Michal Šofer

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.

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.

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.

A Semi-Analytical Prediction of Wear Rate of an Inductively Hardened Wheel Steel under Rolling Contact

Presented paper deals with ratcheting prediction in the field of line rolling contact. The main aim is to simulate the wear process due to the contact fatigue on inductively hardened sample of R7T material. For the stated purposes, the authors have used the wear model based on shear band cracking mechanism and non-linear kinematic and isotropic hardening rule of Chaboche and Lemaitre. Obtained results are subsequently compared with experimental data as well as with metallographic analysis.