| Plastic Deformation and Grain Boundary Dilation. |
Click image to enlarge (~1MB) |
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The next four pictures come from a room temperature, simple shearing experiment, at a shear strain rate of about 5% per minute. The direction of shearing was horizontal on the screen. The field of view is about 1.6mm wide. |
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| 18. The grainy-looking strip along the bottom of the picture is the edge of one of the frosted grips. A nice strain-free foam texture is displayed again. The grain boundaries are dipping steeply into the screen, except around (30,22) where many appear double because of their gentler dips. |
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| 19. After an increment of shear. The intergranular
voids that have dilated are the black gaps between the grains. The
voids have a preferred orientation related to the sense of shear. Is
the sense of shear sinistral or dextral? (ans)
What is the approximate bulk shear strain of lines parallel to the
grips? The faint "plaid" pattern visible in the blue grain at (30,40), and other blue grains, is caused by two sets of deformation bands at right angles to one another. These indicate the activity of basal and prismatic slip systems. See the evidence for grain boundary sliding between the nearly extinguished grain at (80,30) and the blue grain at (63,30). The boundary between these grains has acted like a transform fault between two ridges. |
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| 20. Same as 19 but at higher magnification. Other examples of grain boundary sliding can be seen here. What might happen to voids like these if they formed in a deforming metamorphic rock? How would such features look in thin section? Might they be unrecognizable? (ans) |
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| 21. High magnification view of image 18. |
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