13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- At a far field strain of 0.14, a twin was resolved which is circled in Figure 1. The twin was identified as an extension twin, which forms along the {1012} plane in magnesium. The misorientation profile in Figure 1 displays a rotation of approximately 86˚ about the <1120> direction (a-axis) with respect to the parent grain. This geometrical relationship is characteristic of the extension twin. The twin became a site for crack propagation at a far field strain of 0.246. It has been established that extension twins form at lower stresses to accommodate extension along the <0001> direction (c-axis). However, once nucleated the twins become stress concentrators and eventually become failure sites. Figure 2 shows the SEM image series of a row of holes perpendicular to the tensile axis with a center to center hole distance of 70 µm. EBSD patterns were overlaid in the increments where they were obtained. Figure 2: SEM image series at various far field strains and misorientation profile across the twin circled. A twin was resolved by EBSD at a far field strain of 0.088 which is circled in Figure 2. The twin was characterized as a compression twin, which forms along the {1011} plane in magnesium. The misorientation profile in Figure 2 reveals a rotation which is approximately 56˚ about the a-axis with respect to the parent grain. This is characteristic of the compression twin which accommodates compression along the c-axis. The twin boundary failed at a far field strain of 0.168. It has been established that this twin type requires high stresses to activate. The twin nucleated at a relatively low far field strain. This suggests that a complex stress state occurs within the material. Some grains experience much higher stresses than others due to the mechanical anisotropy of the material. Figures 1 and 2 both show failure at grain boundaries. Grain boundary failure is another dominant fracture mechanism. The effects of void fraction on failure were observed by performing tests on void rows with different center to center holes distances. The center to center hole distance can be related to the void fraction f by λ r f 2 = (1) where r is the radius of the hole and λ is the center to center spacing. The normalized length of the
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