The dynamic strength and fracture properties of dresser basalt

For improvement of drilling and fragmentation techniques, additional data is needed on the strength properties of hard rock under generalized stress conditions, dynamic loading rates and a range in temperatures. This paper describes the development of dynamic test techniques applicable to testing hard rock under high confining pressures, presents the results of an extensive series of tests on Dresser basalt, and correlates the strength data obtained with a failure criterion which incorporates both temperature and strain rate. All tests were performed in uniaxial compression or extension with the radial confining pressure varied from 0 to 100,000 psi, strain rates from 10−4 to 103 sec−1, and temperatures from 80 to 1400°K. The testing was accomplished with hydraulic loading facilities at the lower strain rates with a newly-developed Hopkinson pressure bar apparatus at the highest strain rates. The test results with Dresser basalt show a strong dependence of the ultimate or fracture strength on both temperature and rate of deformation. The combined temperature-rate dependence of the strength was such as to indicate that the fracture was controlled by a thermal activation process. This leads to formulation of a fracture criterion of the form σ1SC(O) + SC(O) − SBC(O)SC(O) SBC(O)σ2 − σ3ST(O)= 1 − βT(A − log ϵ where σ1, σ2 and λ3 are the principal stresses, T is absolute temperature and ϵdota is the strain rate. The constants SC(0), ST(0) and SBC(0) are suitably determined strength values at zero absolute temperature and β and A are constants involving the activation energy, volume and frequency. All of the experimental data obtained correlates well with this fracture criterion. The unconfined compressive strength was found to vary from 41,000 psi to 125,000 psi over the range in temperature and strain rate used in this study, with the higher strengths being obtained at low temperatures and high strain rates. The energy required to fracture basalt in compression is also the greatest at low temperatures and and at high strain rates.