  A Molecular Dynamics approach has been used to compute the shear force
resulting from the shearing of disks. Two-dimensional monodisperse disks have
been put in an horizontal and rectangular shearing cell with periodic boundary
conditions on right and left hand sides. The shear is applied by pulling the
cover of the cell either at a constant rate or by pulling a spring, linked to
the cover, with a constant force. Depending on the rate of shearing and on the
elasticity of the whole set up, we showed that the measured shear force signal
is either irregular in time, regular in time but not in shape, or regular in
shape.
