Transformation of the force - deflection diagram under a three-point-bending into the stress - crack opening relationship

V příspěvku je popsán analytický přechod z diagramu závislosti průhybu vzorku z betonu s příměsí polypropylenových vláken na velikosti zatěžovací síly na vztah mezi napětím a rozevřením trhliny v tahu za ohybu. Transformace je založena na kinematickém popisu chování testovaného vzorku. Napětí je aproximováno kombinací exponenciálních funkcí doplněnou o člen vystihující prokluz vláken přemosťujících trhlinu.In this paper, an analytical form of the transformation from the force – deflection diagram obtained under a three-point bending into the stress – crack opening relationship of concrete with polypropylene fibers is outlined. The transformation is based on the description of the kinematic behavior of the test sample. The stress is approximated by a combination of exponential functions supplemented with a term capturing the slip of fibers bridging the crack

Homogenization of plain weave composites with imperfect microstructure: Part II--Analysis of real-world materials

A two-layer statistically equivalent periodic unit cell is offered to predict a macroscopic response of plain weave multilayer carbon-carbon textile composites. Falling-short in describing the most typical geometrical imperfections of these material systems the original formulation presented in (Zeman and \v{S}ejnoha, International Journal of Solids and Structures, 41 (2004), pp. 6549--6571) is substantially modified, now allowing for nesting and mutual shift of individual layers of textile fabric in all three directions. Yet, the most valuable asset of the present formulation is seen in the possibility of reflecting the influence of negligible meso-scale porosity through a system of oblate spheroidal voids introduced in between the two layers of the unit cell. Numerical predictions of both the effective thermal conductivities and elastic stiffnesses and their comparison with available laboratory data and the results derived using the Mori-Tanaka averaging scheme support credibility of the present approach, about as much as the reliability of local mechanical properties found from nanoindentation tests performed directly on the analyzed composite samples.Comment: 28 pages, 14 figure

Return mapping scheme for the Hoek-Brown model with tension cut-off

The present paper revisits a stress update procedure for the Hoek-Brown plasticity model enhanced by a Rankine type of tension cut-off failure criterion. Limiting tensile stresses not only supports the behavior of weak rock masses with a very low tensile strength but in combination with the original Hoek-Brown model improves robustness of the stress update procedure, i.e. the stress return mapping algorithm. Herein, the primary focus is on the stress return from a space of inadmissible trial stresses for which neither the Hoek-Brown yield surface nor its derivative can be evaluated. All potential scenarios are thoroughly discussed both in the framework of single- and multi-surface plasticity. The presented procedures were implemented and verified with the help of the Geo5 FEM software

Hierarchical Modeling of Mastic Asphalt in Layered Road Structures Based on the Mori-Tanaka Method

We present an application of the Mori-Tanaka micromechanical model for a description of the highly nonlinear behavior of asphalt mixtures. This method is expected to replace an expensive finite element-based fully-coupled multi-scale analysis while still providing useful information about local fields on the meso-scale that are not predictable by strictly macroscopic simulations. Drawing on our recent results from extensive experimental and also numerical investigations this paper concentrates on principal limitations of the Mori-Tanaka method, typical of all two-point averaging schemes, when appliedto material systems prone to evolving highly localized deformation patterns such as a network of shear bands. The inability of the Mori-Tanaka method to properly capture the correct stress transfer between phases with increasing compliance of the matrix phase is remedied here by introducing a damage like parameter into the local constitutive equation of reinforcements (stones) to control an amount of stress taken by this phase. A deficiency of the Mori-Tanaka method in the prediction of creep response is also mentioned particularly in the light of large scale simulations. A comparison with the application of macroscopic homogenized constitutive model for an asphalt mixture is also presented

