Mechanical Behavior of Shale at Different Strain Rates

Abstract

The strain rate-dependent mechanical behavior of shale is characterized using triaxial compression tests under a constant confining pressure of 50 MPa and axial strain rates $\dot{\varepsilon }_{1}$ ranging from 5 × 10⁻⁶ s⁻¹ to 1 × 10⁻³ s⁻¹. This study is conducted on the Longmaxi shale from Dayou in China, which is predominantly composed of brittle minerals including quartz (55%), albite (15%) and cristobalite (3%). The experimental results show that higher axial loading strain rates $\dot{\varepsilon }_{1}$ lead to higher elastic modulus and higher peak shear strength, both following exponential relationships with $\dot{\varepsilon }_{1}$. When $\dot{\varepsilon }_{1} \le 1 \times 10^{ - 5} {\text{s}}^{ - 1}$, failure results in a single linear fracture, whereas a more complex multiple crisscrossing fracture network is formed when $\dot{\varepsilon }_{1} \ge 1 \times 10^{ - 4} {\text{s}}^{ - 1}$. Failure in shale specimens can be described by a damage parameter $D$, which is strongly affected by the axial strain $\varepsilon_{{1{\text{s}}}}$. In addition, the strain rate $\dot{\varepsilon }_{1}$ had different effects on $D$, which also depends on axial strain $\varepsilon_{{1{\text{s}}}}$. Energy accumulation and dissipation are also closely related to $\dot{\varepsilon }_{1}$ with the total absorbed energy $U_{\text{A}}$, the recoverable elastic strain energy $U_{\text{A}}^{\text{e}}$ and the dissipated energy $U_{\text{A}}^{\text{d}}$ at the peak stress increasing with $\dot{\varepsilon }_{1}$. As for the total energy accumulation $U_{\text{A}}$, the recoverable elastic energy $U_{\text{A}}^{\text{e}}$ decreases while the dissipated energy $U_{\text{A}}^{\text{d}}$ increases with increasing strain rate.