The present paper is concerned with studies on initiation and propagation of brittle fracture in rock-like materials under compression. In the first place, theoretical conditions for initiation of fracture are derived from a single open or closed crack inclined arbitrarily to the major principal axis. As the generalization of the Griffith theory the criterion for initiation of fracture from a non-flat elliptical crack is also briefly mentioned.
In the second place, the results of experimental studies are reported on the initiation and propagation of fracture in uniaxial and biaxial compression from a single slit contained in a polymethylmethacrylate plate, in flyash cement paste and in neat cement paste, and also on the initiation and propagation of fracture from a single inclusion made with a sheet of vinylchloride or steel in flyash cement paste.
In the third place, the initiation and propagation of fracture in uniaxial compression in flyash cement paste with multiple slits and/or inclusions are described.
The results indicate that the generalized Griffith criterion is not adequate to describe the initiation of fracture from a single open slit, although it explains that from a single soft inclusion.
The modified Griffith criterion may also be applicable to the latter. Cracks were observed to develop even from hard inclusions. In the model containing multiple flaws, crack is initiated on the boundary of the most vulnerably sized and orientated flaw as if it were a single flaw. At the stage of propagation of this crack, a new crack takes place at the point of the less vulnerable flaw. As the load increases, a large number of cracks are initiated in the similar way, and they so formed propagate over a short distance and align themselves with the major principal axis. In the course of the complex process of fracture propagation, unstabilized zones are locally formed as the result of the intermeshed network of cracks. Collapses or macroscopic fractures occur as an integrated instability over the entire specimen.
It may be said in conclusion that initiation and propagation of fracture in real rock materials under compression are characterized by the phenomena that are similar to what has been described above.
