13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- camera is used to take photographs of the profile of the fractured specimens, as shown in Figure 3 (c). Subsequently, the profile photographs are processed as computer image files. The most important step in this procedure is accurately determining and drawing the tangent line at the beginning of the crack growth path at the middle thickness and the pre-crack line. Finally, the initiation angle is obtained by measuring the angle between the two lines. Based on the above mentioned scheme, the initiation angle of the specimen in Figure 3 is 27.7°. Using the same method, the included angle between the pre-cracked plane and the pre-notched plane (e.g., 5.5° in Figure 3(c)), which is the machining error and which should be distinguished from the initiation angle, also can be recorded. Figure 3: Representative macro pictures of the fracture surface with characteristic regions. A is the fatigue pre-crack, B is the flat fracture region, and C is the shear lip. 3. Fracture test on TC11 titanium alloy material Mixed-mode fracture experiments are conducted on TC11 titanium alloy specimens with varied thicknesses at RT and elevated temperature. The experiments are conducted using the newly developed testing technology, which is based on high-temperature moiré interferometry. Some results are very unusual and remarkable. 3.1. Experimental results at elevated temperature [25] 2 3 4 5 6 7 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 Pmax/b (kN/mm) b (mm) β =0°,300℃ β =0°,500℃ β =27°,300℃ β =27°,500℃ β =53°,300℃ β =53°,500℃ Figure 4: Load capacity versus thickness b and loading angles β at high temperatures (reproduced from Ref. [25]) Figure 4 illustrates the average results for the specimens in the same thickness group at an elevated
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