New paper: Coupled phase-field and plasticity modeling of geological materials: From brittle fracture to ductile flow

Our paper on coupled phase-field and plasticity modeling of geomaterials, in collaboration with Prof. WaiChing Sun at Columbia University, has been accepted for publication in Computer Methods in Applied Mechanics and Engineering.

Read the paper: Choo and Sun, CMAME 2018a

Abstract: The failure behavior of geological materials depends heavily on confining pressure and strain rate. Under a relatively low confining pressure, these materials tend to fail by brittle, localized fracture, but as the confining pressure increases, they show a growing propensity for ductile, diffuse failure accompanying plastic flow. Furthermore, the rate of deformation often exerts control on the brittleness. Here we develop a theoretical and computational modeling framework that encapsulates this variety of failure modes and their brittle-ductile transition. The framework couples a pressure-sensitive plasticity model with a phase-field approach to fracture which can simulate complex fracture propagation without tracking its geometry. We derive a phase-field formulation for fracture in elastic-plastic materials as a balance law of microforce, in a new way that honors the dissipative nature of the fracturing processes. For physically meaningful and numerically robust incorporation of plasticity into the phase-field model, we introduce several new ideas including the use of phase-field effective stress for plasticity, and the dilative/compactive split and rate-dependent storage of plastic work. We construct a particular class of the framework by employing a Drucker–Prager plasticity model with a compression cap, and demonstrate that the proposed framework can capture brittle fracture, ductile flow, and their transition due to confining pressure and strain rate.

The proposed modeling framework can capture a wide range of failure modes of geological materials – from tensile fracture to shear fracture to plastic compaction – and their transition due to confining pressure and strain rate. This figure shows that it can also reproduce the transition of failure modes under tensile loading, from extension fracture (left) to hybrid fracture (middle) to shear fracture (right). This is consistent with the experimental finding of Ramsey and Chester (2004), Nature 428:63–66.

The proposed modeling framework can capture a wide range of failure modes of geological materials – from tensile fracture to shear fracture to plastic compaction – and their transition due to confining pressure and strain rate. This figure shows that it can also reproduce the transition of failure modes under tensile loading, from extension fracture (left) to hybrid fracture (middle) to shear fracture (right). This is consistent with the experimental finding of Ramsey and Chester (2004), Nature 428:63–66.