  When metals are plastically deformed, the total density of dislocations
increases with strain as the microstructure is continuously refined, leading to
the strain hardening behavior. Here we report the fundamental role played by
the junction formation process in the connection between dislocation
microstructure evolution and the strain hardening rate in face-centered cubic
(FCC) Cu, as revealed by discrete dislocation dynamics (DDD) simulations. The
dislocation network formed during [001] loading consists of line segments whose
lengths closely follow an exponential distribution. This exponential
distribution is a consequence of junction formation by dislocations on
different slip planes, which can be modeled as a one-dimensional Poisson
process. We show that, according to the exponential distribution, the
dislocation microstructure evolution is governed by two non-dimensional
parameters, and the hardening rate is controlled by the rate of stable junction
formation. By selectively disabling specific junction types in DDD simulations,
we find that among the four types of junctions in FCC crystals, glissile
junctions make the dominant contribution to the strain hardening rate.
