Vladimír Hrbek

Numerical Optimization and Practical Implementation of the Tube Extrusion Process of Mg Alloys with Micromechanical Analysis of the Final Product

The paper is devoted to the development of a process of tubes extrusion from MgCa08 magnesium alloy. For optimization of extrusion process the Qform software was used. The numerical model of flow stress and fracture criterion for MgCa08 were obtained based on tension/compression measurements performed in a universal testing machine Zwick Z250. Predictions of the flow stress and deformations were modeled as well as the ductility of material. The process was optimized according to the plasticity and temperature criterions. In the optimization process, temperature of the billet and the speed of extrusion were determined. Based on the optimal parameters the extrusion of tubes with external diameter of 5 mm was performed in the laboratory press. On top of the macroscopic testing and calculations, investigations of the material microstructure and the micromechanical behavior of the material after the extrusion were performed by a combination of SEM and nanoindentation analyses. Micromechanical properties of the alloy were detected with the aid of statistical nanoindentation. Samples were characterized in terms of their microstructural defects, distribution of elastic modulus and hardness. Good particle dispersion and homogeneous-like distribution of micromechanical properties was found showing the efficiency of the extrusion process.

MICRO-MECHANICAL PERFORMANCE OF CONCRETE USED AS RECYCLED RAW MATERIAL IN CEMENTITIOUS COMPOSITE

The reduction of industrial pollution is recently one of main goals over all fields. In civil engineering, re-cycling of structural waste provides wide opportunity contributing this effort. This paper focus on re-use of concrete waste, which after further processing can be used in new constructions as partial supplement to the mixture. To investigate the impact of re-cycled concrete addition, it is necessary to determine mechanical and structural parameters of individual phases in the “raw” material. For this purpose, grid indentation and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM, EDX) are combined to determine properties of concrete sample.

DETERMINING THE ROLE OF INDIVIDUAL FLY ASH PARTICLES IN INFLUENCING THE VARIATION IN THE OVERALL PHYSICAL, MORPHOLOGICAL, AND CHEMICAL PROPERTIES OF FLY ASH

The properties of fly ashes vary because of the differences in the properties of their individual particles, and the determination of variation in these properties is of interest to the industries which use pulverized raw fly ash in applications, such as in cementitious materials and in the recovery of certain rare elements from raw fly ash. To investigate the differences in individual particles, four pulverized raw fly ashes from thermal power plants of the Czech Republic were used in this research. It was observed from FE-SEM that all four fly ashes consist of glassy hollow spherical, solid spherical, porous spherical, bright spherical, porous slaggy and compact slaggy particles. Box and whisker diagrams were plotted from the data of EDX individual particle analyses, which showed that the data of percentages for the Si, Al, and Fe elements is more scattered as compared to other elements. It was further observed from ternary phase diagrams and pseudo coloured images, that nature of fly ash particles changes from alumino silicate glassy to alumino silicate calcite metallic to pure ferro-metallic,where glassy particles showed high percentages and pure calcite particles were absent in fly ashes. Furthermore, a comparison between the XRF, the EDX total area analyses, showed that the EDX individual particle analysis gives more realistic and reliable data with median, mean, and the standard deviation for percentages of each element present in the fly ashes.

Influence of Sample Preparation on Determined Nanomechanical Properties of Metastable Calcium Carbonate Polymorphs

Synthetically prepared metastable calcium carbonates (CaCO3) polymorphs, vaterite and aragonite, were used for nanoindentation testing. Nanomechanical measurements were done on samples embedded in epoxy resins. Hardness and reduced modulus were determined and both values were found to be higher for vaterite. Hardness was determined to be in the ranges 1.1 to 0.2 and 0.5 to 0.1 GPa for vaterite and aragonite, respectively. Reduced modulus was found to be in the ranges 32 to 5 and 18 to 4 GPa, for vaterite and aragonite, respectively. Reduced modulus of vaterite was found to be approximately three times lower in comparison with vaterite sample prepared as pressed pellets. These results may be helpful for designing new products containing CaCO3 for applications.

Fracture Properties of Mg-Alloys Assessed with Nanoindentation

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·m1/2.

Grid Indentation and Statistic Deconvolution: Limitations and Accuracy

The evaluation of heterogeneous materials used in civil engineering on the microscopic level has become important aspect in proper understanding of the macroscopic behavior of the material. This contribution aims at advantages and problems related to the technique of grid indentation and at the evaluation of mechanical properties.

USING HOMOGENIZATION AND NANOINDENTATION FOR MICROFIBRIL ANGLE DETERMINATION OF SPRUCE

This paper is concerned with the evaluation of microfibril angle of spruce. The microfibril angle is defined as an inclination of microfibrils from the longitudinal axis, the direction of lumens. It is well known and further supported by the present study that the microfibril angle has a great influence on the final mechanical properties of wood. This angle could be measured either directly using, e.g. polarisation microscopy, X-ray diffraction, infrared spectroscopy, or indirectly, as used in this study, by employing the nanoindentation measurements. Therein, the measured indentation modulus is compared with that obtained numerically using the anisotropic theory of indentation. The latter one depends on the entire stiffness matrix derived through homogenization and the searched microfibril angle. In view of the cell wall microstructure, the effective cell properties were found using the two-step micromechanical homogenization adopting both the Self-consistent and Mori-Tanaka methods.

EVALUATION OF EFFECTIVE PROPERTIES OF BASALT TEXTILE REINFORCED CERAMIC MATRIX COMPOSITES

The present paper is concerned with the analysis of a ceramic matrix composite, more specifically the plain weave textile fabric composite made of basalt fibers embedded into the pyrolyzed polysiloxane matrix. Attention is paid to the determination of effective elastic properties of the yarn via homogenization based on the Mori-Tanaka averaging scheme and the 1st order numerical homogenization method adopting a suitable representative computational model. The latter approach is then employed to simulate the response of the yarn when loaded beyond the elastic limits. The required mechanical properties of individual material phases are directly measured using nanoindentation with in-build scanning probe microscopy. Applicability of the proposed computational methodology is supported by the analysis of a unidirectional fibrous composite, representing the yarn, subjected to a macroscopically uniform strain.

Early Stage Microstructure Development of Cement Paste Modified by Crystalline Admixture

The hydrophobicity enhancement of structural materials is a contemporary topic of discussion. This paper deals with the effect of crystalline admixture (CA) on the microstructure of the cementitious composite in first four weeks after the production. Previous investigation was performed on the referential specimens consisting only from pure cement. The samples of pure CA and cement modified by 1% of CA with same w/c ratio were examined and compared in this paper. By investigation of these materials on the micro-scale in the time, it is possible to identify the development of the microstructure of each and determine the impact of the modification. For this purpose, backscattered electrons microscopy (BSE) with energy-dispersive X-ray spectroscopy (EDX) was used for phase analysis as well as instrumental nano-indentation to obtain the micro-mechanical properties. The presented results show the evolution of mechanical properties and microstructure in time and the impact of the crystalline admixture on modified cement.

Enhancing Engineered Cementitious Composite by External and Internal Hydrophobization

Engineered cementitious composites (ECC) are characterized with increased ductility and strain hardening due to its internal structure design. ECC is especially useful for applications where common steel reinforced concrete is not applicable and the structural members undergo large strains or dynamic action. Such conditions are often combined with environmental effects where structures are partly or fully immersed in water possibly containing some harmful substances such as chloride or sulfuric ions. To maintain sufficient durability of the composite it is necessary to decrease its water absorbability. One of the very efficient ways to do this is to use external or internal hydrophobization of the composite as shown in this paper.