The effect of specimen shape and strain rate on uniaxial compressive behavior of rock material

The mechanical properties of rock material with length/diameter ratios varying from 1.0 to 3.0 were determined using a newly developed servo-hydraulic machine at a wide range of strain rates. The uniaxial compressive strength, initiation and dilatancy stresses, peak axial strain, and strain energy gradually decreased with increasing length/diameter ratios at the same loading condition; for the same length/diameter ratio, these properties increased with increasing strain rate. The elastic modulus increased with increasing specimen shape and strain rate, whereas the Poisson’s ratio was independent on these two factors. The fracture modes were significantly dependent on both strain rate and specimen shape. When the strain rate was below 10⁻³ s⁻¹, splitting was the main fracture mode for the short specimens while the shearing fracture mode dominates the longer specimens; when the strain rate was above 10⁻³ s⁻¹, the fracture mode changed directly from cone-shaped fractures to shearing fractures. The recommended length/diameter ratio was 2.5 at strain rates of 10⁻⁵–10⁻² s⁻¹.