抄録
Plain specimens made of low carbon steel S20C were tested under tensile fatigue load under the conditions of σmin=5.3kg/mm2 and of σm=22kg/mm2. The stress-strain hysteresis loop in each stress cycle was reproduced on a synchroscope, and its variation was observed throughout the test on each specimen, in which the stress applied to the specimen was picked up from the loop dynamometer, and the strain of the specimen was picked up from the specimen itself, both by using the wire resistance strain gauges. Permanent strain occurring in the specimen was also measured by means of a dial indicator.
The following conclusions were obtained through the discussions of the experimental results.
As a whole, the variation of the plastic strain amplitude Δεp, which was obtained as the half width of hysteresis loop, showed a similar tendency to that obtained previously on low carbon steels under completely reversed axial load. In some of the specimens, however, some amount of fluctuation of Δεp was observed through the test period, but no systematic periodicity nor any regularity could be found. There existed a linear relationship between the plastic strain amplitude Δεp and the number of cycles to failure N, when plotted on a log-log paper. The linear relationship was also found when the total hysteresis energy W was plotted against N on a log-log paper, where W was defined as W=w·N, in which w was an area of the hysteresis loop. The increase of the permanent strain εp coincided in its tendency with those observed in many previous investigations on fatigue deformation under simultaneous actions of static and repetitive loads. However, no correlation between the variation of Δεp and the increase of εp could be found.
