Abstract
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∼3×102 cycles).
(2) In one and the same specimens, many microcracks occurred mainly in non-metallic inclusions, lath boundaries (α'B), prior austenite grain boundaries (γB) and bundle boundaries (βB).
(3) The microcracks along α'B originate at the distorted resions like slip bands, but the microcracks along γB 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 γB, and that across the lath boundaries is slower than that along γB.
(5) The microcracks that occurred at γB 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 γB, 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.
