## 3 Results  
### Breakage Process and Mode  
Particle breakage modes vary with particle shape and external conditions [29]. For cone-hemispherical particles, the particle breakage undergoes two processes under external force, which is similar to the work of [PERSON] [30]. A local crushing occurred at the contact area when the normal force was relatively small. With increasing normal force, the particle breakages turn out to be three types: peeling, peeling-splitting and splitting, as shown in Figure 6. Peeling is that the particle is crushed at the contact interface, and a few fragments are peeled off the particle body (see Figure 6a). Another breakage mode is considered as peeling-splitting (see Figure 6b). Some fragments are peeled off at a relatively low load, but the particle is crushed into two pieces with increasing load. The third breakage mode is called splitting (see Figure 6c). Few fragments are peeled off, and the particle is crushed into two pieces directly. Interestingly, a conical core was formed during the breakage process of peeling-splitting and splitting (see Figure 7). The conical core consists of a round platform (the platform is the contact interface) and an inverted cone. The formation of splitting is caused by the conical core inserting into the particle and the tensile stress reaching the tensile strength [31; 32]. Therefore, the occurrence of the conical core is considered as the precondition for particle crushing as splitting eventually.  
Table 3 listed the breakage modes for the cone-hemispherical gypsum particles. Particle breakage mode gradually transfers from peeling to splitting with the increase in contact diameter \(d\) for particles of the same sphere diameter \(D\) and cone angle \(\theta\). For the same sphere diameter \(D\) (here, 50 mm) and cone angle (here, 90\({}^{\circ}\)), particles crush as peeling (\(d=5\) mm), peeling-splitting (\(d=7\), 8 mm) and splitting (\(d=10\) mm), respectively, as shown in Figure 8. The explanation is that for a smaller contact diameter, the particle is more likely to crush at the contact due to the stress concentration before the stress transfers to the inside of the particle. For the same sphere diameter \(D\) and contact diameter \(d\), the  
Figure 5: Experimental instruments and normal contact: (**a**) rock rheological testing system; (**b**) normal contact; (**c**) steel groove base.  
Figure 6: Photography of different breakage modes: (**a**) peeling; (**b**) peeling–splitting; (**c**) splitting.  
larger the cone angle \(\theta\) is, the more likely the particle crushes as splitting. For the same sphere diameter \(D\) (here, 50 mm) and contact diameter \(d\) (here, 7 mm), the breakage modes are peeling, peeling-splitting and splitting for particles of cone angle of 60\({}^{\circ}\), 90\({}^{\circ}\)and 120\({}^{\circ}\), respectively, as shown in Figure 9. However, particles of the same cone angle \(\theta\) and contact diameter \(d\) tend to crush as peeling instead of splitting with the increase in sphere diameter \(D\). Figure 10 shows the breakage modes of particles of the same cone angle (here, 90\({}^{\circ}\)) and contact diameter (here, 8 mm). It can be seen that for sphere diameter \(D\) 30 mm, 50 mm and 70 mm, the breakage modes are splitting, peeling-splitting and peeling, respectively.