  A heterogeneous quasi-brittle granular material can withstand certain levels
of internal damage before global failure. This robustness depends not just on
the bond strengths but also on the topology and redundancy of the bonded
contact network, through which forces and damage propagate. Despite extensive
studies on quasi-brittle failure, there still lacks a unified framework that
can quantify the interdependent evolution of robustness, damage and force
transmission. Here we develop a framework to do so. It is data-driven,
multiscale and relies solely on the contact strengths and topology of the
contact network for material properties. Using data derived from discrete
element simulations of concrete specimens under uniaxial tension, we uncover
evidence of an optimized force transmission, characterized by two novel
transmission patterns that predict and explain damage propagation from the
microstructural to the macroscopic level. The first comprises the optimized
flow routes: shortest possible paths that can transmit the global transmission
capacity. These paths reliably predict tensile force chains. The second are the
force bottlenecks. These provide an early and accurate prediction of the
ultimate pattern, location and interaction of macrocracks. A two-pronged
cooperative mechanism among bottlenecks, enabled by redundancies in
transmission pathways, underlies robustness in the pre-failure regime.
Bottlenecks take turns in accommodating damage, while member contacts spread
the forces to confine damage to low capacity contacts which leave behind a web
of strong contacts to support and curtail the failure of tensile force chains
in the region. This cooperative behavior, while serving to minimize the
inevitable reduction in global transmission capacity, progressively heightens
the interdependency among these contacts and elicits the opposite effect.
