  A novel phase-field for ductile fracture model is presented. The model is
developed within a consistent variational framework in the context of
finite-deformation kinematics. A novel coalescence dissipation introduces a new
coupling mechanism between plasticity and fracture by degrading the fracture
toughness as the equivalent plastic strain increases. The proposed model is
compared with a recent alternative where plasticity and fracture are strongly
coupled. Several representative numerical examples motivate specific modeling
choices. In particular, a linear crack geometric function provides an
"unperturbed" ductile response prior to crack initiation, and Lorentz-type
degradation functions ensure that the critical fracture strength remains
independent of the phase-field regularization length. In addition, the response
of the model is demonstrated to converge with a vanishing phase-field
regularization length. The model is then applied to calibrate and simulate a
three-point bending experiment of an aluminum specimen with a complex geometry.
The effect of the proposed coalescence dissipation coupling on simulations of
the experiment is first investigated in a two-dimensional plane strain setting.
The calibrated model is then applied to a three-dimensional calculation, where
the calculated load-deflection curves and the crack trajectory show excellent
agreement with experimental observations. Finally, the model is applied to
simulate crack nucleation and growth in a specimen from a recent Sandia
Fracture Challenge.
