13th International Conference on Fracture June 16–21, 2013, Beijing, China 4 The morphologies of fracture surfaces were examined by a scanning electron microscope (SEM, HITACHI S-4700) and an optical microscope. 3. Results and discussion 3.1 Shear load versus cross-head displacement curves Fig. 3 displays the shear load versus cross-head displacement curves obtained at 1173K in air. In the initial loading stage, nonlinearity exists for all curves. Except the curve of 2.5D-C/SiC along the warp direction, the slopes of curves keep increasing until to peak load followed by a sudden load drop when the specimens failed, although the test was conducted under a constant rate of 0.595 mm/min. It suggests that the process containing a stable propagation of cracks in the interlaminar region is impossible, and that shear failure occurs whenever the inherent ILSS is reached partly anywhere in the shear plane. It can be found for the curve of 2.5D-C/SiC along the warp direction that the stage prior to failure is similar to those mentioned above, while several steps occur after reaching the peak load, which is related to the special weaving structure. 3.2 ILSS at 1173K ILSS of the tested specimens at 1173K in air are shown in Fig. 4. It clearly shows that ILSS of 2D-C/SiC is higher than that of 2.5D-C/SiC along the warp direction，but considerably lower compared with that of the weft direction for 2.5D-C/SiC. The great shear anisotropy between the warp and weft direction for 2.5D-C/SiC can be understood by the characteristics of the fiber preform architecture, shown in Fig.1. When the specimens were subjected to a shear load along the Figure 3. Interlaminar shear load versus cross-head displacement curves from the compression of DNS specimens for the three kinds of ceramic matrix
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