Abstract
Using a self-designed hydraulic impact drilling test-bed and rock core drill, six groups of cylindrical granite specimens (93 mm dia. × 200 mm) containing central axial holes formed either by impact or nonimpact drilling methods were tested in uniaxial compression to failure on an Instron 1346 universal testing machine to investigate their mechanics and damage properties. The longitudinal acoustic wave velocities were measured before testing. The rock specimens were grouped according to the method of drilling the central hole (impact load exerted by different impact power and different frequencies for an approximately identical impact power, or nonimpact drilling). In this study, a statistical constitutive damage model based on Weibull distribution was used to calculate the degree of rock damage after drilling center holes. The experimental curves were measured to analyze the damage evolution process and the radius of rock damage. These indicate that rock damage increased with the increase of impact power and decreased with increasing impact frequency at constant impact power. This was also verified by the measured longitudinal wave velocity in all rock specimens. These results have significance for guiding the design of composite rock drilling tools that are dedicated to improving rock-breaking efficiency.
Highlights
Gu et al [40] proposed a damage constitutive model that takes the crack compaction effect into account based on accumulated acoustic emission (AE) counts
Rock damage induced by a combination of dynamic and static loading in the process of tunneling was investigated by Zou et al and Tao et al [46,47,48], using the split Hopkinson pressure bar (SHPB) apparatus to study the compressive strength and elastic modulus of rock specimens with boreholes, and the properties of the damage induced in the surrounding rock
Wu et al [56] studied the nonlinear characteristics of salt rock at the creep stage, from which they proposed a constitutive model of creep damage in salt rock, and verified the validity of the model
Summary
If the stress in the rock surrounding the drill hole within a certain range exceeds the ultimate rock strength, it will sustain damage, causing cracks or deformations Once this has occurred, it cannot withstand any further excessive stress. Original rock zone Figure 3: Elastic and plastic zones in the rock surrounding the drill hole. Take one rock element in the plastic zone for stress analysis; according to the equilibrium principle of radial stress and shear stress in a rock unit, equation (10) can be obtained: zσr + σr − σθ 0. Combining equation (9) with equation (10), according to boundary condition σrp|r r0 σi, the radial normal stress in the plastic zone, σrp, is given by r 2 sin φ/1− sin φ σrp σi −.
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