## 3 Experimental Results and Discussing
### Experimental Results
#### 3.3.3 Observation of the Defect with a Microscope  
As introduced in Section 2, there will always be a critical moment when the temperature jump occurs. The temperature jump could be captured by thermal camera. As shown in Figure 8, a laser scanning confocal microscope OLS4100 was used to observe the fibers distribution at the critical moments in order to observe the actual damage inside the specimen, which consisted with the time of Figure 4d, Figure 5d, Figure 6d, and Figure 7d. At this critical moment (1350 s for A1, 942 s for A2, 1260 s for A3,1311 s for A4), the test was stopped and the specimen was taken off the test machine. The fracture surface was observed by microscope, and then Figure 9 was obtained. As shown in Figure 9a, a fracture surface of A1 was photographed by a microscope to show the situation of fiber fracture. The fracture occurred at the interlayer of the laminated fibers, which indicated that the bonding between the fractured fiber and the matrix was weak. Interface debonding occurred and the debond length increased with increasing load. Finally, the entire fiber was pulled out from the matrix. The fractured fiber that was released the stress at the crack tip and the stress field at the  
Figure 8: Microscopic experimental setup. Figure 9: Micrograph of specimens when a temperature jump occurred. cross section was redistributed. The temperature continued to increase. Some energy was consumed by friction during the process of pulling. The fracture of fiber was accompanied by load transfer. The diameter of fiber did not substantially change, which indicates that the fiber instantaneously fractured and did not go through an elongation process. The reinforcement was disabled due to the hollow tubular structure of surrounding matrix. The interface misalignment can be caused by a small load, which caused that the bubble to evolve into crack. The distribution angles of some fibers changed after fracture, which indicated that the fibers had been pulled out from the matrix completely and they had lost their carrying capacity. Bending deformation of some fibers was seen in the pull-out section, which was local and occurred in a small range around the peeling position. Thus, its influence on the deformation and balance of the whole structure can be ignored. First crack and single-fiber fracture occurred in the weakest area under the tensile load. The specimen made a collapsing sound. As the load increased, more and more adjacent fibers fractured and the specimen continued to make a sound until the final failure occurred. Multiple bundles of fibers simultaneously fractured with a split-filamentary form. In Figure 9b, a micrograph of the bubble in specimen A2 was captured, which showed that the fibers around the bubble were completely fractured. The bubble volume expanded and the internal air moved. A new bubble occurred when the bonding between fibers and matrix became fragile, which caused the fibers to fracture and propelled the bubble to evolve into crack. The meso-structure was loose and the degrees of fibers fracture were not uniform, due to the bubble. Some of the fibers became thinner and the bubble was easier to extend due to that a lot of fibers were pulled out. Some fibers around the bubble also fractured, which indicated that some of the load was dispersed on the surrounding fibers and the stress distribution was not uniform. As shown in Figure 9c, a micrograph of fracture surface of A3 was captured after the test. Most of the fibers were still distributed at a 45\({}^{\circ}\) angle. Some fibers were fractured and some fibers changed the orientation. In some region, the interface debonding and fibers deformation occurred. There was no defect in A3. Thus, the internal stress distribution was mainly determined by the carrying capacity of the fibers and the bonding between the fibers and matrix. When the fibers began to sustain stress, the deformation was mainly the stretching of the fiber macro-molecular chain. The deformation of the fiber was almost in a straight line.