Efficient quasi-brittle fracture simulations of concrete at mesoscale using micro CT images and a localizing gradient damage model

In this work, a localizing gradient damage model (LGDM) based on the generalized micromorphic theory is adopted to investigate the quasi-brittle fracture behaviour of concrete at mesoscale for the first time. Micro Computed Tomography (CT) image-based realistic concrete models are generated including aggregates, mortar, pores, and aggregate-mortar interfacial transition zones (ITZs). In the LGDM, an additional micro-force equilibrium equation is formulated with a damage-dependent interaction function to prevent spurious energetic interactions during the softening stage. The damage driving forces are evaluated as history variables through the maximum nonlocal equivalent strains at the integration points. The stiffness matrices and residual vectors for displacement and nonlocal equivalent strain fields are formulated through the conventional shape function and strain matrices of finite elements, by using the user-defined element (UEL) subroutine implemented with the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm to improve numerical performance. Typical nonlinear fracture benchmarks are presented to investigate the variability of mesoscale fracture evolution, crack trajectories, and load-displacement curves as well as numerical performance. It is found that the LGDM significantly improves the computational efficiency over the conventional phase-field cohesive zone model (PFCZM) with 60% savings of CPU time, and holds great potential for multiscale fracture evaluation of quasi-brittle composite materials, through the high-fidelity representation of real materials' microstructures, and flexible simulation of the complicated nonlinear fracture without the need for re-meshing or mesh enrichments.(c) 2022 Elsevier B.V. All rights reserved.