## 2 Experiment Description
### Experimental Results of the Fracture Toughness  
Figure 3 only shows the eight typical load-displacement curves of the CT specimens at various experiment temperatures. When the temperature was below 1173 K, the typical brittle fracture mode of the specimen was seen from the load-displacement curves. At 1223 and 1273 K, the softening phenomenon of the specimen was clearly observed. For example, two of the load-displacement curves both contained a nonlinear region before the load peak reached the peak, induced by crack tip blunting, caused by the viscous flow deformation of the fibers and the bonding between the fibers [25]. When the load reached the peak, the load gradually decreased. The millimeter scale crack tip blunting needed to dissipate the extensive energy absorption. Then, the crack caused by the fiber failure initiated from the blunting region. In addition, the similar crack tip blunting deformation of the specimen, at various temperatures, is observed in [25]. For example, using the SEM images of the crack tip of the CT specimens, with a porosity of 89% at different temperatures, we see that the crack tip sizes almost kept constant, and without crack tip blunting deformation, from 299 K to 773 K. After 773 K, the crack tip blunting was obvious, based on the remarkable residual crack sizes of the CT specimens. Those morphological characteristics of the crack tips are similar with CT specimens with a porosity of 87%. In this work, the fracture toughness \(J_{C}\) was calculated by [13]. \[J_{C}=\frac{2U_{C}}{B(W-a)}g(a/W),\ g(a/W)=1+0.261(1-a/W) \tag{1}\]  
where \(U_{C}\) means the area under the load - displacement curve at the maximum load, as indicated in Figure 4. \(g(a/W)\) is a dimensionless constant. Figure 4: The area under one load – displacement curve of a CT specimen at the maximum load. Figure 3: Typical load – displacement curves from the CT specimen experiments at various temperatures (a / W = 0.4). Based on Equation (1), the fracture toughness \(J_{C}\) was calculated and plotted in Figure 5a at eight temperatures. Below 1173 K, the fracture toughness with two ratios (a/W=0.4 and 0.5), as the temperature increased, exhibited almost the same variation trend (Figure 5a), and their values were close to each other. For the crack length-to-width ratio of 0.5, the fracture toughness \(J_{C}\) increased from 44.92 to 280.09 J/m\({}^{2}\) from 299 K to 1173 K, increasing by approximately 524%. For the crack length-to-width ratio of 0.4, the fracture toughness increased by approximately 574% from 47.32 to 328.28 J/m\({}^{2}\) between 299 and 1273 K. As temperature increased, the fracture process of the specimen dissipated more energy (Figure 3). This energy dissipation process was attributed to the crack tip blunting caused by the viscous flow deformation in the 3D RF material [25]. In addition, it can be seen that the specimen brittlely fractured at 299 K (Figure 5b) and showed obvious residual crack tip blunting deformation at 1123 K (Figure 5c). In fact, our experimental results and further analysis have confirmed that the viscous flow gave rise to the crack tip blunting. For the crack-tip blunting process, it was a challenge to catch this process in the furnace at elevated temperatures (Figure 5d). However, based on the residual crack size of the specimen (Figure 5b,c), we can still draw the conclusion that the crack tip blunting of the specimen occurred when the temperature was above 773 K [25; 27]. When the temperature went beyond 1173 K, two of the temperature dependence of fracture toughness \(J_{C}\) curves exhibited different variation trends. For example, the fracture toughness \(J_{C}\) from the specimen with a crack length-to-width ratio of 0.5 decreased. However, the fracture toughness \(J_{C}\) with a ratio of 0.5 was within the error bars of the fracture toughness \(J_{C}\) with a ratio of 0.4. Two facts may explain this phenomenon. On one hand, the 3D RF material had a high fiber spacing and the fibers in the material were distributed randomly.