The relation between failure modes of the specimens of
rock types under Brazilian test conditions and corresponding
Brazilian tensile strengths is presented in Fig. 15.
Central and central multiple fracture types are common
modes of failure in granite and sandstone specimens across
the entire range of Brazilian tensile strength; central multiple
type is the dominant failure mode in the case of high
tensile strength (Fig. 15a, c). This implies that central
multiple type of fracture patterns develop in order to
release the high stored strain energy for both granite and
sandstone where the capacity for sustaining strain energy is
different for these two rocks. In the case of schists, layer
activation is the principal failure mode across the complete
range of Brazilian tensile strength (Fig. 15b). In other
words, compression-induced tensile stresses exploit the
foliations for the strain energy to be released earlier than
they can do so by forming central or non-central fractures.
Understandably, when such exploitation is not possible, the
specimens fail in pure central or non-central mode resulting
in high tensile strength.