The phonon viscosity mechanism, as it applies to screw dislocations
moving on the {111} planes of face-centered-cubic crystals,
is examined. Formulas are derived for changes in the elastic stiffnesses
of cubic crystals with the impression of an arbitrary elastic
strain. The result is specialized to the case of the strain field of a
screw dislocation in a face-centered-cubic crystal. Lattice energy
absorption from a moving screw dislocation is then considered
through the relaxation of the elastic stiffnesses. Using the formula
for energy absorption, a dislocation damping coefficient, B, is
found. B is independent of temperature above the Debye temperature,
and at lower temperatures has different forms for anisotropic and
isotropic crystals. The damping coefficient for an edge dislocation
is qualitatively similar to that of the screw dislocation.
The results are consistent with recent measurements
of the damping coefficient for dislocations in copper