomechanical testing (Depth sensing indentation) provides a straightforward solution for quantitatively characterizing each of these phases because it is very powerful technique for characterization of materials in small volumes. Measuring the intrinsic properties of each phase separately in multiphase materials gives information that is valuable for the development of new materials and for modelling [1]. In this work, depth sensing indentation has been used to reveal mechanical properties of different phases in steel sheets.
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Authors: Pavol Zubko, Marek Vojtko, Ladislav Pešek, Miroslav Német, Pavel Bekeč
Abstract: Dual phase (DP), interstitial free (IF) and advanced high strength low alloy steel (HSLA)sheets have been successfully used for different components of car body. DP and HSLA are used ascrash resistant and IF as cover or “skin” of car body. The development of new vehicles nowadays isbeing driven by the need to simultaneously reduce mass and increase of passenger and pedestriansafety as well as costs saving through cold forming instead of hot forming. Limited publishedinformation is available on the changes in microstructure of these steel grades at different highstrain rates [1-3].This paper deals with changes of mechanical properties, microstructure and fractography of threesteel grades, which were tested at quasi static (10-3 s-1) and high strain rate (3000 s-1). Themicrostructures were characterized in terms of ferrite grain size, aspect ratio of ferrite andelongation of constituent phases. Deformed and undeformed specimens were compared to assess thechanges in the microstructure. The fracture appearance analysis indicates that the fracture patternunder high strain rates is mainly ductile, regardless of steel grades. The microstructure changessignificantly during the deformation process under both quasi-static and dynamic tension in allinvestigated steels. The plastic deformation in ferrite dominates in this process.
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Authors: Pavol Zubko, Ladislav Pešek
Abstract: The contribution deals with estimation of tensile properties and fatigue behavior based on hardness measurement. First of all the database of tensile and fatigue properties vs. hardness data was created for a group of steels, from literature survey and performed experiments. Tensile strength, yield strength, ductility and parameters of Ludwig-Hollomon equation in static and cyclic loading were extracted and fitted in relation to the hardness HB. The experimental materials were API 5 L grade steels – X60 and X70 after different deformation exposition. Measured tensile curve (SC) and cyclic deformation curve (CDC) were compared with predicted curves. Hardness was measured in-situ during cyclic loading. The maximum possible hardness values were experimentally determined. The results give a good agreement between estimated and measured data of both static tensile test and fatigue properties.
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Authors: Peter Burik, Ladislav Pešek
Abstract: The work experimentally analyses the effect of various factors on hardness measured values on thin steel sheets using Ultrasonic Contact Impedance (UCI) technique. The conditions experimentally used are compared with that according the ASTM A 1038-08 standard. UCI is an experimental technique for indirect hardness measurement. The equipment uses a Vickers indenter and the hardness measurement is based on the change of the resonance frequency during indenter ́s penetration [.The UCI hardness may depend on some factors, therefore optimal measurement conditions must be determined. The effect of distance between indents, zinc coating, sample weight, sample mounting and adhesive material for sample fixation were determined.
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Authors: Peter Burik, Ladislav Pešek, Zuzana Andršová, Pavel Kejzlar
Abstract: Nanomechanical testing using depth sensing indentation (DSI) provides a straightforward solution for characterizing of mechanical properties (indentation hardness HIT, Young’s modulus EIT, indentation energy: total Wtotal, elastic Welast, plastic Wplast) of homogeneous (bulk) materials. However, real materials such as multi-phase steels are a heterogeneous material on the microscopic scale (microstructure). We need to know the local mechanical properties of each phase separately in those materials for reasons development of new materials and for modeling. Mechanical properties of each phase separately in multiphase materials are difficult or even impossible to examine in bulk material ex situ.In this paper we describe the technique for measuring the mechanical properties of each phase separately in multiphase steel by two-dimensional mapping tool. This approach relies on large arrays of nanoindentations (known as grid indentation) and statistical analysis of the resulting data [1, 2]. The aim of this investigation is to optimize the parameters of the grid indentation for a given microstructure of steel sheets.
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Authors: Peter Burik, Ladislav Pešek, Lukáš Voleský
Abstract: Mechanical properties by depth sensing indentation are derived from the indentation load-displacement data used a micromechanical model developed by Oliver & Pharr (O&P). However, O&P analysis on the indentation unloading curve is developed from a purely elastic contact mechanics (sink-in). The applicability of O&P analysis is limited by the materials pile-up. However, when it does, the contact area is larger than that predicted by elastic contact theory (material sinks-in during purely elastic contact), and both hardness H and Youngs modulus E are overestimated, because their evaluation depends on the contact area deduced from the load-displacement data. H can be overestimated by up to 60 % and E by up to 30 % depending on the extent of pile-up [1,2]. It is therefore important to determine the effect of pile-up on obtained mechanical characteristics of the material by depth sensing indentation. The work experimentally analyses the effect of pile-up height on mechanical characteristics H and E, which are determined by O&P analysis. Pile-up height was measured by atomic force microscopy (AFM).
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Authors: Peter Burik, Ladislav Pešek, Zuzana Andršová, Pavel Kejzlar, Pavol Zubko
Abstract: The macroscopic mechanical properties of steel are highly dependent upon microstructure, crystallographic orientation of grains and distribution of each phase present, etc. Nanomechanical testing using depth sensing indentation (DSI) provides a straightforward solution for quantitatively characterizing each of phases in microstructure because it is very powerful technique for characterization of materials in small volumes. Measuring the local properties of each microstructure component separately in multiphase materials gives information that is valuable for the development of new materials and for modelling. The work experimentally analyses the effect of strain history on the mechanical properties of individual components in steel sheets by depth sensing indentation. The measurements were carried out on broken tensile specimens.
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Authors: Ladislav Pešek
Abstract: The article is focused on local stress simulation as well as local deformation analysis in a cylinder of piston combustion engine. Design of the exhaust pipe system has to fulfill special requirements with regard to the combined loading due to high pressures and temperatures. There is a special role of the exhaust pipe barrier, which is situated in the exhaust gas pipe. This barrier separates the main air hole from the secondary lateral auxiliary air holes. The exhaust pipe barrier seems to be a critical point of the cylinder. The finite-element analysis applied on the silumin-cast cylinder is presented in the paper in order to prove the critical places with high stress concentration. The local strength of the cylinder material was estimated using a material model based on relation between the strength and the mean distance between the secondary dendrite arms (SDAS) in the as-cast microstructure. Due to local heterogeneities there are differences in the local strength value at the various places of the microstructure. The simulation of stress distribution was performed by use of the finite-element method. Based on the relation „local stress/local strength“ the individual critical places in the cylinder were defined and located.
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Authors: Peter Zimovčák, Tibor Köves, Ján Dusza, Ladislav Pešek, Francis Chalvet, Goffredo de Portu
Abstract: Micro- and macro-hardness of the individual layers of an Al2O3 / Al2O3 + ZrO2 layered composite have been investigated. The indentation load size effect has been studied in a range of indentation load of 25 mN – 100 N using depth sensing indentation technique and conventional Vickers hardness testers. Fracture toughness values of the layers have been measured using indentation fracture method. The effect of residual stresses on the KIC value has been analyzed.
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