  The nucleation and propagation of hydrofractures by injection of over
pressured fluids in an elastic and isotropic medium are studied experimentally.
Non-Newtonian fluids are injected inside a gelatine whose mechanical properties
are assumed isotropic at the experimental strain rates. Linear elastic theory
predicts that plastic deformation associated to breakage of gelatin bonds is
limited to a small zone ahead of the tip of the propagating fracture and that
propagation will be maintained while the fluid pressure exceeds the normal
stress to the fracture walls (Ch\'avez-\'Alvarez,2008) (i.e., the minimum
compressive stress), resulting in a single mode I fracture geometry. However,
we observed the propagation of fractures type II and III as well as nucleation
of secondary fractures, with oblique to perpendicular trajectories with respect
to the initial fracture. In the Video (http://hdl.handle.net/1813/14122)
experimental evidence shows that the fracture shape depends on the viscoelastic
properties of gelatine coupled with the strain rate achieved by fracture
propagation.
