Force-penetration curves of a button bit generated during impact penetration into rock

• We developed an impact penetration tester built from the same components in an actual rock drill.
• A data correction method was proposed to calculate a force-penetration curve.
• Force-penetration curves were obtained from more than 40 impact penetration tests.
• We investigated the force-penetration relationship of a button bit.

To improve the performance and efficiency of percussion rock drills, it is essential to understand the impact penetration behavior of a bit drilling into rock. However, little knowledge to date has been published about the penetration behavior of the button bit, which is commonly used for hard rock. In this study, the impact penetration behavior of the button bit was investigated in laboratory tests. An impact penetration tester built from the same components used in an actual rock drill was developed and used in single-blow impact penetration tests. Unnatural fluctuations were observed in the force-penetration curves calculated using a two-point strain measurement method, which were probably due to not only the difference in rod stress as measured at two points on the rod, but also to the mismatch between the actual bit and the calculation model. A data correction method, in which the bit force calculated in a free end test is subtracted from that in the impact penetration test, was proposed using a numerical simulation. The method was applied to the measured rod stress, and force-penetration curves with a button bit were obtained from more than 40 impact penetration tests. Graph curves showed that peak force, initial slope and elastic penetration increased, while the maximum and final penetrations decreased with an increase in the secant slope. The crushing and elastic strain energies changed, but the sum of the energies was constant with an increase in the secant slope. The variations in the force-penetration curves are caused by the contact conditions between the bit and rock, the rock properties, and damage to the rock caused with each blow, not by the test conditions. The findings of this study will contribute to progress in the performance and efficiency of percussive rock drilling.