3.2.1.6 Chevron Bend Test
The test resembles a three-point bend test with a V shape or notch cut perpendicular to the axis of the specimen. The notch is positioned directly below the loading point. The test can be Level I consisting of just the yield load, and Level II which requires continuous load and displacement results. Care is taken to observe any anisotropic characteristics and hence any planes of weakness are positioned parallel or perpendicular to the loading axis. Mode I fracture toughness (KIC) can be estimated using the relationships given in Equations (1) and (2) below:-
(1) 
(2) 
Where :
KIC = Mode I fracture toughness. MN/M^1.5
Amin = Dimensionless factor.
Fmax = Maximum Load KN
D = Core diameter cm
a0 = Chevron tip distance from specimen (0.15D)
S = Distance between support points (0.33D)
The ASTM has standards laid down for similar tests that are conducted on concrete samples.
A test based on a similar principle is also given by the ISRM and this ’short rod test’ comprises the same notch, but the use of smaller lengths of sample is possible. Ouchterlony and Zongqi [1983] investigated the short rod test and its accuracy of determining the KIC of rock samples. It was concluded that loading rate played no significant role in the results and that the method gives good results.
Bearman et al [1989] investigated the comminution potential or rock using the chevron bend test and worked towards a ‘comminution index’. The standards set down by the ISRM were adhered to. It was concluded that the only loading test to come close to the performance of the CB test was the UCS test.