Gorazd Fajdiga

Computational fatigue analysis of contacting mechanical elements

Racunarska analiza zamora mehanickih elemenata u dodiru Izvorni znanstveni clanak U radu se ispituje utjecaj razlicitih parametara na raspodjelu naprezanja u dodirnoj povrsini mehanickih elemenata. Cilj je rada odrediti konkretna mjesta u dodirnim tijelima gdje bi se trebale pojaviti male pukotine zbog velikih naprezanja nastalih razlicitim uvjetima dodira. Dvodimenzionalni ekvivalentni kontaktni model zajedno s Hertzovom teorijom dodira upotrebljen je za sve racunarske analize primjenom metode konacnih elemenata. Raspodjela maksimalnog ekvivalentnog von Mises naprezanja na dodirnoj povrsini najprije je određena za prave Hertzove uvjete dodira, gdje se maksimalna naprezanja javljaju na određenoj dubini ispod povrsine dodira. Tada je parametrijskom analizom određen utjecaj opterecenja, zakrivljenosti povrsine dodira, dodirnog trenja, zaostalih naprezanja i elasto-hidro-dinamickog podmazivanja na raspodjelu naprezanja. Sve to nastoji povecati naprezanja i maksimalno se razviti uz ili vrlo blizu dodirne povrsine, sto može dovesti do iniciranja pukotine ispod povrsine ili na njoj. U ovom se radu predstavlja nastajanje pukotine na povrsini i njezino sirenje na bocnoj povrsini zubi zupcanika. Metoda Virtual crack extension (VCE) (virtualno sirenje pukotine), implementirana u metodi konacnih elemenata, primijenjena je za simuliranje sirenja pukotina zbog zamora materijala od zacetka pukotine do stvaranja supljina na povrsini. Usporedba rezultata numericke simulacije i rezultata eksperimentalnog testiranja pokazuje dobru korelaciju. Kljucne rijeci: nastanak pukotine; numericka analiza; problem dodira; strojni elementi; sirenje pukotine; tribologija;

Modelling of Rolling Contact Fatigue of Rails

Rail dark spot defect, also termed as squat failure or shelling, is a rolling contact fatigue failure which occurs frequently on the high speed traffic railway rails. The main goal of this study is to develop a computational model for simulation of the squat phenomena on rails in rail-wheel contact. The proposed computational model consists of two parts: (i) Contact Fatigue Crack Initiation (CFCI) and (ii) Contact Fatigue Crack Propagation (CFCP). The results of proposed unified model enable a computational prediction of a probable number of loading cycles that a wheel-rail system can sustain before development of the initial crack in the rail, as well as the number of loading cycles required for a crack to propagate from initial to critical length, when the final fatigue failure (squat) can be expected to occur.

Computational Analysis Of Stress Field InContact Area Of Gear Flanks Influenced ByEHD-lubrication

The influence of elastohydrodynamic (EHD)-lubrication on stress field in contact area of rolling and sliding mechanical elements is presented in this paper. Computational analysis is performed using the equivalent model of two contacting cylinders that have the same radii as are the curvature radii of contacting mechanical elements at any point of a real contact. The equivalent cylinders are subjected to normal and tangential tractions that are determined with the Hertz contact theory with consideration of additional external contact forces that arise from the EHD-lubrication conditions. The finite element method is used for numerical computations of the stress fields for different lubrication conditions. The results from such computations provide the basis for the influence determination of typical lubrication parameters, like viscosity of lubricant and mean surface velocity of contacting cylinders, on the stress field in contact area.