3.2.1.3 Brazilian Test
The Brazilian test is one that compresses a sample diametrically inducing a stress that causes the sample to yield in tension. The standard procedure can be found in the ISRM [Brown (Ed), 1981] and a similar method is recommended by the ASTM. The relationship between applied forces and yield loads is given by:-
Where P is the yield load in Newtons, D is the disc diameter in mm and t is the thickness of the disc in mm.
Authors have investigated the method both practically and theoretically and have found the two to be comparable. The method of breakage is well documented and has had the term ‘hourglass’ associated with it. Figure 3.3.2.2 shows the typical fracture patterns that are observed in samples.
Figure 3.3.2.2 Typical Brazilian Test Fracture Pattern
Clark [1993] found that the crack initiates in the centre of the specimen and that stress concentrations built up around the loading platens effectively hindering crack propagation in that area.
Fairhurst [1964] and Wijk [1978] looked at the validity of the method from a theoretical perspective and favoured this method to that of the point-load test.
Gunsallus and Kulhawy [1984] conducted an extensive study into the test and compared it to other methods including the UCS, the point load test and KIC. It was concluded that the Brazilian method was more variable in its results than that of the KIC test, was similar in variation to the UCS, but had less variation than the point-load test.
Berry et al [1984] also compared the Brazilian to the UCS method for a range of materials to investigate the effect of rock morphology on comminution behaviour. They found the Brazilian test to give higher positive correlation values (r = 0.93) than that of the point load test