Propagation of Long Fatigue Cracks under Remote Modes II and III in Ferritic-Pearlitic Steel

The aim of this research was to determine the 3D crack path and the related growth micromechanism in specimens made of the ferritic-pearlitic steel loaded in modes II and III. Crack closure effects were minimized at the beginning of the experiments by a special technique of generation of precracks in order to measure effective values of fatigue thresholds. Dependences of the crack growth rate on the stress intensity factor range were plotted for the near-threshold region. The effective mode III threshold $\DeltaK_text{IIIeff,th}=4.4 MPam^{1//2}$ was found to be higher than that for mode II ($\DeltaK_\text{IIeff,th}=2.9 MPam^{1//2}$, the theory predicts $3.1 MPam^{1//2}$) which is in agreement with the results for other metallic materials and available theoretical models. Fractographical observations in three dimensions revealed that the remote mode II cracks deflected towards the opening mode I by the mean angle ≈50°, which is significantly different from the low-angle deflections (coplanar growth) previously observed in the ARMCO iron. The simple linear elastic fracture mechanics based criterion for growth of a mode I branch at the crack front loaded in mode II was fulfilled.