Modeling of fiber-reinforced cement composites: Discrete representation of fiber pullout

The discrete modeling of individual fibers in cement-based materials provides several advantages, including the ability to simulate the effects of fiber dispersion on pre- and post-cracking composite performance. Recent efforts in this direction have sought a balance between accurate representation of fiber behavior and computational expense. This paper describes a computationally efficient approach to representing individual fibers, and their composite behavior, within lattice models of cement-based materials. Distinguishing features of this semi-discrete approach include: (1) fibers can be positioned freely in the computational domain, irrespective of the background lattice representing the matrix phase; (2) the pre- and post-cracking actions of the fibers are simulated with little computational expense, since the number of system degrees of freedom is independent of fiber count. Simulated pullouts of single fibers are compared with theory and test results for the cases of perfectly-plastic and slip-hardening behavior of the fiber-matrix interface. To achieve objective results with respect to discretization of the matrix, pullout forces are distributed along the embedded lengths of fibers that bridge a developing crack. This is in contrast to models that lump the pullout force at the crack surfaces, which can lead to spurious break-off of matrix particles as the discretization of the matrix is refined. With respect to fracture in multi-fiber composites, the proposed model matches theoretical predictions of post-cracking strength and pullout displacement corresponding to the load-free condition. The work presented herein is a significant step toward the modeling of strain-hardening composites that exhibit multiple cracking.