  Materials are often heterogeneous at various length scales, with variations
in grain structure, defects, and composition which has a strong influence on
the emergent macroscopic plastic behavior. In particular, heterogeneities lead
to fluctuations in the plastic response in the form of jerky flow and
ubiquitous strain bursts. One of the crucial aspects of plasticity modeling is
scale bridging: In order to deliver physically correct crystal plasticity
models, one needs to determine relevant microstructural length scales. In this
paper we advance the idea that continuum descriptions of dislocation mediated
plasticity cannot neglect dynamic correlations related to the avalanche
behavior. We present an extensive weakest link analysis of crystal plasticity
by means of three-dimensional discrete dislocation dynamics simulations with
and without spherical precipitates. We investigate strain bursts and related
length scales and conclude that while sufficiently strong obstacles to
dislocation motion tend to confine the dislocation avalanches within
well-defined sub-volumes, in pure dislocation systems the avalanches may span
the system, implying that the dynamic length scale is, in fact, the size of the
entire sample. Consequences of this finding on continuum modeling are
thoroughly discussed.
