# Experimental Research on In Situ Uniaxial Tensile Response of Silica-Based PLC Optical Splitters
## 3 Results and Discussion
### Observed Failure Mode  
Since the fracture locations of all the specimens appear at the adhesively-bonded joints of PLC optical splitters, it is necessary to understand the failure modes of the adhesive joints. Generally, if the failure of the adherend is not considered, the failure modes of adhesive joints can be classified as [13]: (1) adhesive failure (sometimes referred to as interfacial failure), which occurs at the interface between the adherend and the adhesive; (2) cohesive failure, which reveals good compatibility between the adhesive and adherends and it happens inside the adhesive layer [29]; or (3) mixed failure, which is characterized by the above two different failure modes of superposition. The microstructure and morphology of the fractured surfaces were observed by optical microscopy to investigate the failure mechanisms of the specimens. Typical optical microscopic morphologies of the fracture surfaces for Type I and Type II fractures that were mentioned in Section 3.1 are shown in Figures 3 and 4, respectively. Figures 3 and 4 reveal that the failure modes of the two types of fractures are both mixed failure, that is, both cohesive failure and adhesive failure occurred. Similar phenomena were observed for the other specimens. Figure 2: (**a**) Typical load-displacement curves under different loading rates until their ultimate failure. (**b**) Load-time curves corresponding to (**a**). In addition, by observing the failure surfaces of multiple specimens, we noticed that adhesive/interfacial failure are usually prone to occur at the edge of the bonding area (similar to Figures 3c and 4), while cohesive failure tends to occur in the vicinity of the V-groove and the optical fiber (similar to Figures 3a and 4a). These phenomena indicate that cracks first occurred at the edge of the bonding area and rapidly extended until the adhesively-bonded joint failed completely. The reason may be related to the stress distribution of the adhesively-bonded joints. Previous works have shown that the edge of the bonding area usually has a high degree of stress concentration, and cracks are easy to form and propagate from here [30; 31]. Furthermore, noting that the end face structure of the 8-channel fiber array in Figure 3a is obviously more complex than that of the 1-channel fiber array (Figure 4a), it is not difficult to understand why almost all the specimens were Type I fracture under tensile loading. described in Section 2.2, during the uniaxial tensile test of the specimen, the IL at 1.55 \(\upmu\)m wavelength of its eight output ports was monitored in situ by an optical power meter. In order to better compare and analyze the change of the optical performance of the eight ports with the damage degree of the PLC optical splitter, take the 15 s before loading, that is, the sample is in the clamping and static state, as the initial stage of the \(\Delta\)IL-time curves. Typical \(\Delta\)IL-time curves that were obtained under displacement and load control mode are shown in Figures 5 and 6, respectively. Figure 5 shows the \(\Delta\)IL-time curves of a sample from stable standing to the beginning of uniaxial tensile loading to fracture in displacement-controlled mode. In Figure 5, the \(\Delta\)IL-time curves can be clearly distinguished into three stages: stable standing Stage I, uniaxial tensile loading Stage II, and post-fracture Stage III. In Stage I, the \(\Delta\)IL of each port was relatively stable and close to zero; in Stage II, the change of \(\Delta\)IL of each port was basically similar, i.e., it dropped slowly to around \(-1\) dB at the beginning, then dropped sharply to around \(-15\) dB, and after a short stay (different from the load control mode, in the displacement control mode with a loading rate of 0.01 mm/s, even if the specimen has been fractured, the two fracture surfaces are still very close in the direction of the tensile axis in a short time, so the IL does not change much), the \(\Delta\)IL of the eight ports dropped steeply to around \(-60\) dB almost at the same instant.