Dynamic Mohr-Coulomb and Hoek -Brown strength criteria of sandstone at high in rates

Among the available strength criteria used in the analysis of rock mechanics, Mohr-Coulomb and Hoek-Brown strength criteria are commonly used. Unfortunately, validating the applicability and reliability of the above criteria have been a great challenge. This can be attributed to the assumption that the parameters in the strength criteria are constant under static loading. In the recently published literature, rocks are equally subjected to dynamic loads that exhibit different mechanical response from that of static loads. Therefore, under dynamic rock failure, the above strength criteria with constant parameter under static loading do not take into consideration the strain rate effects and thus are unsuitable for the rock mechanics analysis. As such development of alternative dynamic strength criteria for rocks taking into account the rate effects are highly desirable.

Up to now, various researchers have performed a series of dynamic uniaxial compression tests on various materials including granite and marble under different strain rates and confining pressures. For instance, dynamic M-C strength criterion showed good applicability in low confining pressure conditions while H-B strength criterion exhibited good applicability in both low and high confining pressure conditions. However, determining rock dynamic strength criterion at high strain rates is still a challenge. Presently, the split Hopkinson pressure bar (SHPB) devise is used to evaluate rock strength at high strain rates. This has pioneered the establishment of an M-C strength criterion taking into consideration the loading rate and the uniaxial compression strength. As such, recent studies have shown the importance of dynamic uniaxial and triaxial compressive strength of rocks under various confining pressures and high strain rates in developing the dynamic strength criterion.

To this note, Central South University researchers: Xuefeng Si (PhD candidate), Associate professor Dr. Fengqiang Gong, Professor Xibing Li, and Song Luo (PhD candidate) together with Professor Shanyong Wang at the University of Newcastle in Australia investigated the dynamic M-C and H-B strength criteria under high strain rates and low confining pressure. Fundamentally, the triaxial compression tests and the dynamic uniaxial of sandstone were conducted with a modified triaxial SHPB and a conventional SHPB system respectively. The experimental data were analyzed under varying strain rates and confining pressure ranging from 40-160s^-1 and 5.0-15.0MPa respectively. The work is published in International Journal of Rock Mechanics and Mining Sciences.

The authors observed that the strain rate logarithm exhibited a positive and negative linear correlation with the cohesion and internal friction angle respectively. However, change in the cohesion was more dominant than the internal friction. Additionally, the rock parameter m, internal friction, and cohesion changed with an increase in the strain rate because they were not constant. To validate the study, the established expression of the dynamic M-C and H-B strength criteria was used to determine the strength of the sandstone. Interestingly, it proved effective for applications at both high strain rates and low confining pressure. This was also justified by the similarities in the theoretical and actual experimental strengths. Altogether, the study can be extended for analysis of various types of rocks.