  Torsional oscillator experiments show evidence of mass decoupling in solid
4He. This decoupling is amplitude dependent, suggesting a critical velocity for
supersolidity. We observe similar behavior in the elastic shear modulus. By
measuring the shear modulus over a wide frequency range, we can distinguish
between an amplitude dependence which depends on velocity and one which depends
on some other parameter like displacement. In contrast to the torsional
oscillator behavior, the modulus depends on the magnitude of stress, not
velocity. We interpret our results in terms of the motion of dislocations which
are weakly pinned by 3He impurities but which break away when large stresses
are applied.
