  This paper presents an adaptive strategy for phase-field simulations with
transition to fracture. The phase-field equations are solved only in small
subdomains around crack tips to determine propagation, while an XFEM
discretization is used in the rest of the domain to represent sharp cracks,
enabling to use a coarser discretization and therefore reducing the
computational cost. Crack-tip subdomains move as cracks propagate in a fully
automatic process. The same computational mesh is used during all the
simulation, with an $h$-refined approximation in the elements in the crack-tip
subdomains. Continuity of the displacement between the refined subdomains and
the XFEM region is imposed in weak form via Nitsche's method. The robustness of
the strategy is shown for some numerical examples in 2D and 3D, including
branching and coalescence tests.
