Crack initiation stress in low porosity crystalline and sedimentary rocks

The stress–strain data from 376 laboratory tests carried out on samples of igneous, sedimentary and metamorphic rocks were analyzed to establish the onset of microcracking in compression, referred to as the Crack Initiation (CI) stress. A statistical approach was used to find the geological parameters influencing crack initiation stress. Among various rock properties such as grain size and mineralogy, the proportion of the hardest constituent mineral were found to correlate with CI stress. Foliation induced anisotropy was found to affect the peak strength but its effect on CI stress was less pronounced. The CI stress to peak stress ratio ranged from 0.42 to 0.47 regardless of the material properties in uniaxial compression whereas this ratio ranged from 0.50 to 0.54 when confined. The crack initiation parameters for the Hoek–Brown spalling criterion for igneous rocks can be expressed in terms of the CI stress ratio and the tensile strength. A comparison of tensile strength from the Brazilian and Direct tension tests showed that the Direct tensile strength was approximately 0.77 of the Brazilian tensile strength.