Abstract
The authors have applied X-ray diffraction techniques to the study of low-cycle fatigue in order to discuss its macroscopic behavior from the microscopic point of view. In previous papers, it was reported that the formation and development of subgrain were the main features of microstructural change caused by strain-cycling till its failure. And a good correlation was obtained between the particle size measured by the profile analysis of the X-ray diffraction peaks and the damage fraction in low carbon steel under constant strain-cycling.
Further investigation on microstructural behavior of annealed low-carbon steel under cyclic straining was made by conducting two-step strain-cycling tests, and the following was obtained as the results of this study:
(1) Under a constant strain range, the stress range increased rapidly during the first dozens of cycles and, later, tended gradually to saturate. In the two step strain-cycling, the stress range increased with the increase of strain range and decreased with the decrease of strain range.
(2) During the two-step strain-cycling, the mode of change of integral breadth of X-ray diffraction peak was very similar to that of the stress range. However, a small value of the integral breadth was obtained from the failured specimen.
(3) The abrupt change in the integral breadth consequent on the change of strain range is interpreted by the profile analysis of the diffraction peaks that it was occasioned by the change in the extent of micro strain.
(4) However, little effect was observed of the change of strain range on the particle size.
(5) A fairly straight line was obtained between the stress range and the integral breadth of higher angle diffraction peaks.
(6) The value of integral breadth at failure was generally lower than the value obtained from the above plotting of stress range versus integral breadth. However, the value of integral breadth inside of the failured specimens was higher than that on the surface.
