13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Fig. 3. SN curves of A and B specimens. Fig. 4. Dependence of fatigue strength on RH. then a low-stress-level plateau above 75% RH. It is obvious that these different fatigue behaviors cannot be explained only by the effect of humidity. 3.2 Crack initiation and growth SEM observations conducted in previous studies [3-7] showed different morphologies of fatigue cracks in extruded 7075 T6 Al alloy, which are depicted schematically in Fig. 5 (a)~(d). These morphologies are macroscopically classified into (a) tensile-mode-initiation followed by tensile-mode-propagation (TT-type), (b) tensile-mode-initiation and shear-mode-propagation (TS-type), (c) shear-mode-initiation and shear-mode-propagation (SS-type), and (d) multiple-shear-mode-initiation followed by coalescence of cracks leading to apparent tensile fracture (MST-type). Typical examples of fracture surfaces are also shown in Fig. 5. In A specimens RB-tested at 50 Hz, crack initiation takes place in tensile manner and their macroscopic fracture mode becomes TT-type in all of the humid conditions and TS-type in N2 gas. On the other hand, the application of US loading to A specimens changes the crack initiation from tensile mode to shear mode, except at low humidity. Hence US-tested A specimens show TS-type fracture at low humidity and SS-type or MST-type fracture for other conditions. This tendency appears in B specimens RB-tested at 50 Hz, too. The common feature to US-tested A specimens and RB-tested B specimen is that MST-type fracture is induced at 85% RH and in water at low stress level. This fact suggests that many cracks are initiated at the surface exposed to such high humidity for longer time. Both of the initiation and propagation of S-type cracks take place at an angle of about 35o inclined from the loading axis in both of the RB and US tests. In particular, the morphologies of initial cracks are important, since they may be indicative of the slip prior to the initiation as well as the slip associated with the initiation. Fig. 6 shows typical examples of T-type and S-Type short cracks observed on the surfaces of B specimens RB-tested at 25% RH and 85 % RH, respectively. The T-type crack exhibits some irregularities in propagation path, some of which are inclined by ±35o from the loading axis, but it develops in a direction perpendicular to the loading axis. On the other hand, the S-type crack develops in a shape of "V" of which edges are symmetrically inclined by ±35o from the loading axis. The symmetry of crack initiation with a definite geometry, which can be microscopically observed in the zigzag path of T-type crack, indicates that slip leading to the crack initiation may take place 0 50 100 150 200 250 300 350 105 106 107 A nitrogen A RH25% A RH50% A RH85% A water B nitrogen B RH25% B RH50% B RH85% B water Stress amplitude σa (MPa) Number of cycles to failure Nf Rotating bend test (50Hz) 0 50 100 150 200 250 300 0 20 40 60 80 100 A, RB(50Hz) A, US(20kHz) B, RB(50Hz) B, RB(6Hz) σao (MPa) Relative humidity RH (%) Nf = 10 7
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