Unified analysis of phase-field models for cohesive fracture

We address in this work unified analysis of phase-field models for cohesive fracture in order to alleviate the difficulty in selecting proper models and for further improvement. Aiming to regularize the Barenblatt's cohesive zone model, all the discussed models are distinguished by three characteristic functions, that is, the geometric function dictating the crack profile, the degradation function for the constitutive relation and the dissipation function defining the crack driving force. The latter two functions coincide in the associated formulation, while in the non-associated one they are designed to be different. Distinct from the counterpart for brittle fracture, in the phase-field model for cohesive fracture the regularization length parameter has to be properly incorporated into the dissipation and/or degradation functions such that the failure strength and traction–separation softening curve are both well-defined. Moreover, the resulting crack bandwidth needs to be non-decreasing during failure in order that imposition of the crack irreversibility condition does not affect the anticipated traction–separation law (TSL). With a truncated degradation function that is proportional to the length parameter, the Conti-Focardi-Iurlano model and the latter improved versions can deal with crack nucleation only in the vanishing limit and capture cohesive fracture only with a particular TSL. Owing to a length scale dependent degradation function of rational fraction, these deficiencies are largely overcome in the phase-field cohesive zone model (PF-CZM). Among many variants in the literature, only with the optimal geometric function, can the associated PF-CZM apply to general non-concave softening laws and the non-associated PF-CZM to (almost) any arbitrary one. Some mis-interpretations are clarified and representative benchmarks are presented.