Effect of Static Strengthening on Fatigue Limit of Smooth and Notched Specimens in Low Carbon Steel

The influence of the strengthening mechanism on the non-propagating behavior of fatigue cracks was discussed for low carbon steels which were strengthened by silicon or copper solid solution and copper precipitation. The notched round bar specimens with different stress concentration factors were fatigue-tested under uni-axial loading at stress ratio R=-1 (tension-compression). Fractographic observation of non-propagating cracks at stress levels below the fatigue limit were performed, and the correlation between the stage of non-propagating crack and the strengthening mechanism was investigated. Non-propagating cracks could be classified as Stage I or Stage II cracks, depending on the strengthening mechanism and notch root radius. In the case of smooth specimens, the fatigue limit was governed by the resistance of the microstructure to Stage I crack initiation or Stage II crack initiation. Therefore both strengthening mechanisms which prevent the dislocation movement in the ferrite phase are effective in improving fatigue strength. On the other hand, the fatigue limit for notched round bar specimens depends on the balance of microstructural resistance against to Stage I crack initiation, Stage II crack initiation and Stage II crack propagation. Therefore the effective strengthening mechanism varied with the stress concentra-tion factor.