  Experiments of pure tensile fracture in brittle gels reveal a new dynamic
oscillatory instability whose onset occurs at a critical velocity, Vc = 0.87
Cs, where Cs is the shear wave speed. Until Vc crack dynamics are well
described by linear elastic fracture mechanics (LEFM). These extreme speeds are
obtained by suppression of the micro-branching instability, which occurs when
sample thicknesses are made comparable to the minimum micro-branch width. The
wavelength of these sinusoidal oscillations is independent of the sample
dimensions, thereby suggesting that a new intrinsic scale exists that is
unrelated to LEFM.
