  The lifetime and performance of any engineering component, from nanoscale
sensors to macroscopic structures, are strongly influenced by fracture
processes. Fracture itself is a highly localized event; originating at the
atomic scale by bond breaking between individual atoms close to the crack tip.
These processes, however, interact with defects such as dislocations or grain
boundaries and influence phenomena on much larger length scales, ultimately
giving rise to macroscopic behavior and engineering-scale fracture properties.
This complex interplay is the fundamental reason why identifying the atomistic
structural and energetic processes occurring at a crack tip remains a
longstanding and still unsolved challenge.
  We develop a new analysis approach for combining quantitative in-situ
observations of nanoscale deformation processes at a crack tip with
three-dimensional reconstruction of the dislocation structure and advanced
computational analysis to address plasticity and fracture initiation in a
ductile metal. Our combinatorial approach reveals details of dislocation
nucleation, their interaction process, and the local internal stress state, all
of which were previously inaccessible to experiments. This enables us to
describe fracture processes based on local crack driving forces on a
dislocation level with a high fidelity that paves the way towards a better
understanding and control of local failure processes in materials.
