13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Fig. 5. Macroscopic features of fatigue fracture. preferentially in this direction. One can see that the crack propagation in macroscopic scale shows the same tendency in certain circumstances. Thus, the evolution of plastic zone in front of crack tip may also have orientation-dependence intrinsic to the microstructure. Fig. 7 shows typical relations of crack length (l) and number of cycles (N) in B specimens RB-tested at 50 Hz. The values of crack length were measured in transverse direction. Thus the length of a pure S-type crack developing with an angle of 35o, for example, becomes l/sin35o = 1.74l. As shown in Fig. 5, there are some mixtures of T- and S-type crack propagations so that the real length of crack along its path is always larger than the measured value of l in such mixtures. Reminding this effect, one can see from Fig. 6 that the growth of S-type cracks formed at 85% RH occurs much faster than that of T-type cracks at 25% RH. It is also recognized that the initiation of S-type cracks at such high humidity is induced at very small cycles compared with the initiation of T-type cracks at low humidity. On the other hand, A specimens tested in the same test condition show T-type crack initiation irrespectively of environment, while its sensitivity to humid environment appears from low humidity. US tests with 20 kHz enables A specimens to exhibit the S-type cracking so that the crack growth as a function of N becomes much larger than RB-tested A specimens. This result suggests that the formation and propagation of cracks may be rate-controlled. This was also demonstrated in B specimen; the reduction of frequency from 50 Hz to 6 Hz in RB tests transformed the fracture mode from S-type to T-type [7]. A specimen Rotating bend test (50 Hz) B specimen Rotating bend test (50 Hz) in water (225 MPa) in N2 (275 MPa) 25%RH (249 MPa) 65%RH (230 MPa) 85%RH (221 MPa) A specimen Ultrasonic fatigue test (20 kHz) ~35o ~35 o ~35o (a) TT-type (b) TS-type (c) SS-type (d) MST-type 25%RH (280 MPa) 85%RH (280 MPa) 85%RH (200 MPa)
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