Indentation of Berea sandstone under different confining stresses are simulated by flat-joint model in PFC3D in order to investigate the influence of confining stress on rock fragmentation. The results show that there is a critical confining stress during confined indentation. When the confining stress is below the critical value, micro tensile cracks will coalesce and propagate to the boundary of the specimen, ending up splitting the sample which shows a typical brittle mode of failure. While the confining stress is over the critical value, the failure mode will change from brittle to ductile with the macro tensile crack hampered by confinement. Besides, the expanding damaged zone becomes flattened with the increasing of the confining stress and is followed by surface striping. The contour plots of maximum principle stress reveal that the compression area occupies most internal part of the sample except the side of the damaged zone, therefore the crack can only propagate upward and finally induces surface striping.

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