Jiří Němeček

Nanoindentation of Alkali-Activated Fly Ash

Low-calcium ground fly ash was activated with a sodium silicate solution and cured under ambient and heat conditions. Nanoindentation was employed for the characterization of reaction products, mainly alumino-silicate (N-A-S-H) gel. Samples were indented by a series of grids consisting of hundreds of indents each. The intrinsic Youngs modulus of the N-A-S-H gel was found to be around a mean value of 17.7 GPa regardless of the curing procedure. Other characteristic phases were also identified, namely partly activated slag, nonactivated slag, and fly ash particles. Results of pore size distributions measured by mercury intrusion porosimetry were systematically observed depending on the curing procedure. Additional measurements using the same methodology were performed on cement paste samples. The synthesized N-A-S-H gel was found to be mechanically close to calcium silicate hydrate gels in matured cement pastes.

Characterization of Alkali-Activated Fly-Ash by Nanoindentation

Nanoindentation was employed for the characterization of reaction products, mainly N-A-S-H gel, within alkali-activated fly ash samples. Heat and ambient-cured samples from ground fly ash were indented in a grid of hundreds of indents. The intrinsic Young’s modulus of N-A-S-H gel was found around the mean value 17.70 GPa, regardless on the curing procedure. Such finding elucidates intrinsic stiffness of mature N-A-S-H gel with different origin. Partly-activated slag, slag and fly-ash particles were further distinguished by histogram deconvolution.

Electrochemical Injection of Nanoparticles into Existing Reinforced Concrete Structures

This paper deals with the chloride extraction from concrete and shows possibilities of injection of selected nanoparticles and a hydrophobic agent into existing reinforced concrete structures by means of electromigration technique. Accelerated chloride penetration tests are employed to simulate natural chloride exposure. The developed chloride profile is then removed by electroextraction. Similarly, specimens are injected with different concentrations of colloidal nanosilica particles and a siliconate. It is shown in the paper that chlorides can be effectively extracted from the concrete using small voltage lasting for several days. Solutions of nanosilica and siliconate can be transported into concrete via the electric field. Once injected nanosilica can act as a microstructure densifier and further reduce chloride penetration due to the decreased diffusivity of the treated concrete. Siliconate acts on hydrohobization of the outer specimen layer.

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.

Mechanical Properties of Single and Double-Layered PVA Nanofibers

Multi-layered nanofiber textiles can be utilized in many applications. In such case the individual layers are laid in more stages and the question arises whether the connection is perfect. Two kinds of samples of PVA nanotextiles having the weight of 1.3 g/m2 (single-layered, AI) and 2.8 g/m2 (double-layered, AII), respectively. It was shown that mechanical properties, in particular the average tensile strength (24 N/mm for AI and 51 N/mm for AII) and stiffness (950 N/mm for AI and 1600 N/mm for AII), are independent of the number of layers, only their weight per unit area matters. This indicates that the bond between the individual layers is perfect.

Application of Micromechanics on Alkali-Activated Materials

Research of alkali-activated materials has been a traditional domain of chemists. This paper exploits contribution of micromechanics to the subject. A new model for volumetric evolution of chemical phases is formulated. The first homogenization level identifies elasticity on the scale of N-A-S-H gel. Nanoindentation sensing technique yielded the intrinsic Youngs modulus of N-A-S-H gel as ~18 GPa, which was further downscaled to the solid gel particles. Percolation theory had to be introduced to match an early-age elasticity. The second homogenization level takes into account an unreacted fly ash. Homogenization models match well the experimental elasticity and demonstrate stiffening of N-A-S-H gel, induced by increasing packing density of the solid gel particles. The percolation model explains a long setting time of alkali-activated materials.

Micromechanical Homogenization of Ultra-High Performance Concrete

Mechanical properties and durability of Ultra-High Performance Concretes (UHPC) are closely associated with composition and microstructure of tested samples. In this work, determination of effective elastic properties of UHPC composite was performed for a representative volume element using combination of microstructural investigations (scanning electron microscope imaging, image analysis of back scattered electron micrographs and nanoindentation) and analytical methods of micromechanics. Based on the volumetric content and micromechanical behavior of individual components an effective elastic modulus of the whole composite was predicted and compared with macroscopically measured value with good agreement within 5%.

Local Mechanical Characterization of Metal Foams by Nanoindentation

This paper deals with microstructure and micromechanical properties of two commercially available aluminium foams (Alporas and Aluhab). Since none of the materials is available in a bulk and standard mechanical testing at macro-scale is not possible the materials need to be tested at micro-scale. To obtain both elastic and plastic properties quasi-static indentation was performed with two different indenter geometries (Berkovich and spherical tips). The material phase properties were analyzed with statistical grid indentation method and micromechanical homogenization was applied to obtain effective elastic wall properties. In addition, effective inelastic properties of cell walls were identified with spherical indentation. Constitutive parameters related to elasto-plastic material with linear isotropic hardening (the yield point and tangent modulus) were directly deduced from the load–depth curves of spherical indentation tests using formulations of the representative strain and stress introduced by Tabor.

Fracture Process in a Fine-Grained Cement-Based Composite Monitored with Nanoindentation and Acoustic Emission

The contribution is devoted to investigation of the fracture process zone (FPZ) in a fine-grained cement-based composite made from hydrated Portland cement. Particularly, experimental investigations and description of the stable crack propagation using fracture mechanics model are conducted. Three-point bending tests on small composite beams with a central edge notch were performed. The damage due to fracture was monitored by means of nanoindentation performed around the macroscopically observable crack. Acoustic emission events were recorded during the three-point bending test and correlated with load–displacement data. The beneficial effect on the fracture resistance of fine-grained mortar specimens compared to plain cement pastes was quantified.

Numerical modeling of aluminium foam on two scales

The paper deals with computational modeling of aluminium foams on two distinct scales. The microscopically heterogeneous cell walls are modeled with continuum micromechanics models. Several analytical schemes and FFT-based homogenization are applied to predict elastic properties at the first level. Nanoindentation with sharp Berkovich tip is utilized to obtain input parameters for the homogenizations. Plastic properties are assessed directly from spherical nanoindentation at this level.Several geometrical simplifications are studied to model the upper foam level. At first, two dimensional models based on beam analogy and plane strain finite element (FE) models are studied for their ability to predict effective elastic and plastic foam properties. Finally, the behavior of the three dimensional voxel based FE model derived from micro-CT imaging is investigated. Models are compared in terms of their ability to predict experimental results and in terms of their computational demands. It is shown in the paper each model type has difficulties to quantitatively match experimental data in the whole tested range. Two dimensional beam models are capable to predict elastic properties but fail to predict plastic ones. Plane strain FE models are very compliant and lack three dimensional confinement. Three dimensional voxel model has the largest potential to predict experimental measurements but it is the most computationally demanding. It was found the performance of all models on the foam level is very much dependent on their porosity which is the main controlling parameter of the model behavior. Any deviations from experimentally assessed porosity leads to large deviations in the model prediction. Mutual model comparisons and possible solutions are provided in the paper along with computational aspects and requirements.