13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- The observation of fracture surfaces caused by the propagation of a macroscopic tensile mode crack in low humidity, e.g. (F) and (L), was difficult because of the sever contacting at crack tips. However, striations were recognized on the fracture surface of extruded Al alloy 7075-T6 under ultrasonic loading [8], which means that cracks propagated in a ductile manner regardless of humidity. As stated above, fracture mechanism is different depending on humidity when the crack length is smaller than ℓ0, but there is no difference in fracture mechanism when the crack length is larger than ℓ0 in both alloys. These findings on the effect of humidity on fracture mechanism agree well with those on crack propagation rate as shown in Fig. 4. From the results mentioned above, it is concluded that the shear mode crack propagated along slip planes, therefore, cracks in the extruded alloy propagated in shear mode macroscopically because of Figure 6. Etch pit figure observed at fracture surface in drawn Al alloy (a-1) RH25% (a-2) N2 gas (a) Extruded alloy (b-1) RH25% (b-2) N2 gas (b) Drawn alloy Figure 7. Fracture surfaces under rotating bending
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