Observations of crack growth in hard rock loaded by an indenter

Abstract

A description is given of the processes of rock fragmentation induced by circular flat-bottomed punches from 5 to 20 mm dia loading orthogonal to the flat surface of cylindrical specimens of Sierra granite of 89 mm dia and confined by a steel belt, and 100 mm cube specimens confined in a biaxial frame. It is found that the rock deforms elastically until the applied punch load exceeds 45% of the maximum load that the rock cna sustain. At loads greater than 45% of the maximum, a crack is initiated around the perimeter of the punch and this crack propagates in the well-known conical Herizian manner. When the crack begins to propagate, the location of its front is reasonably well defined. However, as it extends with continued application of the load, an intense zone of microcracks surrounds and conceals the crack tip. Fractures initiating within this microcracked region propagate to the surface and form rock chips. The effects of punch size and confining stress, orthogonal to the punch, on the rock indentation strength are examined. Some decrease in punching strength with increasing punch size is noted; there is an increase in strength with increasing confining stress. At low confining stress, specimens split by cleavage through the axis of the punch. The theory of elasticity is used to show that this could occur with a punch loading against a semi-infinite rock surface and may therefore be an important phenomenon, not just an artifact of finite laboratory specimens.