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The following is the report of axial fatigue tests carried out in the plastic range on smoothed specimens with uneven cross section of 0.29% carbon steel. In the completely reversed and pulsating straining tests and the completely reversed stressing tests, we made investigations of the fatigue lives and the variations of stress-strain hysteresis curves during the fatigue tests. Measuring the diametral changes on the minimum cross sectional portion of the specimens, we dealt with the results of the experiment in true stress and true strain.Since the smoothed specimens with uneven cross section did not present buckling even under high compression strain, it was possible to make tests over the wide range of strain amplitude.For the sake of comparison completely reversed straining tests were made on smoothed specimens with uniform cross section.The main results of the experiment are as follows.(1) The relations between the true strain amplitude and the number of cycles to fracture are expressed in the following formulas.(a) For the completely reversed and pulsating straining tests on smoothed specimens with (b) For the completely reversed straining tests on smoothed specimens with uniform cross (2) The fatigue life of smoothed specimens with uneven cross section is longer than that with uniform cross section in the completely reversed straining tests. The larger the strain amplitude is, the greater the difference of the lives is.(3) Two types of fractures were observed in the completely reversed stressing tests, contracted fracture in tests with the fatigue life of less than about 100 cycles and fatigue fracture in tests with the fatigue life of above as many cycles.

ABSTRACT

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The fatigue tests of a low carbon steel were made in rotatory bending at room temperature. Both annealed and strain-aged specimens were prepared to vary the effectiveness of initial dislocation locking and of strain aging for fatigue limit. The latter specimen, subjected to a cyclic stress a decrease in strain aging potential in the preliminary tensile tests.to evaluate the characteristic values Cw and kw. The following results were obtained from the fatigue tests.increases in Cw and kw were about 7% and 65%, respectively.(2) The increase in Cw may be interpreted as a result of the increased frictional stress due to cyclic strain hardening by prestress and precipitation hardening by precipitates formed during cyclic (3) When kw is regarded as a measure of barrier at grain boundary against crack-propagation of a micro-crack initiated in a grain, the increase in kw may be attributed to the increase in initial dislocation locking rather than the effect of relocking of dislocation by cyclic strain aging.It is concluded that the initial locking of dislocations by solute atoms makes an important contribution to the existence of a fatigue limit at room temperature.