Engineering ductile fracture in brittle-matrix composites

Abstract

This letter describes a newly discovered phenomenon: ductile fracture in cementitious materials. The brittle nature of materials such as concrete and ceramics has prompted the development of various approaches to enhance their fracture toughness. Fibre reinforcement has been the common focus in the toughening of the brittle materials. Two types of fracture have been observed to date in cementitious materials and ceramics: brittle and quasi-brittle fracture. Brittle fracture can be observed, for example, in hardened cement paste material. It is characterized by a very small microcrack zone at the crack tip (Fig. la), typically of submillimetre scale, low fracture energy of the order of 0.01 kJm -2, and a linear load versus load-point displacement curve from a fracture test. Quasi-brittle fracture can be observed in concrete and in most fibre-reinforced cements or concretes (FRC). It is characterized by a bridging process zone in addition to the small microcrack zone at the crack tip (Fig. lb). The bridging action provides additional energy absorption through aggregate and/or ligament bridging in concrete, and through fibre bridging in FRC, in the wake of the crack front. For quasi-brittle materials the fracture energy extends a large range, from 0.1 kJm -2 in concrete to several kJ m -z in the case of FRC. Correspondingly, the process zone size extends from the mm scale to the cm scale. The load versus load-line displacement curve from a fracture test may involve a small non-linear region near the peak load, with a significant post-peak tension-softening behaviour. Research into quasi-brittle materials in the past decade has been extensive (see, for example [1, 2] for a recent collection of papers on the subject). It must be mentioned that the classification of the fracture characteristics for cementitious materials described above may also be applicable to current monolithic and fibre-reinforced ceramics.