  The strength of quasi-brittle materials depends on the ensemble of defects
inside the sample and on the way damage accumulates before failure. Using large
scale numerical simulations of the random fuse model, we investigate the
evolution of the microcrack distribution that is directly related to the
strength distribution and its size effects. We show that the broadening of the
distribution tail originates from the dominating microcracks in each sample and
is related to a tendency of crack coalescence that increases with system size.
We study how the observed behavior depends on the disorder present in the
sample.
