Rotating bending fatigue tests in a high cycle region have been performed on smooth cylindrical specimens of low-carbon steel (S15C) and austenitic stainless steel (SUS 304) which have different slip characteristics. Many surface cracks were successively observed, and their states of initiation and distribution as well as their behaviors of coalescence and propagation were investigated.
The main results obtained are summarized as follows;
(1) More surface cracks initiated as the stress amplitude became higher, and they grew individually with coalescence in each other and propagated to failure. The propagation of cracks by coalescences was remarkable in S15C, but rare in SUS 304.
(2) In the moment of coalescence, the crack grew at a jump, and for a while its propagation rate reduced transiently after that. However, the rate approached that of the previous independent crack as progression of fatigue.
(3) The limit condition of coalescence was expressed as h=C1l1(l1≤l1cr), h=hcr(l1>l1cr), where, l1 is the longer crack length in the adjacent two cracks and h the spacing between them, and the values of l1cr, hcr and C1 were 1.5mm, 260∼300μm and 0.18, respectively, for S15C.
(4) The average aspect ratio of depth (b) to length (l), (b/l) of surface cracks were 0.36 and 0.33 in S15C (for l≤5.0mm) and SUS 304 (for l≤10.0mm), respectively.
(5) The relation between the propagation rate of main cracks and the stress intensity factor parameter was shown as dl/dN=C(σa√πl)m.