Gradual fracture of layers in laminated glass plates under low-velocity impact

Modeling of gradual fracture of individual layers of a laminated glass produced by successive low velocity impacts is presented. This subject is first examined experimentally focusing on fracture sequence of the laminate composed of three or four glass layers bonded by PVB interlayers. While both laminate geometries, limited to four test samples, show a significant scatter in impact energies and breakage forces causing fracture as well as differences in fracture sequence with increasing impact height, they consistently support ability of partially damaged samples to withstand contact forces exceeding those leading to fracture of the previous glass layer. Also, the four-glass layer laminates seem to provide the post-fracture response consistent with the concept of a sacrificial-glass-ply design. Apart from experimental work the present topic is addressed also computationally. To this end, the results obtained via the LS-DYNA software are compared to those derived using the phase-field damage model. It is illustrated that properly tuned computational model may provide results, which are in accord with experimental observations both in terms of contact forces and the resulting fracture patterns

Application of the prediction model on the control of hydropower function of selected multi-reservoir system

Diplomová práce je zaměřena na ověření vlivu délky předpovědního období a přesnosti předpovídaných průměrných měsíčních přítoků vody ze zdrojů při řízení hydroenergetické funkce vybrané vodohospodářské soustavy. Pro stanovení předpovídaných přítoků do soustavy nádrží je uplatněn zonální pravděpodobnostní předpovědní model (ZPPM). Při řešení je použit simulační model, který je ručně sestaven v programu Microsoft Excel a optimalizační model, který je automatizovaně sestaven v programu SOMVS a je uplatněn princip adaptivního řízení.The diploma sthesis is focused on verifying the influence of the length of the prediction period and the accuracy of the predicted mean monthly inflows of water sources in the management of hydropower function selected water management system. To determine the predicted inflows into the reservoir system is applied zonal prediction model. When the solution is used a simulation model that is hand-built in Microsoft Excel and optimization model, which is automatically built into the program SOMVS and is applying the principles of adaptive control.

Simulation and optimization model of reservoir system in Svitava river basin

Bakalářská práce je zaměřena na sestavení simulačního a optimalizačního modelu vybrané vodohospodářské soustavy. Oba modely jsou vytvořeny v programu SOMVS. V simulačním modelu jsou na základě volby nalepšených odtoků z nádrží hledány hranice tříd zabezpečenosti. Simulační model simuluje tok vody systémem. Není hledáno optimální řešení pomocí optimalizačních metod. V optimalizačním modelu je použita optimalizační metoda diferenciální evoluce. Optimalizační model hledá maximalizované hodnoty kriteriální funkce pro odběr vody z vybraného profilu.The bachelor´s thesis is focused on the construction of a simulation and optimization model selected water management system. Both models are created with program SOMVS. The simulation model is based on the choice improved outflow from reservoir searching of boundaries Assurance classes. The simulation model simulates the flow of water through the system. Not finding the optimal solution using optimization methods. In the optimization model is used optimization method of differential evolution. The optimization model seeks maximized value of the objective function for water withdrawal from selected profile.

COMPARING THE HOEK-BROWN AND MOHR-COULOMB FAILURE CRITERIA IN FEM ANALYSIS

This paper revisits the issue of a potential substitutions of the Hoek-Brown failure model by the standard Mohr-Coulomb model in the stability analysis of rock masses. The derivation of equivalent shear strength parameters of the Mohr-Coulomb proposed by Hoek et al. [1] is addressed with emphases on the suitable range of stresses for which the equivalence of the two failure criteria applies. To that end, a simple numerical analysis of the oedometric test is carried out. It is seen that a correct choice of the upper limit of the minimum compressive principal stress is crucial for the Mohr-Coulomb model to provide predictions comparable to the Hoek-Brown model. This issue is addressed next in the light of the solution of slope stability problem. All the presented results were derived with the help of the GEO5 FEM finite element software [2]

CORRELATION RELATIONSHIPS FOR THE HYPOPLASTIC MODEL FOR FINE GRAINED SOILS

The paper is concerned with our ongoing research effort devoted to the development of reliable computational tools for the calibration of advanced constitutive models of soils. At present, such software is available for the hypoplastic model of clays applicable to soft soils. This software provides a stepping stone for the determination of potential links between individual model parameters and fundamental characteristics of soils. Identifying such links would allow for tuning the model without performing time consuming experiments, particularly in the case of an initial design. Some preliminary results are presented in the paper