Experimental R-Curves for Assessment of Toughening in Fiber Reinforced Cementitious Composites

Procedures to obtain the experimental R-Curves using a compliance calibration technique are examined. R-Curves provide a convenient means to study the process of fracture and the brittle-ductile transition in materials. Single edge notched beam specimens are tested under closed loop rack mouth opening control. The procedure to obtain the R-Curves using loading/unloading compliance and the residual displacements are discussed. An elastically equivalent toughness K(superscript)R as a function of crack extension is defined to compare the R-Curves with the available data in the literature. The developed test method is applied to fiber reinforced concrete (FRC) composites with up to 8 percent by volume of short, chopped alumina, carbon, and polypropylene (PP) fibers. Significant strengthening of the matrix due to the addition of short carbon and alumina fibers are observed. R-Curves in these composites are characterized by an increase in the steady state fracture toughness. In PP-FRC composites, energy dissipation due to fiber pullout increases the ascending rate of the R-Curve well after the main crack has formed. The work of fracture is calculated from the cyclic loading/unloading tests and the results are compared with the R-Curves.