13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- [25]. Figure 1: Mixed-mode fracture test fixture and corresponding arrangement of U-V mirror set 2.2. Evaluation of fracture parameters Typical moiré fringes seen during the test are illustrated in Figure 2. The fringes tend to be closer to each other and move towards the crack tip with the load. Once the load increases, a small dark spot appears around the crack tip on the surface. This means a plastic zone forms and develops as the load increases. The crack mouth open displacement (COD), which is the relative displacement of point A and B in Figure 2, is measured using a v-displacement field moiré fringes based on the principle of moiré interferometry. Special image processing techniques, such as smoothing, filtering, and refining, are used to reproduce the moiré fringe for counting the number of fringes and reproducing the profile of plastic zone. The stress intensity factors KI, KII, Keff are determined based on load, loading angle, and crack length when the fracture test is finished [25]. Load-COD curves and Keff-COD curves then can be drawn. Figure 2: Representative moiré fringe patterns during the test and reproduced images 2.3. Measurement of crack initiation angle From 2D fracture mechanics, the crack initiation angle is usually defined as the included angle between the initiation orientation and the original crack orientation at half-thickness (middle) plane. The included angle is the one between the flat region B and the pre-cracked plane A in Figure 3. It is an important parameter for mixed-mode fracture but is often difficult to measure. To obtain the angle, as a common methodology, the specimen is first cut into two separate parts from the middle plane, and then the angle is measured [19]. However, it does not work well for metallic material and thin samples. The following method is employed to measure the initiation angle. First, a CCD
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