Font Size: 

The rupture speeds obtained by fracture experiments of monolithic brittle linear
elastic materials are usually by far lower than those predicted by theories and
inferred from inversions of seismograms: The theoretical limiting propagation
speed of a (continuously accelerating) crack tip is the Rayleigh wave speed cR of
the material under typical mode-I and II remote loading conditions. For mode-III
loading, the theoretical limit is larger, the shear wave speed cS of the medium, and
some seismic inversions even suggest the existence of supershear rupture speeds
(i.e., rupture propagating faster than the relevant shear wave; see Table 1) [1-5].
On the contrary, laboratory experiments and observations suggest that when a
crack extends in brittle materials under suitable stress conditions and its velocity
exceeds a certain limit, it oscillates (surface roughening) and subsequently divides
into two or more branches.