13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- temperature. It shows that in addition to some individual cases, the loading capacity increases with the specimen thickness. This observation is contrary to common knowledge. Increasing temperature enhances the fracture load capacity for thick specimens, but reduces it for thin specimens. Detailed analysis is given in Ref. [25]. When fracture toughness is examined, Keffi (the stress intensity factor corresponding to the crack initiation load Pi) with loading angle β and specimen thickness b appears at high temperatures, as shown in Figure 5. Effects of thickness existed in Keffi, although regularity is not obvious. These results again confirm the complex coupled effects of thickness and temperature in the mixed-mode fracture of the alloy. 2 3 4 5 6 7 35 40 45 50 55 60 β =0°,300℃ β =0°,500℃ β =27°,300℃ β =27°,500℃ β =53°,300℃ β =53°,500℃ K effi (MPa√m) b (mm) Figure 5: Results of Keffi versus thickness b and loading angles β at high temperatures (reproduced from Ref. [25]) The thickness effect is significantly coupled with the temperature effect in mixed-mode fracture of TC11 alloy, which is important for the development of 3D fracture theory under complex loading and temperature conditions. Why did this happen? Is this the peculiarity of titanium alloy or is this a shared property of metals or alloys? To answer the questions, further studies on fracture mechanism of high-temperature fracture should be conducted. 3.2. Qualitative comparison with finite element data To compare the test results with the numerical solutions, specimens with the same dimensions as in the experiment are modeled using the finite element (FE) software ANSYS. In the 3D simulation, the Ramberg-Osgood constitutive relation [28] is adopted to represent the material nonlinearity. The corresponding parameters are determined through fitting of experimental data. Accordingly, the stress-strain relation is expressed as 38 19.9578 9.9879 10 ( ) 120000 120000 σ σ ε= + × (1) at RT, and 11 6.6095 9.089 10 ( ) 98000 98000 σ σ ε= + × (2) at 500℃. The contours of Mises stress on the front surface at 500℃ are plotted in Figure 6, where the domain bounded by red lines conforms to the visual region of moiré fringe. In Figure 7, moiré fringe of the same specimen from the tests also is presented. In the first row of Figures 6 and 7, the same load is
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