Direct measurements of the structure of the nonlinear zone in the vicinity of the tip of a dynamic crack

We present experimental measurements of the displacement and strain fields surrounding the tip of a dynamic crack propagating in mode I through brittle elastomers made of polyacrylamide gels. Both the detailed dynamics and fractography of these materials are identical to that of typical brittle amorphous materials, such as soda-lime glass. We demonstrate that linear elastic fracture mechanics (LEFM) does not provide an accurate quantitative description of these fields over fracture velocities ranging between 0.2 to 0.8c _S. Even for moderate strains in the vicinity of the crack tip these discrepancies include: the stress intensity factor is ill-defined, the crack tip opening displacement significantly deviates from its predicted parabolic form, and the strains ahead of the tip are more singular than the r^-1/2 divergence predicted by LEFM. In an accompanying paper we present a rigorous dynamic theory that provides excellent quantitative agreement with these measurements.