13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- During the hold period the strain measured had some noise present in the ~730 data points used for each cycle. This may be due to the thermal effect on the air between the camera and the sample. Nevertheless, a small positive slope was the trend for both cycles. Therefore a crack tip creep rate for this 120 second period was determined. During cycle 9 the creep rate was 1x10-6 and it was 5x10-7 during the hold period of cycle 62. This at least indicates that a crack tip creep rate can be measured on this material at 650ºC for a 120 second hold period using digital image correlation. Also, the results showed that the magnitude of the strain is higher for cycle 9 than cycle 62. It is not clear what has produced this result. It may be that the images were taken with a resolution too low to accurately see the crack tip. Therefore, when the local selection was made to extract the data from the crack tip, the exact location may have differed between the two cycles. 3.2. Crack Opening Displacement (COD) COD measurements were also measured for cycles 9 and 62. These measurements were made relative to the unloaded reference image. A linear measurement of COD was performed across a small region within the selected area shown in Figure 4. Figure 4. The crack opening displacement measurement was made along a linear region shown in the red rectangular box. COD data (in pixels) was then extracted at a location across the EDM notch (i.e. the far left point in the red rectangle shown in Figure 4). The COD data was taken from the beginning (the first two seconds), the middle (seconds 60 and 61), and the end (seconds 119 and 120). The twelve data points at each location were then averaged. The results are shown in Figure 5.
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