Short cracks at notches and fatigue life prediction under mode I and mode III loadings

摘要

Accurate life predictions of notched components require the computation of the short crack propagation life spent after crack initiation at the microstructure scale. Several experimental studies have shown that the notch stress intensity factor (NSIF) provides a unifying parameter for crack propagation lives at different severe V-notches under axial loading. The analysis can be extended to blunt notches by adding the crack initiation life predicted by the notch stress concentration factor (NSCF). The present work makes a further step to test the NSIF approach under torsional loading. Both axial and torsional fatigue tests were performed on plain cylindrical specimens made of 1045 steel with two different circumferential V-notches. S-N data were analyzed and compared to those previously obtained with smooth tubular specimens. The mode I and mode III NSIFs were computed for both notches by finite element analysis. In all cases investigated, the fatigue behavior is controlled by the material resistance to crack propagation. The short crack propagation lives at V-notches under under axial and torsional loadings are well correlated by the mode I NSIF and the mode III NSIF respectively. The fact that the fatigue notch factor is close to unity whatever the notch root radius is due to a very high resistance to mode III crack propagation.