František Fojtík

Memorization and other Transient Effects of ST52 Steel and its FE Description

This contribution presents main results of experimental tests realized on smooth specimens made of ST52 steel under strain controlled uniaxial cyclic loading and compares these results with numerical simulations performed using a modified Chaboche model by finite element method. The effects of NonMasings behavior, memorization, cyclic hardening/softening and mean stress relaxation have been studied at room temperature. The experiments were realized on the reconstructed hydraulic fatigue testing machine INOVA 100 at the Laboratory of modern materials testing and defectoscopy of VŠB-TU Ostrava. Developed mixed hardening material model includes a memory surface stated in stress-space, which makes possible to significantly improve prediction of effects studied.

Experimental and Numerical Study of Uniaxial and Multiaxial Stress-Strain Behaviour of R7T Wheel Steel

This paper is focused on the correct description of stress-strain behavior of the R7T steel. An experimental study on the wheel steel specimens including uniaxial as well as multiaxial tests has been conducted. The main attention was paid to such effects as ratcheting and nonproportional hardening of the material. A cyclically stable behavior of the steel under higher amplitude loading was found. The MAKOC model, which is based on AbdelKarim-Ohno kinematic hardening rule and Calloch isotropic hardening rule, has been applied in subsequent finite element simulations. The numerical results show very good prediction of stress-strain behaviour of the wheel steel.

Efficient lifetime estimation techniques for general multiaxial loading

In this paper, we discuss and present our progress toward a project, which is focused on fatigue life prediction under multiaxial loading in the domain of low-cycle fatigue, i.e. cases, where the plasticity cannot be neglected. First, the elastic-plastic solution in the finite element analysis is enhanced and verified on own experiments. Second, the method by Jiang describing the instantaneous damage increase by analyses of load time by time, is in implementation phase. In addition, simplified routines for conversion of elastic stresses-strains to elastic-plastic ones as proposed by Firat and Ye et.al. are evaluated on the basis of data gathered from external sources. In order to produce high quality complex analyses, which could be feasible in an acceptable time, and allow the period for next analyses of results to be expanded; the core of PragTic fatigue solver used for all fatigue computations are being re-implemented to get the fully parallelized scalable solution.

Computer Aided Evaluation of the Stress Tensor in the Two-Dimensional Photoelasticity

Photoelasticity is a whole field experimental technique for the measurement and visualizing of the stress and strains in the loaded members. This work is intended to create a unique procedure that allows the use of a computer aided technique in the evaluation of the measurement results. The specimens of the thin annular disk, the thin disk and the shaped beam were tested. The PATRAN program is used for the finite element solution of stress analysis in the investigated specimens. The excellent agreement is established between the results of experimental technique based on the created computer procedure and the results of numerical computations.

A method for predicting ratcheting and wear in rolling contact fatigue, taking technological residual stresses into consideration

Glavni cilj nasega rada je pokazati utjecaj zaostalih naprezanja na trosenje i plasticnu deformaciju (ratcheting) povrsine u slucaju 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 koristeni za pokuse izrađene su od celika R7T. Metoda busenja rupe i metoda rendgenske difrakcije primijenjene su kako bi se odredila zaostala naprezanja uzrokovana ponavljajucim dodirnim opterecenjem i tehnoloski zaostalim naprezanjima. Za simulaciju plasticne deformacije i trosenja primijenjen je Mazzuov polu-analiticki pristup, koji koristi punu integraciju Armstrong - Frederick modela. Predložena metodologija kalibriranja modela na temelju inverznog pristupa, omogucuje određivanje parametara modela izravno iz izmjerenog trosenja. Rezultati ovog istraživanja potvrđuju da promatrana tehnoloska tlacna zaostala naprezanja dovode do smanjenja plasticne deformacije, stoga i do nižeg stupnja trosenja.

Full Field Evaluation of the Stress Tensor Components in 2D Photoelasticity Via Computer Software

Abstract The aim of this work is to find out the components of stress tensor in plane specimens. For this purpose the photoelasticity methodology is used. In order to make this technique more comfortable for use, there was developed an algorithm in MATLAB program. The results are compared with numerical solution. The main advantages of the developed algorithm are the speed and the capabilities to extend to analyze the plastic deformation and strain conditions in the material during forming processes.

Locking Compression Plate for Distal Tibia Fractures

This article represents a multidisciplinary approach to biomechanics (engineering + medicine) in the field of distal tibia fractures. The objective of the present study was to carry out a strength and reliability assessment of a medial plate for the treatment of distal tibia fractures. This was performed via numerical modelling (FEM) and experimental methods (compression test and Electronic Speckle Pattern Interferometry – ESPI method). The plate is used for internal fixation in orthopaedics and traumatology. Analyses were performed for non-fused bone with comminuted fracture in the distal metaphysis (i.e. unsuccessful treatment, where all loads in the metaphysis are carried by the plate). An anatomical tibia model (based on CT images in Mimics software) was used for FE analysis. In the experiments, the bone was replaced with a shaped piece of spruce wood (based on Mimics software) and the plate was loaded until its failure (major plastic deformation). Numerical and experimental results were evaluated and compared.

About the Verification of External Fixators Applied in Traumatology and Orthopaedics

In this paper, the authors draw attention to biomechanics connected with the possibilities of treatment of complicated bone fractures. They present information about their own design, laboratory tests and numerical solutions (i.e. strength analyses and reliability assessments) of the various types of external fixators applied in traumatology and orthopaedics (i.e. intended for fractures of limbs and pelvis and its acetabulum). The new design of external fixators is based on the development of Ilizarov and other techniques and satisfies new demands of science.

The scalar, vector and tensor fields in theory of elasticity and plasticity

This article is devoted to an analysis of scalar, vector and tensor fields, which occur in the loaded and deformed bodies. The aim of this article is to clarify and simplify the creation of an understandable idea of some elementary concepts and quantities in field theories, such as, for example equiscalar levels, scalar field gradient, Hamilton operator, divergence, rotation and gradient of vector or tensor and others. Applications of those mathematical terms are shown in simple elasticity and plasticity tasks. We hope that content of our article might help technicians to make their studies of necessary mathematical chapters of vector and tensor analysis and field theories easier. Abstrakt Přispěvek je věnovan analýze skalarnich, vektorových a tenzorových poli, ktere vznikaji v zatižených a deformovaných tělesech. Hlavnim cilem přispěvku je usnadnit vytvořeni nazorne představy o některých zakladnich pojmech a velicinach z teorie poli, jako jsou např. ekviskalarni hladiny, gradient skalarniho pole, Hamiltonův operator, divergence, rotace a gradient vektoru a tenzoru aj. Použiti těchto matematických prostředků je ukazano při řeseni zakladnich uloh teorie pružnosti a plasticity. Obsah clanku snad přispěje technikům k usnadněni studia potřebných matematických kapitol z vektorove a tenzorove analýzy a teorie poli.

Application of Selected Multi-Axial Fatigue Criteria on the Results of Proportional Fatigue Experiments

The contribution describes the experimental results obtained from the combined loading of the specimens in the field of high-cycle fatigue. Those specimens were manufactured from common construction steel 11523.1, melt T31052. The following experiments were performed: The first set of the specimen was loaded by the alternating bending amplitude. The second set was loaded by the amplitude of the bending in combination with constant inner overpressure. The results were evaluated by the conjugated strength criterion and another generally used multiaxial fatigue criteria. The stress-strain analysis of the specimens by FEM was performed to determine parameters (constants) of particular strength criteria.