When the material inside this characteristic zone is stiffened due to hyperelastic properties, cracks propagate faster than the longitudinal wave speed.
The research group of Gao has used this concept to simulate the Broberg problem of crack propagation inside a stiff strip embedded in a soft elastic matrix.
The challenge arises of designing experiments and interpretative simulations to verify the energy characteristic length.
Confirmation of the concept must be sought in the comparison of experiments on supersonic cracks and the predictions of the simulations and analysis.
While much excitement rightly centres on the relatively new activity related to intersonic cracking, an old but interesting possibility remains to be incorporated in the modern work: for an interface between elastically dissimilar materials, crack propagation that is subsonic but exceeds the Rayleigh wave speed has been predicted for at least some combinations of the elastic properties of the two materials.