Abstract: To investigate the mechanical properties of shale under different strain rates, then to provide scientific guidance for the exploration of shale gas, the uniaxial compression experiments of shale samples have been conducted under strain rates of 5×10-4/s, 1×10-4/s, 1×10-5/s and 1×10-6/s, respectively. The experimental results show that the strain rates affect obviously on elastic modulus, peak strength and fracture morphologies, etc. of shale. Elastic modulus and peak strength gradually decrease along with the inclining of strain rate from high to low, but the decreasing speed becomes gradually smaller and smaller. The fitting relationship of elastic modulus (also peak strength) versus minus logarithm of strain rate largely adapts to the power function, whereas a higher fitting degree occurs if the mean values are implemented. Relatively speaking, the elastic modulus has a higher strain rate reaction. The fracture modes are obviously affected by the strain rate: under high strain rate, sample rapidly split into several large pieces and the entire skeleton ruptured; with the decreasing of strain rate, the fracture mode gradually transfers to main splitting failure with local shear failure; under the lowest strain rate, the main fracture mode is still splitting fracture but with many transverse cracks inserting the lateral surface; that is to say, the fracture mode under lowest strain is characterized by occurrence of cracks-net. In a word, the strain-rates affect the fracture morphologies most obviously, which performs mainly the relatively mean fracture and cracks net. Thus, these features of strain rate effect can provide important enlightenment for the parameters determination and hydro-fracture plan-design of shale.

Key words: shale, strain rate, mechanical properties, elastic modulus, peak strength, fracture morphologies, cracks net