# Crack Propagation Behavior of Fused Silica during Cyclic Indentation under Incremental Loads
## 3 Results and Discussion  
### Observation of Indent Surface  
Figures 4 and 5 show the SEM images of indents formed by the Berkovich indenter. Borderline cracks in curved shapes were observed around all the indents, as reported in previous studies [13; 25]. Borderline crack traces were also observed inside the indents, which had been formed during the early cycles. However, there was no surface spalling due to crack propagation. This fact implies that incremental-load cyclic indentation by a Berkovich indenter induces multiple borderline cracks but does not propagate the cracks formed during the early cycles. Incremental-load cyclic indentation by the cube-corner indenter caused various types of brittle fractures, as shown in Figures 6 and 7. For all indents, long radial cracks were observed around the indent edges. For multi-cyclic indentations (Figures 6b-d and 7c-e), surface spalling was confirmed around the indents, which was not observed for single indentation (Figures 6a and 7a) and two-cycle indentation (Figure 7b). This aspect is distinctly different from that of the indents made by the Berkovich indenter. As the maximum load increases, four different kinds of surface spalling were identified. The most popular surface spalling is fan-shaped, the profile of which is composed of a large arc, two radial cracks and an indent edge (for example see Figure 6b). This type of spalling is termed deep spalling, as the large arc indicates that it is caused by deep lateral crack propagation. The second type is small internal spalling which occurs inside the indent (for example, see Figure 6c). This type of surface spalling has an irregular shape and might be a result of the detachment of a plastically deformed layer near the indent surface. The third type, termed secondary spalling, has an elliptical shape which is closely connected to first type (for example, see Figure 6d). It is presumable that the secondary spalling is caused by the propagation of the first type. In addition, the fourth type of surface spalling was confirmed for some indents such as the one in Figure 7e. This type of spalling takes place inside the first type, near the edge of indenter. The size of spalling is small, indicating that it is caused by shallow lateral cracks, and thus, it is named shallow spalling. Figure 5: SEM images of indents generated by a Berkovich indenter at 50 mN: (**a**) single; (**b**) 2 cycles; (**c**) 5 cycles; (**d**) 10 cycles. Surface spalling is a result of lateral crack propagation during unloading [11]. The process of crack initiation, propagation and spalling was observed by using a CCD camera. Figure 8 shows CCD images of the same indents as those shown in Figure 6. For a single indentation (Figure 8a), although lateral cracks were initiated beneath the surface, the cracks did not propagate further. As the indentation cycle increases, the lateral cracks propagate in the subsurface region, causing the size and brightness of the shining areas to increase. Interference fringes are observed (Figure 8b), indicating the formation of big gaps beneath the surface. As indentation cycle further increases, complete spalling occurs, and the material pieces are removed from the surface, leaving dark craters (Figure 8d). From these results, it is concluded that incremental cyclic indentation causes significant lateral crack propagation, which in turn leads to surface spalling. For quantitative evaluation of the effect of loading methods, the incidence of complete spalling at each indentation condition was calculated as shown in Figure 9. In the calculation, spalling with partial connection to the bulk, like that shown in Figure 6c, was not regarded as a complete spalling. The single indentation was not listed in Figure 9 due to there being no occurrence of complete spalling. Figure 9 demonstrates that incremental-load cyclic indentation with more cycles promoted spalling regardless of the maximum load. After 10 cycles, there was no difference between maximum loads of 100 and 50 mN.