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

Abstract

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 eects were minimized at the beginning of the experiments by a special technique of generation of precracks in order to measure eective values of fatigue thresholds. Dependences of the crack growth rate on the stress intensity factor range were plotted for the near-threshold region. The eective mode III threshold ∆K IIIeff,th = 4.4 MPa m 1/2 was found to be higher than that for mode II (∆K IIeff,th = 2.9 MPa m 1/2 , the theory predicts 3.1 MPa m 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 deected towards the opening mode I by the mean angle ≈ 50• , which is signicantly dierent from the low-angle deections (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 fullled.

Influence of Secondary Phase on Intrinsic Threshold and Path of Shear-Mode Fatigue Cracks in Metals

“…Unlike in the case of mode I effective thresholds, the secondary phase can significantly change the mode II and mode III thresholds due to a change of the growth mechanism. Indeed, the previously published results [4] showed that the intrinsic behaviour of the ferritic-pearlitic steel was very different from that of the pure ferritic steel. Here, the influence of secondary phase type is studied also for the fully pearlitic steel and the Ti6Al4V titanium alloy in more detail.…”