Fracture behaviour at the sharp notch tip of high strength rail steels - Influence of stress triaxiality

This work explores the possibility of using mechanical properties of two types of notch-free specimens to evaluate crack growth resistance of a sharp notch tip. Equivalent plastic fracture strain (εeqf,ηav) of pre-cracked single-edged-notched bending (SENB) specimen is estimated using a model that correlates εeqf,ηav for notch-free specimens with their average stress triaxiality. Finite element modelling is used to establish constitutive equations and variations of stress triaxiality in the two types of notch-free specimens and the pre-cracked SENB specimen. All of the information is then used to predict the critical strain energy density factor (Sc) at a characteristic distance ahead of the sharp notch tip and the corresponding stress intensity factor (KSc) for three high strength rail steels. The predicted KSc values are compared with the experimentally measured mode I critical stress intensity factor (KIc) at temperatures 23, −10, and −40 °C. The results show good agreement between KSc and KIc in their variation with temperature, possibly because Sc takes into account both distortional and dilatational strain energy density of the sharp notch tip under small-scale yielding. Results from the study also suggest that a correlation should exist among mechanical properties for notch-free and notched specimens, though their apparent trend of change with temperature may not bear any similarity.