Microstructurally small fatigue crack initiation behavior of fine and
coarse grain simulated heat-affected zone microstructures in low carbon
steels
Abstract
Successive observation of fatigue crack initiation process was conducted
on six types of thermally simulated HAZ microstructures to clarify the
physical significance of fatigue life difference between coarse grain
(CG) and fine grain (FG) heat-affected zone (HAZ) microstructures. The
results showed the fatigue life-decreasing mechanism in the CGHAZ to be
successfully explained by acceleration of the shear mode fatigue crack
growth rate provided the crack is smaller than the prior austenite grain
size, based on the following experimental results. In other words, the
fatigue cracks initiated along the shear stress plane. In addition, the
prior austenite grain size of crack initiation life with the FG and
CGHAZ were comparable, since the shear mode fatigue crack growth rate of
the CGHAZ was faster than that of the FGHAZ. As a result, the remaining
fatigue life after the grain size crack initiation of the CGHAZ become
shorter than that of the FGHAZ as a function of grain size difference.