Some new results on fracture

Rocks and ceramics are the typical prototypes for brittle materials. When they are very strongly sheared they can also flow plastically if the hydrostatic pressures are strong enough. Both cases have been studied extensively experimentally and numerically. The cracks in the brittle case and the shear bands in the plastic case from fractal networks that reflect the detailed stress field and the history of the deformation. The observed patterns are due to instabilities generated by the non-linearity in the constitutive behaviour. In both cases also a rich internal structure is found: power-law distributed gouge in faults and multifractal strain rate distributions in shear bands. We also discuss the process of fragmentation which occurs inside a shear zone