A theoretical model is developed to study the effect
of dwell time on the junction growth during tangential loading and on static
friction of a creeping polymer sphere in contact with a rigid flat under full
stick contact condition. A rapid normal loading into the elastic-plastic
contact of a creeping polymer sphere is followed by a rest period. During this
period (dwell time) creep takes place causing increase of the contact area and
stress relaxation. At the end of the dwell time a rapid increasing tangential
loading is applied to the flat till sliding inception occurs. During the
tangential loading step, further increase of the contact area (junction growth)
and reappearing of a plastic volume in the sphere take place. The reappearing
of the plastic volume and its increase during the tangential loading step
decreases the tangential stiffness of the sphere. Sliding inception, which
determines the end of the tangential loading step, is defined as the moment
when the tangential stiffness of the sphere decreases significantly compared to
its initial tangential stiffness (ITS). The minimum tangential force causing
that sliding inception is defined as the static friction force. The friction
coefficient is defined as the ratio of the friction force over the normal
preload. An increase of the static friction with increase of the dwell time is
clearly demonstrated and explained.
