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 at 375°C prior to the test at room temperature, showed an increase in initial dislocation locking and a decrease in strain aging potential in the preliminary tensile tests.
The relationship between the fatigue limit σw0 and grain size d, σw0=Cw+kwd-1/2, was applied to evaluate the characteristic values Cw and kw. The following results were obtained from the fatigue tests.
(1) Both the characteristic values Cw and kw increased with cyclic prestress at 375°C. The 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 strain aging at 375°C.
(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.
