1.  PDC drill bit is pressed into rock under the weight on bit when drilling hard formations, which makes the rock in contact with PDC cutters show plastic under ultra-high stress.

2.  In plastic (or the rock show plastic under ultra-high stress) formations, the inside parts of the rock ahead will be broken or flow plastically under the torque action of the PDC bit when PDC cutters cutting into the formations, finally the inside parts will disconnect from the rock matrix and form debris.

3.  In brittle formations, the rock is broken by shear force and strain as the rock will show brittle break under the weight on bit and the stress caused by the torque action, the ROP of the PDC bit will be high and the broken volume of the rock will be much more than the displacement volume after the PDC cutters cutting into formations in this situation.