Abstract
Abstract Fatigue test bars fabricated from a SA508 (low carbon steel plate) were cyclicly deformed to crack initiation (100% cumulative damage, CD) and to the factors, 75, 50, and 25% CD. XRD (X-Ray Diffraction line-broadening) was performed on the cross sections of the bars. Thin foils (∼0.lμm thick) were prepared from each cross section, and used for TEM and SAD (Selected Area Diffraction) study. The half-value line breadth change measured by the XRD increased with the CD up to 50%, beyond which a significant reduction was observed for the 75, and 100% CD sample. By the TEM, the undamaged material (0% CD) was characterized by high angle boundaries, small carbide precipitates, and dislocation cell net works in grains. Micro-orientation changes of dislocation cells studied by SAD of the foils and a statistical data analysis clearly demonstrated that the mean orientation difference in the cells, and its standard deviation increased gradually as the CD increased.
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