  Irreversible evolution is one of the central concepts as well as
implementation challenges of both the variational approach to fracture by
Francfort and Marigo (1998) and its regularized counterpart by Bourdin,
Francfort and Marigo (2000, 2007 and 2008), which is commonly referred to as a
phase-field model of brittle fracture. Irreversibility of the crack phase-field
imposed to prevent fracture healing leads to a constrained minimization
problem, whose optimality condition is given by a variational {\em inequality}.
In our study, the irreversibility is handled via penalization. Provided the
penalty constant is well-tuned, the penalized formulation is a good
approximation to the original one, with the advantage that the induced {\em
equality}-based weak problem enables a much simpler algorithmic treatment. We
propose an analytical procedure for deriving the {\em optimal} penalty
constant, more precisely, its {\em lower bound}, which guarantees a
sufficiently accurate enforcement of the crack phase-field irreversibility. Our
main tool is the notion of the optimal phase-field profile, as well as the
$\Gamma$-convergence result. It is shown that the explicit lower bound is a
function of two formulation parameters (the fracture toughness and the
regularization length scale) but is independent on the problem setup (geometry,
boundary conditions etc.) and the formulation ingredients (degradation
function, tension-compression split etc.). The optimally-penalized formulation
is tested for two benchmark problems, including one with available analytical
solution. We also compare our results with those obtained by using the
alternative irreversibility technique based on the notion of the history field
by Miehe et al.\ (2010).
