Abstract
The elementary process of microstructural fatigue crack growth in single-packet structures with different habit-plane orientations to the loading axis was examined using miniature compact-tension specimens to comprehend the intrinsic resistance to fatigue crack growth in lath martensite of low carbon steel. When the angle between the loading axis and the normal to the habit plane (φ) was within ~45°, the cracks propagated nearly parallel to the habit plane. In packets with their habit planes nearly perpendicular (φ > ~70°) to the notch direction, the crack grew across the laths because of damage accumulation through the activation of the out-of-plane slips, which increased the fatigue crack growth resistance. Post-fatigue-test electron microscopy revealed that the microstructural inhomogeneity of the low carbon steel enhanced the strain localisation in coarse laths, which probably led to the premature fatigue crack growth. When the crack grew perpendicular to the habit plane, the intrinsic resistance to fatigue crack growth was higher in the fine-lath region than in the coarse-lath region.
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