The crack propagation behavior in fretting-fatigue was investigated with type SUS 304L stainless steel. To induce fretting-fatigue the specimen which was pressed by the fretting-pads made of SUS 304 was subjected to the cyclic direct stress. The conditions of the fretting-fatigue tests were the stress amplitude of σa=120MPa, the contact pressures of p=40, 80, and 120MPa, and the stress ratios of R=-0.33, 0, and 0.33. The unfretting-fatigue tests of p=0MPa were also carried out for the same values of σa and R so as to compare with the results of the fretting-fatigue tests.
The fretting-fatigue crack initiated at the site on the specimen which was in contact with the edge of the fretting-pad, and propagated at the early stage to the direction inclined to the normal of the cyclic stress axis. The effect of the contact pressure on the crack propagation rate da/dN in the fretting-fatigue under R=0 appeared in the short crack region to about 1mm in crack length. Although the da/dN in the short crack region showed the tendency to increase with increasing p, the da/dN for p=120MPa was the same as that for p=40MPa in the cause of the contact of the opposite surfaces of the oblique fretting crack. The effect of the stress ratio on the da/dN in the fretting-fatigue was mainly owing to the existence or inexistence of the crack surface contacting. Under R=-0.33, the heavy contacting occured at the oblique crack surfaces so that the crack propagation was retarded to the long crack region compared with the case of unfretting-fatigue. Under R=0.33, however, since the contacting did not occur, the da/dN in the fretting-fatigue always surpassed that in the unfretting-fatigue.