  Failure of amorphous materials is characterized by the emergence of
dissipation. The connection between particle dynamics, dissipation, and overall
material rheology, however, has still not been elucidated. Here, we take a new
approach relating trajectories to yielding, using a custom built interfacial
stress rheometer, which allows for measurement of shear moduli (G',G'') of a
dense athermal suspension's microstructure while simultaneously tracking
particle trajectories undergoing cyclic shear. We find an increase in total
area traced by particle trajectories as the system is stressed well below to
well above yield. Trajectories may be placed into three categories: reversibly
elastic paths; reversibly plastic paths, associated with smooth limit cycles;
and irreversibly plastic paths, in which particles do not return to their
original position. We find that above yield, reversibly plastic trajectories
are predominantly found near to the shearing surface, whereas reversibly
elastic paths are more prominent near the stationary wall. This spatial
transition between particles acting as solids to those acting as liquids is
characteristic of a 'melting front', which is observed to shift closer to the
wall with increasing strain. We introduce a non-dimensional measure of plastic
dissipation based on particle trajectories that scales linearly with strain
amplitude both above and below yield, and that is unity at the rheological
yield point. Surprisingly, this relation collapses for three systems of varying
degrees of disorder.
