Abstract
Longitudinal hypersonic velocities in PMMA under uniaxial tensile stress at room temperature were studied. It was observed that the velocity of local thermal phonons along the stress direction showed a gradual softening with the applied stress, and this softening took place even at the stress level where a macroscopic tensile measurement would reveal a relatively linear stress-strain relationship. Present results suggest that the local softening leads to the craze formation in PMMA. However, the local phonons perpendicular to the stress showed no softening. These facts explain the craze formation profile appearing on the sample.