Papers by Author: Ladislav Pešek

Abstract: 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 (indentation hardness H_IT, indentation modulus E_IT, indentation energy: total W_total, elastic W_elast, plastic W_plast) of each microstructure component separately in multiphase materials gives information that is valuable for the development of new materials and for modelling. The mechanical properties of materials measured by DSI are affected by the experimental procedure, by the measurement conditions and factors which result from the material characteristics and device construction. We have to determine the effect of individual factors on the measurement in order to reach the repeatability and to allow the comparing the mechanical properties of the material. The aim of this investigation is to determine the measurement factors that affect indentation hardness of individual microstructural components and global mechanical properties of thin steel sheets. We investigated the factors which result from the material characteristics (crystallographic orientation of grain, grain boundary and anisotropy), preparation of the sample surface (roughness of sample surface) and method of measurement (pile-up, ISE).

Abstract: Depth-sensing indentation (DSI) is used in this work to determine the change of local mechanical properties by annealing of hot-dip galvanized IF steel sheets. The influence of annealing conditions (temperature and time) on: (i) coating composition; (ii) local mechanical properties and (iii) roughness of the coating surface was quantified. Annealing of steel samples (Ti-IF steel and Ti-Nb-P-IF steel alloyed with phosphorus) was performed with different holding times (10, 60, and 300 s) by both temperatures 450 and 550 °C. The zinc in coating transformed during annealing to the intermetallic phases Zn_xFe_y. Annealed Zn-Fe coatings, wherein the iron concentration falls towards the surface, consist of different intermetallic phases.

Abstract: Nanomechanical testing using depth sensing indentation (DSI) provides a straightforward solution for characterizing of mechanical properties (indentation hardness H_IT, Young’s modulus E_IT, indentation energy: total W_total, elastic W_elast, plastic W_plast) 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.

Abstract: The Oliver–Pharr method has extensively been adopted for measuring hardness and Young’s modulus by indentation techniques. However, the method assumes that the contact periphery sinks in, which limits the applicability to the materials pile-up [1]. In this work, we characterize the pile-up (shape and height) in steel sheets with different mechanical properties and propose an improved methodology to calculate the real mechanical characteristics of steel sheets with significant pile-up. Pile-up correction of mechanical characteristics is based on ratio of pile-up height and contact depth. Pile-up height was measured by atomic force microscopy (AFM).

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.

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.

Abstract: The Oliver–Pharr method has extensively been adopted for measuring hardness and elastic modulus by indentation techniques. However, the method assumes that the contact periphery sinks in, which limits the applicability to the materials pile-up [1]. This study proposes an improved methodology to calculate the real mechanical characteristics of individual phases in various steels with significant pile-up. Pile-up correction of mechanical characteristics is based on ratio of pile-up height and contact depth. Pile-up height was measured by atomic force microscopy (AFM). The effect of grain boundaries on the shape and size of the pile-up lobes was also discussed.

Abstract: This paper deals with description of properties of selected natural materials mainly theirs shelter function. The investigated materials are horsetail and walnut shell. Both natural materials have porous shell/tubular structure. Walnut shell provides natural shield cover for fruit with gradient distribution of porosity and membrane function. In case of horse tail, except the gradient distribution of porosity there is also the gradient change of chemical composition along the cross-section.

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.

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.