This investigation has been undertaken to determine the influence of microstructural factors (non-metallic inclusions, prior austenite grain boundaries, laths of martensite, bundle boundaries) on the characteristics of low-cycle fatigue microcracks (initiation sources, initiation period, propagating paths, propagation rate) by conducting optical microscopic observations of the surface of S 38 C smooth specimens tempered at 200°C. The changes in dislocation density during the low-cycle fatigue test have also been studied. The fatigue tests were carried out at 20°C under constant total strain range of ±1.3% by alternating bending. The results obtained are as follows:
(1) There were found early fatigue microcracks more than 10μ at the stage of about 10% of the total life (2∼3×10
2 cycles).
(2) In one and the same specimens, many microcracks occurred mainly in non-metallic inclusions, lath boundaries (α'
B), prior austenite grain boundaries (γ
B) and bundle boundaries (β
B).
(3) The microcracks along α'
B originate at the distorted resions like slip bands, but the microcracks along γ
B seem to have originated in the manner of brittle fracture in grain boundaries.
(4) The propagation rate (other than the joining rate) of microcracks along α'
B is almost equal to that along γ
B, and that across the lath boundaries is slower than that along γ
B.
(5) The microcracks that occurred at γ
B tend to grow into larger cracks than what occurred at α'
B, which often remains non-propagating.
(6) In the extended cracks (by growing and joining), the paths along α'
B are almost equal in length to that along γ
B, and the additional length of their paths occupies about 80-90% of the whole crack length.
(7) During the low-cycles fatigue, dislocation dencity decreases very rapidly in a few cycles, and after that, decreases slowly as the cycles increase in number, and this changing mode is inverse to the increasing distorsion resions.
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