  This paper discusses the time-dependence of the threshold function in the
perfect plasticity model. In physical terms, it is natural that the threshold
function depends on some unknown variable. Therefore, it is meaningful to
discuss the well-posedness of this function under the weaker assumption of
time-dependence. Time-dependence is also interesting from the viewpoint of the
abstract evolution equation. To prove the existence of a solution to the
perfect plasticity model, the recent abstract theory under the continuous class
with respect to time is used.
