Experimental Study on Effects of Loading Rate and Sample Size on the Mechanical and Failure Characteristics of Mudstone

In this study, to explore the effects of the loading rate and sample sizes on the mechanical properties of rock, uniaxial compression experiments are conducted for mudstone with two different sample sizes and loading rates to obtain its mechanical properties and failure characteristics. Results show that mechanical parameters such as the compressive strength and elastic modulus obtained at a loading rate of 0.01 mm/s are higher than those obtained at a loading rate of 0.001 mm/s. Moreover, the mechanical parameters of rock samples in group A (Φ50 × 100 mm2) are higher than those in group B (Φ25 × 50 mm2) at the loading rate of 0.01 mm/s, whereas it is the opposite at the 0.001 mm/s loading rate. At the same loading rate, the strain of rock samples in group A is slightly higher than that of group B in the fracture compaction stage, while the difference is trivial for rock samples with the same size under different loading rates. The dispersion degree of the stress–strain curve under the 0.001 mm/s loading rate is greater than that under the 0.01 mm/s loading rate for both groups, and the local failure characteristics can be observed from the pre-peak curve. A positive and logarithmic relationship is observed between the pre-peak strain energy and compressive strength. Under the 0.01 mm/s loading rate, the rock samples show only shear failure, whereas the mixed shear-splitting failure is observed under the 0.001 mm/s loading rate with extensive fracture development and multiple failure surfaces, indicating that the internal energy release is more stable and completed.