A force-penetration relationship is considered to be closely related to the deformation and fracture mechanism at the bit-rock interface. In this study, a series of percussive long-hole drilling tests using Inada granite were carried out to examine the effects of hole length on drilling performace. Stress wave was measured by a pair of strain gauges, and then force-penetration curve of a button bit was calculated by the measured stress wave. The main results are summarized as follows:
1) It can be said that characteristic impedance of hard metal chips has negligible effect on the calculated force-penetration curve, but characteristic impedance of the bit and travelling time from strain gauges to the bit have considerable effect.
2) With increase of hole length, the energy transmitted to the rock decreases, damage around the bottom of the hole decreases, and then the variation in slope of each force-penetration curve decreases.
3) Average advance per blow of rock drill can be expressed by following equations:
(Average advance per blow of rock drill)
= 0.26× (Maximum penetration)
= 0.51× (Final penetration) .
