To investigate the effect of cyclic strain aging on the fatigue life distribution of an annealed low carbon steel, rotating beam fatigue tests were made in the temperature range from room temperature to 450°C. The results obtained were analyzed by means of the Weibull distribution with three parameters.
It was found that the following Weibull distribution of mixed type was well fitted to the fatigue life distribution, as the cyclic strain aging plays an important role on fatigue behavior of the carbon steel.
F(N)=P1F1(N)+P2F2(N), P1+P2=1,
where F1(N) is the distribution which is independent of the cyclic strain aging and has an increasing failure rate. F2(N) is the distribution which is concerned with the cyclic strain aging and has a decreasing failure rate. P1 and P2 are the probabilities of occurrence of F1(N) and F2(N), respectively. The shape parameters of F1(N) and F2(N) were about 2.63 and 0. 86, respectively.
At room temperature, the distribution at higher stress levels (i.e. in the finite life region) is expressed by only F1(N), because the strain aging cannot be expected, while the distribution at lower stress level (i.e. around the endurance limit) must be expressed by the mixed type, because the cyclic strain aging becomes effective during the cyclic stressing over a long period.
At 375°C, since the cyclic strain aging occurs so rapidly that the multiplicated dislocations are relocked at each cycle, the distribution in all the stress levels is expressed by only F2(N), and P1 is negligible.
