Papers by Author: Jiří Němeček

Abstract: The paper investigates deformations and plastic properties received from different material volumes and tests of magnesium samples. Small volume characteristics gained on single Mg crystals are compared to polycrystalline AZ31 alloy. Results of tests employing nanoindentation, focused ion beam milling and electron backscatter diffraction techniques are presented. Large differences were found between micro-beam testing and spherical indentation tests having the volume one order of magnitude apart. The plastic strength scaling factor was found 1.7 for the studied grain configurations and volumes.

Abstract: The paper presents microscale experimental investigation performed on microtubes that were prepared with a novel laser dieless drawing (LDD) technique from difficult-to-work AZ31 magnesium alloy (nominally 3wt.%Al-1wt.%Zn-0.3wt.%Mn-Mg balanced). A microstructure analysis was performed via various microscopic techniques. Mechanical response of individual grains with various orientations was tested using instrumental nanoindentation and the results were compared with the microstructure. Distributions of elastic modulus, hardness and visco-elastic properties were analyzed. In addition, microtubes were also characterized in terms of their surface roughness and morphology based on different modes of surface treatment. The grain size is practically not changed when LDD is applied after extrusion showing low thermal and straining effect of the process. Local mapping of mechanical properties does not show weak spots and imply feasibility of the novel production technology.

Abstract: Mechanical fracture properties of interfacial transition zone (ITZ) of fine-grained composite based on cement matrix with different types – basalt, granite, marble and amphibolite – of rock inclusion were studied. Specimens with the initial stress concentrator were tested in standard three-point bending configuration. Fracture surfaces were examined with light and electron microscopes. Local ITZ response was characterized by nanoindentation in the vicinity of rock inclusions. Local elasticity, hardness and viscous properties were assessed. It has been shown that the ITZ is mechanically weaker compared to the bulk matrix in the region of ca. 0–20 μm from the inclusion for all specimen’s types. It exhibits gradual increase of elastic modulus and hardness, which can be approximately expressed by a power law. On the other hand, the creep in ITZ was found to be higher compared to the bulk matrix. The results of nanoindentation measurements are in a good agreement with overall mechanical properties, fracture response and microstructure measurements done by scanning electron microscopy.

Abstract: The paper shows results of microscale experimental tests performed on cement paste specimens fabricated by focused ion beam milling. The specimens are prepared in the form of cantilever beams and loaded in bending by nanoindenter. The dimensions of specimens are in the order of a few micrometers which corresponds to the single phase size. Intact and notched specimens with a stress concentrator are tested. Tensile strength and fracture energy are derived for the hydration product by analyzing the nanoindentation data and with the aid of analytical and numerical modeling. Although small number of tests is performed good correlation of the results is reached with respect to the available literature and molecular dynamic simulations.

Abstract: This paper describes results of experimental investigation of chloride ion penetration in concrete. Chloride concentration have been studied in accelerated conditions. Part of concrete samples were treated with colloidal nanosilica solution to increase their resistance to further chloride penetration. Measurement of chloride profiles was performed and compared on samples subjected to accelerated electrical migration tests lasting for a few days. The chloride ion profile in samples was determined in layers of 5 mm thickness up to a total profile thickness of 45 mm. The developed chloride concentrations reached values found after several years of exposure in natural conditions. It was found that the resistance to chloride penetration is substantially improved by nanosilica injection and the resistance to chloride penetration is increased by 60%.

Abstract: This paper shows a micromechanical study of interfacial transition zone (ITZ) around steel fiber in cement paste. It investigates microstructure and mechanical performance of the ITZ by a combination of nanoindentation and scanning electron microscopy (SEM). The investigated specimens were made from cement CEM I 42.5R paste with dispersed reinforcement in the form of steel fiber TriTreg 50 mm. The SEM demonstrated larger porosity and smaller portion of clinkers in the ITZ. Nanoindentation delivered values of elastic modulus, hardness and creep parameters around the fiber. An average value of elastic modulus in ITZ was at the level of 67% in comparison with cement bulk and the width of ITZ was about 40 µm. The value of hardness was found to be 60% of the average hardness of the bulk cement paste. The measured load-displacement curves were used for calculation of creep indentation parameter (CIT) and the creep compliance function. An average value of the creep compliance in the ITZ was found to be two times higher than in the cement bulk.

Abstract: This study deals with experimental determination of tensile properties of cement paste hydration products at micro-scale. Cantilever micro-beams with length of about 16 µm and pentagon cross section with micrometer dimensions were fabricated by focused ion beam milling on hydrated cement paste samples. Nanoindentation was used for evaluating elastic properties while tensile properties were derived from beam bending tests. Displacement controlled micro-scale tests give access to both tensile strength and estimates of fracture energy based on the load-displacement curves measured with the nanoindenter. The mean tensile strength and the fracture energy of inner hydration product were assessed as 791 MPa and 16.7 J/m², respectively. The huge difference of the micro-scale properties when compared to macroscopic values comes from the scaling properties of concrete.

Abstract: The paper summarizes utilization of nanoparticles (NP) in concrete for enhancing its mechanical, transport or other properties. The first part is devoted to review of NPs used in fresh concrete mixtures that have been reported previously in the literature while the second part shows original results from utilization of nanoparticles as healing agents for reparing of existing concrete structures. In the second case, nanoparticles are delivered to concrete via electrokinetic principle by using their surface charge provided in colloidal solutions. Particles are transported in the pore liquid of concrete due to electric field applied between the concrete surface and steel reinforcement. In this way, distressed concrete can be repaired, cracks sealed and risk of corrosion diminished. The injection of NPs can be complemented with extraction of chlorides that are the main deteriorating chemicals in reinforced concrete structures. Finally, the paper shows a framework developed for the electrokinetic transport of ionic species in concrete applied to chloride extraction from concrete and how it can be modified also for transport of nanoparticles.

Abstract: This paper deals with micromechanical properties of biocompatible magnesium alloys MgCa0.8 prepared by extrusion. At first, the microstructure, elastic and hardness properties are assessed at the scale of one micrometer by means of a nanoindenter. Further, determination of fracture mechanism, fracture area and fracture toughness of the alloy is studied with sharp indentation, optical microscopy and SEM and by means of energetic methods. Based on the examination of optical images it was concluded that the Mg matrix is not vulnerable to distinct cracking around indents even for high indentation loads. By computations, it was found the fracture area is very small, about 0.7-3% of the tip contact area based on the assumption of limit values of the target fracture toughness 10-20 MPa·m^1/2.

Abstract: The early stage development of the cement microstructure is a crucial aspect affecting the overall performance of cementitious materials. The formation of hydration products depends on the presence of water and carbon dioxide, as well as on the distribution and re-crystallization of the cement particles. In this paper we evaluate the micromechanical properties evolution of cement phases over first 28 days after casting by the electron microscopy and static areal indentation.