Application of the ΔK, ΔJ and ΔCTOD parameters in fatigue crack growth modelling

U radu su izloženi modeli rasta zamornih pukotina, temeljeni na parametrima mehanike loma ΔK, ΔJ-integral i ΔCTOD koji su utvrđeni koristeći linearno elastičnu i elasto-plastičnu analizu metodom konačnih elemenata. Prag napredovanja pukotine, koji predstavlja graničnu vrijednost relevantnog parametra mehanike loma ispod koje ne dolazi do napredovanja pukotine, uzet je u obzir u modelima. Proveden je test zamora s konstantnom amplitudom cikličkog rasteznog opterećenja za uzorak ploče sa središnjom pukotinom. Test je proveden za uzorak od mekog konstrukcijskog čelika u laboratoriju na sobnoj temperaturi s frekvencijom 5 Hz. Za manje duljine pukotine vrijednosti izračunate u linearno elastičnoj analizi ΔJle poklapaju se s ΔJep vrijednostima izračunatim u elasto-plastičnoj analizi. Za veće duljine pukotina parametar ΔJep raste brže od linearno elastičnog parametra ΔJle, što pokazuje da je pretpostavka linearno elastične mehanike loma o malim plastičnim deformacijama u ligamentu zadovoljena samo za manje duljine pukotine. Modeli temeljeni na elasto-plastičnim parametrima ΔJep i ΔCTOD daju bolje slaganje simuliranog životnog vijeka s eksperimentalnim rezultatima nego linearno elastični modeli temeljeni na parametrima ΔK i ΔJle.This paper presents the developed fatigue crack growth models based on parameters ΔK, ΔJ-integral and ΔCTOD determined by using linear elastic and elastic plastic FE analysis. The fatigue crack growth threshold values of the considered crack growth driving force parameters, below which crack propagation does not occur, were taken into account. Fatigue crack growth tests with constant load amplitude were carried out on a centrally cracked plate specimen until collapse occurred. The tests were performed on mild steel specimens at room temperature in laboratory air, at a loading frequency of 5 Hz. For the analyzed specimen, the calculated ΔJ-integral range values obtained by linear elastic analysis, ΔJle, and the elastic-plastic values, ΔJep, were close at lower crack growth rates. As the crack length increases the ΔJep values became higher as compared with the linear elastic values, ΔJle, This indicates that small scale yielding conditions prevailed only for smaller crack lengths. The models based on the EPFM parameters ΔJep and ΔCTOD provide better agreement with experimental results for higher crack growth rates, compared with the models based on the LEFM parameters ΔK and ΔJle

Interaction of oxygen with the stable Ti5Si3 surface

The atomic structure and surface energies of several low-index surfaces (0001), (1100) and (1120) of Ti5Si3 in dependence on their termination were calculated by the projector augmentedwave method within the density functional theory. It was revealed that the mixed TiSi-terminated (0001) surface is stable within the wide range of change in the Ti chemical potential. However, the Ti-terminated Ti5Si3(0001) surface is slightly lower in energy in the Ti-rich limit. The oxygen adsorption on the stable Ti5Si3(0001) surface with TiSi termination was also studied. It was shown that the three-fold coordinated F1 position in the center of the triangle formed by surface titanium atoms is the most preferred for oxygen adsorption on the surface. The appearance of silicon as neighbors of oxygen in other considered F-positions leads to a decrease in the adsorption energy. The factors responsible for the increase/decrease in the oxygen adsorption energy in the considered positions on the titanium silicide surface are discussed