Stanovení míry opotřebení v oblasti vysokocyklové kontaktní únavy s využitím semianalytického přístupu

Presented paper deals with ratcheting prediction in the field of contact fatigue for the case of line contact. For the stated purposes, the wear model proposed by A. Mazzu [1] is used, which stands out for its simplicity and time efficiency. The model is based on non-linear kinematic and isotropic hardening rule of Chaboche and Lemaitre. Mazzu´s approach is used in order to estimate the wear of wheel specimen as well as plastic shear strain accumulation in case of rolling/sliding contact after cycles in range from 4.105 to 1.106. Obtained results are compared with experimental data, available on author´s department.Předložený článek se zabývá predikcí ratchetingu v oblasti kontaktní únavy pro případ liniového kontaktu. K daným účelům byl využit model opotřebení A. Mazzu [1], který vyniká svou jednoduchostí a s tím související výpočetní časovou efektivitou. Model je založen na nelineárním kinematickém a izotropním zpevnění autorů Chaboche a Lemaitra. Mazzuův model je využit k predikci opotřebení a rovněž k predikci evoluce plastické smykové deformace pod kontaktním povrchem pro vzorek kola v rozmezí 4.105 až 1.106 cyklů. Dosažené výsledky jsou posléze porovnány s experimentálními daty, získanými na autorově pracovišti

Modeling droplet impact on solids obstacle of various shapes

This paper deals with the modeling of the phenomenon of a droplet hitting an obstacle. The numerical model of the multiphase flow is based on the Navier-Stokes differential equations and the finite volume method. During the numerical analysis, the volume of fluid (VoF) method was used as a free surface modeling technique. Various shapes of the analyzed solid obstacles were considered in the numerical model. Two cases of multiphase flows with different surface tensions were analyzed: water-air and gasoil-air. The results were presented in the form of volumetric fraction graphs.Web of Science139550950

Experimentální analýza bandáží válců válcovacích stolic

The goal of the submitted paper is to introduce briefly results of experimental stress analysis which was focused on bandage of a back-up roll of rolling mill. Measurements have been carried out during technological process of manufacturing particular bandage roll as well as during running in the mill line. At first, residual stresses had been measured from previous heat-treating process and also after following press mounting on the roll. After press mounting a superposition, from the residual stresses and the stresses caused by press mounting, occurred. Stresses from the press mounting have been also numerically analyzed. Manufactured rolls were consecutively built into mill line and after particular time of running, residual stresses on the surface of bandage had been measured. For the purpose of comparison, we have also measured residual stresses on forged roll as well as on cracked bandage of broken out roll. Hole drilling method has been used for residual stress measurement.V příspěvku jsou stručně uvedeny výsledky experimentální analýzy napětí v bandáži opěrného válce válcovací stolice. Měření se prováděla jak v průběhu technologického procesu výroby bandážovaného válce, tak i při provozním nasazení ve válcovací trati. Nejdříve se měřila zbytková napětí po tepelném zpracování bandáže a pak po jejím nalisování na válec. Po nalisování došlo k superpozici zbytkových napětí a napětí od nalisování. Napětí od nalisování byla analyzována i početně. Vyrobené válce byly postupně zabudovány do válcovací tratě a po určité době provozu se měřily hodnoty zbytkových napětí na povrchu bandáže. Pro srovnání se měřilo zbytkové napětí i na kovaném válci. Rovněž se proměřila zbytková napětí v prasklé bandáži vyřazeného válce. K měření zbytkových napětí se použila odvrtávací metoda experimentální analýzy napětí

Partial energy transfer model of Lamb waves scattering in materially isotropic waveguides

The scattering phenomena of the fundamental antisymmetric Lamb wave mode with a horizontal notch enabling the partial energy transfer (PET) option is addressed in this paper. The PET functionality for a given waveguide is realized using the material interface. The energy scattering coefficients are identified using two methods, namely, a hybrid approach, which utilizes the finite element method (FEM) and the general orthogonality relation, and the semi-analytical approach, which combines the modal expansion technique with the orthogonal property of Lamb waves. Using the stress and displacement continuity conditions on the present (sub)waveguide interfaces, one can explicitly derive the global scattering matrix, which allows detailed analysis of the scattering process across the considered interfaces. Both methods are then adopted on a simple representation of a surface breaking crack in the form of a vertical notch, of which a certain section enables not only the reflection of the incident energy, but also its nonzero transfer. The presented results show very good conformity between both utilized approaches, thus leading to further development of an alternative technique.Web of Science1110art. no. 450

Metoda za predviđanje plastične deformacije i trošenja kod zamora materijala zbog dodira valjanjem uzimajući u obzir tehnološka zaostala naprezanja

