ABSTRACT
The measured strength of brittle solids, e.g., glass, ceramics, cement-base materials, etc., depends on the size of existing crack-like defect and three material properties: modulus of elasticity, E, Poisson's ratio, v, and the surface free energy, g, of the new crack surfaces (1). The Griffith's concept based on an energy balance conditions for catastrophic fracture resulted in expression relating these values. In the energy balance equation the surface energy has been defined by the energy per unit surface required to cut an infinite body and separate it into halves. However, it has long been known that for many brittle materials the apparent surface energy required to
propagate a crack greatly exceed the surface free energy, because the fracture process of real solids including even the "most brittle" crystals and glasses are actually accompanied by "irreversible" processes at the crack tip, which may substantially increase the work for creating new surfaces, as compared with the intrinsic surface energy (2,3). This is the reason why the more universal approach based on an concept of the critical stress intensity factor at the crack tip widely used in materials research and engineering (4). However, the concept of the critical stress intensity factor is limited by the crack-tip situation at the moment of some critical state of crack, e.g. crack initiation, and cannot describe how the situation changes during the crack propagation process. The irreversible
processes may be developed progressively with crack extension resulting in so-called increased R-curve of crack propagation resistance, where R is an energy characteristic of the crack propagation resistance. Hence, the critical stress intensity factor cannot be considered as the only one fracture resistance parameter to describe whole fracture process in nonline(\r solids. As a result, a concept of the work-of-fracture has been introduced to characterize the fracture resistance, in addition to the common fracture mechanics criteria. The aim of this review is to discuss the principles of the work-of-fracture approach, the techniques of determination, and to give some examples of usefulness of the work-of-fracture measurements in research, development and engineering with ceramics and ceramic-base composites.
