Measuring fracture toughness of crystalline marbles under modes I and II and mixed mode I–II loading conditions using CCNBD and HCCD specimens

Fracture toughness of three crystalline Calcite marbles, which are different only in grain size and distribution, is determined under modes I and II and mixed mode I–II loading conditions using Cracked Chevron-Notched Brazilian Disc (CCNBD) and Hollow Centre Cracked Disc (HCCD) specimens. The results show that mode I fracture toughness ( K I C ) is correlated negatively with grain size. For each marble, HCCD yields lower values of fracture toughness, compared with CCNBD. This difference is negligible under mode I loading condition; while it becomes larger as loading condition transits from mode I to mode II. Measured values of P-wave velocity ( V P ), Brazilian tensile strength ( σ tB ) and Schmidt hammer hardness are in direct relation with K I C of the marbles. The obtained results are compared with three fracture criteria, in which the Minimum Strain Energy Density (MSED) criterion has provided better correlations with different critical combinations of modes I and II Stress Intensity Factors. ► In this study we examined the effect of grain size distribution on rock fracture toughness. ► We measured all fracture toughness modes for three crystalline marbles. ► We compare other rock characteristics with fracture toughness. ► Compared to MTS and MERR criteria, MSED criterion provided better correlations with the obtained results.