Finite Element Simulations of Fiber Pullout Toughening in Fiber Reinforced Cement Based Composites

The role of fibers in fiber reinforced cement based composites was studied by means of finite element method. The study was conducted in two steps. The first step simulated the fiber pullout from a cementitious matrix and resulted in pullout force vs. slip displacement response. The pullout response was used in the second step as the bridging pressure applied over the crack length in composite specimen. The contribution of fiber’s closing pressure was quantitatively measured through calculation of the J-integral and the effective stress intensity factor. The interfacial zone was characterized as a third phase with a lower stiffness and strength as compared to matrix and fiber. The debonding criterion was based on a yield surface defined by normal and shear strength of the interface. After debonding, Coulomb friction was introduced in the debonded zone. Effects of interfacial adhesional strength, clamping pressure, and fiber length on the fiber pullout response were studied. In the composite response simulations, the fibers across a prescribed crack length were modeled as nonlinear spring elements. The pullout force vs. slip displacement was used for the stiffness of the spring elements. J-integral was evaluated for the two cases of with and without fibers, and the difference between the two was used as the toughening contribution of fibers. The fiber toughening effect was studied for different fiber lengths and interface parameters. Results were compared with analytical simulations of crack growth using R-curves and a simplified approach based on linear crack opening–closing pressure relationship.