The two chevron notched rock fracture specimens, CB and SR, recommended by the ISRM
to determine rock Mode I fracture toughness have several disadvantages, such as low loads
required to initiate failure, a large amount of intact rock core is required, complex loading
fixture, difficult sample preparation for SR. The Cracked Chevron Notched Brazilian Disc
(CCNBD, Fig. 1.c) and the Cracked Straight Through Brazilian Disc (CSTBD, Fig.l.b)
specimen geometries overcome these problems and are suitable for mixed fracture mode testing.
The general case for the cracked Brazilian disc problem is when the sample is loaded
diametrically with the crack inclined at an angle 0 to the loading direction, (Fig. 1.a).
Different combinations of mode I and mode II fracture intensities can be obtained simply by
changing this angle. For the special case when 0 =0° the problem is reduced to the mode I
Former research has only solved the mixed mode CSTBD problem for cases with crack length
á(a/R)<0.6 . However á=0.65-0.80 is more appropriate for practical applications .
For the CCNBD specimen, the valid geometrical .range should be given so that users can have
the choice to select their own geometry according to the availability of rock samples.