13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- The Correlated Solutions Vic-2D digital image correlation system was used to capture images and analyze crack tip displacements. Images were captured at a rate of ~6 images per second using a 5MP Point Grey camera. Images were captured for only a few cycles at a time collecting ~730 images per cycle. 3. Results and Discussion 3.1. Crack tip strain The displacement measurements and strain calculations presented herein were performed for cycles number 9 and number 62 during the creep-fatigue test. The measurements were made with respect to an unloaded reference image. The strain measurements were made on a small region around the crack and crack tip as is shown in Figure 2. Figure 2. Representative image of the digital image correlation strain calculation. Inside the red circle is the region around the crack tip that was used for the strain measurements. The crack tip strain was measured for the 120 second hold period of cycle 9 and cycle 62 of the creep-fatigue test using the region contained by the red circle in Figure 2. The strain was calculated for the y-direction using the Lagrange tensor, and the results for cycles 9 and 62 are shown in Figure 3. Figure 3. The crack tip strain during the creep-fatigue hold period for cycles 9 and 62.
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