X-Ray Investigation of Low-Cycle Fatigue in Annealed Low Carbon Steel
On the Microstructure Difference between the Surface and Inside of the Specimen Caused by Strain-Cycling
1969 Volume 18 Issue 195 Pages 1093-1099
Details
1969 Volume 18 Issue 195 Pages 1093-1099
From several reports of scientific experiments that have been recorded in the literature of science it appears that the practice of removing layer after layer of the specimen surface during the low-cycle fatigue testing has an effect of lengthening the fatigue life, and it is surmised as possible that this phenomenon is due to some microstructural difference that is created by cyclic straining between the surface and the inside of the specimen. And detecting of this structural difference would provide more resources to the study on low-cycle fatigue behavior. From this point of view, the authors applied the X-ray diffraction techniques to observation of the microstructural change in the surface and the inside of the specimen during constant strain-cycling.
The following facts have been revealed as the result of this investigation:
(1) It is revealed by the X-ray microbeam diffraction technique that there is subgrain formation and development not only on the surface layer but also inside the specimen in the process of low-cycle fatigue before visible cracks occur.
(2) The period of the low-cycle fatigue process from distribution of the integral breadth of its X-ray diffraction peaks from the surface in the direction of depth to the occurrence of visible cracks is divided into two stages. The first stage marks larger increase, with increase in the number of strain cycling, in the integral breadth on th surface of the specimen than inside. During the second stage the integral breadth tends to decrease on the surface of the specimen, but keeps increasing inside. The integral breadth shows smaller value on the surface than inside around the failure point.
(3) Microscopic cracks are found to occur toward the end of the first stage, and develop in the direction of depth from the surface during the second stage.
(4) The average value of integral breadth obtained from inside the specimen has a linear relation with the stress range during the low-cycle fatigue before fracture.
