  The Brazilian test has been extremely popular while prompting significant
debate. The main source of controversy is rooted in its indirect nature; the
material tensile strength is inferred upon assuming that cracking initiates at
the centre of the sample. Here, we use the Griffith criterion and finite
element analysis to map the conditions (jaws geometry and material properties)
that result in the nucleation of a centre crack. Unlike previous studies, we do
not restrict ourselves to evaluating the stress state at the disk centre; the
failure envelope of the generalised Griffith criterion is used to establish the
crack nucleation location. We find that the range of conditions where the
Brazilian test is valid is much narrower than previously assumed, with current
practices and standards being inappropriate for a wide range of rock-like
materials. The results obtained are used to develop a protocol that
experimentalists can follow to obtain a valid estimate of the material tensile
strength. This is showcased with specific case studies and examples of valid
and invalid tests from the literature. Furthermore, the uptake of this protocol
is facilitated by providing a MATLAB App that determines the validity of the
experiment for arbitrary test conditions.
