  We study the genesis and the selective propagation of complex crack networks
induced by thermal shock or drying of brittle materials. We use a quasi-static
gradient damage model to perform large scale numerical simulations showing that
the propagation of fully developed cracks follows Griffith criterion and
depends only on the fracture toughness, while crack morphogenesis is driven by
the material's internal length. Our numerical simulations feature networks of
parallel cracks and selective arrest in two dimensions and hexagonal columnar
joints in three dimensions, without any hypotheses on cracks geometry and are
in good agreement with available experimental results.
