## 3 Results and Discussion  
### Effect of Jet Pressure on Rock Breaking  
Figure 4 is a comparison diagram showing the rock-breaking effect on erosion specimens with no bit rotation (0 rpm) and with the bit rotating at 100 rpm for different water jet pressures. When the jet pressure was 3 MPa under either rate of rotation, the specimens showed only scour marks and no obvious crushed pit was formed. This indicates that the jet energy was below the critical energy needed for specimen failure; the water jet could not break the specimen effectively but the continuous action of the water jet left scour marks on the specimen's surface. When the jet pressure was 6 MPa or higher, more energy was transferred to the specimen and a crushed pit was formed on the surface of the specimen. A ring-shaped crushed pit was formed on the specimen's surface when the bit was rotated at 100 rpm. When the jet pressure was increased further, the crushed pit in the specimen was gradually enlarged, but when the bit did not rotate, no hole was drilled. When the jet bit was rotated at 100 rpm, the volume of the ring-shaped crushed pit increased, the boss was formed, and most of the 18-100-0 specimens were holed. Figure 5 shows the rock-breaking volume fitting curves for two erosion conditions. The volumes of crushed pits for specimens eroded without bit rotation (that is, eroded under \"fixed erosion\") show gradual linear growth, whereas the rock-breaking volumes for specimens under rotary erosion show exponential growth. Within the range of our experiments, the volume of pits crushed by an 18 MPa  
Figure 4: Diagram comparing the rock breaking of water jets during tests with no bit rotation (0 rpm) and with 100 rpm bit rotation. water jet under fixed erosion reached 2.25 mm\({}^{3}\), but the volume of pits crushed by the same 18 MPa jet under rotary erosion reached 27.11 mm\({}^{3}\). This indicates that for the same jet energy, the average crushed pit volume eroded under a rotating jet is greater than the pit volume eroded under a fixed jet. The main reason for this difference in the volume of rock eroded is that the tensile and shear strength of rock are much lower than its compressive strength; hence, rock is more likely to be broken by shear and tensile stresses than by compressive stress. The rotating water jet has a tangential velocity and where the jet contacts the rock, the oblique impact of the jet shears the rock and the rock surface is damaged by the shear stress. A ring-shaped crushed pit is generated. When the rotating water jet contacts the rock, the radial and tangential stresses in the rock generated by the jet impact are converted into tensile stress and enter a three-dimensional stress state. Thus, a rotating jet is more efficient at breaking the rock than a non-rotating jet [20]. In addition, the rebounding water from a rotating jet flows back along the outside of the jet stream and does not contact the incoming water to reduce the force needed for the subsequent crushing of the rock. This reduces energy losses. The volume of each crushed pit was defined by the depth and diameter of the crushed pit. Figure 6 shows curves for crushed pit depths and diameters for different jet pressures for both rotating and nonrotating jets. For a nonrotating jet, the depth of the crushed pit was 2.63 mm/MPa, but for a jet rotation at 100 rpm, the depth of the crushed pit was 6.33 mm/MPa. The diameters of the crushed pits formed by the two kinds of jets increased quadratically. Under fixed jet erosion the rate of crushed pit diameter growth gradually decreased as the pump pressure increased. When the pump pressure reached a certain value, the diameters of the crushed pits stopped increasing. However, the growth rate of the crushed pit diameters under rotary jet erosion continued to increase slightly as the pump pressure increased. This behavior was consistent with the fixed erosion results reported by [PERSON] et al. [21]. The depth of the crushed pits under rotary erosion was the dominant factor for the volume of the crushed pits.