Crack initiation and damage thresholds in some brittle metamorphic rock types

In this study 60 unconfined compression laboratory tests performed on three brittle metamorphic rock types were analysed in order to evaluate the stress thresholds associated with the onset of stable and unstable microcracking which are known in literature as the "crack initiation stress" and the "crack damage stress", respectively. The crack initiation stress was identified applying the lateral strain response (LSR) methodology while the crack damage stress was detected where total volumetric strain reversal occurs. Although the mineralogical and textural features were quite similar among the investigated rock types, the results of this study showed that the crack initiation and damage stress values were affected by variability. However, when the normalized stress values sci/sUCS and scd/sUCS were considered, the scatter in results decrease consistently suggesting that the stable and unstable crack growth begin at similar stress levels in the investigated rock types.