Mixed-mode fracture toughness of high strength FRC: a realistic experimental approach

Abstract

Unfortunately, fibrous composite materials' mixed-mode fracture toughness (Keff) was measured using inappropriate through-thickness cracked (TTC) specimens. The problem with such specimens is the ignorance of the fibers in the pre-notch surfaces, i.e., no fiber bridging behind the crack tip. In the present paper, a real Keff of fiber-reinforced concrete (FRC) was experimentally determined using matrix cracked (MC) specimens. Traditional (TTC) specimens were also adopted for comparison. The effect of fiber length (35 mm, 50 mm, and hybrid fibers, 50% from each length) and mode of mixity (Me), Me = 0, 1/4, and 1/2 were studied. Hooked end steel fibers of a volume fraction equal to 1% were used. All cracked beams with a crack-length-to-beam-depth ratio equal to 0.3 were tested under three-point bending in mode I and mixed-mode. The span/depth ratio was equal to two for all specimens. Since there is no equation to predict the Keff of MC specimens and the inapplicability of Griffith's theory to predict the Keff due to the difference in crack paths, new realistic procedures were suggested to overcome this dilemma. The results indicated that MC specimens recorded a lower crack initiation load than the peak load. In contrast, the crack initiation load coincides with the peak load in the case of TTC specimens. This reflected the role of steel fibers behind the crack tip in retarding the specimens to reach their ultimate capacity after crack initiation. Keff increased with increasing Me. Although long fibers recorded higher peak load and energy, their effect on Keff of MC FRC specimens was marginal due to the minor effect of fiber length on the crack initiation loads. The MC specimen is a realistic approach for estimating the Keff of FRC.