Radim Halama

Phenomenological Modelling of Cyclic Plasticity

The stress-strain behaviour of metals under a cyclic loading is very miscellaneous and needs an individual approach for different metallic materials. There are many different models that have been developed for the case of cyclic plasticity. This chapter will address only so called phenomenological models, which are based purely on the observed behaviour of materials. The second section of this chapter describes the main experimental observations of cyclic plasticity for metals. Material models development for the correct description of particular phenomenon of cyclic plasticity is complicated by such effects as cyclic hardening/softening and cyclic creep (also called ratcheting). Effect of cyclic hardening/softening corresponds to hardening or softening of material response, more accurately to decreasing/increasing resistance to deformation of material subjected to cyclic loading. Some materials show very strong cyclic softening/hardening (stainless steels, copper, etc.), others less pronounced (medium carbon steels). The material can show cyclic hardening/softening behaviour during force controlled or strain controlled loading. On the contrary, the cyclic creep phenomenon can arise only under force controlled loading. The cyclic creep can be defined as accumulation of any plastic strain component with increasing number of cycles and can influence the fatigue life of mechanical parts due to the exhaustion of plastic ability of material earlier than the initiation of fatigue crack caused by low-cycle fatigue is started.

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.

Implementation of MAKOC cyclic plasticity model with memory

Abstract This paper deals with the description of implementation of the advanced cyclic plasticity model called MAKOC, which is based on the AbdelKarim–Ohno kinematic hardening rule, the isotropic hardening rule of Calloch and a memory surface introduced in a stress space in accordance with the Jiang-Sehitoglu concept. The capabilities of the MAKOC model are compared with the Chaboche model included in some FE codes. Cyclic plasticity models commonly included in commercial FE software cannot accurately describe the behavior of the material, especially in the case of additional hardening caused by non-proportional loading of the material. This fact is presented on the experimental data set of aluminum alloy 2124T851. Steady state material behavior is studied with regard to the subsequent application in computational fatigue analysis. The cyclic plasticity model developed was implemented into the FE code ANSYS 15.0 using Fortran subroutines for 1D, 2D as well as 3D elements. The integration scheme is described in detail including the method of implementing the model and determining an error map for the proposed MAKOC and Chaboche models. The numerical tangent modulus is proposed to ensure parabolic convergence of the Newton-Raphson method for the MAKOC model. An axisymmetric analysis of 3D Hertz problem was performed to show convergence in the local as well as global iterations.

Numerical modeling of steel fillet welded joint

Abstract The paper is focused on the numerical modeling of steel bearing elements and their verification using experiment. Currently, for the stress-strain analysis of the elements supporting structures it is possible to use many commercial software systems, based on the finite element method - FEM. It is important to to check and compare the results of FEM analysis with the results of physical verification test, in which the real behavior of the bearing element can be observed. The results of the comparison can be used for calibration of the computational model.The article deals with the physical test of steel supporting elements, whose main purpose is obtaining of material, geometry and strength characteristics of the fillet welds. The main aim was defining of tested samples numerical models for using FEM analysis and for the commercial software ANSYS. The pressure test was performed during the experiment, wherein the total load value and the corresponding deformation of the specimens under the load was monitored. The measurements were carried out for a more detailed analysis of stresses and deformations in welds samples using a strain-gauge and a Q100 laser device for measuring the 3D deformation and infrared thermographic non-destructive testing.Obtained data were used for the calibration of numerical models of test samples and they are necessary for further strain analysis of steel supporting elements.

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.

Influence of Induction Hardening on Wear Resistance in Case of Rolling Contact

Abstract The main aim of the presented paper is to show how heat treatment, in our case the induction hardening, will affect the wear rates as well as the ratcheting evolution process beneath the contact surface in the field of line rolling contact. Used wear model is based on shear band cracking mechanism [1] and non-linear kinematic and isotropic hardening rule of Chaboche and Lemaitre. The entire numerical simulations have been realized in the C# programming language. Results from numerical simulations are subsequently compared with experimental data.

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.

Mechanics of Herbert Pendulum Hardness Tester and its Application

The knowledge of classical mechanics gives deeper insight into the Herbert hardness tester applicability for material testing. Elastic materials with low friction presence between contact surfaces are supposed to be investigated in this study. Firstly the dynamics approach is used to obtain simplified solution of swing angle. Then a new solution of the problem is gained by means of an energy approach. Slight decrease of the swing angle is predicted by the new solution as also shown in experiments. After comparison of both solutions a new formulae useful for evaluation of rolling resistance coefficient is applied for measurements performed on some metallic materials and artificial sapphire. Rolling resistance coefficients obtained by the way are always less than maximal estimated value.

Identification of Fatigue Constants by Means of 3D Method

Abstract The conventional method for evaluation of the fatigue constants uses one set of experimental data from strain-controlled uniaxial fatigue tests. However, these constants do not ensure the compatibility conditions. The new 3D method retains the mathematical and physical relationships between curves considered. This paper presents a way of implementation of the identification procedure and shows results obtained for three types of materials.