Energy balance in dynamic fracture, investigated by a potential drop technique

A puzzling question in dynamic fracture has been why cracks in amorphous brittle materials always travel at velocities smaller than the Rayleigh wave speed. The answer is that the energy per length needed for the crack to propagate depends strongly on velocity. As the energy flux to the crack tip increases, the crack chooses new modes of dissipation such as micro-cracking and the creation of subsurface damage zones to dissipate this energy. In this paper we use a potential drop technique to measure length and velocity of a crack with high spatial precision and time resolution so as to investigate the modes of dissipation in Homalite-100 and make qualitative comparisons with PMMA. The technique is capable of resolving crack initiation, run, and arrest. Using this technique we search for a 'forbidden band' of velocities in PMMA, Homalite-100, and glass, and we show that no such velocity gap exists in these amorphous materials at room temperature.