Oscillations in Rapid Fracture
  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. 
Ariel Livne, Oded Ben-David and Jay Fineberg