  The statistics and origin of the first discrete plastic event in a one
dislocation dynamics simulation are studied. This is done via a linear
stability analysis of the evolving dislocation configuration up to the onset of
irreversible plasticity. It is found, via a fold catastrophe, the dislocation
configuration prior to loading directly determines the stress at which the
plastic event occurs and that between one and two trigger dislocations are
involved. The resulting irreversible plastic strain arising from the
instability is found to be highly correlated with these triggering
dislocations.
