抄録
Notched specimens of annealed 0.03%C steel were fatigued under completely reversed in-plane bending. The topography on the fatigue fractured surface was examined by electron and optical microscopys, and the change in topography with the crack propagation rate was investigated. The range of the propagation rate obtained in this study was 10-8∼5×10-3mm/cycle. The results are summarized as follows:
(1) The three features, the facets of grain boundary fracture, the imbricate pattern and the parallel pattern, were observed on the fractured surface when the propagation rate was lower than 10-5mm/cycle.
(2) Striations were observed on the surface fractured with the rate higher than 2×10-5mm/cycle. The spacing between striations was equal to the propagation rate within the range of 10-4∼2×10-3mm/cycle. The spacing was larger than the rate at dl/dN=2×10-5∼10-4mm/cycle, while it was smaller at dl/dN>2×10-3mm/cycle.
(3) Cleavage facets were observed on the fractured surface when the propagation rate was higher than 3×10-4mm/cycle. The summed area of cleavage facets on the fractured surface increased with the propagation rate to about 10% of the total area of the fractured surface at dl/dN=5×10-3mm/cycle.
(4) The tip profile of fatigue cracks that ran within the specimen in accordance with its thickness bent in convexwise in the direction of growth when the propagation rate was higher, while it bent in concavewise when the rate was lower. The change in the tip profile was explained by the difference in growth mechanism with the propagation rate.