The main aim of our study is to show the influence of residual stresses on wear and surface ratcheting in the case of line rolling contact. Experiments were carried out on the innovated TUORS (Technical University of Ostrava Rolling Sliding testing machine) testing rig under free rolling and also under a slip ratio of 0,75 %. All specimens used for the experiments were made from R7T steel. A hole-drilling method and an X-ray diffraction method were applied to determine the residual stresses induced by repeated contact loading and technological residual stresses. Mazzu's semi-analytical approach, which makes use of full integration of the Armstrong-Frederick model, was used to simulate ratcheting and wear. The proposed model calibration methodology, based on an inverse approach, allows the model parameters to be determined directly from the measured wear. The results of this study confirm that the observed technological compressive residual stresses lead to lower ratcheting and subsequently also to lower wear rates.Glavni cilj našega rada je pokazati utjecaj zaostalih naprezanja na trošenje i plastičnu deformaciju (ratcheting) površine u slučaju linijskog dodira valjanjem. Pokusi su provedeni na obnovljenoj TUORS (Technical University of Ostrava Rolling Sliding testing machine) opremi za ispitivanje kod slobodnog valjanja i također kod omjera klizanja od 0,75 %. Svi uzorci korišteni za pokuse izrađene su od čelika R7T. Metoda bušenja rupe i metoda rendgenske difrakcije primijenjene su kako bi se odredila zaostala naprezanja uzrokovana ponavljajućim dodirnim opterećenjem i tehnološki zaostalim naprezanjima. Za simulaciju plastične deformacije i trošenja primijenjen je Mazzuov polu-analitički pristup, koji koristi punu integraciju Armstrong - Frederick modela. Predložena metodologija kalibriranja modela na temelju inverznog pristupa, omogućuje određivanje parametara modela izravno iz izmjerenog trošenja. Rezultati ovog istraživanja potvrđuju da promatrana tehnološka tlačna zaostala naprezanja dovode do smanjenja plastične deformacije, stoga i do nižeg stupnja trošenja.Web of Science24114

Acoustic emission signal characterisation of failure mechanisms in CFRP composites using dual-sensor approach and spectral clustering technique

The characterisation of failure mechanisms in carbon fibre-reinforced polymer (CFRP) materials using the acoustic emission (AE) technique has been the topic of a number of publications. However, it is often challenging to obtain comprehensive and reliable information about individual failure mechanisms. This situation was the impetus for elaborating a comprehensive overview that covers all failure mechanisms within the framework of CFRP materials. Thus, we performed tensile and compact tension tests on specimens with various stacking sequences to induce specific failure modes and mechanisms. The AE activity was monitored using two different wideband AE sensors and further analysed using a hybrid AE hit detection process. The datasets received from both sensors were separately subjected to clustering analysis using the spectral clustering technique, which incorporated an unsupervised k-means clustering algorithm. The failure mechanism analysis also included a proposed filtering process based on the power distribution across the considered frequency range, with which it was possible to distinguish between the fibre pull-out and fibre breakage mechanisms. This functionality was particularly useful in cases where it was evident that the above-mentioned damage mechanisms exhibited very similar parametric characteristics. The results of the clustering analysis were compared to those of the scanning electron microscopy analysis, which confirmed the conclusions of the AE data analysis.Web of Science151art. no. 4

Numerical computation of the damping and stiffness coefficients of the classical and magnetorheological squeeze film damper

Technological solution, frequently used to suppress vibrations in rotating machines, consists in adding damping devices between the rotor and its frame. This is enabled by dampers working on the principle of a squeezing thin classical or magnetorheological fluid film. The Navier-Stokes equations, Reynolds equation, and modified Navier-Stokes equations are used to determine the pressure distribution in the thin fluid film. The damping and stiffness coefficients are computed by the developed procedure presented in this paper. The proposed computational approach is based on the perturbation of the synchronous circular whirling motion. The carriedout computational simulations show that the investigated mathematical models of the squeeze film damper and magnetorheological squeeze film damper allowed computation of the damping and stiffness coefficients. It has been found that the stiffness coefficients computed by the proposed mathematical models may be different

The use of Lagrange multiplier formalism to solve the problem of transverse vibrations of the lower-limb exoskeleton

In the paper, the Lagrange multiplier formalism has been used to find a solution to the problem of free transverse vibrations of the lower-limb exoskeleton. Parts of the exoskeleton were replaced with continuous (beams) and discrete elements. The beams have been circumscribed according to the Bernoulli-Euler theory. The physical model takes into account properties that characterize the interaction between exoskeleton elements, contact with the ground and system load. On the basis of the presented mathematical model, an algorithm and a computational program were created. It permitted the research on the impact of selected model parameters on the transverse vibrations of the analyzed system. The presented model, coupled with the longitudinal vibration model, may constitute a complete description of the vibrations of the lower-limb exoskeleton.Web of Science142113613

Influence of material distribution and damping on the dynamic stability of Bernoulli-Euler beams

The study analyzed the influence of materials and different types of damping on the dynamic stability of the Bernoulli-Euler beam. Using the mode summation method and applying an orthogonal condition of eigenfunctions and describing the analyzed system with the Mathieu equation, the problem of dynamic stability was solved. By examining the influence of internal and external damping and damping in the beam supports, their influence on the regions of stability and instability of the solution to the Mathieu equation was determined