Fatigue Characteristics of Aligned Fiber Reinforced Mortar

It has been suggested that the reinforcing fiber size and the spacing between the reinforcing fibers play a dominant role in determining the fracture properties of cementitious composite materials. In the present study, 48 specimens were tested under fatigue loading to investigate the effect of fiber size and the corresponding spacing between the reinforcing fibers on the fracture behavior of mortar composites. The specimen dimensions of 125 × 600 × 25 mm (5 × 24 × 1 in.) and the volume fraction of the reinforcement were kept constant at 0.04 while the cross-sectional area of the reinforcing fibers was varied at 0.79, 1.77, and 3.14 mm² (0.00122, 0.00274, and 0.00487 in.²) with a corresponding spacing between the fibers of 37.5, 15, and 5.8 mm (1.5, 0.6, and 0.23 in.) and a corresponding number of fibers of 3, 6, and 14. The tension-tension flexural fatigue testing was performed at stress levels of 80%, 65%, and 50% of the ultimate flexural strength of similar specimens tested under static loading conditions. The fatigue testing was performed at a constant frequency of 2 Hz. The crack initiated simultaneously as the loading cycles ensued at 80% and 65% stress levels; however, a large scatter in the number of cycles to failure (N_f) was observed. Number of cycles to failure ranged from 240 to 3,069 at the 80% stress level and from 10,165 to 35,070 at the 65% stress level. At the 50% stress level, none of the specimens failed or had appreciable crack growth for up to 500,000 cycles. As in the static case, fiber size and spacing appear to have an influence on the fatigue crack growth behavior; however, as is typical of concrete fatigue testing, the large scatter in the fatigue crack growth data engulfed any apparent trend. The Weibull probability distribution function compared well with the experimental data. A statistical model based on the inclusion model and the Weibull distribution was proposed to account for the fiber spacing effect.