Rock Failure Mechanism

Drilling bits cut rock by five basic mechanisms

While one mechanism may be dominant for a given bit design, more than one mechanism is usually present.

PDC bits shear rock

PDC bits cut rock primarily by wedging, which is more efficient than crushing and grinding.
In the right formations, the wedging action makes it possible for PDC bits to maintain high rates of penetration with less weight on bit.
This reduces wear on bit while yielding high ROP.

Indenting,

Plowing And Grinding

Natural diamond bits are designed to drill with very small penetration into the formation.
The diameter of sandstone grains may not be much smaller than the depth of penetration of the diamonds.
The drilling action of diamond bits is primarily a grinding action in which the cementing material holding the individual sand grains together is broken by the diamonds

Roller cutter bits designed with small or zero offset angle for drilling hard formations employ the crushing mechanism for rock removal.
This cutting action is inefficient and requires high weight on bit to deliver enough energy to the formation to achieve fast penetration.
The drilling action of rolling cone bits designed with a large offset angle for drilling soft formations is more complex than the simple crushing action.
Since each cone alternately rolls and drags, considerable wedging and twisting action is present in addition to the crushing action.